holding pressure

Injection pressure,holding pressure and back pressure

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1．Injection Pressure

Injection pressure is exerted by the hydraulic mechanism of the plastic injection molding system. This pressure is transferred from the hydraulic cylinder to the molten plastic through the molding screw, under which the molten plastic will be pushed to flow into the mold sprue (also known as the primary runner of some molds), the primary runner and the sub-runner via the nozzle of the plastic injection molding machine, and then finally get into the mold cavity through the gate. This process is referred to as the injection molding process or the filling process. The purpose of the pressure is to overcome the resistance occurring when the molten plastic is flowing; or to put it another way, the resistance occurring in the flowing process needs to offset by the pressure exerted by the injection molding machine, so as to facilitate smooth filling.

the screws for injection pressure of injection machines

During the injection molding process, the injection nozzle features the highest pressure in a bid to overcome the resistance to flow throughout the whole process. After that, the pressure shows a trend of gradual decrease from the nozzle to the melt front as the molten plastic flows further. If the mold cavity vents well, the final pressure on the melt front will be equivalent to atmospheric pressure.

The injection pressure on molten plastic is influenced by a diversity of factors. To sum up, there are 3 categories: (1). Material factors, such a material type, viscosity, etc.; (2). Structural factors, such as type, quantity and location of the runner system, shapes of mold cavity and product thickness, etc.; (3). Molding process factors.

2．Value & Time of Holding Pressure

When the plastic injection molding process is drawing to an end, the screw will stop rotating but only keep moving forward. At this point, the injection molding process enters the pressure holding phase, during which period of time the injection nozzle continuously feed materials into the cavity, to fill up the empty space caused by product shrinkage. If the pressure is not held after the cavity is filled up, the product will shrink for about 25%. In particular, shrink marks will be left near the ribs due to enormous shrinkage. Usually, the value of the holding pressure is about 85% of the top injection pressure, which, of course, is subject to actualities.

3．Back pressure

Back pressure refers to the pressure that the screw has to overcome during its return action after injecting material. The application of a high backpressure helps distribute the pigments and melt the plastic, but at the same time, it also extends the screw’s return time, decreases the length of plastic fibers and raises pressure in the injection machine. As a result, the backpressure should be kept lower, usually not exceeding 20% of the injection pressure. Some injection machines allow backpressure programming to compensate for screw travel decrease, which will reduce heat input, causing the temperature to drop. However, since it is not easy to predict the changeable result, corresponding machine adjustment will a troublesome task.

How to set the hold pressure?

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The setting of hold 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.

hold pressure setting

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 hold pressure will lead to:

  1. Stress in the sprue area;
  2. Difficult mold release;
  3. Tensile stress on the outer layer;

Gradual decrease of holding pressure during the pressure holding time may be able to (multistage holding pressure):

  1. Reduce warpage, as well as shrinkage difference in the product molding section between the gate and the far end;
  2. Reduce internal stress;
  3. 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 hold 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.injection molding tooling company in china

Hold 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 hold 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, hold 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.