Colour Matching of plastics is very different to colour matching printed packaging mediums. Most of us are used to using the Pantone PMS colour guide. This is a Calibrated Print Management System for 4 colour process printing. It is very accurate and used globally. It is very easy for a brand owner in one country to have their printing identical on the other side of the world simply by using the relevant PMS number. The latest guides are printed on both a Matt and a Gloss finish.
Many times, when a new label or Brochure is being printed, you can stand beside the printing press for an “On Press” approval. If the first prints are not exactly what is required, a small amount of one of the 4 primary colour inks can be added and the print run continue for full production.
Colour matching plastic is very different. Most Master batch companies do not recognise the PMS system. They prefer a physical sample of what they are matching to. A label, the new bottle etc.
Before starting the process, we need to identify to them the type of plastic it will be used in so they can use the right carrier resin to bind the colour pigment together. This will also include a degree of slip so that the finished part performs as required. Therefore, for a two-piece cap in HDPE and PP, we may need to have 2 colour matches done, one for use in a PP resin with 3 % slip added, and then try for the exact same colour in a HDPE resin with no slip as HDPE is slippery by nature. You will then get a different gloss look in the finished parts so the PP component will have a high-quality gloss look, while the HDPE part in the same colour will have a slightly milky look by comparison.
Their process is to use various colour powder pigments from all around the world. They weigh out each powder accurately, then melt and blend them in an industrial blender in a molten form before extruding and cutting them into plastic pellets. To enhance the depth of colour they tend to add Calcium Carbonate (a white base) for light colours and Carbon Black for dark colours. These pellets are in a concentrated form and are used in a ratio of between 1 to 3% to natural pellets in our molding machines.
When we mold the first coloured parts, the various colours can cause the part to change size. We have experienced a Purple colour where the cooling part actually grows in size where as a red part will shrink in size. We often validate a new tool first in Natural and then in white to set the baseline of the cap sizing. The molding parameters can be varied to try and set the required part size. If we can’t get the part to the required sizing we then have to get the colour house to reformulate the master batch using alternate input powders. So our new colour matching is not just matching the required colour, but also the part size and the slip of the final part. It is an iterative process we need to do in conjunction with the colour house and requires the mold to be in a press so we can run the revised material. Additionally the thickness of the part can give different looking colours. The colour house will provide us a “stepped” colour chip that will have 3 different thicknesses of the same plastic. Thin parts with a degree of transparency will look a little lighter in colour even though it’s the same colour.
Once the new colour is approved by the customer, we keep the reference colour chip and it becomes the Master, and then with each new batch of master batch, they provide a new colour chip made from the new batch of colourant and we then compare it to the master before it is released for production.
This is why we quote 6 to 8 weeks as an indicative time to do a new colour match.
