The influence of injection molding back pressure on injection molding

author: Universe plastic

2024-04-23

What is injection molding back pressure

During the process of melting and plasticizing plastic materials, the molten material continues to push toward the front end of the barrel, and more and more molten materials gradually form a pressure, which will cause the push screw to retreat.

In order to prevent the screw from retreating too fast and ensure uniform compaction of the molten material, a pressure in the opposite direction needs to be provided to the screw. The pressure in the opposite direction that prevents the screw from retreating is called injection back pressure, also known as plasticizing pressure.

Adjustment of injection molding back pressure

The injection back pressure is controlled by adjusting the oil return throttle valve of the injection cylinder. There is a back pressure valve controller behind the injection cylinder of the screw injection molding machine, which adjusts the speed at which the injection cylinder drains oil when the screw rotates and retreats, so that the cylinder maintains a certain pressure.

Advantages of proper injection back pressure

1. It can compact the molten material in the injection molding barrel, increase the density, and improve the injection volume, product weight and dimensional stability.

2. It can squeeze out the gas in the molten material, reduce the air splash and internal bubbles on the surface of the product, and improve the gloss uniformity.

3. Slow down the screw retreat speed to fully plasticize the melt in the barrel, increase the mixing uniformity of toner, color masterbatch and melt, and avoid color mixing in the product.

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

5. It can increase the temperature of the melt, improve the plasticizing quality of the melt, improve the fluidity of the melt when filling the mold, and ensure that there are no cold glue lines on the surface of the product.

Problems that occur when injection molding back pressure is too low

1. The screw retreats too fast, the density of the melt flowing into the front end of the injection barrel is small, and a lot of air is trapped.

2. The product quality is poor, the amount of glue injected is unstable, and the product weight and product size vary greatly.

3. Undesirable phenomena such as shrinkage, air blooms, cold grains, uneven gloss, etc. may appear on the surface of the product.

4. Bubbles are prone to appear inside the product, and the edges and bones of the product are prone to become dissatisfied with glue.

Problems that occur when injection molding back pressure is too high

1. The molten material at the front end of the injection barrel will flow back in the screw groove due to too high pressure, high temperature and reduced viscosity.

2. For plastics or colorants with poor thermal stability, the temperature of the melt increases and the heating time in the barrel increases, resulting in thermal decomposition, or the degree of discoloration of the colorant increases, and the surface color and gloss of the product becomes worse.

3. 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 melt pressure is high, and the nozzle is prone to melt salivation after injection. The next time the glue is injected, the cold material in the nozzle flow channel will block the nozzle or cold material spots will appear in the product.

5. The mechanical wear of the pre-plasticizing mechanism and screw barrel increases.

Professional adjustment of injection molding back pressure

The adjustment of injection molding back pressure should depend on the performance of the raw materials, drying conditions, product structure and quality conditions. The back pressure is generally adjusted between 3-15kg/cm3.

When the surface of the product has air spots, mixed colors, shrinkage, or product size and weight changes greatly, the back pressure can be appropriately increased.

When the nozzle leaks glue, drools, melts decomposes, discolors the product, or returns material too slowly, the back pressure can be appropriately reduced.

Injection molding back pressure is an important parameter for controlling product quality, and appropriate back pressure plays an important role in improving product quality.

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