Tag Archive for: Plastic Injection Molding Process

Plastic Injection Molding Process

Plastic Injection Molding Process normally including injectioin filling, Injection pressue and injection packing pressure.

Injection Pressure will affect the products quality. So a suitable injection pressure does a big favor to high-quality products.

Injection pressure involves two stages. The first stage is, inject the melt plastic material into mold by a high speed, this pressure called first injection pressure. We always called it injection pressure too. Second stage is, the pressure we put after plastic material filled mold, called second injection pressure or packing pressure. Melt plastic material’s resistance related to the characteristic of melt, the shape of product and the mold structure, therefore, first injection pressure should be set according to the specific requirement. Generally, the first injection pressure is about 80~120 Mpa (800~1200kgf/cm2).

For the material with high viscosity, or for the product which need high size precision, we can use 150~200 Mpa or higher injection pressure. Once, some injection molding machine manufacturer said he had used 400Mpa injection pressure, but under normal condition, the plastic injection machine which on the market cannot reach this injection pressure.

Too low first injection pressure will lead to short filling. Too high first injection pressure can increase the product density, decrease the shrinkage. But if the injection pressure is too high, flash will happen, sometimes, the product cannot drop down easily.

Therefore, when we testing mold, at the beginning, injection pressure should be low, then improved gradually, finally confirm a suitable injection pressure.

In addition, injection pressure adjustable rang is related to the kinds of injection molding machine. For exempla, for small home appliance, in order to increase the injection pressure, we should choose injection molding machine with big screw diameter. Fig 2-3:plastic molds

Fig 2-3: Screw diameter and injection unit performance

                                       Screw typeproject           A             B             C
Screw diameter/mmInjection pressure/ Mpa

Theoretical injection pressure/ Mpa

Injection weigh/g


Injection rate/ (cm3/s)

Plasticizing capability/ (kg/h)



















Even the same injection molding machine manufacture by the same manufacturer, different screw diameter will has different max injection pressure. We should confirm the max injection pressure whether meet our requirement.

Second injection pressure (packing pressure) is the pressure which acts on material. The role second injection pressure played is, on the basic of avoiding burr and excessive filling, using melt material supplemented unceasingly, to avoid shrinkage of product.

From the effect of second injection pressure we known, the pressure of second injection pressure should be lower than first injection pressure. But second injection pressure must guarantee the material in cavity fully cures, that is, material in every runner fully cures too.

Want to know more information about plastic  molds and injection molding cost or plastic injection molding process, go to https://en.wikipedia.org/wiki/Injection_moulding , you will solve your issues there.