What are the main steps in the injection molding cycle under normal conditions? How to improve the injection cycle?

In this increasingly competitive business society, improving production efficiency is a matter of great concern. This article introduces you to a variety of ways to improve the injection cycle using existing or newly designed injection molding machines or molds.

Injection cycle

The injection cycle of a hydraulically driven injection molding machine is from the start of clamping to the next clamping. The clamping is generally divided into four sections: rapid clamping, slow clamping, low pressure clamping and high pressure clamping.

The injection begins after the high-pressure clamping is completed and is also divided into multiple sections. The molten plastic fills the cavity during injection. When the cavity is filled, the pressure rises, so the end of the injection is also called the extrusion section. When the control is not suitable, the finished product will be produced.

The pressure holding begins after the injection is completed. In fact, the cooling starts from the filling of the cavity, that is, from the pressure holding. When the mold cools, the finished product shrinks due to cold. The function of holding pressure is to fill the shrinkage formed by shrinkage through the cold-flow passage which has not been solidified, and the holding pressure is generally lower than the injection pressure, so that the finished product is full (no dent) when demolding.

When the cold runner is solidified, it is meaningless to maintain the pressure again, and the pressure can be terminated. The holding pressure can be divided into multiple sections, and the holding pressure of each section is different (generally decreasing step by step), which is divided by time. The total dwell time is determined by the weight of the finished product or from the finished product without dents. Starting from a short dwell time, a little dwell time is added for each injection, until the weight of the finished product no longer increases or the dent is acceptable, the dwell time does not need to be increased.

The "cooling time" parameter placed on the injection molding machine is a period of time from the completion of the pressure holding to the opening of the mold, but the cooling has begun as soon as the cavity is filled with plastic. The purpose of "cooling time" is to continue to cool and solidify the finished product, and it will not be deformed by ejection when it is ejected. It should be said that the "cooling time" is derived from the test.

Shortest cycle time

The shortest cycle time consists of the time required for clamping, injection, holding, cooling and mold opening. The addition is carried out at the "cooling time" and the opening and closing of the mold, even when the pressure is maintained. Multiple ejections are performed simultaneously at the time of mold opening, and the last retraction is simultaneously performed at the time of mold clamping. In this case, there are up to three actions at the same time, each with an independent driver. It is possible that all three are oil circuits (such as three oil pumps), and three are circuits (electric injection molding machines) or a combination of oil circuits, gas circuits and circuits.

Electric injection molding machines generally have four servo motors that drive injection molding, feeding, opening, clamping and demoulding. The advantage is that parallel actions can shorten the cycle. In fact, the hydraulic injection molding machine can achieve this goal when using three independent oil circuits. Therefore, this advantage is not a patent for electric injection molding machines. Since the injection molding is not possible, the four servo motors cannot be operated at the same time.

Dry running time

Many injection molding machine specifications indicate the idle running time, but generally only the calculated theoretical time, ignoring the acceleration and deceleration of the template, of course, the quality of the moving mold is not calculated, which is shorter than the actual opening and closing time. . According to Euromap's standard, the idle time is the time required for the template to open and close, while the opening and closing stroke is set to 0.7 times the four-column space. In the shortest cycle, only the opening and closing time (empty running time) and the injection time are related to the design of the injection molding machine.

Thin wall injection

Thin wall injection molding is defined as injection molding with a wall thickness of 0.5 mm or less, or a process/wall thickness ratio of 300 or more. In order to prevent the molten plastic from solidifying when it is not filled with the mold cavity, thin-wall injection molding is high-speed (and high acceleration deceleration) injection molding. The so-called high-speed injection molding means that the forward speed of the screw is above 300 mm/s. In high-speed injection molding, the injection time is generally within 0.5s. High-speed injections are all aided by accumulators. The oil pump fills the accumulator at the "cooling time", and can also be filled with a small oil pump at a time other than injection and holding pressure. The stored high-pressure oil is released at the time of injection, generally increasing the injection speed by three times.

Blowing off the mold

If the product can be blown off with a blow, it is easy to blow while opening the mold. Generally, the air is blown after the mold is opened, and the demolding time is hidden in the mold opening time.

Pneumatic demoulding

The pneumatic ejection is realized on a small injection molding machine with a small top mold force, which makes the opening and closing mold parallel to the top ejection, saving the thimble action time of about 1 s, which is considerable on the small injection molding machine. Finally, the cycle time of 2s was reached.

The cost of an injection molding machine with three separate oil circuits is higher. Two parallel-action injection molding machines, generally using a hydraulic circuit for the clamping device and another oil circuit for the injection device, which is based on the general injection molding machine.

Considerations for the clamping plate oil circuit board and the injection device oil circuit board.

The hybrid oil-electric injection molding machine combines the strengths of the two drives and is cleverly matched. The general design is the other action of electric feeding and oil pressure.

Double pump injection

The dual pump design uses only one oil pump for injection. The oil circuit can be changed slightly so that the pump is double pumped, which increases the injection speed by nearly double.

Hot runner mold

Hot runners can shorten the cycle in many ways.

1. If the diameter of the cold runner is larger than the thickness of the finished product, the "cooling time" must wait for the runner to cool to a certain extent to open the mold, but the finished product has already cooled and caused waste. The flow path of the hot runner mold is not cooled, and the finished product determines the cooling time.

2. The amount of plastic in the cold runner is a percentage of the finished plastic. Even if it is heavier than the weight of the finished product, the injection and feeding time will be lengthened. The use of hot runner molds eliminates the injection and feeding time required for runner plastics.

3. The cold runner has a nozzle and the mold opening stroke is increased.

4. When multi-cavity injection molding, the cold runner does not guarantee that the finished product will fall or not. It must be removed by mechanical arm (automatic operation) or manually removed (semi-automatic operation), and the cycle will be slowed down.

If the diameter of the cold runner is larger than the thickness of the finished product, the cooling time is waiting for the runner to cool to some extent to open the mold, but the finished product has already cooled and caused waste. The flow path of the hot runner mold does not need to be cooled, and the finished product feels the cooling time.

The amount of plastic in the cold runner is a percentage of the finished plastic, even heavier than the weight of the finished product, and the injection and feeding time will be lengthened. The use of a meeting hot runner mold saves the injection and feeding time required for the runner plastic. The cold runner has a nozzle, and the mold opening stroke is increased. When the multi-cavity injection molding is performed, the cold runner does not guarantee that the finished product is dropped or not, and is taken out by a mechanical arm (fully operated) or manually removed (semi-automatic).

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