Back pressure is very important for injection molding, which directly affects the filling of melt and the quality of plastic parts.
(1) Proper back pressure can improve the quality of products
① It can compact the molten material in the barrel, increase the density, and improve the stability of the injection amount, product weight and size;
② It can "extrude" the gas in the molten material, reduce the gas bloom and internal bubbles on the surface of the product, and improve the gloss uniformity;
③ Slow down the screw backward speed, make the molten material in the barrel fully plasticized, increase the mixing uniformity of toner, color masterbatch and molten material, and avoid the color mixing phenomenon of products;
④ Properly raising the back pressure can improve the shrinkage of the product surface and the glue movement around the product;
⑤ It can increase the humidity of the melt, improve the plasticizing quality of the melt, improve the fluidity of the melt during mold filling, and make the surface of the product free of cold glue lines.
(2) Influence of high back pressure on products
① The plasticizing efficiency will be reduced if the melt pressure at the front end of barrel is too high, the melt temperature is too high and the viscosity decreases, the reverse flow of melt in screw groove and the leakage flow between barrel and screw increase;
② For plastics (such as PVC, POM, etc.) or colorants with poor thermal stability, thermal decomposition occurs due to the increase of the temperature of the melt and the heating time in the barrel, or the discoloration degree of colorants increases, and the surface color and luster of products become poor;
③ If the back pressure is too high, the screw will retreat slowly, and the time of pre molding will be long, which will increase the cycle time and reduce the production efficiency;
④ When the back pressure is high and the melt pressure is high, it is easy for the nozzle to salivate after the injection. When the next injection, the cold material in the nozzle channel will block the nozzle or the cold material spot will appear in the product;
⑤ In the process of injection molding, due to the excessive back pressure, the nozzle often leaks, which wastes raw materials and causes the heating ring near the nozzle to burn out;
⑥ The mechanical wear of the pre molding mechanism and the screw barrel increases.
(3) Influence of low back pressure on products
① When the back pressure is too low, the screw retreats too fast, the density of molten material flowing into the front end of the barrel is small (relatively loose), and more air is entrained;
② It will lead to poor plasticizing quality, unstable injection amount, and great changes in product weight and size;
③ The surface of products will appear shrinkage, air bloom, cold grain, uneven gloss and other adverse phenomena;
④ Bubbles are easy to appear inside the product, and it is easy to walk around the product and bone without glue.
(4) Guiding method of back pressure setting
The back pressure of injection molding should be adjusted according to the performance of raw materials, drying conditions, product structure and quality conditions. The back pressure is generally adjusted at 3-15kg / cm3. When the product surface has a little air, mixed color, shrinkage and product size, weight change, can be appropriate to increase the back pressure. In case of leakage, salivation, overheating and decomposition of molten material, discoloration of product and slow return of material, back pressure can be reduced appropriately.
