In fact, pressure must be generated in the production of injection molded products. In general, the pressure in the mold cavity is generated by a small contraction, and its effect is cumulative, and this additive effect will eventually lead to the production of defective products. Howards explains that often when a part is first removed from the mold, it is able to resist the effects of in-mold pressure on the structure of the article. Plastic products only cause small deformation or cracks at high temperatures. In practice, all mold parameters cause small deformations, resulting in in-mold pressure. When these forces act together on the same component, large additional stresses are generated. The forces acting on the components cause pressure to be generated. When the pressure is small, there is no noticeable effect. However, if the pressure is large enough, the pressure in the mold becomes a factor that must be considered, because its combined effect with the external force will exceed the maximum that the product can withstand, and the most serious consequence is to cause the product to break.
Gas-assisted analysis
1. Improved melting point
First, pressure-resistant gas assist systems employ methods to improve flowability. If a large deflector rod design can be applied, this molding process is more ideal, as these deflector rods become good gas channels.
Gas channels can also be opened in some inconspicuous areas, such as ribs or some junctions. Traditional deflectors are not large enough to provide sufficient fluidity. Therefore, the new design is to have 2 or 3 gas channels at one junction. For large parts, the gas passage at its corner will have a greater effect on the improvement of the melting point.
Gas assist systems may also change the number of sprues and fusion lines. This is because the fewer sprues, the closer the melting point curve. This is also a factor that affects the way raw materials are deposited and the strength of fusion.
2. Reduce the pouring pressure
With multiple pouring, the gas assist system helps to reduce the pressure necessary for the forming of the article. First, the deflector bar provides a resistance-free channel. In the test, the maximum injection pressure required by the traditional method was 7800 psi, while the new method only required 5300 psi. Secondly, these gases fill the voids, so that the part reaches the maximum injection pressure and the maximum projection area during the injection molding process, so the gas assist system is beneficial to reduce the injection pressure and projection area.
The decrease in the projection area is due to the reduction in the amount of resin injected, while the clamping force is determined by the product of the injection pressure and the projection area. Since the injection pressure required to fully fill the cavity is usually 25% of the maximum pressure, in this example, the air pressure used for filling is 1500 psi, which reduces the clamping force from 175t to 75t.
3. Reduce shear force
High shear forces will cause a larger orientation of the polymer macromolecular chain, which will affect the surface finish of the final product and increase the shrinkage due to thermal deformation. If a low-resistance gas channel is provided, the gas assist system can effectively reduce the shear force.
4. Filling pressure
A traditional molded product is often subjected to considerable filling pressure. In the traditional molding process, it usually accounts for 80% of the maximum injection pressure. In gas-assisted systems, it accounts for only 25% of the maximum injection pressure. Gas injection can be carried out at the end of injection molding or at the beginning of rapid injection molding. When the gas enters the part, it finds the path with least resistance and then extrudes the raw material until all the space is completely filled. In this process, the gas establishes the shortest transfer path between the point where the filling pressure is generated and the point where the filling pressure is required. When using a gas assist system, filling can also continue when there has been cooling and condensation between the gas inlet of the part and the part to be filled, because the gas provides access to the raw material.
Therefore, the application of a gas assist system results in a lower filling pressure and a shorter filling path, while extending the time for the raw material to flow. In addition, the gas assist system can also eliminate small cracks in the core of the part.
5. More balanced temperature
During the gradual cooling process, the parts produced by the gas-assisted system fill up faster. For large parts, this advantage in injection speed can have a great impact on the temperature difference between different parts of the article. In conventional moldings, the hot part produces a large shrinkage, which squeezes the already cooled part. Gas assist can create relatively large voids inside the article, thereby reducing the temperature difference between the hot and cold parts. These voids allow the product to be cooled not only from the outside, but also by the gas passage. Therefore, after the injection molding is completed, the overall temperature of the product will be more balanced, thereby improving the quality of the product.
