November 2011 Design Tip



Being Transparent

After years of experience in mixing colourant into resin pellets, we have noticed that the ratio of transparent colorant added to clear resins has a significant impact on the final resulting colour and transparency.


Different manufacturers of transparent colourants specify different ratios depending on the base material and the desired effect, in this Design Tip we will show you how this ratio can impact the final parts.


Whilst we have a variety of popular transparent colourants in stock including Orange, Green, Blue and Red customers can, of course, provide their own pre-compounded transparent resins. If you have specific requirements you should discuss these with your material compounder in advance.


To demonstrate the different effects caused by the ratio of colourant we have used a transparent Blue in a clear polycarbonate.


The test show that, at higher ratios, the resulting colour can be too dark, particularly in thick areas, and transparency suffers. At a ratio of less than 1%, the mixing of colorant into the base can be too uneven, producing a marbleized or swirling effect. This occurs because colorant pellets are added to base resin pellets to produce a “salt and pepper” mixture in the hopper of the moulding press. As pellets are heated and compressed in the barrel of the press, colour from the dye pellets mixes with the melted base resin pellets. When processing a “salt and pepper” colorant mix at the press, the resin only goes through one extrusion process from hopper to part through the barrel. Pre-compounded resins, where the pellets are mixed to a uniform colour, go through multiple extrusion cycles to help make the colour distribution more uniform. The fewer dye pellets per volume of base, the more dispersed they are and the greater the likelihood that some areas of base resin will receive little colour before injection. At less than 1% colour, this lack of wide dispersion starts to become clearly visible.

1/2%—Inconsistent colour distribution throughout the part affects its appearance and may cause inconsistency from part to part even in the same run.

1/2%—Inconsistent colour distribution throughout the part affects its appearance and may cause inconsistency from part to part even in the same run.

1%—Some swirling is common in

1%—Some swirling is common in "salt and pepper" colorant mixes, but we get a reasonably transparent part and reasonable consistency from part to part.

2%—This creates a darker part that can affect transparency/light transmission through the part. The thicker the part, the darker the colour.

2%—This creates a darker part that can affect transparency/light transmission through the part. The thicker the part, the darker the colour.

3%—These parts begin to look more opaque than transparent.

3%—These parts begin to look more opaque than transparent.


Note that even at any ratio, some colour variation within and between parts can be expected. As suggested earlier, the best colour consistency will always be achieved using customer-supplied pre-compounded resins.


To learn more about the “salt and pepper” process and colorant & colour matching, read this past Protomold Design Tip. If you have any questions, feel free to contact our Customer Service Engineers at +44 (0) 1952 683047.


Read past Protomold Design Tips.