How do you calcuate injection pressure?

Table of Contents

Injection pressure is the pressure at which the mold is filled. In other words, the force exerted by the reciprocating screw pushes the molten plastic resin into the mold cavity to approximately 95% of its capacity.

The Power of Injection Pressure Molding during the filling phase of the injection molding cycle, a higher injection pressure may be required to maintain injection speed at the required level. After the mold is filled, high pressure is no longer needed.

Injection pressure molding is balanced against the clamping pressure of the machine and is calculated based on the size and shape of the part and the size of the gates.

Most injection molding plants will recommend 2.5 times the surface square inches of the part and an additional 10% as a safety factor. If you have a 120-square-inch part, you will need a injection molding machine with 300 tons of pressure.

What is injection molding pressure?

The force exerted by a plastic that is transformed into a melt (viscous flow state) at high temperature and pushed into the mold is called injection pressure.

To accomplish the highly technical process of injection molding plastic components, injection molding machines exert an extreme amount of pressure.

Injection pressure is the pressure of the melt acting on the head of the screw during injection molding. It is used to overcome the resistance to the flow of plastic through nozzles, runners, gates, and mold cavities, and to compact the cavity.

The size of injection pressure is related to many factors such as the variety of plastic, the complexity of the plastic part, the wall thickness of the plastic part, the structure of the nozzle, the size of the mold gate, and the types of the injection part, etc. It is usually 40-200MPa.

What is Tonnage?

The tonnage of an injection molding machine refers to the clamping force rate of the injection mold machine. For example, a 50-ton machine can produce a clamping force that is equivalent to 50 tons. Size is also important in injection-molded machines, as the larger the tonnage rate, the larger the machine.

Classification of injection molding pressure

In the process of injection molding, we often encounter three pressure parameters, which are injection pressure, holding pressure, and backpressure, and directly affect the plasticization of plastic and product quality.

Injection pressure

In the current production, the injection pressure of almost all injection machines is based on the pressure applied to the plastic by the top of the plunger or screw (converted from the oil circuit pressure).

On a spiral injection molding machine, the nozzle pressure (Nozzle pressure refers to the pressure inside the nozzle that causes pressure on the plastic to flow) is about 10% less than the injection pressure. In the case of a piston injection molding machine, the pressure loss can reach up to 50%.

Injection pressure is the pressure of the melt acting on the head of the screw during injection molding. It is used to overcome the resistance to the flow of plastic through the nozzle, runner, gate, and mold cavity, and to compact the cavity pressure.

Holding pressure

Holding pressure means that the screw does not retreat immediately after the injection process, but continues to exert pressure on the melt at the front end.

During the holding pressure stage, the plastic in the cavity becomes smaller due to cooling and shrinkage. If the gate is not frozen, the screw slowly advances under the action of holding pressure, so that the plastic continues to be injected into the cavity to make up for the shrinkage. The general holding pressure is ≤ injection pressure.

Holding pressure is the pressure that is maintained on the melt after the mold is filled and until the gate freezes or the pressure is removed by cycle timer control. In most cases, it’s holding pressure that determines shrinkage, not the initial injection pressure.

Backpressure

When screw injection molding machines are used, the pressure on the top of the screw when the melt is turned back by the screw is called plasticizing pressure, also known as backpressure. The magnitude of this pressure can be adjusted by the relief valve in the hydraulic system.

In injection, the magnitude of the plasticizing pressure varies with the design of the screw, the quality requirements of the product, and the type of plastic to be injected.

If these conditions and the screw speed are said to be constant, increasing the plasticizing pressure will enhance the shearing effect, i.e. it will increase the temperature of the melt, but it will reduce the plasticizing efficiency, increase backflow and leakage, and increase the driving power.

In addition, increasing the plasticizing pressure often leads to a uniform temperature of the melt, uniform mixing of the color, and discharge of gas from the melt.

In operation, the decision of plasticizing pressure should be as low as possible under the premise of ensuring the excellent quality of the product. The specific value varies with the variety of plastic used, but usually rarely exceeds 20 kg/cm2.

Injection pressure calculation method

In the process of new mold trial and injection production, people are accustomed to taking the value displayed on the screen of the injection molding machine as the injection pressure, but in fact, the value on the display of the injection molding machine is not the real injection pressure, but only a proportional value. What is the real injection pressure? This needs to be calculated.

There are two values in the parameter table of the injection molding machine: one is the maximum injection pressure and the other is the maximum pump pressure. The actual injection pressure of the gun nozzle in the production process should be equal to these two parameter values divided by each other and then multiplied by the first injection pressure value on the display of the injection molding machine.

The following is an example of the injection molding machine in our injection molding workshop to calculate the injection pressure.

The injection pressure will affect the injection speed and the product appearance defects such as air pattern, punching pattern, and wavy pattern, therefore, the ratio of injection pressure displayed on the screen of the injection molding machine should not exceed 80.

Note: At present, some of the plastic injection molding machines in the injection workshop are A-type screws, while most of the injection machines are still B-type screws.

How to adjust MA1600/540 injection molding machine technical parameters table
Parameter name Unit Screw type Remarks

ParameterUnitABC
ScrewDiametermm404550
ScrewDiametermm404550
ScrewRatio L/D 22.52018
Theoretical Injection Capacitycm3253320395
Injection weightg230291359
Injection PressureMpa215169137
Plasticizing Capacityg/s13.91821.6
Clamping forceKN1600
Mold transfer Strokemm430
Tie bar inner distancemm470*470
Max.oil pump pressureMpa16
Oil pump powerKw15
Electric heating powerKw9.75
Injection rateg/s117148183

How to adjust the injection pressure?

In our machine tuning, we usually use multi-stage injection. The first stage injection controls the gate, the second stage injection controls the main body, the third stage injection fills 95% of the product, and then the pressure is maintained to beat the complete product.

The injection speed controls the melt filling rate, the injection pressure is the guarantee of the filling rate, the injection position controls the melt flow position, and the holding pressure is used to adjust the product weight, size, deformation, and shrinkage.

Considerations for injection pressure selection

  1. When the mold temperature or storage temperature decreases during the adjustment, it is necessary to set a higher injection pressure.
  2. For materials with good flowability, the lower injection pressure is recommended, while for materials with high glassiness and viscosity, higher injection pressure is recommended.
  3. The thinner the thickness of the product, the longer the process, and the more complex the shape, the higher the injection pressure used, which is conducive to filling molding.
  4. The scrap rate of the product is directly related to whether the injection pressure is set reasonably or not.

Summary

When producing injection molded products, selecting and debugging reasonable parameters is a prerequisite for producing high-quality products.

Therefore, high-quality injection molds + experienced injection molding engineers = high-quality products.

These conditions can be met by ZetarMold. If you have an injection molding project that needs to be made, just contact us and let us take care of the rest.

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