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Injection Mold Knowledge
Aug 03, 2018

Core Tip: Injection Molding Knowledge, Injection Molding (Injection Molding) is a method in which a melted material is injected from a high pressure into a mold cavity and cooled and solidified to obtain a molded product.

 1. What is injection molding

The term "injection molding" refers to a method in which a melted material is injected into a mold cavity by high pressure and solidified by cooling to obtain a molded product. This method is suitable for mass production of complex shaped parts and is one of the important processing methods.

The injection molding process can be roughly divided into the following six stages:

1. Closing

2. Injection

3. Maintain pressure

4. Cooling

5. Open mold

6. Product removal

The above process is repeated and the products can be continuously produced.

2. Injection molding machine

Injection molding machines can be divided into mold clamping devices and injection devices.

The main function of the mold clamping device is to open and close the mold and eject the product. The mold clamping device can be divided into a link type as shown in the figure and a direct pressure type in which mold clamping is directly performed using oil pressure.

The injection device is a device that melts the resin material and then injects it into the mold. The resin was extruded into the cylinder from the head as shown, and the melt was transferred to the front end of the barrel by the rotation of the screw. In that process, heating by the heater heats the resin material in the barrel, and the resin is brought into a molten state by the shear stress of the screw, and the molten resin corresponding to the molded product and the main channel and the runner is retained. At the front end of the barrel (known as metering), the screw continues to advance material into the mold cavity.

When the molten resin flows in the mold, the moving speed (injection speed) of the screw must be controlled, and the pressure (maintenance pressure) is used to control after the resin fills the cavity. When the screw position, injection pressure reaches a certain value, we can switch speed control to pressure control.

The term "Mold" refers to a device in which a resin material is injected into a metal mold to obtain a product having a certain shape. Although not shown in the figure, in fact, in order to control the temperature of the mold, there are cooling holes, heaters, and the like that allow the refrigerant (warm water or oil) to pass through the mold.

The material that has become the melt enters the main channel and is injected into the mold cavity through the runner and the gate. After the cooling stage, the mold is opened and the ejector on the forming machine ejects the ejection rod to push the product out.

4. Molded products

The molded product is composed of a main channel into which the molten resin flows, and a runner and a product that guide the molten resin into the cavity. If only one product can be obtained at one time, the production efficiency is not high. If we use a runner to connect multiple cavities together, several products can be molded at the same time.

At this time, if the lengths of the runners to the respective cavities are not equal, the resin cannot be injected into the cavities at the same time, resulting in a difference in the size, appearance, and physical properties of each product. Therefore, we usually design the length of the runners to be equal.

5. Use of recycled materials

The main channels and runners in molded products are not products, and they are sometimes discarded or crushed and reused as molding materials. We call it back material.

The return material is generally not used as a molding material, but is usually used after being mixed with a new material. This is because after molding, the mechanical properties, fluidity, color and other characteristics of the resin will change. The mixing ratio of new material and recycled material is generally controlled below 30%. If the proportion of recycled materials is too high, the inherent characteristics of the material may be degraded. Even if the amount of recycled materials is controlled below 30%, we must follow the rules. Determine the quality of the product, strength, mold structure, shape of the molded product, etc. to determine the correct amount of recycled materials. Especially for stuffing grades, pay special attention to this point.