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How To Colour Plastic

plastic moulded case

Plastic is intrinsic to our world. Our use of commercial, industrial and retail plastic products is astounding. Commodity plastics, such as polyethylene, polypropylene and polyvinyl chloride each have a wide variety of uses, a few of which are listed below:

Polyethylene – supermarket bags and plastic bumpers

Polypropylene – car bumpers, yoghurt containers, even appliances

Polyvinyl chloride – plumbing pipes, shower curtains, window, and door frames

However, what most of us forget is that the plastic that’s derived from natural, organic materials such as cellulose, coal and, of course, crude oil through polymerisation or polycondensation. Once extracted, organic materials are then then linked together to form long polymer chains. These chains do not have a specific colour hue. Plastic resins must be coloured.

Interested in learning more about how plastic resins are coloured? Keep reading below.

Methods of Plastic Colouring

There are four methods to colouring plastic resins. Each is clearly different from the others, has its own distinct attributes and used for colouring different types of plastics.

Masterbatches

The most common and, arguably, one of the most economical ways of colouring plastics, masterbatches offer minimal wastage and can be introduced in small volumes.

Concentrated pigments are dispersed into the polymer carrier resin and then moulded. At this point the masterbatch is placed into a natural resin and directed into the press at the desired quantity to achieve the colour hue or shade. This process is the most common way of colouring plastic as it allows for maximum compatibility and simplifies processing.

Pre-Coloured Resins

Anyone with strong knowledge of plastic colouring tends to favour pre-coloured resin colouring. Why? It’s easy to do and offers consistent performance. Most pigments are 100% polymerised into the package and then treated as originally supplied by the moulders.

Cube Blends

Moulders tend to favour cube blends as they don’t need to use meter equipment through the press process. Cube blends utilise colour systems which provide a combination of dry-blended master-batches with natural polymer, which is then mixed by to stop any inconsistencies when blending the colours.

Dry Colour

Plastic colourants in powder form are produced by mixing pigments with natural pellets to create colours. This is called dry colour and known to be one of the cheapest colourants because of the simplified production process.

However, despite being cheap the process is not without its flaws. The dry colour can be scattered easily which can create an uneven colour. It’s also difficult to measure and, inevitably, creates much discolouration and dirt in the general environment.

red  plastic pieces

Speciality Colour Compounds

Adding value to custom engineered compounds can be achieved with speciality colour compounding. The process can be more complex than other colouring methods, without altering the physical properties of the material.

However, one of the primary benefits of using speciality colour compounds is that they’re great for identification purposes or creating custom colours. The process is also adept in creating compounds with a cosmetic quality appearance or a signature effect aesthetic for specific purposes.

The Colour Matching Process

Colour matching is needed to pinpoint the desired coloured plastic product or individual part. Once completed, this allows engineers to develop a specific colour concentrate for the individual application. Typically, a chip, plaque or Pantone number provides engineers with a colour hue reference to match. The polymer helps to determine the appropriate formulation for the concentrate.

Various factors must be considered when colour-matching the polymer, including the viscosity and melt processing temperature. Specific guidelines that outline what pigment systems, stabilisers, carrier resins or additives are best suited to the process of matching the colour of the polymer.

Despite the advances colour matching techniques, it’s worth remembering that colours will look different depending on the gloss and appearance of the polymer and the filler content. For example, nylon colours will appear slightly different to acrylic colours.

Additionally, environmental conditions, chemical resistance and the MHRA (Medicines and Healthcare Products Regulatory Agency), must also be considered and guidelines followed when preparing to colour match plastics.

What to consider prior to the colouring process

There are multiple factors to consider when determining how to colour plastic. Each is just as important as the last and, if not completed correctly, will affect the quality of the hue.

Compatibility

The colourant and polymer must be chemically compatible to successful colour plastic. Some colourants have chemicals that interact with the chemicals of some plastics. This can alter the properties of polymers, such as flexibility, temperature tolerances or impact resistance. The ratio of colour to plastic should be approximately 1:100 up to 5:100.

Pigment Levels

As the volume of the pigment injection increases, the affect the colour has on the plastic increases. The depth of the colouring and the properties of the plastic can be greatly affected with too much pigment.

Colour Properties

Colours have varying chemical properties which can interact with plastic in different ways. When fabricating the same component in multiple colours, it’s possible that parts with different colours may not have the same dimensions. Such variances must be given special attention, especially when product tolerances are a concern.

Colouring Method

The method used for colouring may influence the plastic’s properties. Before choosing the method for any individual application, it’s paramount that there is complete compatibility between the method and the plastic. This will avoid production problems.

Regulatory Requirements

It’s essential to follow any specific regulatory requirements when colouring plastics. Medical applications plastics, children’s toys and plastics for food have specific guidelines on how plastic should be coloured. If any colouring alters the properties of a component, the product may fail to pass critical safety certifications and be unusable.

As you can see, there are a range of methods, techniques and factors that determine how to safely colour plastic products. Choosing the right means of colouring really depends on one simple thing: individual application.

If you’d like to learn more about plastic colouring, plastic injection moulding or why Wheatley Plastics has been a leading supplier of plastic injection moulding to industries for more than 60 years, get in touch with us today.