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Plastic Injection Molding Precautions

Jul 05, 2017

The process of plastic injection molding forces the melting of the plastic on the mold cavity, after the plastic has cooled, becomes the product, partly too complicated or costly in the process, in which many places can be carried out simultaneously on the same mold.

The mold is part of the machine, receiving the appropriate plastic and shape, the mold is cooled continuously, to the resin curing temperature.

In the plastic injection molding process, Plastic Injection Molding there are four typical complications below.

First, burn or burn part

The melt temperature may be too high, Plastic Injection Molding the polymer may be trapped inside the injection nozzle, the cycle time is too long to resin overheating.

Second, the curved part

Mold surface temperature is not uniform, non-uniform wall thickness of the mold design.

Third, the surface defects

The melt temperature may be too high, resulting in resin decomposition and gas escape (foam), Plastic Injection Molding too much moisture in the resin, and low pressure resulting in incomplete mold filling.

Fourth, the incomplete cavity filled

The injection mold does not have enough resin to be injected, or the injection speed may be too slow.

Above are the basis of injection molding, we must focus on the process to ensure high quality products.

Plastic injection molding process common problems include high-speed filling, low-speed filling, injection temperature control, packing phase of the problem, Plastic Injection Molding back pressure screw problem, plastic injection molding mold temperature problems, plastic injection molding process in the previous article also speak To the next, the plastic injection molding process common problems to do the following brief!

① high-speed filling. High shear rate when the high shear rate, the plastic due to the role of shear thinning and the existence of the decline in viscosity of the situation, so that the overall flow resistance decreases; that is, in the flow control stage, due to high-speed filling, Plastic Injection Molding melt shear thinning effect is often Large, and thin-walled cooling effect is not obvious, so the utility of the rate prevailed.

② low speed filling. Heat transfer control low speed filling, the shear rate is low. Local viscosity is higher, the flow resistance is bigger. As the thermoplastic raw material to add the slower rate, the flow is more slow, so that the heat transfer effect is more obvious, the heat quickly taken away for the cold mold wall. Coupled with a smaller amount of viscous heating phenomenon, the thickness of the curing layer is thick, and further increase the wall of the thin flow resistance. So it is easier to fill at the wall thickness at low speed.

③ injection temperature injection temperature must be controlled within a certain range. The temperature is too low, melt plasticization, affecting the quality of molded parts, increase the difficulty of the process; temperature is too high, easy to break down the raw materials. Plastic Injection Molding In the actual injection molding process, the injection temperature is often higher than the barrel temperature, the value of the injection rate and the performance of the material, up to 30c °. This is due to the heat generated when the melt is cut through the injection port. In the case of mold flow analysis, the difference can be compensated in two ways: one is to measure the temperature at which the melt is injected at the time of injection, and the other is to incorporate the nozzle when modeling.

④ pressure in the injection process near the end of the screw to stop the rotation, but to move forward, then injection into the packing phase. The nozzle of the injection molding machine during the packing process is continuously fed to the cavity to fill the volume vacated due to the shrinkage of the part. Plastic Injection Molding If the cavity is not filled with pressure, the parts will shrink about 25%, especially the tendons due to excessive contraction and the formation of contraction traces. Packing pressure is generally about 85% of the maximum pressure filling, of course, according to the actual situation, the holding time is too long or too short are on the formation of negative. Too long will make the pressure is not uniform, the plastic parts of the internal stress increases, plastic parts easily deformed, severe stress cracking occurs. Plastic Injection Molding Short is the pressure is not sufficient, the product volume shrinkage serious, poor surface quality.

⑤ back pressure screw reversing back to the material required to overcome the pressure. The use of high back pressure is conducive to the dispersion of pigment and plastic melting, but at the same time to extend the screw retraction time, reducing the length of the plastic melt section, increasing the pressure of the injection molding machine, so the back pressure of the lower, generally not More than 20% of the injection pressure. When injecting foam plastic material, the back pressure should be higher than the pressure formed by the gas, Plastic Injection Molding otherwise the screw will be pushed out of the barrel. Some injection molding can be programmed to compensate for the reduction in screw length during melting, which reduces the input heat and reduces the temperature.

⑥ cooling and injection molding mold mold temperature control mold temperature for the plastic molding process a great impact. If the injection molding mold temperature is too low, on the one hand the plastic melt cooling becomes faster, the thickness of the solidified layer increases, Plastic Injection Molding the melt viscosity also increases, resulting in forming pressure becomes larger, forming relatively difficult; the other hand, the plastic melt flow is not smooth, Prone to flow traces, the outline of the product is not smooth, while the length of the weld marks longer and clarity becomes higher, the appearance of the product quality decline. Mold temperature on the performance of the products also have a certain impact, especially the crystalline plastic, products, mold temperature directly affect the crystallization of the type of material and crystallization quality.