The impact of plastic injection molding process on product quality

In the molding process of converting plastic particles into plastic products, plastics are often subjected to high temperature and high pressure, and flow molding at high shear rates. Different molding conditions and processes will have different effects on quality. Injection molding has plastic raw materials It consists of four aspects: injection molding machine, mold and injection molding process.

If the plastic raw materials are correctly selected, the technical parameters of the injection molding machine and the mold structure meet the requirements of injection molding conditions, the quality of the processed products is directly related to the injection molding process.

The quality of products includes internal material quality and appearance quality. Internal material quality is mainly mechanical strength, and the size of internal stress directly affects the mechanical strength of the product. The main reasons for internal stress are determined by the crystallinity of the product and the orientation of molecules in plastic molding. of.

The appearance quality of the product is the surface quality of the product, but warping and deformation of the product caused by large internal stress will also affect the appearance quality. The appearance quality of products includes: insufficient products, product dents, weld marks, flash, bubbles, silver wires, black spots, deformation, cracks, delamination, peeling and discoloration, etc., all related to molding temperature, pressure, flow, time and position. related.

1. Molding temperature: including barrel temperature, plasticizing temperature, nozzle temperature, mold temperature and plastic drying temperature, etc.

Barrel temperature: It is the melting temperature of the plastic. If the barrel temperature is set too high, the viscosity of the plastic after melting is low. Under the same injection pressure and flow rate, the injection speed is fast, and the molded products are prone to flash, silver, discoloration and brittleness.

The temperature of the barrel is too low, the plastic is poorly plasticized, the viscosity is high, the injection speed is slow under the same injection pressure and flow rate, the molded products are easily insufficient, the weld marks are obvious, the dimensions are unstable and there are cold blocks in the products.

Nozzle temperature: If the nozzle temperature is set high, the nozzle will easily drool, causing cold filaments in the product. Low nozzle temperature causes clogging of the mold pouring system. The injection pressure must be increased to inject plastic, but there will be cold material in the molded product immediately.

Mold temperature: If the mold temperature is high, the injection pressure and flow rate can be set lower. However, at the same pressure and flow rate, the product will easily flash, warp and deform, and it will be difficult to eject the product from the mold. The mold temperature is low, and under the same injection pressure and flow rate, the product is insufficiently formed, with bubbles and weld marks, etc.

Plastic drying temperature: Various plastics have different drying temperatures. ABS plastics generally set a drying temperature of 80 to 90°C, otherwise it will be difficult to dry and evaporate the moisture and residual solvents, and the products will easily have silver wires and bubbles, and the strength of the products will also decrease.

2. Molding process pressure: There are pre-molding back pressures: injection molding pressure, clamping pressure, holding pressure, mold support and neutron pressure, injection platform pressure, etc.

Pre-molding back pressure: high back pressure and high storage density means more material can be stored within the same storage volume. Low back pressure means low storage density and less storage material. After setting the storage position, and then making a large adjustment to the back pressure, you must pay attention to resetting the storage position, otherwise it will easily cause flash or insufficient product.

Injection pressure: Different types of plastics have different melt viscosities. The viscosity of amorphous plastics changes greatly with changes in plasticizing temperature. The injection pressure is set according to the welding viscosity of the plastic and the plastic process ratio. If the injection pressure is set too low, the product will be insufficiently injected, resulting in dents, weld marks and unstable dimensions. If the injection pressure is too high, the product will have flash, discoloration and difficulty in mold ejection.

Clamping pressure: It depends on the projected area of the mold cavity and the injection pressure. If the clamping pressure is insufficient, the product will easily flash and increase in weight. If the clamping force is too large, it will be difficult to open the mold. Generally, the clamping pressure setting should not exceed 120par/cm2.

Holding pressure: When the injection is completed, the screw continues to be given a pressure called holding pressure. At this time, the product in the mold cavity has not yet frozen. Maintaining pressure can continue to fill the mold cavity to ensure that the product is full. If the holding pressure and pressure setting are too high, it will bring great resistance to the support mold and the pull-out core. The product will easily turn white and warp. In addition, the mold runner gate will be easily expanded and tightened by the supplementary plastic, and the gate will be broken in the runner. If the pressure is too low, the product will have dents and unstable dimensions.

The principle of setting the ejector and neutron pressure is to set the pressure based on the overall size of the mold cavity area, the core projection area of the inserted core, and the geometric complexity of the molded product. size. Generally, this requires setting the pressure of the supporting mold and the neutron cylinder to be able to push the product.