Choosing the right injection molding machine | Daxin

December 23 16:18 2021

Important factors that usually affect the selection of injection molding machines include molds, products, plastics, molding requirements, etc.:

Mold size (width, height, thickness), weight, special design, etc.;

The type and quantity of plastics used (single raw material or multiple plastics);

The appearance dimensions (length, width, height, thickness), weight, etc. of the injection molded product;

Molding requirements, such as quality conditions, production speed, etc.

Select model: select injection molding machine according to molded product and plastic

There are many types of injection molding machines. First, correctly determine which type of injection molding machine or which series the molded product should be produced. According to the product weight, shape, molding cycle, etc., select the appropriate injection volume and clamping force of the injection molding machine.

Select the size and specifications of the mold: determine whether the inner distance of the large column of the machine, the thickness of the mold, the minimum size of the mold, and the size of the mold plate are appropriate from the mold size to confirm whether the mold is suitable.

The width and height of the mold must be smaller than or at least one side smaller than the inner distance of the large column;

The width and height of the mold should be within the size range of the mold plate;

The thickness of the mold must be between the mold thickness of the injection molding machine;

 

The width and height of the mold must meet the minimum mold size recommended by the injection molding machine, and it is too small.

For different materials and different treatments, such as PVC products, the mold should:

1) The surface of the cavity is plated with hard chrome to prevent corrosion.

2) The gate opening should be as short as possible, the diameter should be large, and the cross section of the runner should be as large as possible to keep the runner smooth.

3) After production, clean the cavity with mild lye (such as sodium nitrate, etc.), and then spray with alkaline protectant.

4) In order to get a good appearance, the setting of the cold feed port is more important.

Finished product pickup: The mold opening stroke and the mold supporting stroke are large enough to facilitate the extraction of the finished product.

The mold opening stroke must be at least more than twice the height of the finished product in the direction of opening and closing the mold, and the length of the sprue must be included;

The stroke of the supporting mold must be able to easily eject the finished product.

Choose the clamping force according to the molded product and plastic.

When the raw materials are injected into the mold cavity at high pressure, a mold-supporting force will be generated. Therefore, the clamping unit of the injection molding machine must provide enough “clamping force” to prevent the mold from being opened.

Determine the required injection volume by the weight of the finished product and the number of mold cavities and select the appropriate screw diameter.

Calculating the weight of the finished product needs to consider the number of mold cavities (a mold with several cavities);

For the sake of stability, the injection volume needs to be more than 1.35 times the weight of the finished product, that is, the weight of the finished product needs to be within 75% of the injection volume.

Generally speaking, a screw with a smaller diameter can provide a higher injection pressure.

Screw compression ratio and injection pressure

Some engineering plastics require higher injection pressure and appropriate screw compression ratio design to have a better molding effect. Therefore, in order to make the finished product shot better, the injection pressure requirements and compression ratio must also be considered when selecting the screw. At the same time, the particularity of the material should also be considered. For example, PVC is easy to decompose and is more sensitive to shear, so the screw is best to use: Akebo with a deep groove and a moderate compression ratio, and the HCL precipitated when the PVC is decomposed is acidic. It will corrode the screw, so the screw and three small parts need to be plated with hard chrome. Under the set barrel temperature and injection pressure, if the cavity cannot be completely filled, first consider increasing the injection pressure. The injection pressure is more effective for the adjustment of the injection molding process.

Injection speed

Some products require high injection rate and rapid injection to form stable molding, such as ultra-thin products. In this case, it may be necessary to confirm whether the injection rate and rate of injection of the machine are sufficient, and whether it needs to be equipped with pressure accumulators, closed-loop control and other devices. Generally speaking, under the same conditions, a screw that can provide a higher injection pressure usually has a lower shot speed. On the contrary, a screw that can provide a lower injection pressure usually has a higher shot speed. Therefore, when selecting the screw diameter, the injection volume, injection pressure and injection rate (injection speed) need to be cross-considered and selected.

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