After years of experience in mixing colorant into resin pellets, Proto Labs engineers have developed a standard for the ratio of transparent colorant added to clear resins. This 1% ratio applies to all of the transparent colorants we stock, including:
• Trans Red UN3012TR
• Trans Amber UN2034TR
• Trans Green UN6881TR
• Trans Blue UN5016TR
• Trans Grey UN7903TR
(All of our stocked transparent materials are Omnicolor concentrates, manufactured by Clariant.)
You can, of course, provide your own pre-compounded transparent resins at any color ratio you choose, and current customers who are already having parts made using a different colorant ratio can continue to do so.
The decision to standardize on a 1% ratio for transparent colors was based on experimentation at varying ratios of colorant to clear base resin. The ideal colorant ratio turned out to be significantly lower than the 3% ratio Protomold recommends for coloring opaque resins.
Our tests showed that, at ratios higher than 1%, the resulting color is too dark, particularly in thick areas, and transparency suffers. At 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 molding press. As pellets are heated and compressed in the barrel of the press, color 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 color, go through multiple extrusion cycles to help make the color 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 color before injection. At less than 1% color, this lack of wide dispersion starts to become clearly visible.
Note that even at our recommended 1% ratio, some color variation within and between parts can be expected. As suggested earlier, the best color consistency will always be achieved using customer-supplied pre-compounded resins.