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.