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The plastic injection, also known as thermoplastic injection molding or thermoplastic injection, is an industrial process for manufacturing plastic parts. The raw material used is the plastic in pellet forms. The pellets are melted by heat and friction in a worm. This stage is called lamination. It corresponds to the change of state of the plastic material, which passes from solid to fluid. The resulting plastic is now easier to work. It will be injected under pressure through hydraulic cylinders, in a mold. The mold consists of various channels made of cooled steel in which circulates a cooling liquid. The plastic hardens under the effect of cold which causes solidification of the desired part. It remains only to open the mold and eject the solid plastic. This last step, like all previous ones, is fully automated. The machine which carries out the whole process is called injection molding machine, injection molding or injection press.
For better understanding here are two important elements:
- Explanatory video
- Other Sources
In our daily life, there is a very large amount of injected plastic parts. Chairs, garden tables, toys, parts of household appliances, car bumpers, ... A shrewd eye will easily distinguish an injected plastic part by the following elements: - Parts full plastic - Small round dishes (slot ejectors) - Trapezoidal geometries facilitating the ejection from the mold - Weld Lines - Surface Defects
Not all plastics are injectable. It is however a great number of them that meets the necessary conditions and which may undergo a plastic injection cycle. Each of them has different specific advantages and technical specifications, which can guide us in choosing the best plastic for our final part.
We can group the injection plastics into three categories:
1. Heat hardening plastics
These plastics take their final form in the first cooling. This means they will be very strong, but cannot be reused and recast to make other parts. The reversibility is not possible either. They are used for big and massif parts, like garden furniture for example.
These plastics may be deformed when they rise in temperature. Conversely to the heat hardening plastics, thermoplastics will return to their initial shape by cooling, unless they are heated and cooled many times. Parts made with these plastics are relatively flexible, such as kitchen utensils or construction toys.
Heat hardening plastics and thermoplastic are members of polymers family. There is also another large family of injectable plastics: elastomers. The biggest advantages are their elastic and rubber properties. Elastomers are generally used to manufacture the tires, gaskets, cushions or insulation.
Design of injected plastic parts
Before the manufacturing stage, we must design the piece on computer. CAD specific software or Computer Aided Design is used for this. The design is an important step because it will determine the successful implementation of manufacturing and the quality of the parts obtained. Indeed, before starting the production, we must consider various constraints naturally related to the manufacturing process of plastic injection:
- Production of molds
- Geometry of the plastic part (the same thickness if possible)
- Spoils (to facilitate the ejection)
- Ribbing (to limit the risk of breakage)
IPost-production treatment of injected plastic parts In order to obtain a final result complying with technical specifications, some further steps may be added to the original manufacture. The overmolding is one of the examples. This process is to put a second layer of material on the previously molded part. One can thus obtain two colors or two textures (like toothbrushes). This overlay can also allow the protection of the starting piece for shockproof applications for example. There is a wide variety of post-production surface treatments to work on the color, the properties and appearance of the outside part of the plastic product.