Thin-wall injection molding

Thin wall injection molding is a specialized form of conventional injection molding. The thin wall injection molding process focuses on mass producing plastic parts that are thin and light so that material cost savings can be made and cycle times can be as short as possible. Shorter cycle times means higher productivity and lower costs per part.

The definition of thin wall is really about the size of the part compared to its wall thickness. For any particular plastic part, as the wall thickness reduces the harder it is to manufacture using the injection molding process. The size of a part puts a limit on how thin the wall thickness can be. For packaging containers thin wall means wall thicknesses that are less than 0.025 inch (0.62mm) with a flow length to wall thickness greater than 200.

Processing

Standard tool and melt temperature can also be applied when dealing with thin-walled parts. In order to reduce the filling pressure, however, it is usually recommended to increase melt residence time in the barrel, this can lead to critical material reduction. In order to avoid freezing effects during the filling process, the injection time is rather short. In case of cellular phone covers, standard injection times are less than 0.5 secs.

Markets

The trend towards thin wall molding continues to increase in many plastic industries as plastic material and energy costs continue to rise and delivery lead times are squeezed. 

The following industries make use of thin wall molding:

• food packaging ( eg. food containers and lids)
• automotive (eg. both structural and non-structural car parts)
• mobile telecommunications (eg. mobile phone housings)
• medical (eg. syringes) 
• computing equipment (eg. computer housings)

Benefits

• Cheap, safe and clean plastic parts.
• Thin wall molding reduces resource consumption and cuts weight, reducing fuel usage and carbon emissions in shipping – further supporting sustainability efforts.
• Allows faster cycle times compared with thicker walled plastic parts. This is good for injection molders because it reduces their delivery lead time and cost per part.
• Lighter parts reduce fuel emissions in automotive applications.
• Made from recyclable plastics such as polypropylene (PP) in food packaging.
• Some thin wall parts can be made from sustainable plastics.

Disadvantages

• Environmental litter.
• High capital investment cost for injection molders. Thin wall molding requires specialized molding machines, injection molds and robots that can withstand the high stresses, fast cycle times and relentless 24/7 production schedules.
• To make thin wall parts requires highly skilled molding technicians and these are difficult to find and keep.

Examples

Plastic resins suitable for thin-wall molding should have high-flow properties, particularly low melt viscosity. In addition, they need to be robust enough to avoid degradation from the heat generated by high shear rates (high injection speeds)

Some plastic manufacturers make plastics specifically for thin wall applications which have excellent flow properties inside the mould cavity. For example, plastic manufacturer Sabic, has a polypropylene food contact grade plastic which is specifically designed for thin wall margarine containers and lids.

Another plastic manufacturer Bayer, make a blend of Polycarbonate (PC) and Acrylonitrile butadiene styrene (ABS) specifically designed to make thin wall mobile housings.

Equipment

• Plastic Injection Molding Machine.

Compared to conventional injection molding, thin wall molding requires molding machines that are designed and built to withstand higher stresses and injection pressures. The molding machines computer control should also be precise in order to make quality parts. For this reason these molding machines are more expensive than general purpose machines.

Thin-wall-capable machines usually also have accumulator-assisted clamps to accommodate fast cycle times.

Regular maintenance schedules must be completed so that the machine and part quality does not suffer. These machines usually work 24/7 so they need to be well maintained. 

• Injection Mold Design

As with the injection molding machines, injection molds need to be robust enough to withstand high stresses and pressures. Heavy mold construction with through hardened tool steels will ensure a long lasting mold.

The mold must also have a well designed cooling system so that heat can be quickly extracted from the hot plastic part allowing fast cycle times. To achieve this, cooling channels need to be designed close to the molding surface. Cleaning the mould on a daily basis is also a critical requirement to maintain the part quality.

                                    

STANDARD VS. THIN-WALL PROCESSING
Key Factors	Conventional	Thin-Wall
Typical Wall, in.	0.080-0.120	0.050-0.080	<0.050
Machinery	Standard	High-end	Custom
Inject. Pressure, psi	9000-14,000	16,000-20,000	20,000-35,000
Hydraulic System	Standard	Standard	Accumulators on injection & clamp units. Servo valves.
Control System	Standard	Closed-loop on injection speed, hold pressure, decompression speed, screw rpm, backpressure, and all temperatures.	Same as at left, with resolution of 0.40 in. on speed, 14.5 psi on pressure, 0.004 in. on position, 0.01 sec on time, 1 rpm on rotation, 0.10 ton on clamp force, 2° F on temperature.
Processing
Fill Time, sec	>2	1-2	0.1-1
Cycle Time, sec	40-60	20-40	6-20
Tooling	Standard	Better venting, heavier construction, more ejector pins, better polish	Extreme venting, very heavy construction, mold interlocks, precise surface preparation, extensive ejection features, mold costs 30-40% higher than standard.

Resin Transfer Molding

Principle:

RTM is similar to the traditional transfer molding or reaction injection molding (RIM) with a difference that a reinforcement is molded with the resin. The physical arrangement and type of equipment used in RTM are like RIM.

Material Used:

Thermoset        : Unsaturated polyster resin, Epoxy resin, phenolic resin, polyurethane resin, silicone, alkyd,                

                         DAP, thermoset polyimides.

Thermoplastic   : Nylon, PP, PE, styrenic resin, thermoplastic polyster, flouropolymer, liquid crystal polymers                 

                         and thermoplastic elastomers

Additives Used:

Fillers

Flame retardants
Conductive additives

Catalyst

Accelerator 

U V stabilizers

Release agents-

Reinforcing fibre -Glass fibres in continuous rovings, yarn form and chopped strand mat

They are used as perform which is an arrangement of fibres configured to replicate the shape finished part.

 Process Description:

In the resin Transfer Molding(RTM) process, dry (i.e., unim-pregnated) reinforcement is pre-shaped and oriented into a skeleton of the actual part known as the preform, which is inserted into a matched die mold. The mold is then closed, and low-viscosity thermoset resin is injected into the tool. During this time, the resin "wet out" the fibres and the air is displaced and escapes from vent ports placed at the high point. Heat is applied to the mold to activate the polymerization that solidifies the resin. The resin cure beings during filling and continues after the filling process. Once the part develop sufficient strength, it is removed or de molded.

Process Characteristics:

• Part cost is moderate to high.
• Tooling Cost is low.
• Production Rate is low.
• Part strength is high.
• Parts are easily painted.

Trouble shooting:

Problem	Possible cause
Fibres not fully wetted	Resin viscosity too high, Improper mix ratio of filler
Poor or inadequate cure	Improper temperature Apply proper vacuum to the mold Improper accelerator content Initiator is added to compensate shortage of inhibitor
Part delaminates	Poor resin movement due to high viscosity

Lost Core Molding Process

lost core injection molding, also known as Fusible core injection molding, is a specialized plastic injection molding process used to mold internal cavities or undercuts that are not possible to mold with demoldable cores. Strictly speaking the term "fusible core injection molding refers to the use of a fusible alloy as the core material; when the core material is made from a soluble plastic the process is known as soluble core injection molding. This process is often used for automotive parts, such as intake manifolds andbrake housings, however it is also used for aerospace parts, plumbing parts, bicycle wheels, and footwear.

The most common molding materials are glass-filled nylon 6 and nylon 66. Other materials include unfilled nylons, polyphenylene sulfide, glass-filled polyaryletherketone (PAEK), glass-filled polypropylene (PP), rigid thermoplastic urethane, and elastomericthermoplastic polyurethane.

Process:

The process consists of three major steps: casting or molding a core, inserting the core into the mold and shooting the mold, and finally removing the molding and melting out the core.

Core

First, a core is molded or die cast in the shape of the cavity specified for the molded component. It can be made from a low melting point metal, such as a tin-bismuth alloy, or a polymer, such as a soluble acrylate. The polymer has approximately the same melting temperature as the alloy, 275 °F (135 °C), however the alloy ratios can be modified to alter the melting point. Another advantage to using a metal core is that multiple smaller cores can be cast with mating plugs and holes so they can be assembled into a final large core.

One key in casting metal cores is to make sure they do not contain any porosity as it will induce flaws into the molded part. In order to minimize porosity the metal may be gravity cast or the molding cavity may be pressurized. Another system slowly rocks the casting dies as the molding cavity fills to "shake" the air bubbles out.

The metal cores can be made from a number of low melting point alloys, with the most common being a mixture of 58% bismuth and 42% tin, which is used for molding nylon 66. One of the main reasons its used is because it expands as it cools which packs the mold well. Other alloys include tin-lead-silver alloys and tin-lead-antimony alloys. Between these three alloy groups a melting point between 98 and 800 °F (37–425 °C) can be achieved.

Polymer cores are not as common as metal cores and are usually only used for moldings that require simple internal surface details. They are usually 0.125 to 0.25 in (3.2 to 6.3 mm) thick hollow cross-sections that are molded in two halves and are ultrasonically welded together. Their greatest advantage is that they can be molded in traditional injection molding machines that the company already has instead of investing into new die casting equipment and learning how to use it. Because of this polymer core materials are most adventitious for small production runs that cannot justify the added expense of metal cores. Unfortunately it is not as recyclable as the metal alloys used in cores, because 10% new material must be added with the recycled material.

Molding

In the second step, the core is then inserted into the mold. For simple molds this is as simple as inserting the core and closing the dies. However, more complex tools require multiple steps from the programmed robot. For instance, some complex tools can have multiple conventional side pulls that mate with the core to add rigidity to the core and reduce the core mass. After the core is loaded and the press closed the plastic is shot.

Melt-Out

In the final step, the molded component and core are both demolded and the core is melted-out from the molding. This is done in a hot bath, via induction heating, or through a combination of the two. Hot baths usually use a tub filled with glycol or Lutron, which is aphenol-based liquid. The bath temperature is slightly higher than that of the core alloy’s melting point, but not so high that it damages the molding. In typical commercial applications the parts are dipped into the hot bath via an overhead conveyor. The advantage to using a hot bath is that it is simpler than induction heating and it helps cure thermoset moldings. The disadvantage is that it is uneconomically slow at a cycle time of 60 to 90 minutes and it poses environmental cleanup issues. Typically a the hot bath solution needs cleaning or replacement every year or every half year when used in combination with induction heating.

For thermoplastic moldings induction heating of the core metal is required, otherwise the prolonged heat from a hot bath can warp it. Induction heating reduces the melt-out time to one to three minutes. The disadvantage is that induction heating does not remove all of the core material so it must then be finished off in a hot bath or be brushed out. Another disadvantage is that the induction coils must be custom built for each molding because the coils must be 1 to 4 in (25 to 100 mm) from the part. Finally, induction heating systems cannot be used with moldings that have brass or steel inserts because the induction heating process can destroy or oxidize the insert.

For complex parts it can be difficult to get all of the core liquid to drain out in either melt-out process. In order to overcome this the parts may be rotated for up to an hour. Liquid core metal collects on the bottom of the heated bath and is usable for a new core.

Equipment

Traditional horizontal injection molding machines have been used since the mid-1980s, however loading and unloading 100 to 200 lb (45 to 91 kg) cores are difficult so two robots are required. Moreover, the cycle time is quite long, approximately 28 seconds. These problem are overcome by using rotary or shuttle action injection molding machines. These types of machines only require one robot to load and unload cores and have a 30% shorter cycle time. However, these types of machines cost approximately 35% more than horizontal machines, require more space, and require two bottom molds (because one is in the machine during the cycle and the other is being unloaded and loaded with a new core), which adds approximately 40% to the tooling cost. For small parts, horizontal injection molding machines are still used, because the core does not weigh enough to justify the use of a rotary machine.

For four-cylinder manifolds a 500-ton press is required; for a six- to eight-cylinder manifold a 600- to 800-ton press is required.

Advantages and Disadvantages

The greatest advantage of this process is its ability to produce single-piece injection moldings with highly complex interior geometries without secondary operations. Similarly shaped objects are usually made from aluminium castings, which can weigh 45% to 75% more than a comparable molding. The tooling also lasts longer than metal casting tooling due to the lack of chemical corrosion and wear. Other advantages include:

• Very good surface quality with no weak areas due to joints or welds
• High dimensional accuracy and structural integrity
• Not labor intensive due to the few secondary operations required
• Little waste
• Inserts can be incorporated

Two of the major disadvantages of this process are the high cost and long development time. An automotive part can take four years to develop; two years in the prototype stage and two years to reach production. Not all products take this long, for instance a two-way valve produced by Johnson Controls only took 18 months. The initial cost can be as much as US$8 million to produce a four-cylinder engine manifold. However, computer flow analysis has helped reduce lead time and costs.

One of the difficulties that result from these long development times and high costs is making accurate cores repeatably. This is extremely important because the core is an integral part of the mold, so essentially each shot is into a new mold cavity. Another difficulty is keeping the core from melting when the plastic is shot into the mold, because the plastic is approximately twice the melting temperature of the core material. A third difficulty is the low strength of the core. Hollow plastic cores can collapse if too much pressure is used in the shot plastic. Metal cores are solid so they cannot collapse, but are only 10% as strong as steel cores so they can distort. This is especially a problem when molding manifolds, because the waviness of the core can be detrimental to the airflow within the runners.

Another disadvantage is the need for a large space to house the injection molding machines, casting machines, melt-out equipment, and robots.

Because of these disadvantages, some moldings that would be made via this process are instead made by injection molding two or more parts in a traditional injection molding machine and then vibration welding them together. This process is less expensive and requires much less capital, however it imparts more design constraints. Because of the design constraints, sometimes parts are made with both processes to gain the advantages of both.

Application 

The application of the fusible core process is not limited just to the injection of thermoplastics, but with corresponding core alloys also to thermosetting plastic molding materials (duroplast). The fusible core process finds application, for example, for injection molded passenger car engine intake manifolds. By modifying the equipment, small molded parts like valves or pump housings can be manufactured, as the manufacture of the fusible cores and the injected parts can be carried out on an injection molding machine.

Reaction Injection Molding

Principle

Reaction injection is a process of molding of articles by reaction or curing of reactants inside the mold.

Two reactive components are metered together and mixed inline so that they being to react in either polymerization or cross linking reaction. It is one of the important molding process for making foamed or non foamed parts.

Materials Used:

Mostly RIM system uses urethane formulations. RIM systems utilizing epoxies, polyesters, polyamides, and nylon 6 are currently under development. System utilizing reinforced nylon are approaching.commercialization for use in the automobile industry.

Urethane systems have traditionally been used for RIM because they readily meet processing and performance requirements. The RIM process requires liquid intermediates which can be catalyzed to provide rapid polymerization at low temperatures without producing gaseous by-products. Most urethane systems are comprised of two liquid intermediate feeds. When blowing agents are incorporated in the isocyanate feed, foam products are formed.

Additives:

Antioxidants, Blowing agents, fillers, colorants, 1 lubricants,urethane catalysts, UV stabilizers, Reinforcing agent.

Process Description:

Two liquid reactants-polyisocyanate components and resin mixture-are held in separate temperature controlled feed tanks equipped with agitators. From these tanks, the polyol and isocyanate are fed through supply lines to metering units that precisely meter the reactants, as high pressure, to the mixing head. When injection beings and valves in the mixhead open, the liquid reactants enter a chamber in the mixing head at pressure between 105 and 210  kg/cm²
where they are intensively mixed by high-velocity impingement. From the mix chamber, the liquid flows into the mold at approximately atmospheric pressure and undergoes an exothermic chemical reaction, forming the polyurethane system used. An average mold for an elastomeric part may be filled in one second or less and be ready for demolding in 30-60 seconds. Special extended geltime polyurethane RIM systems allow the processor to fill very large molds using equipment originally designed for molds with smaller volumes.

The schematic diagram of RIM

Advantages:

• Low tool cost.
• Complete design freedom
• Higher strength to weight ratio.
• Improve or eliminate secondary operations.
• No sink marks.
• Lower weight.
• wide range of physical properties 
• In addition to high strength and low weight, polyurethane Reaction Injection Molding (RIM) parts exhibit heat resistance, thermal insulation, dimensional stability, and a high level of dynamic properties.
• They also offer resistance to inorganic and organic acids as well as many other potentially damaging materials and chemicals including a large number of solvents.
• Resistance to weathering and aging is another plus, though extended exposure to the sun's ultraviolet rays typically results in a color shift at the surface.
• Low processing temperatures (35^oC to 65^oC) and low injection pressure (2-7 kg/cm²) make the Reaction Injection Molding (RIM) process more economical than other molding methods for large parts.

Co-Injection Molding

Principle

Co-injection is a molding process for making components of two materials. It is a three step injection process.

1^st Step – Start injection of outer skin into mold.

2^nd Step – Then a core (different material) is injected behind the first material.

 3^rd Step – Finally, the original “Skin” material is injected behind the core material.

This third step is required to purge the system and seal the gate for next shot.

The skin and core material are sequentially injected through the nozzle. The skin material is partially filled in the mold followed by injection of core component and filling of cavity is completed with skin components.

There are 3 options of co-injection

a a) Solid outer skin/foam core.

b b)Solid outer skin/solid core.

c c) Foamed outer skin/solid core.

The gate location is very typical with varying wall thickness. The wall thickness is >4.0mm for foam core/solid skin.

       3-4 mm for solid core/solid skin.

       2mm is minimum.

This technique is specially suitable for EMI shielding and high loading of conductive filler (carbon black) application.

Advantages

·         High strength cores with soft skins

·         Reclaimed core material inside of a high cost skin.

·         Attractive colors.

·         Reduced assembly costs

·         Decorative

·         Innovative solution

Process description and its types:

Co-injection is done in three ways:

·       Machine based

-          Skin and core material is injected through nozzle

·       Mold based

-          Skin and core material is injected through valve gate (hot runner system)

·       Mono sandwich

-          Skin material is extruded into the nozzle and injected into mold

Effects of viscosity

It is always preferable to use a more viscous material as the core. If the core resin is less viscous than the skin resin, it will not be able to push the skin resin ahead and it will “tunnel” through the hot center flow area. This result in low core percentages and inconsistent flow patterns.

Benefits of the Co-injection process

The co-injection molding process is similar to conventional injection molding except for one major difference. Co-injection uses a special valve configuration that enables two separate injection units to inject chemically compatible thermoplastics through the same injection port. This process allows one material, usually the prime material, to form the outer skin of the part while a second material fills the center. Co-injection offers many cost saving and design benefits over the conventional injection molding process. These benefits include the ability to mold larger parts with less clamping pressure, reduced material costs, and eliminating the need to paint glass filled parts.

The best part about this technology is that no special molds are required and existing conventional molds can be used with usually only minor modifications needed.

Thermoset Injection Molding

Principle

Injection molding of thermoset materials is the most automatic method of processing these materials and has become the most common. The main difference between injection and transfer molding is reduce material handling.

In thermoset injection molding, the material is fed directly into the hopper of the molding press, eliminating performing, perform storage, and preheating as in compression molding and then injection molding cycle is completed.

Materials used

Phenolics, MF, UF, Polyster, alkyd, DAP, PU

Additives used

Colorants, Fillers, lubricants, reinforcement, flame retardants

Process Description

A reciprocating screw plasticates the material charge. Material moves along the flight of screw. The material is heated by conductive heat and frictional heat by rotation of screw. The 1:1 compression ratio screw is generally used. The material changes into semi viscous form and screw moves backward due to back pressure and stops as per the charge. The screw moves forward at speed up to 50m /min and apply pressure up to 1400 kg/cm² on material. The material at around 90^oC injected into mold at 175-200^oC. The crosslinking takes place and part cures. Following cure, the mold opens automatically and part ejects out. In this case the barrel is heated by water. There is no check ring on the screw of thermoset molding machine.

An injection molding press consists of two major sections, one is the clamping section and other is material processing section. The clamping section, which is similar to a compression press, is basically a hydraulic cylinder that closes the mold halves and holds them together under pressure. In the case of a toggle press, there is a cylinder and linkage mechanism, that closes the mold halves and holds them together under pressure. In addition to the clamping mechanism, this part of the press also provides the mechanism for removing the parts from the mold.

The mold consists of a cavity side with one or more cavities and a core side. There is pure bushing which is the channel that connects the nozzle of the injection cylinder with the runner system of the mold. It is tapered to facilities its removal of the sprue from the mold. The cavities are connected to the sprue by way of runners and gates.

The mold is heated by either electric cartridge heaters, steam or hot oil to a temperature range of

·         165^oC - 182^oC for phenolic molding compounds

·         150^oC - 177^oC for melamine-phenolic molding compounds

·         163^oC - 182^oC for granular polyester molding compounds

·         143^oC - 171^oC for BMC(Bulk molding compound) polyester molding compounds

The material processing section includes the barrel, the reciprocating screw or sometimes a plunger for BMC materials and the material hopper, which is normally replaced with a stuffer, when molding BMC. A reciprocating screw is always used to process phenolics, melamine-phenolics and granular polyesters. BMC molding compounds are usually processed using a reciprocating screw, but they can also be run on plunger presses.

The reciprocating screw aids in the processing of thermoset materials in a number of ways. The rotational motion advances the material down the screw to where it is plasticized (changed form a solid to semi-viscous state) and then injected into the mold. At the same time that the screw rotation is advancing the material, the screw is being forced backward. This “backing up” of the screw allow the plasticized material to move in front of the screw. Once the pre-determined amount of material is plasticized in front o the screw, the screw is pushed forward the material out of the barrel and into the mold.

Process parameter

The processing of a thermoset molding compound is controlled by three parameter namely temperature, pressure and time. In injection molding, each of these is affected by a number of variables that need to be controlled.

a)  Temperature - The melt temperature of the molding material (stock temperature) is controlled by the barrel temperatures, screw speed, injection speed and back pressure. The water jackets around the barrel regulate the point at which the material will frictional heat. To maintain a consistent and workable melt temperature, all of these variables must to be coordinated and adjusted. The stock temperature cannot be so hot that the material cures before it is able to fill the parts, nor so cold that the cycle times have to be extended in order for acceptable parts to be produced from the mold.

b)  Pressure - The pressure on the material is controlled by the primary injection pressure, which moves the screw forward at a rapid speed to fill the cavities. The holding pressure on the material until it is sufficiently cured.

c)  Time - The time required for each phase of the process should be established and optimized. The high pressure injection phase should be controlled by a limit switch that changes the pressure on the injection cylinder from the primary or high pressure timer should be used to insure that the switch is made from primary to secondary injection pressure, if for some reason the limit switch fails to carry out this function.

Advantages:

·       Material handling is reduced because the press hopper will usually hold sufficient material to mold parts for an extended period of time.

·       Longer and smaller diameter core pins may be used because they can be supported on both can be molded without having material flash.

·       With the mold being closed before any material is injected into it, parts containing metal inserts can be molded without having material flash.

·       Relatively tighter tolerances across parting lines are possible.

·       Parting line flash can be hold to a minimal thickness if the mold is designed properly and well maintained.

·       Injection molding of thermoset materials tends itself to automating the process which can result in lower piece cost.

Disadvantages

·       Warpage can be a problem in injection molding because injection materials have softer flows and higher shrinkages. The forcing of the material through a sprue, runner and gate, can orient the material producing non-uniform shrinkage.

·       The filling of the parts through one or two gates produces parts that have knit lines. These knit lines are usually the weakest areas on the part.

·       The total amount of scrap produced during injection molding will usually be higher than that for compression molding because of the additional scrap created by the sprue and runner.  In the past, thermoset scrap had to be disposed of in a landfill. However, some thermoset materials are now being successfully recycled.

A constant supply of plastic thermoset resin, usually in the form of pellets or granules, is fed into the barrel of the injection unit through a large hopper. As the screw augers the plastic resin through the barrel from the hopper to the nozzle, the resin is heated in two ways:

1.    The Feed Zone – This zone is not directly heated. In this zone, the plastic resin pellets are packing into the screw chamber, forming a long thin “robin” of resin material that wraps around the screw.

2.    The Rear Zone – Heater bands around the barrel heat its middle part, which in turn starts heating the ribbon of resin pellets. In this zone, the resin begins to melt and changes from cold pellets into warm slush.

3.    The Front Zone – More heater bands bring the barrel and the resin fully up to melt temperature. In this zone, the resin becomes a hot, flowing fluid that is ready for injection into mold. This “melt” accumulates in the front zone and nozzle, where it is held at melt temperature until the reciprocating screw has metered the appropriate amount of melt for a full shot. At this point the screw rams the melt through the nozzle into the mold.

A secondary source of heating comes from “shearing” as the resin moves forward through the barrel by the screw. Shearing occurs as the resin is scraped from the inner walls of the barrel by the flights of the screw, and as the resin drags along the inner surfaces of both barrel and screw. All this scraping and dragging creates friction, which in turn creates heat.

Specialized Molding Process

• ·         ThermosetInjection Molding
• ·         Plastic Foam Molding
• ·         Laminates
• ·         ReactionInjection Molding
• ·         Thin Wall Molding
• ·         ResinTransfer Molding
• ·         LostCore Molding
• ·         Co-injectionor Sandwich molding
• ·         Multi component molding
• ·         Gas Assist Injection molding
• ·         Water Assist Injection Molding
• ·         Mucell Technology
• ·         All Electric Injection Molding

Fibre-reinforced plastic

For many applications it is possible to increase the modulus and strength of plastics by means of reinforcement. Reinforced plastics are generally similar to laminates in a number of applications. Basically, they differ in use of pressures as prescribed for laminates. Fibre reinforced plastics are generally consists of a polymer resin matrix and reinforcement along with other additives such as catalyst, initiator, filler, lubricant etc. Both thermoplastics are used but thermosets are used but thermosets are most widely used.

Fibre-reinforced plastic (FRP) (also fibre-reinforced polymer) is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, or aramid, although other fibres such as paper or wood or asbestos have been sometimes used. The polymer is usually an epoxy, vinylester or polyester thermosetting plastic, and phenol formaldehyde resins are still in use. FRPs are commonly used in the aerospace, automotive, marine, and construction industries.

Process definition

A polymer is generally manufactured by Step-growth polymerization or addition polymerization. When combined with various agents to enhance or in any way alter the material properties of polymers the result is referred to as a plastic. Composite plastics refer to those types of plastics that result from bonding two or more homogeneous materials with different material properties to derive a final product with certain desired material and mechanical properties. Fibre reinforced plastics are a category of composite plastics that specifically use fibre materials to mechanically enhance the strength and elasticity of plastics. The original plastic material without fibre reinforcement is known as the matrix. The matrix is a tough but relatively weak plastic that is reinforced by stronger stiffer reinforcing filaments or fibres. The extent that strength and elasticity are enhanced in a fibre reinforced plastic depends on the mechanical properties of both the fibre and matrix, their volume relative to one another, and the fibre length and orientation within the matrix.Reinforcement of the matrix occurs by definition when the FRP material exhibits increased strength or elasticity relative to the strength and elasticity of the matrix alone.

Process description

FRP involves two distinct processes, the first is the process whereby the fibrous material is manufactured and formed, the second is the process whereby fibrous materials are bonded with the matrix during the moulding process.

Fibre process

The manufacture of fibre fabric

Reinforcing Fibre is manufactured in both two dimensional and three dimensional orientations

1. Two Dimensional Fibre Reinforced Polymer are characterized by a laminated structure in which the fibres are only aligned along the plane in x-direction and y-direction of the material. This means that no fibres are aligned in the through thickness or the z-direction, this lack of alignment in the through thickness can create a disadvantage in cost and processing. Costs and labour increase because conventional processing techniques used to fabricate composites, such as wet hand lay-up, autoclave and resin transfer moulding, require a high amount of skilled labour to cut, stack and consolidate into a preformed component.
2. Three-dimensional Fibre Reinforced Polymer composites are materials with three dimensional fibre structures that incorporate fibres in the x-direction, y-direction and z-direction. The development of three-dimensional orientations arose from industry's need to reduce fabrication costs, to increase through-thickness mechanical properties, and to improve impact damage tolerance; all were problems associated with two dimensional fibre reinforced polymers.

The manufacture of fibre preforms

Fibre preforms are how the fibres are manufactured before being bonded to the matrix. Fibre preforms are often manufactured in sheets, continuous mats, or as continuous filaments for spray applications. The four major ways to manufacture the fibre preform is though the textile processing techniques of Weaving, knitting, braiding and stitching.

1. Weaving can be done in a conventional manner to produce two-dimensional fibres as well in a multilayer weaving that can create three-dimensional fibres. However, multilayer weaving is required to have multiple layers of warp yarns to create fibres in the z- direction creating a few disadvantages in manufacturing,namely the time to set up all the warp yarns on the loom. Therefore most multilayer weaving is currently used to produce relatively narrow width products, or high value products where the cost of the preform production is acceptable. Another one of the main problems facing the use of multilayer woven fabrics is the difficulty in producing a fabric that contains fibres oriented with angles other than 0" and 90" to each other respectively.
2. The second major way of manufacturing fibre preforms is Braiding. Braiding is suited to the manufacture of narrow width flat or tubular fabric and is not as capable as weaving in the production of large volumes of wide fabrics. Braiding is done over top of mandrels that vary in cross-sectional shape or dimension along their length. Braiding is limited to objects about a brick in size. Unlike the standard weaving process, braiding can produce fabric that contains fibres at 45 degrees angles to one another. Braiding three-dimensional fibres can be done using four step, two-step or Multilayer Interlock Braiding.Four step or row and column braiding utilizes a flat bed containing rows and columns of yarn carriers that form the shape of the desired preform. Additional carriers are added to the outside of the array, the precise location and quantity of which depends upon the exact preform shape and structure required. There are four separate sequences of row and column motion, which act to interlock the yarns and produce the braided preform. The yarns are mechanically forced into the structure between each step to consolidate the structure in a similar process to the use of a reed in weaving.Two-step braiding is unlike the four step process because the two-step includes a large number of yarns fixed in the axial direction and a fewer number of braiding yarns. The process consists of two steps in which the braiding carriers move completely through the structure between the axial carriers. This relatively simple sequence of motions is capable of forming preforms of essentially any shape, including circular and hollow shapes. Unlike the four step process the two step process does not require mechanical compaction the motions involved in the process allows the braid to be pulled tight by yarn tension alone. The last type of braiding is multi-layer interlocking braiding that consists of a number of standard circular braiders being joined together to form a cylindrical braiding frame. This frame has a number of parallel braiding tracks around the circumference of the cylinder but the mechanism allows the transfer of yarn carriers between adjacent tracks forming a multilayer braided fabric with yarns interlocking to adjacent layers. The multilayer interlock braid differs from both the four step and two-step braids in that the interlocking yarns are primarily in the plane of the structure and thus do not significantly reduce the in-plane properties of the preform. The four step and two step processes produce a greater degree of interlinking as the braiding yarns travel through the thickness of the preform, but therefore contribute less to the in-plane performance of the preform. A disadvantage of the multilayer interlock equipment is that due to the conventional sinusoidal movement of the yarn carriers to form the preform, the equipment is not able to have the density of yarn carriers that is possible with the two step and four step machines.
3. Knitting fibre preforms can be done with the traditional methods of Warp and [Weft] Knitting, and the fabric produced is often regarded by many as two-dimensional fabric, but machines with two or more needle beds are capable of producing multilayer fabrics with yams that traverse between the layers. Developments in electronic controls for needle selection and knit loop transfer, and in the sophisticated mechanisms that allow specific areas of the fabric to be held and their movement controlled. This has allowed the fabric to form itself into the required three-dimensional preform shape with a minimum of material wastage.
4. Stitching is arguably the simplest of the four main textile manufacturing techniques and one that can be performed with the smallest investment in specialized machinery. Basically the stitching process consists of inserting a needle, carrying the stitch thread, through a stack of fabric layers to form a 3D structure. The advantages of stitching are that it is possible to stitch both dry and prepreg fabric, although the tackiness of the prepreg makes the process difficult and generally creates more damage within the prepreg material than in the dry fabric. Stitching also utilizes the standard two-dimensional fabrics that are commonly in use within the composite industry therefore there is a sense of familiarity concerning the material systems. The use of standard fabric also allows a greater degree of flexibility in the fabric lay-up of the component than is possible with the other textile processes, which have restrictions on the fibre orientations that can be produced.

Moulding processes

There are two distinct categories of moulding processes using FRP plastics; this includes composite moulding and wet moulding. Composite moulding uses Prepreg FRP, meaning the plastics are fibre reinforced before being put through further moulding processes. Sheets of Prepreg FRP are heated or compressed in different ways to create geometric shapes. Wet moulding combines fibre reinforcement and the matrix or resist during the moulding process. The different forms of composite and wet moulding, are listed below.

Composite moulding

Bladder moulding

Individual sheets of prepreg material are laid -up and placed in a female-style mould along with a balloon-like bladder. The mould is closed and placed in a heated press. Finally, the bladder is pressurized forcing the layers of material against the mould walls. The part is cured and removed from the hot mould. Bladder moulding is a closed moulding process with a relatively short cure cycle between 15 and 60 minutes making it ideal for making complex hollow geometric shapes at competitive costs.

Compression moulding

A "preform" or "charge", of SMC, BMC or sometimes prepreg fabric, is placed into mould cavity. The mould is closed and the material is compacted & cured inside by pressure and heat. Compression moulding offers excellent detailing for geometric shapes ranging from pattern and relief detailing to complex curves and creative forms, to precision engineering all within a maximum curing time of 20 minutes.

Autoclave / vacuum bag

Individual sheets of prepreg material are laid-up and placed in an open mold. The material is covered with release film, bleeder/breather material and a vacuum bag. A vacuum is pulled on part and the entire mould is placed into an autoclave (heated pressure vessel). The part is cured with a continuous vacuum to extract entrapped gasses from laminate. This is a very common process in the aerospace industry because it affords precise control over the moulding process due to a long slow cure cycle that is anywhere from one to two hours. This precise control creates the exact laminate geometric forms needed to ensure strength and safety in the aerospace industry, but it is also slow and labour intensive, meaning costs often confine it to the aerospace industry.

Mandrel wrapping

Sheets of prepreg material are wrapped around a steel or aluminium mandrel. The prepreg material is compacted by nylon or polypropylene cello tape. Parts are typically batch cured by hanging in an oven. After cure the cello and mandrel are removed leaving a hollow carbon tube. This process creates strong and robust hollow carbon tubes.

Wet layup

Fibre reinforcing fabric is placed in an open mould and then saturated with a wet [resin] by pouring it over the fabric and working it into the fabric and mould. The mould is then left so that the resin will cure, usually at room temperature, though heat is sometimes used to ensure a proper curing process. Glass fibres are most commonly used for this process, the results are widely known as fibreglass, and is used to make common products like skis, canoes, kayaks and surf boards.

Chopper gun

Continuous strand of fibreglass are pushed through a hand-held gun that both chops the strands and combines them with a catalysed resin such as polyester. The impregnated chopped glass is shot onto the mould surface in whatever thickness the design and human operator think is appropriate. This process is good for large production runs at economical cost, but produces geometric shapes with less strength than other moulding processes and has poor dimensional tolerance.

Filament winding

Machines pull fibre bundles through a wet bath of resin and wound over a rotating steel mandrel in specific orientations Parts are cured either room temperature or elevated temperatures. Mandrel is extracted, leaving a final geometric shape but can be left in some cases.

Pultrusion

Fibre bundles and slit fabrics are pulled through a wet bath of resin and formed into the rough part shape. Saturated material is extruded from a heated closed die curing while being continuously pulled through die. Some of the end products of the pultrusion process are structural shapes, i.e. I beam, angle, channel and flat sheet. These materials can be used to create all sorts of fibreglass structures such as ladders, platforms, handrail systems tank, pipe and pump supports.

RTM & VARTM

Also called resin infusion. Fabrics are placed into a mould which wet resin is then injected into. Resin is typically pressurized and forced into a cavity which is under vacuum in the RTM (Resin Transfer Molding) process. Resin is entirely pulled into cavity under vacuum in the VARTM (Vacuum Assisted Resin Transfer Molding) process. This moulding process allows precise tolerances and detailed shaping but can sometimes fail to fully saturate the fabric leading to weak spots in the final shape.

Advantages and limitations

FRP allows the alignment of the glass fibres of thermoplastics to suit specific design programs. Specifying the orientation of reinforcing fibres can increase the strength and resistance to deformation of the polymer. Glass reinforced polymers are strongest and most resistive to deforming forces when the polymers fibres are parallel to the force being exerted, and are weakest when the fibres are perpendicular. Thus this ability is at once both an advantage or a limitation depending on the context of use. Weak spots of perpendicular fibres can be used for natural hinges and connections, but can also lead to material failure when production processes fail to properly orient the fibres parallel to expected forces. When forces are exerted perpendicular to the orientation of fibres the strength and elasticity of the polymer is less than the matrix alone. In cast resin components made of glass reinforced polymers such as UP and EP, the orientation of fibres can be oriented in two-dimensional and three-dimensional weaves. This means that when forces are possibly perpendicular to one orientation, they are parallel to another orientation; this eliminates the potential for weak spots in the polymer.

Failure modes

Structural failure can occur in FRP materials when:

• Tensile forces stretch the matrix more than the fibres, causing the material to shear at the interface between matrix and fibres.
• Tensile forces near the end of the fibres exceed the tolerances of the matrix, separating the fibres from the matrix.
• Tensile forces can also exceed the tolerances of the fibres causing the fibres themselves to fracture leading to material failure.

Material requirements

The matrix must also meet certain requirements in order to first be suitable for the FRP process and ensure a successful reinforcement of itself. The matrix must be able to properly saturate, and bond with the fibres within a suitable curing period. The matrix should preferably bond chemically with the fibre reinforcement for maximum adhesion. The matrix must also completely envelope the fibres to protect them from cuts and notches that would reduce their strength, and to transfer forces to the fibres. The fibres must also be kept separate from each other so that if failure occurs it is localized as much as possible, and if failure occurs the matrix must also debond from the fibre for similar reasons. Finally the matrix should be of a plastic that remains chemically and physically stable during and after reinforcement and moulding processes. To be suitable for reinforcement material fibre additives must increase the tensile strength and modulus of elasticity of the matrix and meet the following conditions; fibres must exceed critical fibre content; the strength and rigidity of fibres itself must exceed the strength and rigidity of the matrix alone; and there must be optimum bonding between fibres and matrix.

Glass fibre material

FRPs use textile glass fibres; textile fibres are different from other forms of glass fibres used for insulating applications. Textile glass fibres begin as varying combinations of SiO₂, Al₂O₃, B₂O₃, CaO, or MgO in powder form. These mixtures are then heated through a direct melt process to temperatures around 1300 degrees Celsius, after which dies are used to extrude filaments of glass fibre in diameter ranging from 9 to 17 µm. These filaments are then wound into larger threads and spun onto bobbins for transportation and further processing. Glass fibre is by far the most popular means to reinforce plastic and thus enjoys a wealth of production processes, some of which are applicable to aramid and carbon fibres as well owing to their shared fibrous qualities.

Roving is a process where filaments are spun into larger diameter threads. These threads are then commonly used for woven reinforcing glass fabrics and mats, and in spray applications.

Fibre fabrics are web-form fabric reinforcing material that has both warp and weft directions. Fibre mats are web-form non-woven mats of glass fibres. Mats are manufactured in cut dimensions with chopped fibres, or in continuous mats using continuous fibres. Chopped fibre glass is used in processes where lengths of glass threads are cut between 3 and 26 mm, threads are then used in plastics most commonly intended for moulding processes. Glass fibre short strands are short 0.2–0.3 mm strands of glass fibres that are used to reinforce thermoplastics most commonly for injection moulding.

Carbon fibre

Carbon fibres are created when polyacrylonitrile fibres (PAN), Pitch resins, or Rayon are carbonized (through oxidation and thermal pyrolysis) at high temperatures. Through further processes of graphitizing or stretching the fibres strength or elasticity can be enhanced respectively. Carbon fibres are manufactured in diameters analogous to glass fibres with diameters ranging from 9 to 17 µm. These fibres wound into larger threads for transportation and further production processes. Further production processes include weaving or braiding into carbon fabrics, cloths and mats analogous to those described for glass that can then be used in actual reinforcement processes.

Aramid fibre material process

Aramid fibres are most commonly known Kevlar, Nomex and Technora. Aramids are generally prepared by the reaction between an amine group and a carboxylic acid halide group (aramid); commonly this occurs when an aromatic polyamide is spun from a liquid concentration of sulfuric acid into a crystallized fibre. Fibres are then spun into larger threads in order to weave into large ropes or woven fabrics (Aramid). Aramid fibres are manufactured with varying grades to based on varying qualities for strength and rigidity, so that the material can be somewhat tailored to specific design needs concerns, such as cutting the tough material during manufacture.

Examples of polymers best suited for the process

Reinforcing Material	Most Common Matrix Materials	Properties Improved
Glass Fibres	UP, EP, PA, PC, POM, PP, PBT, VE	Strength, Elasticity, heat resistance
Wood Fibres	PE, PP, ABS, HDPE, PLA	Flexural strength, Tensile modulus, Tensile Strength
Carbon and Aramid Fibres	EP, UP, VE, PA	Elasticity, Tensile Strength, compression strength, electrical strength.
Inorganic Particulates	Semicrystalline Thermoplastics, UP	Isotropic shrinkage, abrasion, compression strength

Applications of fiber reinforced plastic

Glass-aramid-hybrid Fabric (for high tension and compression)

Fibre-reinforced plastics are best suited for any design program that demands weight savings, precision engineering, finite tolerances, and the simplification of parts in both production and operation. A moulded polymer artefact is cheaper, faster, and easier to manufacture than cast aluminium or steel artefact, and maintains similar and sometimes better tolerances and material strengths. The Mitsubishi Lancer Evolution IV also used FRP for its spoiler material.

Carbon fibre reinforced polymers

Rudder of A310 Airbus

• Advantages over a traditional rudder made from sheet aluminium are:
• 25% reduction in weight
• 95% reduction in components by combining parts and forms into simpler moulded parts.
• Overall reduction in production and operational costs, economy of parts results in lower production costs and the weight savings create fuel savings that lower the operational costs of flying the aeroplane.

Glass fibre reinforced polymers

Engine intake manifolds are made from glass fibre reinforced PA 66.

• Advantages this has over cast aluminium manifolds are:
  • Up to a 60% reduction in weight
  • Improved surface quality and aerodynamics
  • Reduction in components by combining parts and forms into simpler moulded shapes.

Automotive gas and clutch pedals made from glass fibre reinforced PA 66 (DWP 12-13)

• Advantages over stamped aluminium are:
• Pedals can be moulded as single units combining both pedals and mechanical linkages simplifying the production and operation of the design.
• Fibres can be oriented to reinforce against specific stresses, increasing the durability and safety.

Structural applications of FRP

FRP can be applied to strengthen the beams, columns, and slabs of buildings and bridges. It is possible to increase the strength of structural members even after they have been severely damaged due to loading conditions. In the case of damaged reinforced concretemembers, this would first require the repair of the member by removing loose debris and filling in cavities and cracks with mortar orepoxy resin. Once the member is repaired, strengthening can be achieved through the wet hand lay-up process of impregnating the fibre sheets with epoxy resin then applying them to the cleaned and prepared surfaces of the member.

Two techniques are typically adopted for the strengthening of beams, relating to the strength enhancement desired: flexural strengthening or shear strengthening. In many cases it may be necessary to provide both strength enhancements. For the flexural strengthening of a beam, FRP sheets or plates are applied to the tension face of the member (the bottom face for a simply supported member with applied top loading or gravity loading). Principal tensile fibres are oriented in the beam longitudinal axis, similar to its internal flexural steel reinforcement. This increases the beam strength and deflection capacity, and its stiffness (load required to cause unit deflection).

For the shear strengthening of a beam, the FRP is applied on the web or side faces of the member with fibres oriented transverse to the beam longitudinal axis. This is necessary for resisting shear forces, in a similar manner as internal steel stirrups, by bridging shear cracks that form under loading and restricting their growth. The FRP can be applied in several configurations, depending on the exposed faces of the member and the degree of strengthening desired, this includes: side bonding, U-wraps or U-jackets, and closed wraps or complete wraps. As the name suggests, side bonding involves applying FRP to both sides of the beam only. It provides the least amount of shear strengthening due to failures caused by debonding of the FRP from the concrete surface at the free edges. A more desirable strengthening configuration is the use of U-wraps, for which the FRP is applied continuously in a 'U' shape around the sides and bottom (tension) face of the beam. If all faces of a beam are accessible, then the use of closed wraps is desirable to provide the most strength enhancement. Closed wrapping involves applying FRP around the entire perimeter of the member with an overlap of FRP provided, such that there are no free ends and the typical failure mode is rupture of the fibres. For all wrap configurations, the FRP can be applied along the length of the member as a continuous sheet or as discrete strips, having a predefined minimum width and spacing.

Slabs may be strengthened by applying FRP strips at their bottom (tension) face. This will result in better flexural performance, since the tensile resistance of the slabs is supplemented by the tensile strength of FRP. In the case of beams and slabs, the effectiveness of FRP strengthening depends on the performance of the resin chosen for bonding. This is particularly an issue for shear strengthening using side bonding or U-wraps. Columns are typically wrapped with FRP around their perimeter, as with closed or complete wrapping. This not only results in higher shear resistance, but more crucial for column design, it results in increased compressive strength under axial loading. The FRP wrap works by restraining the lateral expansion of the column, which can enhance confinement in a similar manner as spiral reinforcement does for the column core.