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Several injection molding process parameters that affect the strength of injection molded parts

Jan. 01, 70
1. Increasing the injection pressure can increase the tensile strength of PP injection molded parts
Compared with other hard rubber materials, PP material is more elastic, so the density of injection molded parts will increase with the increase of pressure. This characteristic is relatively obvious. When the density of a plastic part increases, its tensile strength will naturally increase, otherwise it will decrease.
However, when the density increases to the maximum that PP itself can reach, and then increase the pressure, the tensile strength will not continue to increase, but will increase the residual internal stress of the injection molded part, making the injection molded part brittle, so it should be adequate.
Other materials have similar situations, but the degree of obviousness will be different.
2. Mould transporting hot oil injection molding can improve the strength of steel parts and nylon parts
Nylon and POM materials are all crystalline plastics. The mold uses a hot oil machine to transport hot oil for injection, which can slow down the cooling rate of the injection molded part and increase the crystallinity of the plastic. At the same time, due to the slowing down of the cooling rate, the residual internal stress of the injection molded part is also reduced. Therefore, the impact resistance and tensile strength of nylon and POM parts injected with hot oil by the hot oil machine will be improved accordingly.
It should be noted that the size of nylon and POM parts injected with hot oil in a hot oil machine is somewhat different from that of water injection molded parts, and nylon parts may be larger.
3. The melting speed is too fast, even if injection at 180℃, the glue will be raw
Normally, PVC 90°C material is injection molded at 180°C, and the temperature is sufficient, so there is generally no problem of raw rubber. However, it is often due to reasons that have not attracted the attention of the operator, or deliberately speeding up the melt speed in order to speed up production, which makes the screw retreat quite fast. For example, it only takes two or three seconds to retract the screw to the maximum melt volume. At more than one part of the position, the time for the PVC material to be heated and stirred is seriously insufficient, causing the melt temperature and uneven mixing of the raw rubber, which makes the strength and toughness of the injection molded part become quite poor.
Therefore, when injecting PVC materials, be careful not to adjust the melt speed to more than 100 rpm. If you must adjust it quite quickly, remember to increase the temperature of the material by 5 to 10°C, or increase the back pressure of the melt to match. At the same time, you must always check for raw rubber problems, otherwise it is very likely to be Cause heavy losses.
Regarding this issue, since most people don't care too much, it is specially mentioned here to remind you to pay attention to it during production and remember to do a good job in inspection.
On the contrary, it should be noted that if the melt speed is too slow, even if it is 180 degrees injection molding PVC 90 degrees, it will also cause burning problems, especially transparent PVC, there will be many black spots and gas lines on the injection molded parts.


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