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    December 7, 2022

    The Secrets to Post Processing Injection Molded Parts


    Once you make an injection-molded part, you might have some extra steps you want to perform. A lot of people want to brand their products, change the aesthetics, and boost the performance of their parts. This is where post-processing comes in.

    However, post-processing plastic injection molded parts aren’t the same as working with metal parts. In this guide, we’ll explain the differences and outline some common post-processing operations you might do on a plastic injection molded part.

    Why Injection Molded Parts are Different

    A lot of post-processing steps are harsh and reserved for metals. Things like traditional polishing and welding generate too much heat for plastic parts. With injection-molded plastic parts, you need to avoid heat and minimize the force that you exert. 

    As a material, plastic is more sensitive to loads and heat. Plastic tends to deform and fail if too much force is applied, or if friction generates too much heat.

    Common Post-Processing Options for Plastic Injection Molded Parts

    If you want to post-process a plastic injection molded part, you have plenty of options. Here are some of the more common options to choose from.


    Painting a plastic part works just like painting anything else. It’s used to change the color and final appearance of the part.

    Some people will paint a part to match their branding, make the unit stand out, or highlight the part’s function. 

    Painting an injection molded part is a little different because you have the option of painting the part before it’s even made. How is that possible? This can be done by using pigmented polymers in the injection molder. Doing this will give you colored parts directly from the machine.

    If you’d rather paint the part afterward, you can choose between spray painting, silk screening, and powder coating. 

    Spray painting is the easiest and least expensive version. Powder coating is the most expensive, but offers the longest-lasting results and flake protection over time.

    Laser Marking

    Laser marking allows you to draw aesthetic designs on your parts after they’re made. Commonly, companies will use laser marking to put their logo, product tracing information, or some branding directly on the part.

    This is done with a laser. The laser will trace a pre-determined path on your part. A reaction occurs between the plastic and laser, and you’re left with gray or black lines wherever the laser just traced. 

    This method is preferred over traditional printing because it doesn’t fade as quickly, and it can work on a wider range of plastics and colors. The downside is that laser marking is typically more expensive than stamping. 

    Gate Trimming

    Injection molding machines utilize cavities that are filled with molten plastic and left to cool and solidify. A byproduct of this process is flashing. Flashing is excess material along the seam of the mold, and it’s especially bad at the gate — the gate is the part of the cavity where the plastic gets injected into.

    Flashing can cause a long list of problems for your product, so it’s best to remove it. As a result, most injection molding companies offer gate trimming or gate removal services.

    This service will get rid of the flashing on each of your parts, and leave you with a smooth part.

    Gate trimming as a post-processing step is unique to injection molding.

    Ultrasonic Welding

    People are always surprised to learn that you can actually weld plastic. The process is very different than traditionally welding two metal parts together, but the theory is identical.

    With ultrasonic welding, very high-frequency sounds are pumped through a horn that makes contact with the plastic parts that you want to weld together.

    The sound waves vibrate very quickly which creates a ton of friction at the end of the horn. Of course, the friction creates heat which physically melts the plastic parts. Once the weld seam cools, it becomes a solid bridge between the two parts, sealing them together just like traditional welding.

    Pad Printing

    Pad printing uses a rubbery surface to stamp ink onto plastic parts that aren’t flat. Since a rubbery surface is used, the stamping surface can mold around the plastic part when the ink is transferred.

    You might see this done on cups, plates, or pens. If you tried using a flat stamp on any of these parts, only a portion of the ink would transfer onto the object.

    As opposed to laser marking, pad printing applies a layer of ink on top of your part. The ink can chip and wear over time, but it’s a much more affordable post-processing step.


    All of the post-processing operations we outlined are perfect for plastic injection molded parts. It’s an easy way to change how your finished parts look and operate without jumping through extra hoops during the design phase. If you want to get started with professional-grade plastics post-processing today, reach out to our team at Rapid Axis. We have full injection molding capabilities, and we offer a number of post-processing steps.

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