Plastic injection molding is a popular manufacturing technique in which thermoplastic pellets are converted into high volumes of complex parts. The injection molding process is suitable for a variety of plastic materials and is a vital aspect of modern life—phone cases, electronic housings, toys, and even automotive parts would not be possible without it. This article will break down the basics of injection molding, describe how injection molding works, and illustrate how it is different from 3D printing.

Add color by melting a few Polly Plastics Color Pellets (sold separately) along with the moldable sheets.  Or you can add powder paint or alcohol dye (available at craft stores) to the molten plastic.  Alternatively, apply acrylic paints or special spray paints for plastics after your part has cooled.  Add a polyurethane top coat to lock in color.  You can even color the surface with permanent markers.

The reciprocating screw forces this melted plastic through the nozzle, which is seated within a depression in the mold known as a mold sprue bushing. The moving platen pressure fits the mold and the nozzle together tightly, ensuring no plastic can escape. The melted plastic is pressurized by this process, causing it to enter all parts of the mold cavity and displacing cavity air out through the mold vents.

The injection molding cycle works by this continuous process—closing the mold, feeding/heating the plastic granules, pressurizing them into the mold, cooling them into a solid part, ejecting the part, and closing the mold again. This system allows for the rapid production of plastic parts, and upwards of 10,000 plastic parts can be made in a workday depending upon the design, size, and material.

Once the plastic has filled the mold including its sprues, runners, gates, etc., the mold is kept at a set temperature to allow uniform solidification of the material into the part shape. A holding pressure is maintained while cooling to both stop backflow into the barrel and reduce shrinking effects. At this point, more plastic granules are added to the hopper in expectation of the next cycle (or shot). When cooled, the platen opens and allows the finished part to be ejected, and the screw is drawn back once more, allowing material to enter the barrel and start the process over again.

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Included below is a table of the common injection molding plastics, each with its own set of advantages and general industry applications:

Polly Plastics Moldable Plastic Sheets are great for crafts, art projects, home repair, model making, toys, cosplay costumes/masks/armor and prototyping.  Use it make custom parts, handles, grips, hooks, brackets and cases.  You get 3 sheets, measuring 8” x 12” x 1/16” thick.  They are just the right thickness for easy flexibility so you can wrap objects or mold into complex shapes.

The basics of plastic injection molding process includes creating the product design, making a tooling a mold to fit the product design, melting the plastic resin pellets, and using pressure to inject the melted pellets into the mold.

In Plastic Injection Molding, plastic injection molds come out with suitable surface quality and are essentially ready for application (with only sprues, runners, gates, and minor defects to manage). Thermoplastics are generally the preferred material used in injection molding, though newer injection molding processes can accept thermosets, impregnated materials, some metal powders, and some types of ceramics.

Xometry offers a full range of injection molding capabilities to help with your production needs. Visit our website to explore the full range of our capabilities or to request a free, no-obligation quote.

3D printing is a separate set of processes from injection molding. With 3D printing, the material is deposited in a layer-by-layer process guided by a 3D model of the desired part. With injection molding, molten plastic is injected into a mold cavity under high pressure, creating a part all at once. Both processes are considered to be "Additive Manufacturing".

Choosing the right plastic for plastic injection molding can be difficult—there are thousands of options in the market from which to choose, many of which will not work for a given goal. Luckily, an in-depth understanding of the desired material properties and intended application will help narrow the list of potential options into something more manageable. When considering the application, it is important to keep in mind the following questions:

Designers (engineers, mold maker businesses, etc.) create a part (in the form of a CAD file or other transferrable format), following fundamental design guidelines specific to the injection molding process. Designers should try to include the following features in their designs to help increase the success of a plastic injection mold:

With 3D printing, the material is deposited in a layer-by-layer process guided by a 3D model of the desired part, while with injection molding, molten plastic is injected into a mold cavity under high pressure, creating a part all at once. To learn more, see our guide on 3D Printing vs. Plastic Injection Molding.

Highly skilled machinists and toolmakers, using the product design, fabricate a tooling mold for the injection molding machine. A tooling mold (also known as simply a tool) is the heart and soul of the injection molding machine. They are carefully designed to contain the negative cavity for the product design and additional features such as sprues, runners, gates, vents, ejector systems, cooling channels, and moving components. Tooling molds are made out of specific grades of steel and aluminum that can withstand tens of thousands (and sometimes hundreds of thousands) of heating and cooling cycles, such as 6063 aluminum, P20 steel, H13 steel, and 420 stainless steel. The mold fabrication process takes upwards of 20 weeks to complete, including both fabrication and approval, making this step the most extended aspect of injection molding. It is also the most expensive part of injection molding, and once a tooling mold is fabricated, it cannot be drastically changed without incurring additional costs.

Start building, Start Making, Modifying and Mending!  Order your Polly Plastics Heat Moldable Plastic Sheets today.   And your satisfaction is our top priority so we offer a money back guarantee on all our products, no questions asked.

After operators obtain the finished mold, it is inserted into the injection molding machine, and the mold closes, starting the injection molding cycle.

Plastic granules are fed into the hopper and into the barrel. The reciprocating screw is drawn back, allowing materials to slip into the space between the screw and the barrel. The screw then plunges forward, forcing the material into the barrel and closer to the heater bands where it melts into molten plastic. The melting temperature is kept constant as per the material specifications so that no degradation occurs in the barrel or in the mold itself.

In 3D Printing, Supports for overhangs and voids are also printed with the part, as well as additional features to reduce warping and other defects. 3D printing is a lengthy process, taking at minimum several hours, and can take a few days depending upon the size and complexity of the printed object. 3D printed parts are commonly produced out of thermoplastics, but other possible materials include thermosets, impregnated plastics, ceramic powder, and metal/alloy powder.

A good pointer to keep in mind is material selection also depends on the product design, both in the application and in complexity. For example, designs calling for bendable sections/living hinges will require a strong yet flexible plastic like polypropylene. Similarly, structural parts that must be resistant to chemicals, abrasions, etc. should prioritize plastics like PEEK, Nylon, and others within these families. Â

“This is honestly my go-to product now. I love it. Easy to work with, reusable, and solid. I use it with a heat gun. Acrylic paints work well on it and it's easy to cut and layer. Honestly, I have been recommending this product to everyone I know.” – Kylie StarEater

The components of an injection molding machine include hopper, a barrel, a reciprocating screw, heater(s), movable platen, a nozzle, a mold, and a mold cavity.

This article presented the basics of injection molding to explain how it works, the materials that can be used, and how the process differs from other techniques such as 3D printing. We hope this article helped readers understand this manufacturing process and how it can add immense value to manufacturing capacity, given a few key considerations.

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Polly Plastics Moldable Plastic Sheets are a great starting shape whether you are making something, mending something or modifying something.  Just heat the sheets with a heat gun or in hot water above 150 degrees F for 2-3 minutes until they turn from white to clear.  Once they are molten, you can mold them into any shape you desire.  Upon cooling, they will be strong and permanent.  If you change your mind, you can reheat and modify until you get it right.  Since this plastic is so strong, you can drill it, sand it, varnish it or superglue it to something else.  Safe to use.