May 20, 2020

3 key process parameters affecting the strength of injection molded parts

1, Increasing the injection pressure can improve the tensile strength of PP injection molded parts. PP material is more elastic than other rigid plastics. Therefore, the density of injection molded parts will increase with the increase of pressure. This feature is relatively obvious. When the density of plastic parts increases, its tensile strength will naturally increase, otherwise it will decrease.

However, when the density increases to the maximum value that PP itself can reach, if the pressure is increased, the tensile strength will not continue to increase, but will increase the residual internal stress of the injection molded part and make the injection molded part brittle, so it should be enough.

Other materials have a similar situation, but the obvious degree will be different.

2, Mold injection with hot oil can improve steel parts

Nylon and POM materials are both crystalline plastics. The mold is heated and injected with a hot oil machine, which can slow down the cooling rate of the injection molded parts and improve the crystallinity of the plastic. At the same time, due to the slowing of the cooling rate, the residual internal stress of the injection molded parts is also reduced. Therefore, the impact resistance and tensile strength of nylon and POM parts that are heated and injected with a hot oil machine will be correspondingly improved.

It should be noted that the size of nylon and POM parts that are heated and injected with a hot oil machine will be slightly different from those of water injection molding, and the nylon parts may be larger.

3, The plasticization speed is too fast

Under normal circumstances, PVC plastic with a hardness of 90 °C is plasticized at 180 °C. The temperature is sufficient, and the problem of raw material (inadequate plasticization) will not occur. However, it is often due to reasons that have not attracted the attention of the operator, or the melt speed is intentionally increased in order to speed up the production, which causes the screw to retreat very quickly. For example, it takes only two or three seconds to retreat the screw to the maximum melt volume. At more than one-half of the position, the PVC material is heated and stirred for a serious amount of time, resulting in melt temperature and uneven mixing of raw rubber problems, resulting in the strength and toughness of injection molded parts will become quite poor.

Therefore, when injecting PVC material, be careful to adjust the melt speed to more than 100r / min. If it must be adjusted fairly quickly, remember to increase the material temperature by 5-10 °C, or appropriately increase the melt back pressure to match, and also pay attention to frequent checks for raw rubber problems, otherwise it is very likely cause significant losses.

Regarding this issue, since most people do not care too much, it is specially proposed to remind the production must pay attention, and remember to do a good job of inspection.

On the contrary, it should also be noted that if the melt speed is too slow, even if the PVC material is injected at 180 °C, it will cause scorching problems. Especially for transparent PVC, there will be many black spots and air marks on the injection molded parts.

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Injection Molding, Plastic Mold Case, Plastic Molding Design, Plastic Parts
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