What must be done before injection moulding?

2021-05-28 11:05

In this issue, I would like to introduce some preparatory work before startup and injection molding, so as to better understand, use and maintain the injection molding machine, develop good injection molding operation habits, and help improve the life of the injection molding machine and production safety.

Before starting the injection molding machine, a preliminary inspection of the machine should be carried out to see whether various safety devices are effective, whether each travel switch is normal, and how much lubricating oil is available to lubricate the machine.

The work that must be done before starting the injection moulding machine

① Check whether there is water or oil in the electric control box. Do not start the machine if the electrical components are wet. Electrical components should be blown dry by maintenance personnel before starting the machine.

② Check whether the power supply voltage meets the requirements, generally it should not exceed 15%.

③ Check whether the emergency stop switch and the front and rear safety door switches are normal. Verify that the rotation directions of the motor and the oil pump are the same. Check for loose electrical connectors.

④ Check whether the cooling pipe is unobstructed, and pass the cooling water into the oil cooler and the cooling water jacket at the end of the cylinder.

⑤ Check whether there is lubricating oil (grease) in each moving part, and add enough lubricating oil.

⑥ Turn on the electric heating switch to heat all parts of the cylinder. When the temperature of each section reaches the requirements, keep the temperature for a period of time to stabilize the temperature of the injection molding machine. The holding time varies according to the requirements of different equipment and plastic materials.

⑦Add enough plastic to the hopper. According to the requirements of different plastics to be injected, some raw materials need to be dried first.

⑧ The heat shield on the hopper should be well covered. This not only saves electricity but also prolongs the service life of electric heaters and current contactors.

Preparation before injection moulding

① Raw material pretreatment

According to the characteristics of the plastic and the material supply, the appearance and process properties of the raw materials should generally be tested before molding. If the plastic used is in powder form, such as polyvinyl chloride (PVC), it should also be quantitatively mixed and dry mixed; if the plastic part has coloring requirements, an appropriate amount of colorant or masterbatch can be added; the supplied granules usually contain different degrees of moisture , solvents and other volatile low molecular substances, especially some plastics with a tendency to absorb moisture, the water content always exceeds the allowable limit for processing. Therefore, they must be dried and the moisture content determined before processing. Polycarbonate (PC) is sensitive to water at high temperature and requires a water content of less than 0.2%, even 0.03% - 0.05%, so it is usually dried in a vacuum drying oven. The dried plastic must be properly sealed to prevent the plastic from reabsorbing moisture in the air and losing its drying effect. For this reason, a drying hopper can be used to continuously provide dry hot material for the injection molding machine, which is conducive to simplifying the operation, maintaining cleanliness, improving quality and improving injection rate. The loading capacity of the drying hopper is generally 2.5 times the hourly capacity of the injection molding machine.

② Blade preheating

Injection molded products require metal inserts to be embedded into the molded part for assembly and strength. During injection molding, the cold metal insert placed in the mold cavity cools with the hot plastic melt, often resulting in significant internal stress around the insert due to the significant difference in shrinkage between metals (especially in rigid chain polymers such as PS) and plastics. The presence of such internal stresses can lead to cracks around the insert, significantly reducing the suitability of the molded part. This can be achieved by choosing a metal with a high coefficient of thermal expansion (aluminum, steel, etc.) and preheating the insert (especially larger metal inserts) for the insert. At the same time, the design of plastic parts is based on measures such as the use of greater wall thickness around the insert.

③ Cleaning of the barrel

Before the newly purchased injection molding machine is used for the first time, or during the injection molding production process, when the plastic variety, raw material, color or plastic is found to be decomposed, the barrel of the injection molding machine needs to be cleaned or disassembled.

The cleaning of the barrel generally adopts the heating barrel cleaning method. Cleaning materials are usually made from plastic materials (or plastic recycled materials). For heat-sensitive plastics, such as PVC raw materials, low-density polyethylene, polystyrene, etc., transition cleaning can be used, and then new materials can be used to replace transition cleaning materials.

④ Selection of release agent

A release agent is a substance that can easily release a plastic product from a mold. Zinc stearate is suitable for general plastics except polyamide; liquid paraffin is more suitable for polyamide plastics; silicone oil is expensive and troublesome to use, so it is rarely used. The amount of release agent should be controlled in an appropriate amount, with less or no use at all. Overspray can affect the appearance of the part and negatively affect the finish of the part.

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