The basic equipment for injection molding is an injection molding machine and an injection mold. Figure 1-2 shows the injection molding process of a screw-type injection molding machine.
Injection molding principle
△Injection molding process flow
The principle is that granular or powdered plastic is added to the injection molding machine barrel, heated and melted, and then the high pressure and high speed of the injection molding machine screw pushes the molten plastic through the nozzle at the front end of the barrel and quickly injects it into the closed mold cavity [Fig. 1-2(a)]. The melt filling the cavity is cooled and solidified under pressure to maintain the shape given by the cavity [Fig. 1-2(b)]. Then the mold is opened and the product is taken out [Fig. 1-2(c)].During the injection molding process, plastic undergoes a series of changes, including softening, melting, flowing, shaping, and solidification.
(Figure 1-2 Injection Molding Principle of Screw-Type Injection Machine)
Softening and Melting:
Figure 1-4 shows the barrel and screw structure of an injection molding machine. Because a circular heater is installed on the outside of the barrel, the plastic melts as it moves forward under the rotation of the screw, and is finally injected into the mold through the nozzle.
(L1-Feeding section; L2-Compression section; L3-Measuring section; h1/h2-Compression ratio; D-Screw diameter)
Plastic undergoes the following changes during the mold filling process:
Before the screw rotates (L2), the temperature and pressure of the screw rod are relatively low due to the reduction in melt volume caused by the material entering the mold cavity (L1). After the screw rotates (L3), the plastic temperature has reached the melting temperature and has become molten. To ensure product quality, the plastic must be fully melted before re-melting. At this time, if the plastic has already entered the compression stage at a certain degree of fusion, its degassing effect will be greatly affected.
Even if the quantity (L3) remains the same, due to different screw groove depths h₀, the plastic will experience different degrees of shear action during the screw rotation process, thus the degree of plasticization will vary.
In summary, under the same molding cycle, the degree and quality of plastic melting will be affected by the screw's gas content and melting quality:
① The effective length of the screw is directly proportional (increases): L/D=22～25.
② The compression ratio of the screw: h₁/h₂=2.0～3.0 (generally 2.5).
③ The compression part of the screw is relatively proportional: L₁/L₂=40%～60%.
Since the value is too large, the material's residence time will also increase, and the screw rotation will continuously send the molten plastic forward. At this time, the plastic will continue to flow in the mold cavity under pressure, and then in one molding cycle (without external intervention before the screw starts to move forward). After the screw rotates, it will move forward under the action of mechanical force, gradually compacting the plastic and injecting it into the mold cavity. In the instant before, its melt will be subjected to rapid compression (called instantaneous compression), which can easily cause crystallization and lead to defects. Using slow injection can avoid crystallization (complete crystallization, making it completely amorphous due to rapid cooling).
Flow：
When the melt is injected into the mold cavity under high pressure and high speed, two phenomena will occur during the injection process. One is that the plastic in contact with the mold wall in the molten state will solidify and form a thin layer due to the rapid cooling caused by contact with the mold cavity surface. This thin layer is called a frozen layer (or called instantaneous frozen layer), which will cause the temperature of the molten plastic itself to decrease (mainly due to the influence of the loss of latent heat of crystallization). For example, in polyethylene, the latent heat of crystallization released during the cooling process of the melt through the mold wall can reach 50℃ or above. Therefore, after the melt fills the entire mold cavity and returns to an urgent state, the temperature will decrease. The second is that a larger portion of the molten plastic will continue to maintain its flow direction and undergo reverse flow.
As can be seen from Figure 1-5, when the melt is in contact with the wall of the mold cavity, it will produce a frozen layer, and a faster flow rate will be generated in the central part away from the cavity. The plastic will flow in a layered manner in the area between the frozen layer and the cavity wall. After the plastic passes through in such a state and is cooled and shaped into a product, the layering will still exist in the molded product in a parallel direction and a vertical direction, resulting in differences in strength and toughness of the product, which will exist during the release and shaping stages of the molded product.
1 - Injection molding machine; 2 - Resin injection mold (actually composed of main runner and gate);
3 - Mold (inside the cavity); 4 - The part with a faster flow rate at the center;
5 - The part with a very slow flow rate along the cavity wall; 6 - Resin molecules that are oriented and stretched out;
7 - Resin molecules that are entangled together.
Shaping and Curing：
When molten plastic is injected, it enters the mold through a nozzle, takes shape, and then cools and solidifies to become the finished product. However, the actual time it takes for the molten plastic to fill the mold is several seconds, making it very difficult to observe the filling process.
American engineer Stevenson used computer simulation to depict the filling process of a polypropylene car door being molded using a hot runner mold with two gates, and calculated the injection time (i.e., filling time), weld line, and required clamping force. Figure 1-6 illustrates the model obtained from his simulation. The flow and filling state of the melt in Figure 1-6 is not significantly different from what was imagined, and may accurately reflect the actual filling process of a car door.
Many methods exist for simulating the flow of injection molding processes (such as the FAN method, CAIM simulation system, and Moldflow simulation system). These simulation methods are currently used to predict the filling process of molten plastic in a mold, aiming for more rational mold design and selection of gate location or type.
After the molten plastic is shaped, it enters the solidification process. The main phenomenon occurring during solidification is shrinkage, which occurs simultaneously due to cooling and crystallization. Figure 1-7 shows the shrinkage of three types of polyethylene with different crystallinities as the temperature decreases.
(a-PE with a relative density of 0.9645; b-PE with a relative density of 0.95; c-PE with a relative density of 0.918; d-Cooling rate curves: C1, C2, C3-all three have the same cooling rate.)