Plastic Injection Molding

Plastic injection molding

Plastic injection molding is the process of injecting melted plastic resin, then forcing it under pressure into a plastic mold, where it cools and hardens.  The plastic mold is then opened, and the part is removed or “ejected”, usually automatically by “ejector pins”, long thin metal rods inside the mould that push through the cavity forcing the plastic part out of the mold cavity automatically as the molding machine opens the mold.

The plastic mold must have a very heavy wall thickness to keep the mold cavity from bulging since the plastic resin, is injected with as much as 20,000 pounds per square inch of pressure. And for the same reason molding machines must be able to clamp the mold shut with tremendous force.

On the top of the molding machine there is usually a hopper which is filled with the plastic resin to be molded, usually in the form of small pellets about 1/3 the size of a grain of rice.  The hopper feeds the pellets into the “barrel” of the machine.

The barrel has a long screw or auger inside, that rotates, forcing the pellets toward the front of the barrel.  The process of auguring the pellets produces a lot of heat, that melts the pellets. Additional heat is supplied to the barrel through the use of heating bands, electric resistance heating elements that wrap around the barrel.

At the end of the barrel there is a nozzle from which the melted plastic resin is ejected into the mold. The nozzle also has electric resistance heating bands wrapped around it.  The temperature of the nozzle is very important, and the temperature is monitored and controlled at the operator control panel.

If the nozzle temperature is not hot enough the plastic can “freeze off”, forming a solid plug that has to be removed before plastic can be injected into the mold.  If the nozzle is too hot, it can burn the resin destroying the plastic before it makes it into the mold.

Most injection molding machines can be run in either manual mode, or automatic mode.  In the manual mode, an operator slides open a door providing access to the mold itself. The operator reaches in and pulls the part out of the mold, then closes the door, and the mold closes, is filled with plastic, cools of for a few moments, and then opens up again. At that point the operator opens the door again, and the whole process repeats.

In automatic operation there is no operator.  The machine just hums along by itself, automatically ejecting the part or parts when the mold opens, and then re closing the mold, injecting the plastic etc.

Plastic injection molds can become quite complex. They usually require water lines to be built into the mold to cool of the melted plastic resin and harden the part, and they have ejector pins that push into the mold cavity as the mold opens, to eject the part out of the cavity.  Often the molded part requires moving metal pieces called “slides” that must pull metal cores out of areas of the part where there is no plastic such as the inside of pipe fittings etc.

There are many different brands and types of plastic injection molding machines that vary in the way they clamp the mold shut and other differences as well. What I’ve talked about is very very basic.

Plastic Injection Mold Manufacturer

Plastic Injection Mold Manufacturer

1, Core and cavity have three-dimensional surface.

Plastic products external and internal shapes are depend on core and cavity and it is very difficult to process this complex three-dimensional surface, especially the process of blind hole. If we use traditional process, it not only requires high technical level of work,but also need long period of processing .

2,High accuracy and surface quality requirement, long service time.

Presently, the plastic products size accuracy requirement is about IT6-7, surface roughness is Ra0.2-0.1μm, while injection mould fitting size accuracy requirement should be achieve to IT5-6 and surface roughness lower than Ra0.1μm. Plastic injection molds long-life for is essential improving efficiency and reducing costs

If you want to improve efficiency and reduce cost, I think you need a long life injection mould. Currently, the general useful time of injection mould is more than 100 million.Precision injection molds need a big stiffness mould frame, heavy template, and many support column or cone positioning device in order to prevent the deformation of the mold. The ejection equipment will affect the products’ deform and size accuracy, so we should choose the best ejection point, then guarantee the even demould.

For the structural design, most of high accuracy injection moulds choose mosaics structure; it will improve the processing accuracy and the interchange ability of mould part.

3, Long process, tight manufacture time.

A complete product always consists of injection product and part components supporting. Because of high requirement for products shape or size accuracy and because there are many characteristics of resin material, so after finishing the mould’s manufacture, we should test and modify the mould repeatedly, reducing developing and delivery time.

4, Remote design and off-site manufacture

Mould manufacture is not the finally destination, mould manufacture’s destination is appeal to customer’s requirement. Ordinarily, customer brings forward finally design, according to the customer’s requirement, mould manufacturer start to produce.

5, Specialize division, dynamic combination

Generally, mould production is always single, but there are many standard elements, such as mould frame and ejection pin. These can’t be finished in one manufacturing process.

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