The pressure in the injection molding process includes plasticizing pressure and injection pressure, which directly affects the plasticization of plastics and the quality of products.
1. Plasticizing pressure:
(back pressure) when the screw injection machine is used, the pressure on the molten material at the top of the screw when the screw rotates and retreats is called plasticizing pressure, also known as back pressure. This pressure can be adjusted by the overflow valve in the hydraulic system.
In injection, the plasticizing pressure needs to change with the screw design, product quality requirements and different types of plastics. If these conditions and the screw speed remain unchanged, increasing the plasticizing pressure will strengthen the shear effect, that is, it will increase the melt temperature, but it will reduce the plasticizing efficiency, increase the countercurrent and leakage flow and increase the driving power.
In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the color mixture uniform and discharge the gas in the melt. In general, in operation, the plasticizing pressure should be determined as low as possible on the premise of ensuring good product quality. The specific value varies with the variety of plastics used, but it usually rarely exceeds 20 kg / cm2.
2. Injection pressure:
In the current production, the injection pressure of almost all injection machines is based on the top of the plunger or screw to the plastic
The applied pressure (converted from oil circuit pressure) shall prevail. The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, give the melt filling rate and compact the melt.
The time required to complete an injection molding process is called molding cycle, also known as molding cycle. It actually includes the following parts:
Molding cycle: the molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, the relevant time in the forming cycle should be shortened as far as possible on the premise of ensuring the quality. In the whole molding cycle, injection time and cooling time are the most important, which have a decisive impact on the quality of products.
The mold filling time in the injection time is directly inversely proportional to the mold filling rate, and the mold filling time in production is generally about 3-5 seconds. The pressure holding time in the injection time is the pressure time on the plastic in the mold cavity, which accounts for a large proportion in the whole injection time, generally about 20-120 seconds (for extra thick parts, it can be as high as 5-10 minutes).
Before the molten material at the gate is frozen, the holding time has an impact on the dimensional accuracy of the product. If it is later, it has no impact. Pressure holding time also has a favorable value. It is known that it depends on material temperature, mold temperature and the size of sprue and gate. If the size and process conditions of sprue and gate are normal, the pressure value with the smallest fluctuation range of product shrinkage shall prevail.
The cooling time mainly depends on the thickness of the product, the thermal and crystallization properties of the plastic, and the mold temperature. The end point of the cooling time shall be based on the principle of ensuring that the product will not change during demoulding. The cooling time is generally about 30 ~ 120 seconds. It is unnecessary to have a long cooling time, which will not only reduce the production efficiency, but also cause demoulding difficulties for complex parts, and even produce demoulding stress during forced demoulding. The other time in the forming cycle is related to the continuity and automation of the production process and the degree of continuity and automation.
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