Blow mold is used in the production of hollow bodies made of thermoplastic. This process is one of the special injection molding processes.
In this manufacturing process, a pre extruded or injection molded preform is formed on the contour of the blow mold by means of internal pressure. In addition, packaging items such as cans, bottles or barrels, and many other technical components can be efficiently produced in this way. They include, for example, shell and half-shells, as well as fuel tanks and ventilation ducts with various cross-sections. According to the manufacture of the preform, there is a difference between extrusion blow molding and stretch blow molding.
Blow molding process
There are differences between the two blowing methods Extrusion blow molding and Stretch bladder. The so-called preform is a prerequisite for performing two types of blow molding. According to the process used, the preform can be produced continuously by extrusion or separately by injection molding. For stretch blow molding process, the preform is usually produced by extrusion blow molding.
Then the preform is put into the mold, in which compressed air is used to expand it. During the expansion process, the workpiece receives its final geometry in the cavity. The premise of filling with compressed air is that the preform is soft enough. This can be ensured by heating with a heating element (such as an infrared radiator) or by direct deformation after extrusion, after which the plastic still has sufficient deformability.
Extrusion blow molding process
Nowadays, the hollow bodies made of thermoplastics are mainly produced by extrusion blow molding process. In the production process, two processes can be performed in parallel using extrusion blow molding process. On the one hand, the continuous extrusion of the preform (so-called one-step forming) and on the other hand, the periodic transfer of the preform and its forming (forming) by blowing air into the tool.
Stretch blow molding process
Stretch blow molding is a special form of blow molding. Similar to film, this method has the following effects: high orientation can be introduced into the material in the process of drawing near the melting temperature of glass or crystal. This leads to a significant improvement in the properties of the material.
The so-called preform is a prerequisite for blow molding. This was produced in the previous process step by conventional injection molding or extrusion blow molding. In the first step of blow molding, the preform is heated first. Infrared radiation is used for this purpose. It can be well automated and has the necessary performance. This makes it ideal for bringing large amounts of heat into the semi-finished product.
After heating, remove the heater from the tool or move the preform into the tool. Which choice to use depends on the type of machine. When the tool is closed, the bottleneck stretches longitudinally. In this way, the preform can be fixed axially. In addition, it will produce a media imperceptible gaze.
In contrast to extrusion blow molding, the preform is stretched not only in the circumferential direction but also in the longitudinal direction during stretch blow molding. This is done mechanically with the help of stamps. At the same time, air blowing is used for circumferential stretching. The process is carried out at very low temperatures, which means that a strong deformation force is required. This is why a blow pressure of 20 bar or more occurs during this process. The hollow body produced after inflation is cooled in the tool. Eject only if it has sufficient dimensional stability