Back when everyone was running injection molding machines with plungers, if you needed to color material, you had to have it pre-colored at a compounder. The compounder would melt the natural material down, blend in raw pigments and other additives and then re-pelletize the material.
The advent of screw type injection machines and color concentrates significantly reduced the cost of coloring plastic materials. However, running material blended with color concentrate has its challenges. It has to be mixed carefully and the material has to be processed carefully to get good results. Sometimes, even if you do everything right, you end up with nothing but problems.
The biggest problem with color concentrate is what a lot of people refer to as color streaking. I’m not sure that’s a good description. I think that I would call it color swirling. It presents as an area (often by the gate) that is discolored, usually a light colored area with some darker swirls around it. This is a sign that the color concentrate is not dispersing properly in the molding machine. People sometimes blame it on poor blending of the color concentrate but this is not true. A poor blend will give you parts that are too light (or translucent) and then parts that are too dark within the same box.
If you are experiencing poor color dispersing or color swirling, there are some things that you can try to fix it:
- Use as much back pressure as you can while still getting the screw time to retract in a reasonable amount of time.
- Although using 40 to 80% of barrel capacity is usually acceptable, I have found that having more shots in the barrel helps disperse the pigment through the material. I would say using 40 to 60% of your barrel capacity is preferred when running color concentrate.
- Having the carrier resin in the concentrate match the material you are using will help. For instance, use an ABS carrier for ABS instead of PE carrier. So called “universal” or “general purpose” color concentrates are usually PE based and work great in polyethylene and pretty good in polypropylene but should not really be used for any other type of material.
- Using a color concentrate designed for a higher let down ratio can help. If you are using a 50:1 ratio concentrate, try a 25:1. Don’t run the 50:1 at 25:1, but have your color concentrate supplier provide a new formulation designed for use at the different ratio.
- I have found that mixing nozzles really help. There are a number on the market.
- I have not had any experience with them, but I understand that people have had great results using mixing screws.
Sometimes you can do all of the above and still have high scrap rates. It may be the tool design or you might not have a machine with enough shot size to run with 40-60% of your shot. Also, some materials are more difficult than others to run with color concentrate:
Easy to color with concentrate:
More difficult to color:
Very difficult to color:
There are times when you just cannot get good colored parts or your scrap rate is so high that it just doesn’t make sense anymore. I once saw an injection molder that was running acetal parts molded in color. They were running the parts, grinding them up and then re-running the regrind to get good parts. They were essentially running pre-colored material at that point. As a matter of fact, that is a good way to see if your appearance issues are caused by the color concentrate. Grind some parts up and run the regrind at 100%. If the color issues clear up, you know that switching to pre-colored material will solve your issues.
Color concentrates typically cost anywhere from $3.00/lb to $10.00/lb depending on the color, carrier resin and let down ratio. This should add between $.05 – $.15/lb to your material cost. You are displacing some of the material with color concentrate so this actually reduces the cost slightly. Having material pre-colored will typically cost from $.30-$.55/lb. There are small compounders around that will pre-color as little as 1500 lbs. I have even had 50 lbs pre-colored but it was expensive. This is a significant cost difference but constantly struggling with scrap and rejections sometimes make it worth it.
Color concentrates have helped reduced the cost of molding colored parts by not only being cheaper than pre-colored materials but by allowing molders to carry less color inventory. However, there are still times when pre-color material is better in the long run. If you run a lot of color concentrates, find a good colorant supplier who has good technical support for when you have issues. They can help educate your staff about looking at color and processing colored materials. Also, it’s a good idea to find a local compounder that will pre-color materials for you when color concentrates just won’t cut it.