Injection pressure refers to pressure exerted on plastic melt by top of plunger or screw. Its function is to overcome flow resistance of plastic melt flowing from barrel to mold cavity, giving melt a certain filling rate, compacting and feeding melt. During injection process, pressure at nozzle of injection machine is the highest to overcome flow resistance in the whole process of melt. After that, pressure gradually decreases along flow length to front wave front of melt. If inside of cavity is well vented, final pressure at the front of melt is atmospheric pressure. On injection machine, gauge pressure is commonly used to indicate injection pressure, which is generally between 40 and 130MPa.
Size of injection pressure depends on type of injection machine, plastic variety, melt temperature, product shape, mold structure, mold temperature, structure, size and process size of gating system.

 

When selecting injection pressure, injection pressure allowed by injection machine should be considered first. Only within rated injection pressure range of injection machine can injection pressure required by specific product be adjusted. Under other conditions being same, injection pressure used by plunger injection machine should be larger than that of screw type. This is because plunger injection machine needs to overcome a large frictional resistance during injection, resulting in a large pressure loss.

Different plastics have different melt viscosity affected by injection pressure. For example, ABS, whose melt viscosity is not sensitive to temperature, should use higher injection pressure; while PE, PP, etc., whose melt viscosity is more sensitive to temperature, a lower injection pressure can be used. When using plastic with good fluidity to mold thick-walled products with simple shapes, injection pressure can be lower, generally not exceeding 70MP. If melt viscosity of plastic is not too high, shape of product is not too complicated and accuracy requirements are general, injection pressure should not be too high, and it can be selected in range of 70-100MPa. If plastic is of medium or high viscosity, there are certain requirements for accuracy of product, but shape of product is not too complicated, injection pressure should be slightly higher, and preferable range is 100-140MPa. If plastic viscosity is high (such as PPO, PSU) and product is thin in wall, complex in shape, and requires high precision, injection pressure should be high, generally 140-180MPa. For high-quality, precise and micro products, injection pressure can be 180-250MPa or higher.
As injection pressure increases, filling speed of plastic increases, welding strength in product increases, and density of product increases. However, internal stress of product also increases with increase of injection pressure, and it is easy to deform after molding, so product using high pressure injection should be annealed.

During injection process, injection pressure and plastic melt temperature actually restrict each other. When melt temperature is high, injection pressure should be appropriately reduced; otherwise, injection pressure should be appropriately increased. In the case of production conditions and product quality standards permitting, use of high temperature and low pressure process conditions is very beneficial to protect mold, prolong service life of mold, and reduce wear of hydraulic system of machine.

Product has complex structure, thin wall, large area and long process, so a higher injection pressure should be used. Mold structure is simple, gate size is large, barrel temperature and mold temperature are high, and a lower injection pressure should be used.
In addition, 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 errors in external dimensions of products and dents on the surface, it means that injection pressure is insufficient, and injection pressure should be appropriately increased.
2) When injection molding products, if it is difficult to demold and product has flash, it means that injection pressure is too high, and injection pressure should be appropriately reduced.
3) Like other injection process conditions, selection and control of injection pressure are not isolated. It affects and interacts with other process conditions, which should not be ignored when selecting any process conditions. In short, injection pressure is a very important factor in parameters of injection process conditions. There are many influencing factors and relationship is more complicated. Before formal production, injection molding should be started from a lower injection pressure, then increase or decrease should be determined according to quality of product, and finally a reasonable injection pressure should be determined. Table 1 lists available injection pressure ranges for some plastics for reference.
Table 1 Optional injection pressure range for some plastics

Holding pressure refers to pressure that continues to be applied to melt in mold within a period of time after injection pressure completes melt filling (cooling and condensation time after melt filling).
Function of pressure holding is to solidify plastic under pressure, provide sufficient melt supplement when melt condenses and shrinks, so as to ensure shape density and dimensional accuracy of injection product after molding.
Two main parameters to be controlled during holding process are holding pressure and holding time. With increase and extension of holding pressure and holding time, cavity pressure increases, density of product also increases, and shrinkage rate of product decreases.
Choice of holding pressure is affected by structural characteristics of product. In production, holding pressure is generally equal to or less than injection pressure. When holding pressure is large, products with high density, low shrinkage rate, high dimensional accuracy and good mechanical properties can be obtained. If holding pressure is too small, molding will be insufficient