Plastic mold injection molding process

1) Injection process action selection:
Generally, injection molding machines can be operated manually, as well as semi-automatic and fully automatic operations.
Manual operation is achieved by the operator switching the operation button switch during a production cycle. Generally, it is only selected during machine testing and mold adjustment.
During semi-automatic operation, the machine can automatically complete a work cycle, but after each production cycle is completed, the operator must open the safety door, remove the workpiece, and then close the safety door before the machine can continue production for the next cycle.
During fully automatic operation, the injection molding machine can automatically enter the next work cycle after completing one work cycle. During normal continuous operation, there is no need to stop the machine for control and adjustment.
During normal production, semi-automatic or fully automatic operation is generally selected. At the beginning of the operation, the operation mode (manual, semi-automatic, or fully automatic) should be selected according to the production needs, and the manual, semi-automatic, or fully automatic switches should be switched accordingly.
2) Pre molding action selection
Injection molding machines generally have three options based on whether the injection seat before and after pre molding feeding is retracted, that is, whether the nozzle is away from the mold. (1) Fixed feeding: The nozzle is always attached to the mold before and after pre molding, and the injection seat does not move. (2) Pre feeding: The nozzle is placed against the mold for pre molding feeding. After pre molding, the injection seat is retracted and the nozzle leaves the mold. The purpose of choosing this method is to use the injection hole of the mold to support the nozzle during pre molding, to prevent molten material from flowing out of the nozzle when the back pressure is high. After pre molding, it can avoid heat transfer caused by prolonged contact between the nozzle and the mold, which affects the relative stability of their respective temperatures. (3) Post feeding: After the injection is completed, the injection seat moves back, the nozzle leaves the mold and is pre molded. After pre molding, the injection seat moves forward. This action is suitable for processing plastic with extremely narrow molding temperatures. Due to the short contact time between the nozzle and the mold, it avoids the loss of heat and also avoids the solidification of molten material in the nozzle hole.
At the end of injection and the completion of the cooling timer, the pre molding action begins. The rotation of the screw melts the plastic and pushes it to the front of the screw head. Due to the one-way valve function of the stop ring at the front end of the screw, molten plastic accumulates in the front end of the barrel, forcing the screw backwards. When the screw retreats to a predetermined position (determined by a travel switch or electronic ruler to control the distance of the screw's retreat and achieve quantitative feeding), the pre molding stops and the screw stops rotating. Next is the ejection (also known as rubber extraction) action, which involves the screw making a slight axial retreat. This action can relieve the pressure of the molten material gathered at the nozzle and overcome the "salivation" phenomenon caused by the imbalance of internal and external pressure in the barrel. If there is no need for ejection, the ejection stop switch should be adjusted to the appropriate position (using an electronic ruler to set the last storage position to the same as the ejection position), so that at the same time as the pre molded stop switch is pressed, the ejection stop switch is also pressed. When the screw performs a ejection action and moves back to the stop switch, the ejection stops. Then the seat begins to retreat. When the injection seat retreats to the stop switch, the injection seat stops retreating. If a fixed feeding method is used, attention should be paid to adjusting the position of the travel switch.
Generally, fixed feeding methods are used in production to save the time of injection seat forward and backward operations and accelerate the production cycle.
3) Injection pressure selection
The injection pressure of the injection molding machine is adjusted by a proportional pressure regulating valve. When the pressure is set, the injection pressure in the front and rear stages is controlled by switching between high and low pressure oil circuits.
There are three pressure options available for injection molding machines of ordinary medium size and above, namely high pressure, low pressure, and high pressure followed by low pressure.
4) Selection of injection speed
The injection speed of the injection molding machine is adjusted by a proportional flow valve, and sometimes there is a large flow oil pump and a small flow pump running simultaneously in the hydraulic system to supply oil. When the oil circuit is connected to a large flow rate, the injection molding machine achieves rapid mold opening and closing, rapid injection, rapid material storage, etc. When the hydraulic oil circuit only provides a small flow rate, various actions of the injection molding machine slowly proceed.
5) Selection of Ejection Form
There are two types of ejection forms for injection molding machines: mechanical ejection and hydraulic ejection. Some are also equipped with pneumatic ejection systems, with single and multiple ejection times. The ejection action can be manual or automatic.
6) Temperature control
Using temperature measuring thermocouples as temperature measuring elements, coupled with a temperature measuring millivolt meter as a temperature control device, it commands the on/off of the current of the material barrel and the mold electric heating coil, and selectively fixes the temperature of each section of the material barrel and the mold temperature.
7) Mold closing control
Clamping is the process of using a huge mechanical force to tighten the mold and resist the high pressure injection and filling of molten plastic during the injection molding process, which causes the mold to experience a huge opening force.
The clamping structures of injection molding machines include fully hydraulic and mechanical linkage types. Regardless of the structural form, the clamping force is ultimately implemented by the fully extended connecting rod. The straightening process of the connecting rod is the process of spreading the movable plate and tail plate, and also the process of stretching the four connecting rods under force.
The magnitude of the clamping force can be determined by the highest value raised by the oil pressure gauge at the moment of tightening the mold. If the clamping force is large, the highest value of the oil pressure gauge will be high, and if not, it will be low. Smaller injection molding machines do not come with a clamping oil pressure gauge, so it is necessary to determine whether the mold is truly tight based on the straightness of the connecting rod. If a certain injection molding machine easily straightens the connecting rod during mold closing, or "almost" fails to straighten it, or if one of the several connecting rods is not fully straightened, mold expansion will occur during injection molding, and the workpiece will have flash or other problems.
8) Mold opening control
After the molten plastic is injected into the mold cavity and the cooling is completed, the mold opening action is followed to remove the product. The mold opening process is also divided into three stages. The first stage is to slowly open the mold to prevent the part from tearing in the mold cavity. The second stage is rapid mold opening to shorten the mold opening time. Slow mold opening in the third stage to reduce impact and vibration caused by mold inertia