How to find the balance point between back pressure and loosening in injection molding process?
- time:
- 2019-05-13
Back pressure and loosening are two very important parameters in injection moulding process, which will directly affect the quality of products. Although there is no intrinsic relationship between the two, their roles are different, but they will complement each other when debugging the process, so the performance of the two in the process and product appearance quality is mutually influential.
How to balance the relationship between the two in the actual process debugging, first of all, we need to know what is back pressure, loosening, and the role of both in the process debugging.
Definition, formation and function of back pressure
One
Definition of Backpressure
In the process of screw storage, the screw rotates backwards, and more and more materials are accumulated in the front of the screw. These materials accelerate the screw to squeeze backwards. But in order to compact the material in front of the screw, it is necessary to give the screw a push forward force, which is back pressure (also known as plasticizing pressure).
Two
Formation of Back Pressure
In the process of plastic melting and plasticizing, the melt is continuously conveyed by screw to the front end of the barrel (metering room), and more and more, gradually forming a pressure to push the screw back. In order to prevent the screw from retreating too fast and ensure the uniform compaction of the melt, it is necessary to provide the screw with a pressure in the opposite direction, which is called back pressure to prevent the screw from retreating. Its control is realized by adjusting the return throttle valve of the injection cylinder. Back pressure valves are installed at the back of injection cylinder of pre-plasticized screw injection machine to adjust the speed of oil release from injection cylinder when the screw rotates backward so as to maintain a certain pressure on the cylinder.
Three
The role of back pressure
3.1. Increase density to improve the stability of weight and size of products.
3.2. It is beneficial to remove the gas from the solution in the barrel and reduce the gas sparks and bubbles on the surface of the product.
3.3. Slow down the speed of screw retreat and make the melt in the barrel fully plasticized.
3.4. The shrinkage of product surface can be improved by properly raising back pressure (suitable for the mould and product of non-sequential valve gate)
3.5. It can increase the temperature of the melt, improve the plasticizing quality of the melt and improve the fluidity of the melt during filling.
Effect of too low back pressure on process and product quality
1. When the back pressure is too low, the screw retreats too fast, the melt density flowing into the front of the barrel is low (loose), and more air is clamped into it.
2. It will lead to poor plasticization, unstable material quantity, and great changes in product quality and product size.
3. The surface of products will be shrinkage, cold grain and uneven gloss.
4. Bubbles are easy to appear in the interior of the product, and the outline and tendon of the product are easy to be missing.
5. Low temperature of solution, high viscosity and poor liquidity of solution.
Effect of too high back pressure on process and product quality
1. The melt pressure at the front end of the barrel is too high, the material temperature is high, the viscosity decreases, and it is easy to fill, but it will increase the product contour flying edge.
2. The higher the back pressure is, the longer the storage time will be, so the thermal decomposition of the solution will be caused by the increase of the heating time in the barrel, and the change of the surface color/gloss of the product will affect the apparent quality.
3. Excessive back pressure, slow screw back and long storage time will increase the forming cycle time, resulting in a decline in production efficiency.
4. High back pressure, high melt pressure and easy salivation of non-sequential valve gate result in cold material blockage in gate runner or cold material spot on product surface during next injection.
5. The mechanical wear of screw and barrel increases.
Definition and function of loosening
Definition of retirement
One
Before sending back
At the end of holding pressure, before the screw rotates and discharges start, properly withdrawing the screw can reduce the pressure of the front end solution in the die (reduce the stress of the product). This is the former loosening, which is seldom used in general.
Two
Loose back
When the screw is rotated and stored, the proper withdrawal of the screw can reduce the pressure of the solution at the front end of the screw. This is backward loosening. Back loosening is a commonly used parameter, which can prevent the nozzle from salivating and the tapping of the material head.
Effect of loosening on process and product quality
1. Too little loosening will cause salivation of nozzle and gate, serious tapping of material head, which will result in cold material blocking gate or cold material mark on product surface during next injection, but when screw withdraws, it will not inhale a large amount of air and will not form net-like scattering silver thread on product surface.
1. Excessive loosening will prevent nozzle and gate from salivating and tapping. However, when screw withdraws, a large amount of air will be inhaled, which will easily lead to mesh silver lines on product surface, especially on gate accessories.
Adjustment of Back Pressure and Loosening
1. When increasing back pressure, we should observe whether there is salivation at the gate of fixed die or whether the tapping is serious. If there is, we should increase the loosening position appropriately, but also prevent the appearance of craze near the gate of product surface.
1. When the loosening position is increased, it is necessary to observe whether there are net scattering crazes near the gate on the surface of the product. If there are any, it is necessary to increase the back pressure properly, but also to prevent the salivation of the gate and the tapping of the material head.
Therefore, the adjustment of back pressure is proportional to the size of loosening.
