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this post was submitted on 21 Jun 2024
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Asklemmy
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For die casting, it is post processing. After the part comes out of the mold there is the sprue, overflow tabs, and flash (extra material at the parting line) that needs to be cleaned up. Then the parts are often chromate conversion coated and then finished with either a wet paint or powder coat. Plating is also an option.
For injection molding, parts typically come out of the mold fully finished (or just requiring gate removal). The vast majority of parts are only textured where the final finish is part of the mold. You can do anything from a high polish to rough textures, or even any crazy pattern or simulated material you can imagine. For graphics you can apply in-mold decoration where a film is placed in the mold before the plastic is injected. There are some post processing finishes that are pretty frequently done such as wet painting and plating.
How are finishes so durable and thin?
My assumption of a lack of post processes is because I come from a background of automotive refinishing and repair, where I've owned a shop and painted for many years along with getting into custom art graphics and airbrushing. The only finishes I know of that provide a similar durability are two part urethanes. Those are far too thick by comparison. When cutting into plastics that have been moulded, the finish shows no signs of mechanical layering or bonding like a post process finish in most cases. Often a cleanly broken or cut part shows a similar type of penetrating surface alteration I associate with a polishing operation, where the surface transitions in color and grain structure with in millimeter or few (in cases where the break is clean and does not appear to be influenced by stress alterations like ABS where it whitens under tension).
How does chromate conversion work with a prep regime and what kind of wet paint can offer similar durability to a 2k urethane when it is impossibly thin? Like I know the limitations of urethane well when it comes to corners and pointy bits where it will thin from surface tension. There is not a chance in hell that the buttons on the side of my phone could be painted with such a finish with an even conformal coating and remain durable for years of constant abrasion. Is there a name for this class and type of finish? Where are they sourced? What is the scale of the industry? Is there a way to access the process and products at a small scale?
Im not a coating scientist (I'm a design engineer) so I can't really answer your questions in detail. But powder coats are typically polyurethane or polyester. There are different types of wet paints as well. Some surfaces also get an additional UV top coat for abrasion resistance.
For a phone button you might be looking at something like PVD coatings or platings. Chromating wouldn't be used in those cases as it is not terribly durable but it provides a good surface for the coating that goes on top or it. All of thses coatings are on the scale of a few mils thickness.
Most of these finishes are done on industrial scales and even the manufacturers send them out to external places for finishing (e.g. in Asia that type of facility is strictly controlled for environmental reasons) . At the end of the day I just speak with my supplier and tell them the finish I want and they take care of the rest.