It is usually slow and inefficient to mold thermoplastics using the compression molding techniques described above. In particular, it is necessary to cool a thermoplastic part before removing it from the mold, and this requires that the mass of metal making up the mold also be cooled and then reheated for each part. Plastic Injection Molding is a method of overcoming this inefficiency. Injection molding resembles transfer molding in that the liquefying of the resin and the regulating of its flow is carried out in a part of the apparatus that remains hot, while the shaping and cooling are carried out in a part that remains cool. In a reciprocating screw injection molding machine, material flows under gravity from the hopper onto a turning screw. The mechanical energy supplied by the screw, together with auxiliary heaters, converts the resin into a molten state. At the same time, the screw retracts toward the hopper end. When a sufficient amount of resin is melted, the screw moves forward, acting like a ram and forcing the polymer to melt through a gate into the cooled mold. Once the plastic has solidified in the mold, the mold is unclamped and opened, and the part is pushed from the mold by automatic ejector pins. The mold is then closed and clamped, and the screw turns and retracts again to repeat the cycle of liquefying a new increment of resin. For small parts, cycles can be as rapid as several injections per minute.
One type of network-forming thermoset, polyurethane, is molded into parts such as automobile bumpers and inside panels through a process known as reaction PEEK Injection Molding, or RIM. The two liquid precursors of polyurethane are a multifunctional isocyanate and a prepolymer, a low-molecular-weight polyether or polyester bearing a multiplicity of reactive end-groups such as hydroxyl, amine, or amide. In the presence of a catalyst such as a tin soap, the two reactants rapidly form a network joined mainly by urethane groups. The reaction takes place so rapidly that the two precursors have to be combined in a special mixing head and immediately introduced into the mold. However, once in the mold, the product requires very little pressure to fill and conform to the mold—especially since a small amount of gas is evolved in the injection process, expanding the polymer volume and reducing resistance to flow. The low molding pressures allow relatively lightweight and inexpensive molds to be used, even when large items such as bumper assemblies or refrigerator doors are formed.
The importance of Mold Design And Making on the productivity of a tool is often overlooked in the design of a mold. Several areas in the mold design exist where the molder must work with the mold builder in order to optimize the productivity of the mold. A good standard for mold productivity is saleable parts out of the press per hour. Cycle time and part quality are the critical aspects of saleable parts per hour. The areas of design found to be most important for increased productivity are the sprue bushing, runners and gates, hot manifold, venting, cooling, and ejection. While each of these items is specific to the mold being built, good design for each can contribute to improved part quality and optimum cycle time.
Too often the mold maker is left to decide the sizes of the sprue, runners, and gates and only when running the first samples does the molder learn that the sizes are not optimal. Much of this can be resolved beforehand by following the principles of runner and gate design found in the Injection Molding Handbook, as well as other reference materials. Again, runners sized too small affect the heat and pressure of the Plastic Mold and runners too large may slow the cycle for cooling time and cause unnecessary regrind.
