Thanks Trisha.
I try to keep thicknes of acrylic to a minimum when doing S&P. This cuts down on grinding, which cuts down on heat. Heating acrylic can cause it to warp slightly when it cools.
Also....thick in the S&P can contribute, distorts a little in the pot. Thick acrylic S&P may cause internal structures of the acrylic to develope stress while curing. It can hold this stress till thinned down by grinding.
Then,warpage can be induced by the heat of grinding and making an appliance a little to warm. No to mention the torque added by holding it and grinding.
Long winded version of exactly what Trisha said.
Check out the S&P video here. After the spring aligner, I do a regular lower. Watch the thickness of it. I place dry polymer over the whole thing when I'm happy with the thickness, keeps the acrylic from slumping. Slumping can be a major cause of S&P getting way to thick.
Thinner is better for many reasons. Patient comfort is the most important. Keep them tick enough to be strong, thin enough so the patient will wear it.
Average thickness??? I try to keep my wires of the model by half the thicknes of the wire I'm using for a given appliance.
Then, acrylic over those wires not more than the thickness of the wire for a given appliance.
I did in the past (way past) have a problem with distortion. Then I started doing things the way I just described.
As an aside......
Grinding of lens and mirrors for telescopes.
When a lens or mirror gets close to its final "figure", more care is required. The glass heats when under "fine" polish, as well as "coarse" grinding.
Readings of the curve of the glass are taken during the whole process. Obviously the heat in coarse grinding is greater, but, the heat involved in the fianl polish is sometimes the most crucial.
The glass has to be allowed to cool for about 45 minutes after a 1 or 2 minute polish. This allows it to lose heat and assume its relaxed tempertature. Then the lens is measured for true finish.
Far more touchy than acrylic, but, the principles apply.
I hope this is as clear as mud!
Bottom line??? Keep it thin, keep it room temperature, don't apply any torque.....ever...even in packing and shipping.
Mike.
