Plastic Manipulation Tools for The Exploratorium

Vacuum forming activity at The Exploratorium

I built a simple vacuum forming rig for The Exploratorium consisting of a sealed wooden box with a peg board top and gasket, and a laser cut frame to hold the vacuum forming stock (PETG) while heating and forming it.

After the stock was heated over the hot plate for three or four minutes it was ready to form, a shop vac drew the air out of the box pushing the plastic down over the objects as a result of the pressure differential. I found that with my fairly low powered vacuum hot plate only provided enough heat to form the major shapes, but that a great deal of detail could be captured by going over the plastic with a heat gun.

Below are several examples of vacuum formed sheets visitors made while I had machine out on the museum floor.

A heat gun was used in this sheet to clearly capture the writing on the lower gear.

This is a crude water lens made by vacuum forming a glass lens with a small dowel to allow a filling port and hot gluing it to a flat sheet of stock to make a vessel. When filled with water it make a surprisingly low distortion magnifying glass.

Acrylic molding "press"

Preparation to cast the negative of the bowl: I made studded the top plat with screws hang down into the paster securing it to wood, and sprayed to the bowl with mold release.

Pouring the plaster positive through the top plate into the bowl.

The resulting casting.

Felt was stretched over the positive and adhered with a tacky non-permanent glue to prevent the acrylic from sticking to the plaster. After the felt was firmly in place I was able to easily trim off the excess felt using a razor blade, making the mold ready to use.

After about 15 minutes at 315f in a convection toaster oven the acrylic is ready to press. It has the consistency of probably most similar to slight dried out Provolone cheese, it sags and bends very easily but does not stretch.

Pressing the acrylic.

The first acrylic bowl came out a little small given I underestimated the amount of material necessary to cover the curve, but I have since dialed in the disk diameter and can produce bowls which are the same size as the original. I tried this activity a few time on the museum with some success though it is a time consuming process. In an effort to deal with the long plastic heating time for the bowls I laser cut a bunch of acrylic fork blanks and made a fork press from laser cure plywood again covered in felt to produce a nice s curve. The forks proved a much more realistic activity for the museum floor as it still displayed the behavior of thermo plastic only took about 8 minutes per fork . I'll try to post pictures of this some time in the future.

Simple blow molding flask

The mold flask was made by casting plaster around an acrylic tube with a plastic ball stuck it, in some old shipping tubes capped with plywood.

I used some cheap 1/4'' ply to make a linkage to bring the halves of the mold together and lock them shut. To turn a tube in to a ball I heated the acrylic tube for about 15 minutes in a toaster oven (which happened to have a convenient rotisserie function) at around 315f. When the tube achieved the proper provolone texture it was closed in the flask, one end was crimped off with players and a stopper with a schrader valve was inserted into the other side allowing it to be inflated by a bike pump. Inflating the bulbs is correctly surprisingly hard, the stopper always leaked a little so you had to keep pumping constantly. If you pumped to much the vessel could explode out of the seam in the flask. If you pumped to little you would never get the tube to expand to the sides of the mold as the acrylic is very elastic and would try to return to its tube form as soon as the pressure was released.

Here is mold opened just after the acrylic tube as been inflated and allowed to cool.

The resulting blow molded vessel is fairly strong but will actually tear not crack if you squeeze the bulb too much. Its not surprising the acrylic is not used to make thin blow molded objects given this odd behavior.