Injection Molding Painting Processes

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By Jeremy Losek, Aug 25th, 2022

Can You Paint Injection Molded Plastic?

Yes. Painting injection-molded parts is a fast and cost-effective way to ensure a uniform look and protect the newly manufactured parts. After injection molding, the plastic parts are ejected from the machine. These parts are bare – the finished texture and color will be determined by the characteristics of the plastic resin. The parts can be sent directly to the customer if no further post-processing is required.

Some injection-molded parts will require post-processing to achieve the desired texture and color. Painting is a fast and cost effective way to ensure uniformity in texture and color over an extended run of parts. Painting also offers several benefits including covering flaws, chemical and stain resistance, protection from UV light, scratch and abrasion resistance, and protection from the elements.

The experts at ICOMold by Fathom can help customers choose the right painting process to achieve the desired finish.

Painting Injection Molded Plastic

Painting injection molded plastic parts requires an in depth knowledge of the chemistry and characteristics of the various plastic materials. It also requires knowledge of the mold and all the variables involved in the plastic manufacturing process. Several factors, including the molding process, mold type, mold surface, and part surface preparation must be considered and understood to achieve long-lasting adhesion between paint and plastic. The experts at ICOMold have several decades of combined experience and can answer any questions customers may have about the plastic painting process.

The Pros and Cons of Painting Injection Molded Plastic

Weighing the pros and cons can help customers decide if painting injection-molded parts is the right choice. The experts at ICOMold by Fathom are always ready to advise customers on their post-processing options.

Pros of Painting Injection Molded Parts

Silk Screen prep for injection mold partColor. The plastic parts painting process ensure uniform color throughout the manufacturing run. This means that the first piece made and the last piece ejected will look exactly the same, even if the color of the plastic resin varies over the course of molding. In many cases, it is less expensive to paint every piece than it is to dye the plastic resin to match the desired color

Cover Imperfections. Paint will cover most imperfections that result from the injection molding process. These imperfections can be caused by the mold itself or by the design geometry. Paint will also cover up inconsistencies in the resin. Plastic resins with glass and carbon fill will show fibers near the surface of the part.

Finish. The finish of a bare plastic injection molded part is determined by the chemical characteristics of the resin. Plastic resins have finishes varying from satin to semi gloss. Painting injection molded plastic with ensure the correct finish. Customers can chose from a dull matte finish all the way to high gloss.

Stain and Chemical Resistance. Painting injection molded plastic will help the finished part resist staining from environmental factors and contact with certain chemicals. The plastic painting process will protect and lengthen the life span of injection molded parts.

Easy Clean Up. Painted surfaces are much easer to clean than unpainted surfaces. As noted above, paint will protect the integrity of the part from staining and chemicals. The same paint will make clean up a breeze should the part become soiled.

Scratch and UV Resistance. Plastic injection molded parts can be used in a variety of environments. The harshest environment is typically exposure to the elements. Parts being used in an outdoor setting must be able to stand up to all weather conditions and anything that gets thrown at it, figuratively and literally. The plastic parts painting process will add an extra layer of protection, making parts better able to withstand physical abuse and long exposure to sunlight.

The Cons of Painting Injection Molded Parts

Extra Cost. Painting is a post-processing procedure and will cost extra. Skipping any post-processing will reduce cost, especially if you are happy with the color and texture of the bare plastic. Beyond the added cost, there are no other downsides to painting injection-molded parts. Painting plastic injection molded parts is an inexpensive and easy way to protect new parts.

Types of Plastic Painting Processes

There are several types of plastic painting processes to choose from. The process that will best fit your project will depend on how the part is used, where the part is used, and what environmental factors may impact the part.

Spray Painting. Spray painting is the simplest and most cost-effective painting process used to add color or character to plastic parts. Some paints are two-part and self-curing. Other plastic paints require UV curing to increase durability. An ICOMold Project Manager can help you determine the best type of spray paint for your project.

Powder Coating. The powder coating process starts with a powdered plastic that is sprayed onto the parts. A UV light is then used to cure the paint and adhere it to the surface. The chemistry of both the powdered plastic and the plastic injection molded part must be taken into account. This is to ensure the powder will bond electrostatically to the plastic before the UV curing process. Powder coating can provide a tough, long-lasting finish on plastic injection molded parts.

Silk Screening. Silk screening is used when more than one color is desired. This painting process also provides a way to apply detailed designs, in multiple colors, onto the part. There are some limitations to where and how silk screening can be used. Silk screening requires a flat surface where the paint will be applied. The process involves making a screen — a thin plastic sheet with a screen. A negative of the design is printed on the screen. The screen is laid on the part, paint is applied to the screen, and the screen is then removed, leaving behind the design. A separate screen is required for each paint color.

Stamping. Stamping is simple, quick, and affordable painting process for adding color to plastic injection molded parts. A large, soft pad is created with a raised design that will pick up the paint, which is then applied it to the plastic part. The pad is dipped in paint and then placed on the part. Removing the pad leaves behind the desired design. Stamping is a versatile painting process that is more precise than spray painting and has more options for placement than silk screening.

In-Mold Painting  – In-mold painting involves applying paint to the injection mold cavity before the plastic is injected, allowing for color transfer via a chemical bond during the injection molding process. In-mold painting creates exceptionally strong adhesion between the plastic and the paint. This is 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 procedures to obtain optimal results. Virtually any color can be achieved in gloss or satin. Textured surfaces that resemble wood or stone can also be created.

Preparing for the Plastic Painting Process is Crucial

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

Surface energy is another variable to consider. Surface energy refers to how receptive the plastic will be toward paint. The chemical makeup of the plastic material will determine the molecular force of attraction, which in turn will determine how likely the paint will adhere to the surface. The higher the surface energy, the more receptive the plastic is to the paint. Plastics like polyethylene and polypropylene have a low polarity of molecules, making these materials hard to paint. By increasing the surface energy of these materials, we can improve paint adhesion.

ICOMold knows from experience that the chemical nature of these additives will often change how or if the paint will adhere to the finished plastic part. Speak with an expert at ICOMold to determine the best plastic painting process for your next injection-molding project.

Cleaning Parts Before the Plastic Painting Process

Surface preparation is key to a long-lasting paint job. Injection molded plastic parts need to be free of soil, residue, and other debris before painting. ICOMold will take precautions in every painting process to keep surfaces free of fingerprints, static, dust, and other contaminants. Paint room air is filtered and kept at 50% relative humidity. We also incorporate water-soluble mold releases that are easily removed from the plastic surface with a detergent. The mold release agent will not interfere with paint adhesion once it has been removed.

Plasticizers are sometimes added to injection molding resins to increase impact strength, but these can negatively affect paint adhesion. Neglecting to take all these factors into consideration when developing a manufacturing and plastic paint process can result in costly field failures and liability claims down the road. ICOMold has the expertise to help safeguard your products against these possibilities.

Powder coating on plastic injection partSurface Etching for Better Paint Adhesion

Plastic parts are inherently smooth and smooth surfaces are not always easy to paint. In order to facilitate the plastic painting process, parts need to have their surface roughed up, or etched, using a chemical agent. The chemical agent will create micro-roughness and affixing sites that will provide a more suitable surface to ensure paint adhesion.

Ideally, solvents should be included in the paint to accomplish surface etching during the plastic painting process. Selecting the right solvent is important because over-etching and under-etching can result in a less than durable paint job or even a ruined part. Effective and safe etching with solvents requires knowledge, care, and a delicate touch.

A properly applied chemical reaction can be used when solvent etching is not possible. Polypropylene and polyethylene are an example of low polar plastic materials. Low polar plastic can be treated with oxidation. In this case, the plastic may be briefly exposed to an open flame from a gas burner. This starts an invisible oxidative chemical reaction that forms enough polarity on the surface of the part to provide good paint adhesion.

Plasma conditioning can be used to increase surface energy to boost paint adhesion. ICOMold also uses light-sensitive chemicals called photosensitizers. After applying the photosensitizers, the plastic part is exposed to ultraviolet light or to sent through an electrical corona discharge that generates ozone. Both processes act on the microscopic level to improve paint adhesion on plastics with low surface energy.

Adhesion is the Key to Plastic Painting Processes

Excellent paint adhesion on plastic can often be achieved with the same paint used on metals. This can be an important feature when a product includes both plastic and metal parts. The metal and plastic can then be painted at the same time using the same paint.

ICOMold specializes in proper material selection, strict surface preparation, and appropriate paint selection. We have developed extensive adhesion testing procedures to ensure excellent results over the life of the plastic parts.

Injection Molding Production Highlights

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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


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