Plastic injection molding processes are the most common process in the thermoplastic part production industry. According to the used material type and production parameters, there are various kinds of plastic injection processes are developed. In here, we will give summary information about;
- Thermoset plastic injection processes,
- Multi-injection molding process,
- Reaction injection molding process,
- Thermoplastic foam injection process.
What Are The Thermoset Injection Processes?
You know that polymer materials are classified into three groups; thermoplastics, thermosetting, and elastomers. Most plastic injection machines are produced for thermoplastics. But also, plastic injection machines are made for thermoset plastics. There are major differences in the tooling and design between thermoplastic injection machines and thermosetting injection machines. But the most important differences between them is the length of the barrel and screw, and heating temperatures.
This difference is related to the curing of thermoset materials at certain temperatures. We do not want to cure thermoset material outside of the mold. Curing of thermosets is not desired inside the barrel or between the barrel and mold.
So, screw and barrel temperatures are held at 50 degrees to 125 degrees. This temperature changes according to the thermoset material being molded.
The most important thermoset plastics that injection molded are; urea-formaldehyde, epoxies, melamines, unsaturated polyesters, and phenolics. Most thermoset plastics are injected with fillers. So, composite materials are also injection molded.
Typical mold temperatures range from 150 degrees to 230 degrees. Injection cycle times change according to the curing time of injected polymer.
Multi Injection Molding Process
The multi injection process is a commonly used process to produce unitary parts made from different kinds of plastics. When these different kinds of plastics are combined, useful products that include different plastics combined are produced.
The used injection nozzles are more complex than other conventional injection molding processes. There are different methods to produce different parts;
- Bi-injection molding: Multi injection process that combines different colors of plastics. Automotive taillights are the most important examples. In general, two positioned molds are used. When one type of plastic is injected inside the first section of the mold, the second section opens. Then the injection process goes on with other kinds of plastic. After solidification, these two plastics combined to constitute the whole part.
- Sandwich Injection Molding: In sandwich molding, structural foams are generally produced. In general application, foam is first molded inside mold as core material, and another thermoplastic coating material is injected to surround the core material. Different nozzle configurations are used to accomplish this kind of plastic injection process.
Explanation Of Reaction Injection Molding Process
As you understand from its name, in reaction injection molding, there is a reaction of mixed polymers before the injection to mold. This reaction takes place in relatively short times with the addition of catalysts. Epoxies, urethanes, and urea-formaldehyde are the most important examples.
In the general application of reaction injection molding, two reactive ingredients are held in separate containers. At the beginning of the injection cycle, required precise amounts of reactive materials are mixed inside the mixing head. Then this mixed and reacted polymer is pumped inside the mold cavity. The reaction also continues inside the mold cavity.
The most important advantage of the reaction injection molding process is the requirement of lower injection pressures compared with other plastic injection processes. Because of that, the requirement for tooling is also lower and cheaper.
The automotive industry is one of the most important markets of this reaction injection molding industry that various kinds of automotive parts such as bumpers are produced with reaction injection molding processes.
Thermoplastic Foam Injection Molding Process
Thermoplastic foam injection is a very basic process that polymer melt mixed with gas, and injected inside the mold. This gas and polymer melt mixture creates a cellular internal structure, which is also called foam.
This foam is lightweight. And the outer surface of this foam is formed when injected foam touches the cold mold surface. The surface characteristics of produced foam are not generally good. Some voids can occur on the surface and because of this, some additional surface improvement processes needed to be applied.
Because of these additional processes, the production cost can increase. But in thermoplastic foam injection molding, there is no requirement of high injection pressures like other plastic injection machines. This factor can reduce the costs of the thermoplastic foam injection molding process.
Also because of low injection pressures in thermoplastic foam injection molding, large parts can be produced. So, structural foam parts are produced generally with thermoplastic foam injection molding processes. Machine housings and structural foams are important application areas of the thermoplastic foam injection process.
Conclusion For Plastic Injection Process Types
Also, there can be other plastic injection processes to produce very specific types of plastic parts in the plastic industry. Maybe, if you are a good engineer, you can develop your plastic injection process to produce a specific part to market.
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