Injection defects: formation of back pressure in injection molding
- Formation of back pressure
In the process of plastic melting and plasticization, the molten material continuously moves to the front end of the barrel, and more and more, gradually forming a pressure to push the screw backward. In order to prevent the screw from retreating too fast and ensure uniform compaction of molten material, it is necessary to provide the screw with a pressure in the opposite direction, which is called back pressure.
Back pressure, also known as plasticizing pressure, is controlled by adjusting the return throttle valve of the injection cylinder. The back pressure valve is set at the rear of the injection cylinder of the pre plasticizing screw injection molding machine to adjust the oil discharge speed of the injection cylinder when the screw rotates and retreats, so as to keep the cylinder under a certain pressure; The backward moving speed (resistance) of the screw of the whole motor is controlled by the AC servo valve.
- Benefits of properly adjusting back pressure
1. It can compact the molten material, increase the density, and improve the stability of glue injection amount, product weight and size.
2. It can “extrude” the gas in the molten material, reduce the air flower on the product surface and internal bubbles, and improve the gloss uniformity.
3. Slow down the screw backward speed, fully plasticize the melt, increase the mixing uniformity of toner, masterbatch and melt, and avoid color mixing of products.
4. Properly raising the back pressure can improve the shrinkage of the product surface and the glue walking around the product.
5. It can raise the temperature of the melt, improve the plasticization quality of the melt, improve the fluidity of the melt during mold filling, and there is no cold glue grain on the product surface.
3、 When the back pressure is too low, the following problems are easy to occur
1. When the back pressure is too low, the screw retreats too fast, the density of molten material flowing into the front end is small (relatively loose), and there is more air.
2. It will lead to poor plasticization quality, unstable glue injection, and large changes in product weight and product size.
3. The surface of the product will shrink, blow, cold material lines, uneven luster and other adverse phenomena.
4. Bubbles are easy to appear inside the product, and the periphery and bone positions of the products are easy to walk away from the glue.
4、 If the back pressure is too high, the following problems are easy to occur
1. The melt pressure at the front end is too high, the material temperature is high and the viscosity decreases. The reverse flow of the melt in the screw groove and the leakage flow between the barrel and the screw increase, which will reduce the plasticizing efficiency (the amount of plasticized material per unit time)
2. For plastics with poor thermal stability (such as PVC, POM, etc.) or colorants, the temperature of molten material increases and the heating time in the barrel increases, resulting in thermal decomposition, or the discoloration degree of colorants increases, and the surface color / gloss of products becomes worse.
3. If the back pressure is too high, the screw retreats slowly and the pre plastic return time is long, the cycle time will be increased and the production efficiency will be reduced.
4. With high back pressure and high melt pressure, the nozzle is prone to melt salivation after glue injection. The cold material in the nozzle channel will block the nozzle or appear cold material spots in the product during the next glue injection.
5. In the process of beer molding, glue leakage often occurs in the nozzle due to excessive back pressure, which wastes raw materials and burns out the heating ring near the nozzle.
6. The mechanical wear of prefabricated mechanism and screw cylinder increased.
5、 Adjustment of back pressure
The adjustment of injection back pressure shall depend on the performance, drying condition, product structure and quality of raw materials. The back pressure is generally adjusted at 3-15kg / cm3. When the surface of the product has a little airiness, color mixing, shrinkage, and the size and weight of the product change greatly, the back pressure can be increased appropriately. In case of glue leakage, salivation, overheated decomposition of molten material, product discoloration and too slow material return at the nozzle, it can be considered to reduce the back pressure appropriately.
Back pressure is one of the important parameters to control melt quality and output in injection molding process. Appropriate back pressure plays an important role in improving output, which can not be ignored.
