Although it may seem similar to the extrusion, there is no confuse injection molding with extrusion. In this case, molds are used instead of the die, although the first part of the procedure may seem similar to extrusion.
In a injection molding machine, what is done is to have a material in a hopper or tank, as in extrusion. And it will also be carried by an endless screw or similar to generate pressure towards a nozzle. So far everything is the same ...
The difference is that, instead of the die in the tip, what is done is injecting the material into a closed mold. In this way, it is possible to fill the entire mold 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 dealt with materials in a liquid state so that you can work with them. In the case of plastics or metals, it will be done hot. In the case of other materials such as cements, etc., it can be done cold.
When I knowe solidifies the material injected into the mold through a hole (known as a gate), then the mold is opened to extract the formed part. That is why it is not a continuous or semi-continuous process like extrusion, therefore, you must wait and it takes longer to form parts, since you will have to have empty molds to continue.
Despite that, it has a big advantage. And it is that you can create shapes that could not be generated with extrusion, such as the most complex geometries or hollow pieces and closed 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, lego pieces and the Playmobil are created in this way. Also many plastic elements such as other types of toys, components for vehicles such as the plastic dashboard, and a long etc.
An injection molding machine consists of some fairly elementary parts:
- Injection unit: it is the element that is responsible for 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 prevent them from deteriorating or corrupting 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 coming out when it is injected under pressure. In this way, the material will occupy the entire volume of the interior of the mold and will form the part, with a very high surface quality.
- Molde: it can be unitary, normally composed of two shells 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 to room temperature, undergo extensive temperature control, or quenched by immersing them in water, etc. That will depend on the type of piece you are working with and what you want to achieve.
As you can see, the process depends on molds not so fast as in extrusion. But the shapes achieved will be more complex, with better surface finishes.
Finally, finishes with molds will depend in part on the type of material with which you work, as it happened with extrusion. But they will also depend on the quality of the finish of the molds. In addition, the molds have to be kept in good condition, and that means a good cleaning also of the remains that may remain.
Even so, a number of defects during injection molding, such as:
- Empty holes- Finish defects occur when cooling is too fast, as some materials require slow cooling. It can also occur when the design of the part, the injection pressure is low, or the mold is not adequate, or the material is at a low temperature and cannot flow well to all areas.
- Leakage: If the seal is not hermetic, material may leak, which can also cause damage. Also, that can cause burrs.
- Roughness: If there are roughnesses on the surface such as orange peel, they are usually due to too high a temperature. They can also be due to the mold, which is in bad condition.
- Surface degradation: It can occur due to a bad mold seal, humidity, or other additives.
- Fractures or cracks: When the mold is too cold, the temperature contrast between the hot material and the mold can cause breakage.