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How to choose the right injection molding machine?

Update:23-01-2021
Summary:

Important factors that usually affect the selection of […]

Important factors that usually affect the selection of PET Bottle Blow 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.

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

Since there are many types of injection molding 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 injection 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 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.

3. Obtained: Determine whether the "mold opening stroke" and "mold stroke" are sufficient to take out the finished product from the mold and 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 mold support stroke must be sufficient 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) × mold cavity number × mold pressure (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".

To calculate the weight of the finished product, the number of mold cavities (a mold several cavities) shall be considered;

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.

6. Good injection: The conditions of "screw compression ratio" and "injection pressure" are determined by the plastic.

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: and 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 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, injection volume, injection pressure and injection rate (injection speed) need to be cross-considered and selected.

In addition, a multi-loop design can also be used to shorten the molding time with synchronized compound actions.

There are some special issues that may also have to be considered:

Size matching problem:

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.

The concept of fast machine 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 injection: increase the motor and pump, or add a pressure accumulator (preferably closed loop control);

Increase the feeding speed: increase the motor motor and pump, or reduce the feeding hydraulic motor to increase the screw speed;

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

Increase the mold waterway and improve the cooling efficiency of the mold.

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