This chapter explains each molding condition (injection, temperature, and ejector)
Molding conditions differ depending on each molding material. A good part is made only with the proper setting of each condition.
Air-tightness is required of the mold to prevent development of flashes during the molding process.
On the other hand, good venting is necessary in order to emit the air inside the mold cavity. Setting conditions for injection molding is said to be difficult. It is often the case that there are few defects if conditions are set based on experience. The following are general molding conditions for your reference.
Injection Conditions
Injection pressure
The injection pressure required for molding differs considerably depending on the molding material, shape, and preciseness of dimension.
There is primary injection pressure with which molding material is filled in the mold cavity. There is also a secondary injection pressure (which is also called holding pressure) applied to prevent sink mark after mold filling.
At the initial setting of conditions, only primary pressure is applied to satisfy the filling condition, without considering secondary pressure. This method helps specify the role of primary and secondary pressure. This method enables us to recognize what is necessary for the ultimate molding condition.
The pressure inside the cavity changes depending on several conditions, such as the type of molding material, the structure and dimension of the clamping device, cavity shape and molding condition. Therefore, it is difficult to obtain by calculation or measurement.
Injection pressure is said to decrease by 30 – 50% within the cavity due to loss of pressure.
[Pressure inside cavity of plastics]
Resin Name |
Resin
Temperature [
 ] |
Injection
Pressure [kgf/
 ]
|
Pressure
Inside the
Cavity [kgf/
] |
Polyethylene (PE) |
180-300 |
600-1400 |
230-320 |
Polypropylene (PP) |
200-300 |
600-1400 |
220-320 |
Polyvinyl chloride (PVC) |
150-180 |
1000-1500 |
280-290 |
Polystyrene (PS) |
180-315 |
700-1700 |
260-320 |
Polycarbonate (PC) |
280-320 |
800-1500 |
270-300 |
Acrylonitrile butadienstylene (ABS) |
200-280 |
700-1500 |
330-440 |
Polyamide (PA) |
230-300 |
800-1500 |
240-450 |
Injection Speed
The faster the injection speed, the better the molding condition. However, excessive speed will result in the following inconveniences: ”
– During injection of the molding material into the mold cavity , molding material may compress the air within the mold, and the resulting high air pressure hinders further filling of the molding material.
– The temperature of the molding material rises to combustion temperature and considerably deteriorates the quality of the molded parts.
– The molten plastics is filled within the cavity before the air inside the cavity is ejected. This will cause the molding material to mix with the air, which deteriorates the surface of the part.
Accordingly, it is necessary to adjust the injection speed by checking the part condition. Generally, the thinner the part is, the higher the injection speed should be.
Temperature Condition
Resin Temperature
In the case of thermoplastics resin, the volume becomes larger as the temperature of the molten plastics gets higher. Accordingly, the shrinkage rate becomes higher. However, as the temperature drops down below a certain level, the flow of the molten plastics becomes less smooth, and the shrinkage rate becomes higher.
Mold Temperature
Effects of mold temperature differ among the types of molding material. With a rise of the mold temperature, thermosetting plastics becomes harder, and the shrinkage rate becomes lower.
Mold temperature has effects on the cooling speed of thermoplastics resin. The higher the mold temperature, the larger the heat expansion and molding shrinkage.
In general, the lower the mold temperature, the shorter the molding cycle becomes. Therefore, it is advised to start with a lower temperature and then adjust to the appropriate one by raising it gradually.
Resin Name |
Resin
Temperature [
] |
Mold Temperature [
] |
Polyethylene (PE) |
180-300 |
15-75 |
Polypropylene (PP) |
200-300 |
40-60 |
Polyvinyl chloride (PVC) |
150-180 |
35-65 |
Polystyrene (PS) |
180-315 |
20-60 |
Polycarbonate (PC) |
280-320 |
85-125 |
Acrylonitrile butadienstylene (ABS) |
200-280 |
40-85 |
Polyamide (PA) |
230-300 |
20-90 |
Ejection Condition
There are two types of ejectors: mechanical ejector and hydraulic ejector.
Currently, the hydraulic ejector is popular. In general, the higher the ejection speed, the easier to remove the part from the mold.
However, if the draft angle of the product is small, and mold-removing resistance is higher, the part could be damaged or broken under high ejection speed. Therefore, in such a case, ejection pressure needs to be raised by slowing the ejection speed.
