Selection of injection pressure and holding pressure during injection molding

2025-09-28

Injection pressure refers to the pressure exerted by the plunger or screw tip on the plastic melt. Its function is to overcome the flow resistance of the plastic melt from the barrel into the mold cavity, allowing the melt to fill at a certain rate and compacting the melt to compensate for shrinkage. During the injection process, the pressure at the injection machine nozzle is highest to overcome the flow resistance throughout the entire melt flow path. Subsequently, the pressure gradually decreases along the flow length to the front of the melt wave. If the cavity is well vented, the final pressure at the front of the melt is atmospheric pressure. Injection pressure is often indicated by a gauge pressure on injection machines and is generally between 40 and 130 MPa.

The size of the injection pressure depends on the type of injection machine, plastic type, melt temperature, product shape, mold structure, mold temperature, structure and size of the pouring system, and process size.

(1) When selecting the injection pressure of the injection machine type, the first thing to consider is the injection pressure allowed by the injection machine. Only within the rated injection pressure range of the injection machine can the injection pressure required for the specific product be adjusted. Under the same conditions, the injection pressure used by the plunger injection machine should be greater than that of the screw injection machine. This is because the plunger injection machine needs to overcome greater frictional resistance during injection, resulting in greater pressure loss.

(2) The melt viscosity of different plastics is affected differently by injection pressure. For example, the melt viscosity of ABS is not sensitive to temperature, so a higher injection pressure should be used; while the melt viscosity of PE, PP, etc. is more sensitive to temperature, so a lower injection pressure can be used. When using plastics with good fluidity to form simple thick-walled products, the injection pressure can be lower, generally not exceeding 70MPa. If the melt viscosity of the plastic is not too high, the shape of the product is not too complex, and the precision requirements are average, the injection pressure shouldn’t be too high, and can be selected within the range of 70-100MPa. If the plastic has medium or high viscosity and there are certain requirements for product precision, but the product shape is not too complex, the injection pressure should be slightly higher and the range can be 100-140MPa. If the plastic has high viscosity (such as PPO, PSU) and the product has thin walls, complex shapes, and high precision requirements, the injection pressure should be higher, generally 140-180MPa. For high-quality, precision and micro-products, the injection pressure can be 180-250MPa or higher. As the injection pressure increases, the plastic filling speed increases, the weld strength in the product increases, and the density of the product increases. However, the internal stress of the product also increases with the increase of injection pressure, and it is easy to deform after molding. Therefore, products injected with high pressure should be annealed.

(3) Melt temperature During the injection process, the injection pressure and the plastic melt temperature are actually mutually constrained. When the melt temperature is high, the injection pressure should be appropriately reduced; otherwise, the injection pressure should be appropriately increased. When production conditions and product quality standards permit, the use of high temperature and low pressure process conditions is very beneficial for protecting the mold, extending the mold life, and reducing the wear of the machine hydraulic system.

(4) Product shape and mold structure: For products with complex structure, thin wall, large area and long process, higher injection pressure should be used. For products with simple mold structure, large gate size, high barrel temperature and mold temperature, lower injection pressure should be used.

In addition, the following issues should be noted when selecting and controlling injection pressure:

(1) When injection molding plastic products, if there are quality problems such as large dimensional errors in the product and dents on the surface, it means that the injection pressure is insufficient and the injection pressure should be increased appropriately.

(2) When Injection Molding products, if demoulding is difficult and the product has flash, it means that the injection pressure is too high and the injection pressure should be appropriately reduced.

(3) Like other injection process conditions, the selection and control of injection pressure is not isolated. It interacts with other process conditions and shouldn't be overlooked when selecting any process condition. In short, injection pressure is a very important factor in injection process parameters. Many factors influence it, and the relationship is complex. Before formal production, injection molding trials should begin at a low injection pressure, then increase or decrease it based on the quality of the product, and finally determine the appropriate injection pressure.

Holding pressure

Holding pressure refers to the pressure applied to the melt in the mold for a period of time after the injection pressure has completed melt filling (the cooling and condensation period after melt filling). The purpose of holding pressure is to solidify the plastic under pressure and provide sufficient melt replenishment during melt shrinkage and solidification to ensure the density and dimensional accuracy of the finished injection molded product. The two key parameters that require control during the holding process are holding pressure and holding time. Increasing and extending holding pressure and holding time increase cavity pressure, product density, and shrinkage. The choice of holding pressure is influenced by the structural characteristics of the product. In production, holding pressure is generally equal to or less than the injection pressure. High holding pressures produce products with higher density, lower shrinkage, higher dimensional accuracy, and better mechanical properties. However, these pressures can lead to higher residual stresses within the product after demolding, which can easily cause flash and flaking. Too low holding pressures can result in incomplete molding.