Injection Molding Painting Processes

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By Jeremy Losek, Sep 14th, 2021

Injection Molding experts offering the most advanced painting processes

The injection molding process produces plastic parts that are often left bare. This is the most cost-effective option, if you can find a bulk material that is the color you want, and if the texture of the plastic as it comes out of the mold is acceptable.

If, on the other hand, you can’t achieve the color and texture you need straight from the mold, there are several possible plastic painting processes available.

The painting of plastic parts created through injection molding requires knowledge of the chemistry and characteristics of various plastic materials, of the mold and all the variables involved in the plastic manufacturing process. Several factors, including the molding process and mold type used, mold surface, and part surface preparation must be considered and understood if good, long-lasting adhesion is desired.

When IMD or overmolding won’t do the job

While we offer IMD (In-Mold Decoration) and overmolding, which have become common, there are projects on which these methods won’t deliver the desired results. Many of the colored designs and accents we see on plastic parts are added after the injection molding process is complete.

There are many ways to enhance a part’s appearance and performance. Films, paint, powdered plastics, even metallic vapors can be used.

Below are some of the plastic part painting processes we offer.

Injection molding painting processes

Silk Screen prep for injection mold part

Our plastic injection molding design experts can help you choose the most efficient painting process that will deliver the results you need for your parts and end product. With consistent success, we’ve utilized the following methods on injection molding projects for our customers.

Spray Painting – This is the simplest and most cost-effective painting process used to add color or character to plastic injection molded parts. Some paints are two-part and self-curing. Other plastic paints require UV curing to increase durability. Which is best for your project can be determined between you and your ICOMold Project Manager.

Powder Coating – This painting process involves a powdered plastic that is sprayed onto the plastic parts, then UV cured for adhesion to the surface.  The chemistry of both the powder and the plastic part must be factored to ensure the powder will bond electrostatically to the plastic before UV curing can take place. Done correctly, powder coating can provide a tough finish that will last on plastic injection molded parts.

Silk Screening – When the desired result requires more than one color applied over a plastic injection molded part, this is one painting process that can provide a way to apply more detailed designs and multiple colors. Silk screening involves the creation of a thin plastic sheet with a screen area design that allows the paint to get through to the part in the desired pattern. A separate screen is required for each paint color. The screen is placed over the part, paint is moved across it, then the screen is removed, leaving a thin layer of paint.

Stamping – This is a very simple, quick and affordable painting process for adding color to plastic injection molded parts. Simply put, a large pad is created with a raised design that will pick up the paint, then apply it to the plastic. The drying or curing method chosen will be determined by the many variables in material and injection molding process selected.

In-Mold Painting  – This process involves painting the injection mold cavity before the plastic is injected, allowing for color transfer via a chemical bond during the injection molding process. This creates an exceptionally strong adhesion. Because the paint moves and flexes with the part, in-mold painted parts are more resistant to chipping, cracking and flaking than those painted after injection molding.

As with all painting processes, in-mold painting requires the correct chemistry and procedure to achieve optimal results. Virtually any color can be achieved in gloss or satin, plus textured surfaces that resemble wood or stone can be created. An added benefit: with this painting method there’s no need for a separate painting area.

Preparing parts for painting is crucial

Silk screen printing on plastic partWhen painting plastic injection molded parts, it’s important to understand the extreme variation in the properties of various materials. Even within a single family of plastics, the paintability of each can be surprisingly varied. The differences in molecular weights of the plastics and the incorporation of additives in the resin manufacturing process can alter painting results. The plastic’s base composition could be comprised of a single resin or might include two or more resins. Other variables are fillers, extenders, plasticizers, and additives that are often mixed in to produce desirable physical and chemical properties. Colorants are also added to adjust gloss or provide a precise color.

What we know from experience is that the chemical nature of these additives will often change the paintability of the finished plastic part. The injection molding process used to produce the part can also affect paint adhesion.

One of the things we determine before proceeding with painting is the surface energy of the plastic. The higher the surface energy, the more receptive the plastic usually is to the paint. When the painting process is performed correctly, adhesion will be better. The low polarity of the molecules in plastics, including polyethylene and polypropylene is the reason the surface energy is low in these materials. By increasing the surface energy, we can improve paint adhesion. This is one of the main reasons we pretreat plastics that are going to be painted.

Powder coating on plastic injection part

Cleaning plastic parts for painting

Any painter worth his rate knows that surface preparation is key to a long-lasting paint job. Before painting, injection molded plastic parts need to be free of soil, residue and other debris. We take precautions in every painting process to keep surfaces free of fingerprints, static, dust and other interferences. Paint room air is filtered and kept at 50% relative humidity. We also incorporate water-soluble mold releases which are easily removed from the plastic surface with a detergent that won’t interfere with paint adhesion.

Plasticizers are sometimes added to injection molding resins to increase impact strength, but these can negatively impact paint adhesion, just as mold releases do. Failure to take all these factors into consideration when developing a manufacturing and paint process can result in costly field failures and liability claims down the road. We know how to safeguard against these things.

Surface etching for better paint adhesion

Plastic parts are inherently smooth. To overcome this fact, the surface of the part can be roughed-up, or etched, using a chemical agent to generate micro-roughness and affixing sites that will provide adhesion for the paint.

Ideally, solvents can be included in the paint to accomplish this. Selecting the right solvent is important because over-etching and under-etching can both result in a less-than durable paint job or even a ruined part. Effective and safe etching with solvents requires knowledge and care.

When solvent etching is not a good choice, a chemical reaction, properly applied, can be used. Polypropylene and polyethylene are two examples of low polar plastics that can be treated oxidatively. In this case, the plastic may be briefly exposed to an open flame from a gas burner. This starts an oxidative chemical reaction that is invisible yet forms enough polarity on the surface of the part to provide good paint adhesion.

Plasma conditioning allows plastics to be painted with good adhesion as well. Two other surface preparation options we have in our arsenal are light-sensitive chemicals called photosensitizers, followed by exposure to ultraviolet light, or to pass plastic parts through an electrical corona discharge that generates ozone.

Adhesion is the name of the game

Often, excellent paint adhesion can be achieved with the same paint used on metals. This can be an important feature when a product includes both plastic and metal parts that can be painted at the same time using the same paint.

In addition to proper material selection, stringent surface preparation and appropriate paint selection, we’ve developed very extensive adhesion testing procedures to ensure excellent results in the long run.

The very important point here is that plastic part surface preparation is vital to any successful painting process. At ICOMold, we have options available that will bring you the best possible end result and products you can be proud of.

Injection Molding Production Highlights

>  Instant mold and part quote
>  Low cost, quick build and quality production
>  ICOMold’s Lifetime Warranty on Production Molds
>  Online project management
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>  No size limitations
>  Any commercially available material and surface finish

What is the process for injection molding production?

ICOMold’s instant online plastic injection molding quote and mold frame sharing technology enables us to simplify and shorten both the quoting and tooling manufacturing process for custom plastic injection molding.

  1. Load your 3D CAD file to get an instant mold and part quote
  2. Upon order confirmation, ICOMold starts the mold and part order process
  3. Tooling design review by ICOMold engineers
  4. Upon design approval, ICOMold begins building your injection mold
  5. Customer examines samples for approval
  6. Part production begins

Family Injection Molding Case Study
Go to our plastic injection molding and CNC machining case studies page to see how we helped customers on their projects.

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