The effect of back pressure on 注塑 is very important, and it directly affects the filling of the melt and the quality of the plastic parts.
(1) Proper 背壓 can improve the quality of products
①It can compact the melt in the barrel, increase the density, and improve the injection volume, product weight and dimensional stability;
②The 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;
③Slow down the retreat speed of the screw to 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;
④ Properly increasing the back pressure can improve the shrinkage of the product surface and the glue running around the product;
⑤It can increase the humidity of the melt, improve the plasticization quality of the melt, and improve the fluidity of the melt when filling the mold, and the surface of the product has no cold glue lines.
(2) The impact of excessive back pressure on products
①The melt pressure at the front end of the barrel is too high, the material temperature is high, and the viscosity decreases. 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;
②For plastics with poor thermal stability (如PVC, POM, 等。) or colorants, the temperature of the molten material increases and the heating time in the barrel increases, resulting in thermal decomposition, or the degree of color change increases, and the surface color and luster of the product Get worse;
③The 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;
④The 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;
⑤During the 注塑機 working process, 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 the screw barrel is increased.
(3) The impact of low back pressure on products
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 fan barrel is small (relatively loose), and there is more air;
②It will lead to poor plasticization quality, unstable injection volume, and large changes in product weight and product size;
③The surface of the product will appear shrinkage, air blooms, cold material lines, uneven gloss and other undesirable phenomena;
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.
(4) Guidance method for back pressure setting
這 back pressure of injection molding should be adjusted according to the properties of the raw materials, dry conditions, product structure and quality conditions. The back pressure is generally adjusted at 3-15kg/cm3. When the product surface 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, you can consider appropriately reducing the back pressure.
