Different Types of Plastic Injection Molding

Injection molding is a process that uses heated plastic to form parts. These parts are then cooled to set them.

The sprue is the first channel that delivers the melt from the injection unit to the front of the mold. It also contains a gate, which directs the molten plastic into the cavities.


Thermoplastics are used to make a wide range of products. They are flexible and can be reshaped as needed. They are also less expensive than molded metal. They can be molded into complex shapes and are very durable. They are ideal for prototyping and can complete manufacturing runs within a few days. They also have a high turnaround rate and are more accurate than plastic machined parts.

Thermoplastics become viscous when heated, which makes them pliable enough to take on the shape of a mold. The plastic is injected into the mold under pressure. The screw then draws back, releasing the pressure and allowing the plastic to cool in the mold. This process can take a few seconds to several minutes, depending on the complexity of the part. Thermoplastics can be blended with additives to meet strict smoke, flammability, and toxic gas release requirements. They are also more eco-friendly than other materials. They do not emit any volatile organic compounds (VOCs) or create hazardous waste.


Thermosetting materials are used in injection molding for a variety of products. They are highly resistant to electricity, water, and chemicals. They are also heat-resistant and can withstand high temperatures. They are ideal for medical device injection molding, because they can be molded into permanent parts that will not melt or soften when exposed to heat.

The thermoset plastic material is fed from a hopper into a heated barrel with a reciprocating screw. The screw delivers the raw material forward, mixes and heats it to lower its viscosity by reducing Van der Waals forces between the molecules. This is the most crucial step of plastic injection molding.

The molten plastic is then transferred to the mold cavities, where it solidifies and cools. Once the molded product has hardened, valves close to isolate the injection system and chemical precursors and the mold opens to eject the molded part. Injection pressure is replaced by holding pressure during this process to compact the molded part and control cooling and shrinkage.

Cellulose acetate

Cellulose acetate is made from the cellulose found in wood and cotton fibers. It is a bioplastic that can be used for eyeglasses, film, and other applications. It replaced nitrate film in photography because it is less flammable. It also can be molded into frames and can be dyed to produce different colors.

To make cellulose acetate, the raw starch must be dried and reacted with acetic acid to produce the acetate molecule. The acetate is then mixed with other plasticizers to create various plastics. The plastics made from this mixture can be used in injection molding.

The acetate can be blended with polylactic acid (PLA) to increase its elongation and reduce its density. The blends also have good mechanical properties. The acetate/PLA composites have a lower water content than other starch injection molded plastics and can be recycled easily. They are also more biodegradable than general-purpose resins and can degrade in soil, compost, or even the ocean.


Polypropylene is a thermoplastic that’s used in dozens of industries, including medical and automotive. It’s particularly popular for medical applications due to its incredible chemical and bacterial resistance and its ability to withstand stringent sterilization procedures.

It also has a high resistance to moisture, making it an ideal resin for products that need to be resistant to dampness. Additionally, it has a low level of electrical conductivity, making it useful for electronic parts.

Xometry offers a wide variety of injection molding machines that can handle polypropylene, allowing us to produce your custom plastic components and parts in the most efficient manner possible. Polypropylene’s isotactic structure allows it to revert to its liquid form easily, which reduces material waste and cycle times. It’s also a great choice for creating living hinges, which are common in plastic bottle caps and other consumer items. Lastly, PP’s low melt viscosity makes it easy to mold thin features like living hinges.

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