Finding a Balance with Resin Colourants

Injection-moulded parts, for all their many shapes and sizes, all start out pretty much the same—as small plastic pellets. The base colours of those pellets vary somewhat depending on the resin, but they all fall somewhere on a monochromatic scale from clear through various shades of natural to black (see Figure 1). Turning those dull little pellets into the rainbow of colours that comes out of moulding presses requires the addition of colourant, which can be a fairly simple or fairly involved process. The complexity depends on how particular you are about your colour.

If all you want, for example, is red, Protolabs can add a stocked colourant to your base resin at no charge. If you want a specific colour, and you know the RAL or Pantone reference then we can purchase a specific colourant for you for a small additional charge, though the exact colour you end up with may depend somewhat on the shade of the base resin.

There are basically two ways to get coloured injection-moulded parts. The simplest is to mix dye pellets into a batch of base resin pellets (see Figure 2). As the pellets are heated and compressed for injection into the mould, the base resin and dye pellets melt and mix together before being injected into the mould. The other way is to order pre-compounded pellets in the exact colour you want; for example, a standard PMS colour or match to a sample swatch.

There are several things you should keep in mind if you choose to have Protolabs add the colorant.

1. As stated above, Protolabs does not offer exact match colours.

2. The effect of base resin shade on final colour is greater on light colours. In a pre-compounded colour, this can be adjusted for in the compounding process, but may be noticeable when you use Protolabs' stock colours.

3. Because dye pellets are mixed with base resin pellets in a “salt and pepper” mix, there is a chance of "swirling” in the resulting parts (see Figure 3). The degree of swirling depends on a number of factors. “Hot” colorants—red, orange, yellow—tend to exhibit a higher swirling risk than cool colours like blue and green. Also, large parts, because they use more of the contents of the chamber in which resin pellets are melted and mixed, are less likely to show swirling than small parts.

4. Unless you use pre-compounded coloured resins, there is a risk of inconsistency from lot to lot and from part to part. This is one of the reasons that we recommend our standard ratio of two percent (2 %) colourant.

5. Certain base resins “don’t play well with others” when it comes to colourants. For these—high-temperature nylon and polycarbonate, for example—Protolabs stocks a limited variety of resin-specific colorants. These allow you to achieve colour but further limit your choice of hue.

6. Finally, certain resin characteristics—UV protection, flame retardant, and medical or food compatibility—can be affected by the choice of colourant. Protolabs does not stock colourants specifically made to maintain these characteristics.

If you require an exact match to the blue of your corporate logo or the colour-matched cap of a spray-paint can, the process becomes more complicated however there is a solution: the use of pre-compounded resins purchased from specialized vendors. Pre-compounding consists of mixing colourant with base resin, melting and extruding the resin, re-pelletizing the resulting mix, and then repeating the process until a thorough mix is achieved. If necessary, the mix can be adjusted during the process to match the customer’s exact specifications. Vendors like Albis and Plastribution can match virtually any colour in a wide variety of resins, and can preserve critical resin characteristics in the process (see full contact list below). Because the colour is evenly distributed among the pellets, there are no issues of swirling or part-to-part inconsistency. Protolabs can use these pre-compounded resins to produce parts, delivering all the benefits of rapid injection moulding along with the exact colour and characteristics you need.

In short, if you need approximate colours, for general appearance or any other reason, Protolabs can provide a range of colourants at no charge. If you need a specific RAL or Pantone colour Protolabs can purchase a specific colourant for you for a small one off charge, but we cannot guarantee an exact colour match for the reasons above.

If you need exact specific colours, maximum consistency, and protection of specialized resin characteristics, outside vendors can provide the coloured resins and Protolabs can deliver the parts. If you have any questions, feel free to contact our Customer Service Engineers at +44 (0)1952 683047.

Property-Based Material Search Engines:

• Matweb: Material property database with data sheets of thermoplastic and thermoset polymers, metal alloys and more
• IDES: Free plastics datasheets for OEMs

Resin Supplier Links:

• Abbey Polymers : Source of thermoplastic Elastomer (mouldable rubber)
• Albis: Makrolon^® polycarbonate, Bayblend^® polycarbonate/ABS blend
• Dupont Plastics: Delrin acetal, Zytel nylon, etc.

Colorant and Color Matching Supplier Links:

• Colour Tone Masterbatch: Custom formulated colouring systems and additive masterbatchs for plastics
• Plastribution: Plastic additives and specialty compounded resins.