How important is back pressure in the injection molding process?
Formation of back pressure
In the process of plastic melting and plasticization, the molten material continuously moves to the front end of the barrel (in the measuring chamber), and more and more, gradually forming a pressure, pushing the screw backward. In order to prevent the screw from retreating too fast and to ensure uniform compaction of the melt, it is necessary to provide a pressure in the opposite direction to the screw. The pressure in the opposite direction to prevent the screw from retreating is called back pressure.
Back pressure is also called plasticizing pressure, and its control is achieved by adjusting the return throttle valve of the injection cylinder.
Back pressure pre-plasticized screw injection molding machine is equipped with a back pressure valve at the rear of the injection cylinder, which adjusts the speed of the injection cylinder when the screw rotates back, so that the cylinder maintains a certain pressure; the screw back movement speed (resistance) of the full motor is determined by AC servo valve control.
The benefits of properly adjusting back pressure:
1. It can compact the melt in the barrel, increase the density, and improve the injection volume, product weight and dimensional stability.
2. The gas in the melt can be "squeezed out" to reduce air blooms and internal bubbles on the surface of the product, and improve the uniformity of gloss. Slow down the retreat speed of the screw, make the melt in the barrel fully plasticize, increase the uniformity of the mixing of toner, masterbatch and melt, and avoid color mixing in the product.
3. Slow down the retreat speed of the screw to fully plasticize the melt in the barrel, increase the mixing uniformity of toner, masterbatch and melt, and avoid color mixing in the product.
4. Properly 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 plasticization quality of the melt, and improve the fluidity of the melt when filling the mold. There is no cold glue pattern on the surface of the product.
When the back pressure is too low, the following problems are likely to occur:
1. When the back pressure is too low, the screw retreats too fast, the density of the melt flowing into the front end of the barrel is small (relatively loose), and there is more air trapped.
2. It will lead to poor plasticization quality, unstable injection volume, and large changes in product weight and product size.
3. Undesirable phenomena such as shrinkage, air bloom, cold material lines, uneven gloss, etc. will appear on the surface of the product.
4. Bubbles are easy to appear inside the product, and the periphery and bones of the product are easy to move and dissatisfied with the glue.
Excessive back pressure is prone to the following problems:
1. The melt pressure at the front end of the barrel is too high, the material temperature is high, and the viscosity is reduced. The reverse flow of the melt in the screw groove and the increase in the leakage flow between the barrel and the screw will reduce the plasticization efficiency (plasticization per unit time) The amount of material).
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 of the product /The gloss becomes 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 reduce the production efficiency. 4. The back pressure is high and the melt pressure is high. After the glue is injected, the nozzle is prone to melt drooling. When the glue is injected next time, the cold material in the nozzle runner will block the nozzle or cold material spots will appear in the product.
5. In the process of beer molding, the nozzle leaks due to excessive back pressure, which wastes raw materials and causes the heating ring near the nozzle to burn out.
6. The mechanical wear of the pre-plastic mechanism and screw barrel increases.
Adjustment of back pressure:
The back pressure of injection molding should be adjusted according to the properties of the raw materials, drying conditions, product structure and quality conditions. The back pressure is generally adjusted at 3-15kg/cm3. When the surface of the product has a little air bloom, color mixing, shrinkage and large changes in product size and weight, the back pressure can be appropriately increased. When the nozzle leaks, salivates, melts overheated and decomposes, the product discolors, and the material return is too slow, consider appropriately reducing the back pressure. Back pressure is one of the important parameters for controlling melt quality and product quality in the injection molding process. Proper back pressure plays an important role in improving product quality and cannot be ignored!
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