How to choose plastic injection molding machine
Typically, the important factors when choosing plastic injection molding machine include mold, product, molding requirements and so on. Therefore, you must collect or have the following information before making a choice: mold dimension (width, height, thickness), weight and special design; types and amount of the plastic (single or multiple plastic materials); dimensions and weight of injection molding product (length, width, height, thickness); molding requirements, such as quality condition, production speed and so on.
In obtaining the above information, you can follow these steps to choose a suitable plastic injection molding machine:
- Choose the right type of plastic injection molding machine: make decision by the products and plastic
The plastic injection molding machine has a lot of types, so at the beginning, you should determine that what kind or which series of plastic injection molding machine is fit for the product. For example, for thermoplastic plastic or Bakelite resin or PET, the product is single color, two-color, multi-color, laminated or blending. In addition, some product requires high stability (closed loop), high precision, high injection speed, high injection pressure and rapid production (multi-loop) and so on.
- Able to put it on: the size of mold determine whether the internal spur of large column, the mold thickness, the minimum mold dimension and the mold plate size of plastic injection molding machine are suitable or not, to make sure that it is able to put mold on.
The mold width and height must be less than or at least less than the internal spur of large column; The mold width and height must in the mold plate size range; the mold thickness should be between the thickness of plastic injection molding machine; The mold width and height must be fit with the minimum mold size recommended by the plastic injection molding machine.
- Able to put it out: the mold and product judge whether the “Opening Stroke” and “Ejector Stroke” are enough to remove the finished product.
The opening stroke should at least larger than the product height along the opening direction more than twice, including the length of vertical sprue; ejector stroke should long enough to eject the finished product.
- Able to maintain locked: the product and plastic determine the tonnage of “clamping force”.
When the raw materials inject into the mold cavity in a high pressure, it will create a bulging force, so the clamping unit of the plastic injection molding machine should provide sufficient “clamping force” so that the mold was not subject to distraction. The need of clamping force is calculated as follows:
Clamping force = projected area of product in the direction of opening and closing mould (cm2) ×the number of mold cavity×in-mold pressure (kg/cm2);
Projected area of product in the direction of opening and closing mould is generated by the product appearance size.
In-mold pressure is different with different materials, generally take 350-400 kg/cm2.
The machine clamping force is usually greater than 1.17 times of the bulging force.
- Enough injection quantity: the weight of product and the number of mold cavity determine the “injection quantity” and select the appropriate “screw diameter”.
For stability purpose, the injection volume required for more than 1.35 times of the product, which means the product weight should less than 75% of the injection amount.
- Good injection: plastic determine “screw compression ratio” and “injection pressure” and other conditions.
Some plastic requires a higher injection pressure and appropriate screw compression ratio for a good molding. So when choosing screw, it should considered about the demand of injection pressure and the problems of compression ratio. In general, smaller diameter screw provides higher injection pressure.
- High injection speed: confirm the injection speed.
Some product requires a high injection speed to stabilize the injection molding, such as ultra-thin wall product. In this case, it is necessary to make sure that whether the injection speed and injection rate are enough or not, and whether accumulator and closed-loop control are necessary or not. Generally speaking, in the same condition, a screw in the plastic injection molding machine with high injection pressure has a lower injection speed, on the contrary, a screw with low injection pressure has a higher injection speed. So when choosing the screw diameter, injection volume, injection pressure, and injection rate (injection speed) should be cross considered.
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(edit by DAKUMAR)