The setting of holding pressure is aimed to prevent resin backflow, while at the same time compensating for resin shrinkage caused during the cooling process, so as to achieve the optimal molding outcome. If the holding pressure is set too high, the product will be prone to flash, over filling or stress concentrating near the gate, etc.; on the other hand, if the holding pressure is too low, excessive shrinkage and dimensional instability will be likely to occur.
Holding pressure only works well along with the settings of pressure switchover point and holding time in the plastic injection molding process.
Insufficient holding pressure will lead to: 1. dents; 2. bubbles; 3. increased shrinkage rate; 4. decreased product dimensions; 5. larger dimensional fluctuation; 6. inner-layer orientation caused by melt backflow, etc.
Excessive holding pressure will lead to:
- Stress in the sprue area;
- Difficult mold release;
- Tensile stress on the outer layer;
Gradual decrease of holding pressure during the pressure holding time may be able to (multistage holding pressure):
- Reduce warpage, as well as shrinkage difference in the product molding section between the gate and the far end;
- Reduce internal stress;
- Reduce energy consumption.
The setting of pressure holding time is aimed to control the duration of the holding pressure effect. An insufficient holding time will result in product dimensional and weight instability. However, if the holding time is set to be too long, molding efficiency will be affected. A proper pressure holding time should last till the gate solidifies. In the meantime, appropriate coordination between the value and time of holding pressure is able to bring the effect of the procedural holding pressure into full play. The purpose of holding pressure is to seal the sprue and compensate for material shrinkage after injection is completed. As a result, the holding pressure must be greater than the internal pressure.
If the holding time is set to be shorter than the maximum effective pressure holding time, i.e. insufficient holding time, the following results may occur: 1. dents; 2. bubbles; 3. underweight; 4. smaller dimensions; 5. internal orientation caused by melt backflow; 6. greater warpage, especially for semi-crystalline materials; 7. larger dimensional fluctuations; 8. increased shrinkage, etc. The set holding time must effectively last till the sprue solidifies. Usually, a sufficient holding time is approx. 30% of the cooling time.
In general, injection pressure control is composed of first-stage pressure, second-stage (holding) pressure or more stages of injection pressure control. An appropriate pressure switchover plays an important role in the avoidance of overpressure, overflow or incomplete filling. The specific volume of a molded product is dependent on the melt pressure and temperature during the pressure holding time when the gate is closed. Every time when switching from pressure holding to product cooling, if the pressure and temperature can be kept consistent, the specific volume of the product will remain unchanged. Under consistent mold temperature conditions, the value of holding pressure is the most important parameter that determines product dimensions, while the value of holding pressure and temperature are the most important variables that influence product dimensional tolerance. For example, after injection is completed, the holding pressure decreases immediately, and when the surface layer reaches certain thickness, the holding pressure will rise again. This way, thick-walled large products can be molded with a low clamping force, so as to eliminate dents and flash etc.
Holding pressure and speed are usually 50% – 65% of the top injection pressure and speed. That is to say, the holding pressure is approx. 0.6 – 0.8MPa lower than the injection pressure that feeds plastic into the mold cavity. Since the holding pressure is lower than the injection pressure, during the relatively long holding time, the hydraulic pump will be working under a low load, so its service life will be accordingly extended. At the same time, power consumption of the pump motor will also be brought down.
***While facilitating smooth and complete mold filling, holding pressure can also eliminate product defects like weld lines, dents, flash and warpage, etc. It is thus very helpful for the production of various types of parts, including thin-walled parts, multi-headed small parts, long-cycle large parts, as well as parts with an unbalanced cavity or even those with insufficient clamping force.
During the plastic injection molding process, the molten material shrinks due to cooling. However, the screw needs to keep moving forward slowly, so that the molten plastic in the barrel can continue flowing into the cavity, to compensate for the shrinkage. This process is known as pressure holding. To put it simply, its purpose is to compensate for product shrinkage, as well as ensure a stable production process. In addition, the holding pressure is also able to adjust product dimensions, and effectively eliminate weld lines, dents, flash and warpage at the same time.
As a matter of fact, holding pressure and injection mean the same thing – applying a force to push the screw forward. The only difference is that in the injection process, the screw is pushed to move at a set injection speed and the max injection pressure; during the pressure holding time, the screw is pushed to move at a set injection pressure and the top pressure holding speed.
Multistage injection molding is able to adjust the speed and pressure at which the raw material flows into the mold cavity. This way, the defect rate of some complicated structure products will be decreased, and at the same time, the small inserts in the mold will be well protected. For example, when the raw material flows into the mold, its speed and pressure can be reduced when reaching a small insert, so the insert will not be prone to damage. Multistage holding pressure is also able to reduce the occurrence of dents – another benefit that helps reduce product defects.