What are the process parameters of injection molding of precision rubber and plastic components?

The process parameters of injection molding of precision rubber-plastic components are the core elements to ensure the dimensional accuracy, appearance quality and mechanical properties of products, which need to be accurately set and matched according to the characteristics of rubber-plastic materials, product structure and mold design, and are mainly divided into four categories: temperature parameters, pressure parameters, time parameters and speed parameters, as follows:

Temperature parameter

Temperature is the key to determine the melting state and molding quality of rubber and plastic raw materials, and it needs to be accurately controlled in sections to avoid degradation or insufficient plasticization of raw materials.

Temperature of barrel: adopt sectional temperature control, and gradually increase the temperature from the feeding section to the homogenization section to ensure the smooth plasticization of raw materials.

Feed section: The temperature is the lowest, so it is only necessary to soften the raw materials to prevent the raw materials from melting prematurely and causing bridging. For example, the temperature in this section of PP material is about 160-180℃, and that of PC material is about 220-240℃.

Melting stage: the temperature rises, which promotes the complete melting and plasticization of raw materials, with PP at 180-200℃ and PC at 250-280℃.

Homogenization stage: the highest and stable temperature ensures the uniformity of melt, with PP at 200-220℃ and PC at 280-300℃.

Nozzle temperature: 5-10℃ slightly lower than the homogenization temperature, to prevent the melt from salivating, and at the same time to avoid cold material entering the cavity to form defects.

Mold temperature: directly affects the crystallinity, shrinkage and demoulding performance of the product, and needs to be adjusted according to the material type.

Rigid crystalline materials (such as POM, PA): the mold temperature is 60-120℃, which improves the crystallinity and reduces the warping deformation.

Amorphous materials (such as PC, ABS): the mold temperature is 40-80℃, which improves the surface gloss of products.

Flexible elastic materials (such as TPU, TPE): the mold temperature is 20-50℃, which can speed up cooling and setting and improve production efficiency.

Pressure parameter

Injection pressure determines the melt filling ability and product compactness, which needs to be controlled by stages to avoid flash and stress concentration of products caused by too high pressure.

Injection pressure: the pressure to push the melt to fill the mold cavity, which is related to the material fluidity and the wall thickness of the product.

High fluidity materials (such as PE and PP): the injection pressure is 80–120mpa.

Low fluidity materials (such as PC, PMMA) or thin-walled precision parts: the injection pressure is 120–180 MPa.

Holding pressure: the pressure used to compensate the cooling shrinkage after the melt fills the cavity, usually 40%-60% of the injection pressure. Insufficient holding pressure will lead to shrinkage marks and size shrinkage of products; Excessive pressure is easy to produce internal stress.

Back pressure: the reverse pressure when the screw rotates to plasticize, which is used to compact the melt, exhaust gas and improve the plasticization uniformity, with a value of 0.5–3 MPa. If the back pressure is too low, the melt will contain gas and produce bubbles. Too high will increase the plasticizing time and even lead to the degradation of raw materials.

Time parameter

The time of each stage needs to be accurately matched to balance production efficiency and product quality.

Injection time: the total time for filling the cavity with melt, which is divided into primary injection time (filling the cavity 80%–90%) and secondary injection time (filling the rest). The total time is usually 2–10 seconds. Thin-walled parts need to shorten the injection time to prevent the melt from solidifying in advance.

Holding time: The time from the beginning of holding pressure to the back of the screw should cover the solidification stage of the product, usually 5-30 seconds, and the holding time of thick-walled products should be extended to compensate for shrinkage.

Cooling time: the time for the product to cool and set in the mold, which is subject to no deformation after demoulding, usually 10-60 seconds, which is determined by the wall thickness of the product and the mold temperature.

Plasticizing time: the time for completing one plasticizing by screw rotation should be matched with the injection and cooling time to avoid affecting the production rhythm.

Velocity parameter

Injection speed affects the flow state of melt and directly affects the appearance and internal quality of products, so it is necessary to adopt sectional variable speed control.

Injection speed: it is divided into multiple settings and follows the principle of "slow-fast-slow".

Initial stage (at the gate): low speed (20–50 mm/s) to prevent melt injection and avoid crazing and bubbles.

Intermediate section (cavity filling): high speed (50–150 mm/s), rapid cavity filling, reducing melt cooling time, suitable for thin-walled precision parts.

Terminal section (before the cavity is full): low speed (10–30 mm/s) to reduce impact, stress concentration and flash.

Screw speed: it affects plasticizing efficiency and melt uniformity. Too fast speed will lead to shear overheating degradation of raw materials, while too slow speed will lead to insufficient plasticization. It is usually set to 50–150 r/min, and the high-viscosity material (such as PC) takes a lower rotation speed, and the low-viscosity material (such as PP) takes a higher rotation speed.