Have you ever tried to paint or dip straight to a plastic part and the paint or film just would not stick? It is because of the type of plastic you are working with. Both types, polyethylene (PE) and polypropylene (PP) have the same issues with paints and film adhesion and it is because of the chemical make-up. They have a chemically inert, nonporous surface which causes them to have a low surface tension. This is what prevents the paint or film from properly adhering to the part.
There is another issue that can cause this as well. It is from the release agent that is used when producing some plastic parts. This release agent is used to make the injected part pop out of the mold easily when being produced. Sometimes the release agent is sprayed on the mold and others it is mixed in with the plastic material prior to being injected in the mold. This release agent can cause problems with the adhesion/bonding of paints and film to the part because it is designed to prevent the part from sticking in the mold. This is a secondary problem that will cause the lack of adhesion, but still important to know.
It is always a good idea to prep and wash the plastic part properly before trying to paint or apply a film directly too. You can do this by washing the part down well wish dish soap (such as Dawn Dish Soap) and then using a scouring pad to ruff the surface up slightly. This will help to assist and ensure a good bond when you paint or dip the plastic item.
Flame Treating – What is does
Flame treating is a process in which heat and alcohol are introduced to the PE or PP plastic part to make the bonding of the paint or film possible on the plastic part. By applying heat (without going into crazy scientific gibberish), you are changing the surface of the plastic part and creating a polarity which allows the bonding to form from the paint to the part.
Flame Treating – How to flame treat parts
What you will need:
- Heat source (heat gun or propane torch)
- 91% Isopropyl Alcohol (add a spray head to bottle to make things easier)
- Shop towel
Wave the heat over the part and try not to leave the heat in one place for too long, keep it moving around. As you do this, you will notice the part starting to change color slightly and it will start to sweat (look wet). If you are working with large items, it may be better to do this in sections. Once you are done heating the item, spray it with the alcohol and wipe it down well.
Be sure to get the edges of the back of a part as well. When you apply the primer, base and clear it will wrap around the edges and on to the back as well. You want to make sure these back areas have good bonding abilities for the base and clear to be applied along with the front. If not, then the paint could start peeling or flaking off from the back side edges and cause problems for the front after time.