Plastic injection molding is a type of manufacturing process that involves injecting melted plastic into a mold to form a finished product. Injection molding can be used with a wide range of materials such as thermoplastics and thermosetting polymers. It is a highly cost-effective manufacturing process that can produce large quantities of identical parts with high accuracy and repeatability. Plastic injection is most often utilized where there is a need for mass production of a part. The primary advantages of this process over other forms of manufacture are its speed and the low upfront cost for the molds required to make the parts.
The process begins with the pelletized raw material being fed into a heated barrel with a reciprocating screw that serves as both an injection system and plasticizer. The screw shears the polymers, heats them to a molten state and reduces their viscosity by mechanically shearing and frictional heating. The plastic is then injected into the mold through a nozzle that is attached to the screw or plunger.
Older injection machines had separate barrels for melting and injection but the newer, single-barrel injection systems combine these functions in one unit. This allows for faster and more accurate plastic injection. Once the plastic enters the mold, it is injected into the cavity through a nozzle that is aligned with the front of the mold.
After the majority of the molten plastic has been injected into the mold, packing and holding pressure is applied to the cavity until it is 95% filled. Then the molded part is cooled and ejected from the mold. The cooling process can take anywhere from a few seconds to several minutes depending on the size of the finished part.
Some of the most common types of plastics used in injection molding are acrylonitrile butadiene rubber, polypropylene, polyethylene, nylon and styrene acrylonitrile – sarane. All of these have their own unique set of properties and characteristics that make them ideal for different uses.
Injection molding is a relatively automated process that requires very little human intervention. Once the initial costs for the design and creation of the molds are paid, the actual manufacturing time is short. However, it is important to understand that a plastic injection machine is a complex piece of machinery and there are certain factors that can impact the quality of the resulting molded products.
A common cause of defects in molded products is improper injection pressure. Injection pressure must be consistent with the flow characteristics of the plastic. If the injection pressure is too high, it can cause the molten plastic to solidify prematurely in the mold channels before the cavity has been properly packed. On the other hand, if the injection pressure is too low, the molten plastic may not be transferred to the cavities at all. This can result in a void or hollow in the final product. Other factors that can influence the quality of molded products include: