Although it may appear similar to extrusion, injection molding should not be confused with extrusion. In plastic injection, molds are used instead of the die, although the first part of the procedure may seem similar to extrusion.
What is plastic injection molding?
In an injection molding machine, a plastic injection molding service manufacturer dispose of a material in a hopper or tank, just like in extrusion. And it will also be carried by a worm screw or the like to generate pressure towards a nozzle. So far everything is the same.
The difference is that, instead of the die at the tip, you is inject the material into a closed mold. In this way, the entire mold can be filled to create the desired shape. The mold will be closed under pressure so that the material does not come out, in addition to being cold.
Of course, to be possible, they must also be treated with materials in a liquid state so that they can be worked with. In the case of plastics or metals, it will be hot. In case of other materials such as cements, etc., it can be done cold.
When the material injected into the mold solidifies through a hole (known as a gate), then the mold opens to extract the formed part. That is why it is not a continuous or semi-continuous process such as extrusion, therefore, it must be expected and it takes longer to form parts, since empty molds will have to be available to continue.
Despite that, it has a great advantage. And it is that you can create shapes that could not be generated with extrusion, such as the most complex geometries or hollow and closed parts on one of their faces, etc. That is why it is also a very popular process in the industry, especially in the molding of plastics.
For example, many plastic elements such as other types of toys, vehicle components such as the plastic dashboard are made through using this process.
Machinery used for plastic injection molding
An injection molding machine consists of some fairly basic parts:
Injection unit: it is the element that is in charge of melting the material and adapting it to the optical working temperature. Then by means of a hydraulic mechanism or by means of spindles it will be pushed under pressure to inject it into the mold through the gate. The spindles and containers, or parts in contact with the material, will be of one type or another, with different finishes to avoid deterioration or corruption with use. For example, a machine for PVC is not the same as another for metal.
Closing unit: it is a hydraulic or mechanical press that closes the mold with force to prevent the molten material from escaping when it is injected under pressure. In this way, the material will occupy the entire volume of the interior of the mold and form the part, with a very high surface quality.
Mold: it can be unitary, normally made up of two helmets that will form the shape of the final piece. Sometimes you can also have a battery of molds connected by different conduits to each other, so that the material can flow from one to another and fill them all. This means that more pieces can be created in one go. The molds can be allowed to cool down to room temperature, carry out an exhaustive temperature control or suddenly cool down by immersing them in water, etc. That will depend on the type of piece with which you work and what you want to achieve.