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.