Plastic mold opening injection molding process


Author: MULAN –Plastic Molding Manufacturer

Injection molding process of plastic mold​1) Action selection in the injection process: general injection molding machines can be operated manually, semi-automatically and fully automatically. Manual operation is in a production cycle, each action is realized by the operator switching the operation button switch. Generally, it is only used when testing the machine and adjusting the mold.

During semi-automatic operation, the machine can automatically complete a working cycle, but after each production cycle, the operator must open the safety door, remove the workpiece, and then close the safety door, so that the machine can continue the production of the next cycle. In fully automatic operation, the injection molding machine can automatically enter the next working cycle after completing one working cycle. In the normal continuous working process, there is no need to stop the machine for control and adjustment.

During normal production, semi-automatic or fully automatic operation is generally used. 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 switch should be switched accordingly. 2) Selection of pre-molding action According to whether the injection seat moves back before and after pre-molding feeding, that is, whether the nozzle leaves the mold or not, the injection molding machine generally has three options.

(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 presses against the mold for pre-plastic feeding. After the pre-molding is completed, the injection seat moves back 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, so as to prevent the molten material from flowing out of the nozzle when the back pressure is high. The relative stability of their respective temperatures.

(3) Post-feeding: After the injection is completed, the injection seat retreats, the nozzle leaves the mold and then pre-molding, and the injection seat moves forward after pre-molding. This action is suitable for processing plastics with a particularly narrow molding temperature. Due to the short contact time between the nozzle and the mold, heat loss and solidification of the molten material in the nozzle hole are avoided. At the same time that the injection is finished and the cooling timer is finished, the pre-plastic action starts.

The rotation of the screw melts and extrudes the plastic in front of the screw head. Due to the function of the one-way valve played by the stop ring at the front end of the screw, the molten plastic accumulates at the front end of the barrel, forcing the screw back. When the screw retreats to the predetermined position (this position is determined by the travel switch or electronic scale, the distance of the screw retreating is controlled to realize quantitative feeding), the pre-molding stops and the screw stops rotating.

Followed by the ejection (also called glue pumping) action, the ejection means that the screw makes a slight axial retreat. This action can relieve the pressure of the molten material gathered at the nozzle and overcome the pressure caused by the imbalance of the internal and external pressure of the barrel. Caused by "salivation" phenomenon. If ejection is not required, the ejection stop switch should be adjusted to an appropriate position (use the electronic scale to set the last storage position to be the same as the ejection position), so that the pre-molding stop switch is pressed at the same moment, the ejection Back off stop switch is also pressed. When the screw moves backward to press the stop switch, the ejection stops.

Then the injection seat began to retreat. When the injection seat retreats until the stop switch is pressed, the injection seat stops retreating. If the fixed feeding method is adopted, attention should be paid to adjusting the position of the travel switch.

Generally, the fixed feeding method is adopted in the production to save the operation time of injection seat advancing and retreating and speed up the production cycle. 3) Injection pressure selection The injection pressure of the injection molding machine is regulated by the proportional pressure regulating valve. In the case of setting pressure, through the conversion of high pressure and low pressure oil circuit, the level of injection pressure in the front and back stages is controlled. There are three pressure options for ordinary medium-sized and above injection molding machines, namely high pressure, low pressure, and high pressure first and then low pressure.

4) Selection of injection speed The injection speed of the injection molding machine is adjusted by the proportional flow valve, and sometimes a large flow oil pump and a small flow pump are set in the hydraulic system to supply oil at the same time. When the oil circuit is connected to a large flow rate, the injection molding machine can realize 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 will proceed slowly. 5) Choice of ejection form There are two types of ejection forms for injection molding machines: mechanical ejection and hydraulic ejection, and some are equipped with pneumatic ejection systems, and there are two types of ejection times: single and multiple.

The ejection action can be manual or automatic. 6) Temperature control uses the temperature measuring thermocouple as the temperature measuring element, and is equipped with a temperature measuring millivolt meter as a temperature control device to direct the current on and off of the barrel and the mold heating coil, and selectively fix the temperature of each section of the barrel and the mold temperature . 7) Mold clamping control Mold clamping is to close the mold tightly with huge mechanical thrust to resist the huge opening force of the mold caused by the high-pressure injection of molten plastic and filling the mold during the injection molding process.

The clamping structure of the injection molding machine has a full hydraulic type and a mechanical linkage type. Regardless of the structural form, the mold clamping force is implemented by the connecting rod being fully straightened in the end. The straightening process of the connecting rod is the process in which the movable plate and the tail plate are stretched, and it is also the process in which the four pull rods are stressed and stretched.

The size of the mold clamping force can be known from the highest value of the oil pressure gauge when the mold is tightened. If the mold clamping force is large, the maximum value of the oil pressure gauge will be high, and vice versa. Smaller injection molding machines do not have a mold clamping oil pressure gauge. At this time, it is necessary to judge whether the mold is really tight according to the straightening of the connecting rod. If the connecting rod of an injection molding machine is easily straightened when the mold is closed, or "almost" fails to straighten, or one of several pairs of connecting rods is not fully straightened, the mold will bulge during injection molding, and the parts will be damaged. There will be flash or other problems.

8) Mold opening control When the molten plastic is injected into the mold cavity and the cooling is completed, the mold opening action follows, and the product is taken out. The mold opening process is also divided into three stages. In the first stage, the mold is opened slowly to prevent the parts from tearing in the cavity.

In the second stage, the mold is opened quickly to shorten the mold opening time. In the third stage, the mold is opened slowly to reduce the impact and vibration caused by the inertia of mold opening.

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