Introduction: Fire-polish Your Laser Cut Project Items

By www.flickr.com/photos/davidwsmit

There are lots of great projects you can build with laser-cut and hand-cut PMMA plexiglas (acrylic) projects. Sometimes these projects don't look great because of the ends - cut, broken, sanded; these ends aren't completely transparent and professional.

This Instructable describes fire-polishing and shows the results. We accomplished this simple additional task on materials and gadgets we made on laser cutters at TechShop.

Step 1: Prepare Your Edge - Surface Preparation

Most PMMA (Acrylic) edges that are laser cut are automatically smooth, mostly due to the laser beam thinness and the fact that the cut is made with heat from the UV laser light doing the cutting. But, if the cut is uneven or if some finishing or adjustment is needed, or if acrylic is cut by hand, then the edge will exhibit cut marks and other surface roughness. It won't be shiny, but instead matte and uneven.

Project aesthetics is affected by the smoothness of the surfaces when working with acrylics, and if you use the acrylic to carry light (as in another Instructable) then its critical to make that light-carrying edge as smooth and flat as possible. Fire-polishing does just that.

Because the process slightly melts and reforms the rough material, it's absolutely critical to minimize the roughness of the edge before fire-polishing. It's not so different from other types of surface polishing -- to get the final mirror shine, you need the most rough parts sanded down flat.

So, first get some sandpaper or a sanding block. Make sure the surface of the sandpaper or the block are completely flat, and begin sanding slowly against the surface. Since plastic is affected by heat, make sure to sand with a bit of water, which also carries away the waste material and keeps the paper or block clear and continuing to sand. Also, where possible keep the protective film on the main surface of the item so it won't become accidentally scratched as you grip it.

I started with 400 grit material until the surface was evenly flat and matte - I regularly reviewed the edge under magnification and used a light to reflect and reveal the unsanded surfaces. Once that was complete, I moved to 800 grit sandpaper, again maintaining a flat sanding surface. Check the edge regularly, as this process doesn't require a terribly large amount of sanding or great force to achieve the effect. I stopped at 800, but you could easily go on to 1500 grit for an even better and smoother effect.

Once the edge is very flat and regular on its surface, you clean the surfaces with denatured (or 91%) alcohol, and then move to the next step.

Step 2: Fire-polishing!

First, prepare the item by standing or bracing it so that the edge to be polished it facing up on your bench. Make sure your area is safe for torches/high-temp work. If you attempt to hold the piece instead (not recommended), be sure to use non-flammable gloves to protect from the hot plastic.

Next, your tools. The torch should be an adjustable propane type with a medium to small flame (perhaps 1/2" wide or less. I used a Bernz-o-matic Micro-Torch and a Blazer Torch; both worked great. I didn't have as good an experience with larger torches using for sweating copper pipes, but you might be able to adjust the flame lower and get good results.

First, test your process on a piece of scrap material or waste from your cutting -- sand that part first (first step), then test your process on that. DO it over and over, each time with better sanding technique and vary your flame speed along the edge (see below).

Before we start, here's how it works: Flame polishing is a process of melting just the smallest edge surface of the acrylic, smoothing the edge, but not melting or burning the plastic.

  1. Once the piece is ready, set the flame to about 1" in length.
  2. Start your sweep of the flame well before the start of the edge, and end it well past the end of the edge -- it's very much like spray painting, moving deliberately and consistently, from before the material until well after it.
  3. With my 1/4" thick material, I moved about 1" / second along the edge
  4. Review the edge - if you still see roughness then you either went too fast or didn't sand the edge smooth enough

If you are trying your technique on a scrap piece, then try different speeds of motion - you'll know it's too slow when the plastic begins to flame, or browns, or slumps. Once you reach the right speed, the edge will be smooth and slightly rounded.

Step 3: Conclusion - Lovely Edge

Here's a few closing remarks and points:

  1. Use fire-polishing on nearly any PMMA/Acrylic edge to improve it, but be sure to test your technique first, as every material surface type is slightly different.
  2. Most of the time it's very hard to smooth a large flat face, as the flame and heat is hard to make consistent along the entire face
  3. If too much heat is added to a piece it might begin to curl - be sure to let your piece cool between flaming each edge.

Enjoy your great laser- or hand-cut creations and look in to TechShop for great tools and help in creating these items.