The shrinkage rate of POM is very large, up to more than 2.5%. Therefore, the size of injection molded parts is also greatly affected by the injection pressure and injection time.
When the injection pressure increases or the injection time increases, the size of the injection molded parts will increase. Conversely, when the injection pressure is reduced or the injection time is shortened, the size becomes shorter. Usually the pressure is increased by 10% or the injection time is increased or decreased by 2 to 4 seconds. The size of the injection molded parts has changed a lot.
In other words, the shrinkage of the POM parts can improve greatly with the injection pressure and time, so it is easy to adjust the pressure and time of the injection to make the shrinkage of the POM parts within 0.3% to 2.5%. A 100mm long cavity, through adjusting the pressure and time, can obtain injection parts with different sizes of about 97.5 ~ 99.7mm.
Therefore, once the size of the injection molded part is determined, the pressure and time cannot be adjusted casually. Due to the quality of the machine, pressure fluctuations of varying degrees are bound to occur, which will definitely cause unstable effects on the size of the injection molded parts. If the size requirements are relatively stable, the size of the injection molded parts must be regularly checked regularly. If the dimensional accuracy is very high, for example, the tolerance is less than ± 0.02mm, a stable and accurate high-quality injection molding machine is indispensable.
It is important to note that the size of the injection molded parts will continue to shrink and change for a long time after demoulding. It usually takes 6h for the size to be relatively stable. When the dimensional accuracy is high, it must be 24h.
The injection pressure and time have similar effects on the size of other materials, but only the degree of dimensional change varies. In summary, the greater the maximum shrinkage that the material can achieve, the greater the impact, the greater the dimensional change, and the harder it is to control. In addition to POM materials, the shrinkage of PP materials is also very large.
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