First, the formation of back pressure
In the process of plastic melting and plasticizing, the melt continuously moves to the front of the barrel, and more and more pressure gradually increases to push the screw backwards.In order to prevent the screw from retreating too fast and ensure the uniform compacting of the melt, Screw provides a pressure in the opposite direction, the reverse direction to prevent the screw back pressure called back pressure.
Back pressure is also known as plastication pressure, and its control is achieved by adjusting the injection cylinder back to the throttle valve.Pre-plastic screw injection machine injection cylinder rear are equipped with back pressure valve, adjust the screw rotation back injection cylinder discharge Oil speed, the cylinder to maintain a certain pressure; full motor screw back speed (resistance) is controlled by the AC servo valve.
Second, the appropriate back pressure adjustment benefits
1, the barrel can melt compaction, increase density, increase the amount of plastic, product weight and size stability.
2, the melt gas can be 'out' to reduce the gas flow on the surface of the product, the internal bubbles, improve gloss uniformity Slow down the screw back speed, so that the barrel of the plastic melt full plastic, increase the toner, Color Masterbatch and melt mixing uniformity, to avoid mixed color products appear.
3, slow down the screw back speed, so that the barrel of the plastic fully plasticized, increase the mixing of toner, masterbatch and melt uniformity, to avoid mixing products phenomenon.
4, appropriate to enhance the back pressure, can reduce the surface of the product shrinkage and the glue around the product situation.
5, can increase the temperature of the melt, plastic melt plastic to improve the quality, improve melt flow when filling, product surface without cold gel pattern.
Third, the back pressure is too low, prone to the following problems
1, the back pressure is too low, the screw back too fast, into the front end of the barrel melt density is small (more loose), into the air and more.
2, will lead to plasticizing poor quality, shotcrete instability, product weight, product size changes.
3, the product surface will shrink, gas flowers, cold lines, uneven gloss and other undesirable phenomena.
4, prone to bubbles inside the product, the product around the bones and easy to get rid of plastic.
Fourth, high back pressure, prone to the following problems
1, the front end of the barrel melt pressure is too high, the material temperature is high, the viscosity decreases, the melt in the screw slot countercurrent and the barrel and screw gap leakage flow increases, will reduce the plasticization efficiency (per unit time plasticization The amount of material).
2, the thermal stability of poor plastic (such as PVC, POM, etc.) or colorant, due to the temperature of the melt increased and the barrel in the heating time caused by thermal decomposition, or colorant discoloration degree increases, the surface color / Gloss worse.
3, back pressure is too high, screw back slowly, plastic back to the material for a long time, will increase the cycle time, leading to decline in production efficiency.
4, the back pressure is high, melt pressure is high, the nozzle prone to melt after the plastic injection salivation phenomenon, the next injection, the nozzle of the cold runner will block the nozzle or cold blobs appear in the product.
5, in the process of beer molding, often due to back pressure is too large, the phenomenon of leakage of glue nozzle, waste of raw materials and lead to burn nozzle near the heating ring.
6, pre-plastic body and screw barrel mechanical wear increased.
Five, back pressure adjustment
Back pressure injection molding adjustment should be depending on the performance of raw materials, drying conditions, product structure and quality conditions, back pressure is generally adjusted in the 3-15kg / cm3. When the product surface a little gas flowers, color mixing, shrinking and product size , Weight change, the industry that the injection may be appropriate to increase the back pressure.When the nozzle leakage of plastic, salivation, melt overheating decomposition, the product discoloration and back to the material is too slow to consider appropriate to reduce the back pressure.
Backpressure is one of the important parameters controlling melt quality and product quality in the injection molding process. Appropriate backpressure plays an important role in improving product quality and can not be neglected.