The influence of back pressure on injection molding

Back pressure plays a very important role in injection molding, directly affecting the filling of the melt and the quality of the plastic parts.

Appropriate back pressure

1. It can compact the melt in the barrel and increase the melt density. And then, it can also increase the shot size and guarantee the stability of the product weight and size.
2. It can squeeze out the gas in the melt to reduce gas marks and internal bubbles on the product surface. It helps to improve the uniformity of gloss.
3. Slow down the screw retreat speed to fully plasticize the melt in the barrel. It also helps to increase the mixing uniformity of color powder, masterbatch and melt, and avoid color mixing in the product.
4. Improve the shrinkage of the product surface and the flash around the product.
5. Increase the melt humidity to improve the plasticization quality of the melt. Therefore, there is no cold melt mark on the  product surface because the melt fluidity is improved when filling the mold.

Too high back pressure

1. At the front end of the barrel, the melting pressure and temperature will be too high, which will decrease the melt viscosity. Therefore, not only will the melt backflow in the screw groove increase, but the leakage flow in the gap between the barrel and the screw will also increase. In this case, it will reduce the plasticization efficiency.
2. For plastics (PVC/POM) or colorants with poor thermal stability, as the melt temperature and the heating time increases in the barrel, it will be thermal decomposition or high colorant discoloration degree, which will eventually lead to poor color and poor gloss of the product surface.
3. The screw retraction is slow due to too high back pressure. It will extend pre-plastic recycling time, increase the cycle time and reduce production efficiency.
4. The nozzle melt is easy to generate the melt salivation after injection due to the too high back pressure. Under the next injection, the cold slugs in the runner will block the gate or generate cold flow marks in the product.
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.
6. It will increase the mechanical wear of the pre-plastic mechanism and the screw barrel.

Too low back pressure

1. The screw will retreat too fast with too low pressure. In this case, the density of the melt flowing into the front end of the barrel becomes low, resulting in more trapped air inside.
2. It will lead to poor plasticization quality, unstable shot size, and large changes in product weight and product size.
3. The surface of the product will have sink marks, gas marks, cold flow marks, uneven gloss and other defects.
4. It is easy to generate bubbles inside the product and the short shot around the product and the rib.

Guidance method for back pressure setting

The back pressure adjustment of injection molding should be determined according to the property, drying conditions, product structure and quality conditions of the raw materials. Generally, the back pressure is adjusted to 3-15kg/cm3. When there are a few gas marks, mixed colors, shrinkage on the product surface, and large changes in product size and weight, the back pressure can be appropriately increased. If there is nozzle leakage, melt salivation, thermal decomposition of the melt, product color change, delayed screw retraction, it can be considered to reduce the back pressure properly.