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How to choose injection molding machine

Generally speaking, most of the customers who have been engaged in the injection molding industry for many years have the ability to judge by themselves and choose the appropriate injection molding machine for production. However, in some cases, the customer may need the assistance of the manufacturer to decide which injection molding machine to use, or even the customer may only have the product sample or idea, and then ask the manufacturer whether the machine can be produced or which model is compared suitable.

In addition, some special products may need special devices such as accumulators, closed loops, injection compression, etc., to be more efficient. This shows that how to determine the appropriate injection molding machine to produce is an extremely important issue.

The important factors that usually affect the selection of injection machines include molds, products, plastics, molding requirements, etc. Therefore, the following information must be collected or available before making a selection:

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

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

◆ Appearance size (length, width, height, thickness), weight, etc. of the injection molded product;

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

Suitable injection molding machine:

1. Select the right model: The model and series are determined by the product and plastic.

Since there are many types of injection machines, it is necessary to correctly determine which injection molding machine or series the product should be produced from at the beginning, such as general thermoplastics or bakelite raw materials or PET raw materials. Color, two-color, multi-color, interlayer or mixed color, etc. In addition, some products require conditions such as high stability (closed loop), high precision, ultra-high rate of fire, high shot pressure or rapid production (multi-loop), and the appropriate series must also be selected for production.

2. It can be put down: Determine whether the “Large Column Inner Distance”, “Mold Thickness”, “Mold Minimum Size” and “Die Plate Size” of the machine are appropriate according to the mold size to confirm whether the mold can be put down.

◆ 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 of the mold plate;

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

◆ The width and height of the mold must meet the minimum mold size recommended by the injection molding machine.

3. Obtained: Determine whether the “mold opening stroke” and “mold stroke” are sufficient to take out the finished product from the mold.

◆ 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 enough to eject the finished product.

4. Lockable: The tonnage of “clamping force” is determined by the 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.

The calculation of the clamping force demand is as follows:

◆ Calculate the projected area of the finished product in the direction of the switch mold from the appearance size of the finished product;

◆ Die support force = projected area of the finished product in the direction of the switch mold (cm2) × number of mold cavities × pressure in the mold (kg/cm2);

◆ The pressure in the mold varies with the raw material, and the general raw material is 350~400kg/cm2;

◆ The clamping force of the machine must be greater than the clamping force, and for the sake of safety, the clamping force of the machine is usually 1.17 times greater than the clamping force.

At this point, the specifications of the clamping unit have been preliminarily determined, and the tonnage of the model has been roughly determined. Then the following steps must be performed to confirm which injection unit has a screw diameter that meets the requirements.

5. Full shot: Determine the required “shot volume” from the weight of the finished product and the number of mold cavities and select the appropriate “screw diameter”.

◆ The number of mold cavities (the cavities of the mold) should be considered when calculating the weight of the finished product;

◆ For the sake of stability, the injection volume must be 1.35 times the weight of the finished product, that is, the weight of the finished product must be within 75% of the injection volume.

6. Good injection: The plastic determines the conditions such as “screw compression ratio” and “injection pressure”.

Some engineering plastics require a higher injection pressure and a suitable screw compression ratio design to have a better molding effect. Therefore, in order to achieve better injection of the finished product, the injection pressure requirements and compression ratio must also be considered when selecting the screw. Generally speaking, a screw with a smaller diameter can provide a higher injection pressure.

7. Shoot fast: confirm “shooting 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 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, the screw that can provide a higher injection pressure usually has a lower rate of fire, on the contrary, the screw that can provide a lower injection pressure usually has a higher rate of fire. Therefore, when selecting the screw diameter, the injection volume, injection pressure, and injection rate (injection speed) need to be considered and selected.

In addition, a multi-loop design can also be used to shorten the molding time with synchronized compound actions. After the above steps, in principle, an injection molding machine that meets the requirements can be determined, but there are some special issues that may have to be considered, including:

◆ The size:

Under certain special conditions, the customer’s mold or product may have a small mold but a large amount of shot, or a large mold but a small amount of shot. In this case, the standard specifications set by the manufacturer may not be available. To meet customer needs, so-called “size matching” must be carried out, that is, “big wall small shot” or “small wall big shot”. The so-called “large wall and small injection” refers to the original standard clamping die unit with a smaller injection screw. On the contrary, the “small wall and large injection” refers to the original standard clamping die unit with a larger injection screw. Of course, there may be several levels of difference between mold clamping and injection.

◆ Concept of fast or high speed machine:

In actual application, more and more customers will ask to buy the so-called “high speed machine” or “fast machine”. Generally speaking, in addition to the needs of the product itself, most of the goals are to shorten the molding cycle and increase the output per unit time, thereby reducing production costs and improving competitiveness. Generally, there are several ways to achieve the above purpose:

◆ Speed up the injection speed: increase the motor and pump, or add a pressure accumulator (preferably closed loop control);

◆ Accelerate the feeding speed: increase the motor and pump, or reduce the feeding hydraulic motor to speed up the screw speed;

◆ Multi-loop system: adopts double-loop or three-loop design to perform compound actions simultaneously and shorten the molding time;

◆ Increase mold waterway to improve mold cooling efficiency.


Post time: Feb-20-2021