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In the injection molding process, thermoplastic resins are melted and the melt is forced (injected) into a mold. After this melt cools until the polymer solidifies, the parts are removed (ejected) from the mold. Injection molding permits mass production netshape manufacturing of high precision, three-dimensional of plastic parts[1]. One of the most common plastics manufacturing processes, injection molding can produce parts weighing as little as fraction of a gram or as much as 150 kg[2]. The process currently consumes 30% of polymeric resins[3] of which 90% are thermoplastics (i.e., capable of being remelted)1. Major advantages include capabilities to produce parts with: 1) virtually unlimited complexity, 2) fine details and good surface appearance, 3) controlled wall thickness and excellent dimensional stability, and 4) requiring limited or no finishing.
The five important M’s in the injection molding process are the
1. Injection molding machine
2. Mold
3. Material
4 Method
5. Man (i.e., operator).
While there are many variations of each M, this discussion is limited to single-stage reciprocating screw injection machines and the flow of thermoplastic polymer melts in two-plate cold runner molds.
1. Injection Molding Machines
As illustrated in Figure 1, injection molding machines have three major components: the 1) injection unit, 2) clamping unit, and 3) controls. The injection unit plasticates (melts) and injects the polymeric material into the mold. The clamping unit supports to the mold and provides the mechanisms for opening and closing of the mold and for ejection of molded parts.
Figure 1. Injection molding machine[4].
During a thermoplastic molding cycle, the clamp of the injection molding machine closes, thereby closing the mold. Molten plastic located between the nozzle and screw in the barrel of the injection unit is forced (by the screw) into the mold. The controlled volume of melt injected into the mold typically fills the cavities to about 95 to 98% of their total volume. After injection is completed, the screw is pressured for a given period of time. In this packing stage, more melt is forced into the mold to compensate for shrinkage of the melt as it cools. The packing stage is followed by a holding stage, in which a controlled pressure is exerted on the screw for a specific length of time. Holding pressure prevents the melt from flowing back into the runners. When the gate freezes (solidifies), melt can no longer exit the cavity and the holding stage ends. While the melt cools immediately upon entering the cavity, a “formal” cooling stage follows the holding stage. During this period, the part cools until it is capable of withstanding ejection forces. The screw also rotates to melt more plastic and build up the molten plastic shot for the next molding cycle. At the end of the cooling stage, the mold is opened and the part is ejected.
The times associated with a conventio
al molding cycle are shown in Figure 2. For 2 to 3-mm thick parts, filling occurs in less than 5 s, packing requires one-third of the fill time[5], the holding time depends on the gate size, and cooling is longest part of the cycle. Thin-walled parts (i.e., wall thickness is les than 1 mm), however, filling in less than 1 s, typically have no packing or holding stage, and cool rapidly.
Figure 2. Molding cycle diagram[6].
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