One of the tips in our guide is "don't use paint". The main reason for not using pain is that when plastic is painted, the paint acts as a contaminant when that plastic is ground up for recycling. There are methods for removing some types of paint before recycling, but these are sometimes caustic and always expensive.
Eco-Pro: Can hide imperfections that would normally cause parts to be rejected, increasing yield rates. This often comes at the expense of many other trade-offs, however (see Eco-Con), and is usually not worth it.
Eco-Con: Decreases recycleability of plastic parts, energy and water intensive to produce in most cases, manufacturing waste usually results in toxic byproducts, creates large amounts of material waste due to over-spray and parts used for masking, wears off eventually making products look old (and therefore more likely to be thrown away).
Question the premise of the finish: Is a painted surface really conveying the feeling of the design properly? Would a natural finish look higher quality? If I have to paint this material, should I be using another material that naturally looks like the finish I want?
Alternatives: Mold-in color, double-shot molding, pad printing, anodizing, powder coating, pad printed cardboard.
Same finish, better process: Back painting clear parts can give the same effect while making the part less susceptible to wear.
Materials: Almost any material.
NOTES: The best advice for someone looking to design a sustainable product is not to use finishes at all. Unfinished materials are always easier to recycle so designers should use other methods of adding detail to their designs, like embossing, texture changes, etc. But this isn't always practical or in line with the aesthetic requirements of the product, so some finish is often required. We decided to single our paint with the "don't use paint" tip in the Field Guide because we are trying to address the highest impact problems, and paint is definitely one of them. Here are a few reasons why.
Paint is very difficult to seperate from plastics before it is ground up for recycling. this means that painted parts are more expensive to recycle, and therefore less likely to be recycled. Contamination is less of a problem for metal recyclers since metal is melted to temperatures that burn the paint off, but doing this releases some pretty nasty stuff into the atmosphere.
Paint is usually sprayed on, so for each part that receives a shinny coat of paint, there is quite a bit of paint that misses the mark and is wasted.
3. Drying process
Painted parts are often dried in ovens, which can be the size of entire houses, are open ended, and run all day long.
Painting contoured geometry requires a separate part to me molded (when painting plastics) or stamped (when painting sheet metal) in order to mask areas that are to remain unpainted. Sometimes these masks are larger than the part that is being painted! Plastic masks can only be used around three times before the paint buildup makes them useless. Since they're covered in paint, they can't be reground, and are usually shipped off to low quality plastics down-cyclers. Metal part masks can be used many more times, and are then sold to metal recyclers who may or may not remove the paint propoerly before melting the metal down, which burns off the paint and releases dioxins into the atmosphere.