Problems Gluing Laser Cut Acrylic

Aug 06, 2010

Acrylics and similar plastics are commonly glued using “solvent glues”. That is, the glues actually soften or partially dissolve the edges of the materials you are bonding. As the solvent evaporates, the two edges ‘re-set’ themselves, as a single piece. Use the right glue (and technique), and you have a strong bond that is nearly invisible, almost as if it had been cast as a single piece.

Gluing acrylic can pose some unique challenges.  The cracking you see in the picture is called crazing, and it can be a real problem in production until you know what causes it, and how to prevent it. By far the most common cause is the use of an incompatible glue. Use the wrong glue, and you get uneven bonding,  discoloration, or improper curing that results in a bad case of crazing. Luckily, this is easily to prevented by knowing which material you have – acrylic, polycarbonate, polyethylene, polypropylene, or polyurethane – and selecting the appropriate glue. If you’re not sure what the right glue is, ask your plastics supplier.  They almost universally do some form of fabrication in house, so someone there will be able to point you in the right direction.

Even with the right glue, there can still be a problem gluing plastics – especially laser cut acrylic. When the material is made, it is either extruded (passed between rollers) or cast (poured into a mold and allowed to set.) In both cases, it solidifies into a sheet with even internal stresses, keeping it clear and strong over time. However, when you cut it with a laser, the edges get super heated, and then quickly cooled again. This introduces internal stresses to the sheet. Most of the time, you will never notice, as it does not meaningfully compromise the strength or longevity of the material. Apply a solvent however, and that tension is released in the form of thousands of tiny cracks along the cut edges. Due to the differences in manufacturing process, extruded acrylic is far more susceptible to this problem than cast acrylic. Material thickness is a factor as well.

A couple solutions exist:

  • Because the heat damage doesn’t go very deep, you can use a router or joiner to cut away 1/16 or 1/32 of an inch from the edges of the pieces you are gluing.  The machine edge will not be glass-clear like a laser cut (pictured to the right), but because of the way the glue works, as long as it is smooth, it should become clear during the gluing process. Obviously if you’re going to cut away some of the material, you need to anticipate this and make your laser cut part correspondingly larger to get the final size you want.
  • The other option is more involved, but also a more complete solution. You can anneal the part. Annealing is commonly used in glass working to avoid similar internal stresses created by uneven cooling. To do this, you’ll need to buy or build a batch oven large enough for your parts. Then heat the pieces up to 180F slowly (about 1 hour per 1/8″ of thickness – not less than two hours.) While acrylic doesn’t melt until almost 300F, it does soften enough at this temperature that it may pick up texture from whatever it is laying on if you aren’t careful. Special Teflon mats exist for this apparently, but we haven’t gotten to try this solution. Once you reach temperature, allow it to cool back to room temperature gradually over about 2 hours of time. The annealing process relieves the heat stresses in the pieces, and allows you to assemble and glue the pieces normally. If you are using a DIY batch oven, there are a number of projects out there that show you how to replace the cheap thermostat on a toaster oven with a thermostat/timer that is perfect for this job.

Another option is to do what we did, skip the glue and use mechanical means to connect the parts.

Special Note: While this discusses crazing in terms of glues, it can happen via a variety of other compounds as well. Novus II and Novus III plastic polishing compounds contain a solvent in order to soften the plastic slightly for the purpose of removing deep scratches. Rust-Oleum Painters Touch 2X paint contains Acetone which is a plastic solvent. I sprayed it onto some laser etched extruded acrylic and saw massive crazing throughout the etched areas. I purchased some Rust-Oleum Plastic paint that I haven’t tried yet, it doesn’t contain acetone, but does contain other volatile compounds that are solvents to some materials, so we’ll see.