2022.10.20
1. Low pressure foaming method
The difference between low-pressure foam injection molding and ordinary injection molding is that the cavity pressure of the mold is low, about 2 to 7 mpa, while the ordinary injection molding is 30 to 60 mpa. Low-pressure foam injection molding generally adopts the under injection method, that is, a certain amount of plastic melt (including foaming agent) is injected into the mold cavity, and the gas decomposed by the foaming agent expands the plastic and fills the mold cavity.
For low pressure foam injection molding on ordinary injection molding machines, chemical blowing agents are usually mixed with plastic and plasticized in a barrel. Self-locking nozzles must be used. During the injection process, the surface of the product will become rough due to the rapid diffusion of gas, so the injection speed of the EPS molding machine should be fast enough. Typically, a supercharger is used to increase the speed and volume of the injection so that the injection can be done in an instant.
2. High pressure foaming method
The injection cavity pressure of the high-pressure foaming method is between 7 and 15MPa, and the full injection method is used, that is, the injection volume is exactly equal to the cavity volume. To foam and expand the part, the mold cavity can be forcibly expanded, or a portion of the plastic can be separated from the mold cavity. The mold cavity expansion method is usually used.
Compared with the ordinary injection molding machine, the EPS molding machine adopts the method of expanding the cavity to increase the secondary mold clamping and pressure maintaining device. When the molten mixture of plastic and blowing agent is injected into the mold cavity, it is delayed for a period of time, and then the moving mold plate of the mold clamping mechanism moves back a short distance, so that the moving mold and the fixed mold of the mold are slightly separated, the mold cavity expands, and the mold cavity is inflated. The plastic begins to foam and expand.
After the product cools, a dense skin forms on the surface. Since the foam expansion of the plastic melt is controlled by the moving template, the thickness of the dense skin of the product can also be controlled. The moving formwork can be moved in whole or in part for localized foaming to obtain products of different densities. The high-pressure foaming method has high requirements for mold manufacturing precision, high mold cost, and high requirements for secondary mold clamping and pressure holding of EPS molding machines.
3. Two-component foaming method
Two-component foam injection molding is a special high-pressure structural foam injection molding method that uses a special two-component injection molding machine. The EPS molding machine has two sets of injection devices: one set is used to inject the core of the product, and the other set is used to inject the surface of the product. In the injection molding process, the skin material is injected first, and then the core material mixed with the blowing agent is injected through the same gate.
Since the core material flows in a laminar state, it ensures that the core material is evenly wrapped within the cortex, thus allowing the cavity to be completely filled. As the molten material fills the mold cavity, a small amount of molten material without blowing agent is injected to seal the gate. After removing the product, the gate is removed to obtain a lightweight product with a non-foamed dense surface and a foamed core.
The microcellular foam molding of EPS molding machine belongs to the physical foaming method. The cell diameter of traditional foams is usually greater than 50mm, and the cell density (the number of cells per unit volume) is less than 106/cm3. These large-sized bubbles tend to be the source of initial cracks when stressed, thereby reducing the mechanical properties of the material.
In order to meet the industrial requirements of reducing the cost of some plastic products without reducing the mechanical properties, the foamed plastics with a pore size of micrometers were developed, and the foams with a cell diameter of 1mm-10mm and a cell density of 109-1012/cm3 were developed. Foamed plastics are defined as microcellular plastics.
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