Influence of Back Pressure on Injection Molding

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

1. Appropriate back pressure can improve the quality of products

1.1 It can compact the molten material in the barrel, increase the density, increase the amount of glue shot, and ensure the stability of product weight and size.

1.2 It can “squeeze out” the gas in the molten material, reduce the gas spots on the surface of the product, internal air bubbles, and improve the uniformity of gloss.

1.3 Besides, it can 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.

1.4 Appropriately increasing the back pressure can improve the shrinkage of the product surface and the glue around the product.

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

2. The impact of excessive back pressure on the product

2.1 The pressure of the molten material at the front end of the barrel is too high, the material temperature 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.

2.2 For plastics with poor thermal stability (such as PVC, POM, etc.) or colorants, the temperature of the melt increases and the heating time in the barrel causes thermal decomposition, or the degree of discoloration of the colorant increases, and the surface color and gloss of the product worse.

2.3 If the back pressure is too high, the screw retreats slowly, and the pre-plastic material returns for a long time, which will increase the cycle time and lead to a decrease in production efficiency.

2.4 If the back pressure is high, then the melt pressure will be also high. After the injection, the nozzle is prone to melt salivation. When the glue is injected next time, the cold material in the nozzle flow channel will block the nozzle or cold material spots will appear in the product.

2.5 During the injection molding process, the nozzle often leaks glue due to excessive back pressure, which wastes raw materials and causes the heating ring near the nozzle to burn out.

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

Influence of Back Pressure on Injection Molding

3. The impact of overly low back pressure on the product

3.1 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 fan barrel will small (relatively loose), and a lot of air will be trapped.

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

3.3 The surface of the product will have shrinkage, air marks, cold lines, uneven gloss and other undesirable phenomena.

3.4 Air bubbles are prone to appear inside the product, and the surrounding area and bone parts of the product are prone to dissatisfaction with glue.

4. 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, with theproduct size and weight changing greatly, then increase the back pressure 
  • When the nozzle has glue leakage, salivation, overheating and decomposition of the molten material, discoloration of the product and slow return of the material, then consider toreduce the back pressure appropriately.

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