The role of back pressure and how to set it during injection molding

The role of back pressure and how to set it during injection molding.

1. The effect of back pressure on injection molding is very important, directly affecting the filling of the melt and the quality of plastic parts.

2. Appropriate back pressure can improve the quality of products

3. It can compact the molten material in the barrel, increase the density, increase the injection volume, and ensure the stability of product weight and size;

4. It can "squeeze out" the gas in the molten material, reduce the air marks on the surface of the product, internal bubbles, and improve the uniformity of gloss;

5. Slow down the screw back speed to fully plasticize the molten material in the barrel, increase the mixing uniformity of toner, color masterbatch and molten material, and avoid color mixing in the product;

6. Properly increasing the back pressure can improve the shrinkage of the product surface and the resin filling around the product;

7. It can increase the humidity of the molten material, improve the plasticizing quality of the molten material, improve the fluidity of the molten material when filling the mold, and have no cold glue lines on the surface of the product.

The impact of excessive back pressure on the product

1. The pressure of the molten material at the front end of the barrel is too high, the temperature of the material is high, the viscosity decreases, the counterflow of the molten material in the screw groove and the leakage flow between the barrel and the screw increase, which will reduce the plasticizing efficiency;

2. For plastics with poor thermal stability (such as PVC, POM, etc.) or colorants, due to the increase in the temperature of the melt and the increase in the heating time in the barrel, thermal decomposition is caused, or the degree of discoloration of the colorant increases, and the surface color and gloss of the product become worse;

3. The back pressure is too high, the screw retreats slowly, and the pre-plastic return time is long, which will increase the cycle time and lead to a decrease in production efficiency

4. The back pressure is high and the melt pressure is high. After the injection, the nozzle is prone to melt salivation. When injection next time, the cold material in the nozzle flow channel will block the nozzle or cold material spots will appear in the product;

5. During the injection molding process, due to excessive back pressure, the nozzle will leak glue, waste raw materials and cause the heating ring near the nozzle to burn out;

6. The mechanical wear of the pre-plastic mechanism and the screw barrel increases.

Effect of too low back pressure on products

1. When the back pressure is too low, the screw retreats too fast, the density of the molten material flowing into the front end of the barrel is small (relatively loose), and a lot of air is trapped;

2. It will lead to poor plasticization quality, unstable injection volume, and large changes in product weight and product size;

3. The surface of the product will have shrinkage, gas spots, cold material lines, uneven gloss and other undesirable phenomena;

4. Air bubbles are easy to appear inside the product, and the surrounding area and bone parts of the product are easy to have short shot issues.

Guidance method for back pressure setting

The adjustment of injection molding back pressure should depend on the performance of raw materials, drying conditions, product structure and quality status, and the back pressure is generally adjusted at 3-15kg/cm3. When the surface of the product has a little gas, color mixing, shrinkage, and product size and weight change greatly, the back pressure can be increased appropriately. When the nozzle has glue leakage, salivation, overheating and decomposition of the molten material, discoloration of the product and slow return of the material, it can be considered to reduce the back pressure appropriately.