Thursday, September 13, 2007

Surface Preparation for Painting

Buzz: Ran into a question concerning surface preparation for plastic being decorated: “I need help to design a flame unit to treat a flat plastic product. I am building a machine to accurately align an applique on the part. It is a low volume application and has a shoestring budget. It can be a Rube Goldberg type machine so any help would be appreciated.”

Skippy: Flame annealing the surface of PP and PE products will definitely improve the adhesion properties of many adhesive systems and inks - you can prove this in short order just playing the flame from an off the shelf 'sweated copper tubing' burner using a 'paint removal spreader tip' quickly over the area to be 'stuck to'.

The easiest way to accomplish what you want to do uses gravity for the application motion - the parts are 'dropped down an incline', and the surface to be treated is hit with one or more gas burners - you need a hot flame on and off the surface in a very short time span so as not to release the other residual 'stress' in the product as extruded or molded. LP versus Natural gas burners are much better suited and commonly available for this process. Best results will be with a tunable flame that is in the blue color range starting at approximately 1100 degrees F (and could be hotter - check with the burner manufacturer to see optimum and working temperatures) – yellow flame is too cold. Applying the flame UNDER the part is easiest to control although ring type burners for 360 degree application are available.

Buzz – what about safety?

Skippy – Good observation - Be sure to build in the necessary safety devices used to SHUT the unit down immediately if your process goes out of temperature range or allows product to catch fire (which inevitably happens), use Personal Protection Equipment (heat gloves, eye protection, keeping extra skin covered etc). Another hint, stay away from highly flammable clothing as the operator - rayon, polyester, etc are NOT GOOD CHOICES. Use protective clothing/handling gloves tailored to the task and be sure to keep an appropriately rated fire extinguisher handy -

Again, the 'blue flame' concept mentioned is in approximately the 1100 degree F or more range; depends on time and temperature - it does oxidize the surface. The burner at the action point is only one part of the system - you need to use a thermocouple in the flame stream (in the right temperature range,) and a temperature reading and gas control device to get an idea of where you are at temperature wise for repeatability each production cycle. If you need more exposures, the product can be run through successive rings, or passed successively through a single ring or burner. You might need to put a cooling cycle or water source between rings or passes.

Do not scavenge rings from your stove at home - you need to consider the burners, any cooling required, the distribution of flame versus part size and time, etc. take a peek at this or similar sites -

I'm sure these or other people can help you get started.

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