Optical finish for acrylic -- vapor polishing and other techniques

I needed to polish two acrylic lenses that were made on a CNC lathe to the best possible finish (hopefully, optical quality). I made some test coupons and tried three different polishing techniques on three different surfaces to see which combination of techniques would yield the best results. The winner was clearly 2000 grit sandpaper followed by Novus No.2 polish.
Vapor polishing
 http://www.youtube.com/watch?v=bduno3eafcc

Hybrid rocket engine with acrylic and gaseous oxygen

I built a small rocket engine for demonstration purposes. The engine is built from a 2" diameter acrylic rod through which I drilled a 0.5" hole. The oxygen at 80 psi or less is passed through the hole and then is forced through a convergent-divergent nozzle at the tail end. The nozzle's throat is about 0.25" and expands to 0.625". I lit the engine by inserting a burning cotton swab (with wooden stick) while a small amount of oxygen was flowing. The acrylic catches fire very easily in a pure oxygen environment. The engine can be throttled and shut off completely, which is a major benefit to hybrid engine designs. Solid-fuel rockets cannot be throttled or shut off, which makes them difficult to control.

Effect of long-term high pressure CO2 on acrylic

I left my supercritical CO2 chamber charged up with 750 psi liquid CO2 (not supercritical) for about a week. I then depressurized the chamber, and opened it. At first, the acrylic seemed fine with just minor surface crazing. After a few hours, I was surprised to find the acrylic had deformed in a major way and was full of CO2 bubbles. Weird!

Comparing different brands of acrylic and cement to get the best joints

Getting really clear, bubble-free joints in thick acrylic is not easy. I've done quite a lot of practicing with various methods and eventually did some testing on the materials and cements that I was using. The link below shows the results. Since McMaster plastic is a lot less expensive than TAP plastic, I tend to use McMaster to supply the plastic and I use TAP green label cement on it. I've never tried IPS #3, since I've been told it's identical to TAP green label. I'm not convinced about this and may have to try it some day.


http://www.magconcept.com/acrylic/

Acrylic flowers

I built this bouquet of flowers from flat sheets of opaque acrylic plastic. The flower petals were machined on a CNC mill. The flower centers and stems were cut by hand with a hole saw and table saw, respectively. Everything is assembled with TAP quick-setting acrylic cement (similar to Weld-On #3). The label was made on a Brother label printer, and the label edges are actually much less apparent than it seems in this photo.

Thermoforming acrylic

I've worked with acrylic in many different situations, but have never yet put a piece in the oven to thermoform it. I've used strip heaters and heat guns before, but these techniques are more suited to edge bends, and today, I had the task to curve a thick piece in a consistent arc. Here is the machined blank, ready to be formed.
This is 3/8" thick cast acrylic (about 2" by 4") with a few slots cut into it. I put the acrylic on a piece of cardboard, since I was afraid the oven rack or metal pan would stick to the acrylic during heating. As it turned out, the hot acrylic had no tendency to stick to anything, and the cardboard got pretty toasted. The next time that I try this, I will use thin MDF or peg board to support the acrylic.

I put the acrylic and cardboard into my toaster oven, which was preheated to 350*F. After 5 minutes, the piece was getting flexible, but not soft enough. After 7 minutes total, the piece was perfect, and I quickly placed it onto my mold.The mold is made of three 3/4" thick pieces of MDF, which I cut into an arc with bandsaw, then sanded and primed the edge. After the primer was dry, I used double-stick tape to attach a piece of 1/32" delrin. This provides a slick surface that can withstand heat and many chemicals.

I used another piece of 1/32" delrin and a paper towel to push the softened acrylic down onto the mold. It took less than minute for it to hold its new shape. I deburred and flame-polished the edges to finished the piece.