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How to Select Injection Molding Machine

An injection molding machine is a specialized machine for producing plastic products, which mainly consists of an injection system and a clamping system.

According to the different machine structures, commonly used injection molding machines can be divided into two categories: vertical injection molding machines and horizontal injection molding machines. We must choose the most suitable machine model according to the product to achieve higher efficiency.

We encounter many plastic products in our daily lives. Some are made of single plastic materials, such as plastic barrels and covers. Some are products made of metal and plastic, such as power cords, tool handles, etc. Of course, there is also a type of product that combines two or different colored materials, such as a two-color toothbrush handle. For selecting these two types of machines, there is a relatively simple criterion: whether the insert needs to be placed in the mold during product production.

We usually use vertical injection molding machines as long as we remember metal or some plastic products as inserts into the mold. This is because the horizontal injection molding machine places the mold horizontally, and when we set the insert, the insert may fall off, making it difficult to fix the insert in the mold.

This will affect production efficiency and product quality, but vertical injection molding machines can solve this problem well. Of course, horizontal injection molding machines also have significant advantages. Some products that do not require insert placement can be combined with a robotic arm to achieve fully automated production, significantly improving production efficiency.

Based on this criterion, we can quickly find suitable machine suppliers for the product and consult more details. There are many choices for the clamping force of the injection molding machine. The size of the clamping power depends on the specific specifications of the product—for example, the product size, injection weight, number of mold cavities, and mold size.

When the raw material is injected into the mold cavity at high pressure, a force is generated to hold the mold, so the clamping unit of the injection molding machine must provide sufficient “clamping force” to prevent the decay from being forced open. The calculation of the clamping force requirement is as follows:

Calculate the projected area of the finished product in the opening and closing mold direction based on the size of the finished product;

Holding force = projected area of the finished product in the opening and closing mold direction (cm2) × number of mold cavities × mold internal pressure (kg/cm2).

The internal mold pressure varies with the raw material, generally taking 350~400kg/cm2.

The clamping force of the machine needs to be greater than the holding force, and for safety reasons, the clamping power of the machine usually needs to be 1.17 times greater than the holding force.

At this point, the specification of the clamping unit has been preliminarily determined, and the tonnage of the machine model has been roughly defined. Next, the following steps must be taken to confirm which injection unit’s screw diameter suits the needs.

Different screw diameters correspond to different injection weights. We must determine the required “injection volume” and choose the appropriate “screw diameter” based on the product weight and the number of cavities. Injection weight refers to the weight of plastic that needs to be injected into a single product. To produce high-quality products, we need to ensure that the machine’s injection weight exceeds the product’s.

For example, if your product’s injection weight is 30g and your mold cavity is 4 cavities, you must choose a machine with an injection weight greater than 120g. For stability, the injection weight is generally 1.35 times or more than the injection weight.

In addition, we also need to consider the size of the mold. Different machines with different clamping forces have various mold capacities. We need to determine whether the machine’s “distance between tie bars,” “mold thickness,” “minimum mold size,” and “mold plate size” are appropriate based on the size of the mold to confirm whether the mold can fit.

The width and height of the mold should be smaller than or at least one side should be smaller than the distance between tie bars; the width and height of the mold should ideally be within the range of the mold plate size; the thickness of the mold should be between the mold thickness of the injection molding machine. The width and height of the mold should comply with the recommended minimum mold size of the injection molding machine, and it should not be too small.

We need to determine whether the “mold opening stroke” and “ejection stroke” are sufficient to remove the finished product based on the mold and the finished product.

The mold opening stroke should be at least twice the height of the finished product in the direction of mold opening and closing and include the length of the vertical runner (sprue). The ejection stroke should be sufficient to eject the finished product.

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