# Compressive strength of cast acrylic

I have been experimenting with engraving a design into 1/2" cast acrylic and then transferring that design to a 24 gauge piece of well annealed copper and silver. It all works well, but I took the press up to 6,000 lbs and the die cracked. This was not unexpected, but I am curious what the maximum amount of force I can apply is and still keep the die from cracking. Tables tell me that the compressive strength is 18,000 psi, but my physics knowledge is having a hard time converting this to plain force. Maybe that isnâ€™t possible. The area over which I applied that force was about a square inch, so that tells me I was applying a pressure of 6,000 lbs/square inch. Can anyone help with this calculation. In the end, I will just make another die and slowly experiment with increasing the force until I get good results and donâ€™t crack the die. Thanksâ€¦Rob

You have to calculate the area that is subjected to the pressure.
Is it a positive (rised) or a negative (depressed) form?
If it is a positive the actual area might be quite small.

The actual area of the metal that is moved with a small piece of urethane is very small. Letâ€™s say .25 square inch. The force of the press is exerted over the area of the piece of urethane, maybe .75 square inch. The die cracked in places removed from the area that was covered by the urethane. As a physic teacher who hasnâ€™t taught in years, the math eludes me. Thanks for any help. In the end, I will solve the problem by trial and error, I have a lot of cast acrylicâ€¦Rob

I wonder if your engraving process might leave micro defects that could grow under pressure. Perhaps consider a quick flame-polish prior to applying pressure? Also, your engraving ensures that some of your compressive force becomes tensile.

Canâ€™t help with calculations, though. The dust layer is too thick.

1 Like

I failed to mention earlier that once the engraved lines are laid down, I deepen them with 1/16" dental ball bur using my air turbine. It spins at up to 50,000 rpm without a load, so it goes through the acrylic fairly easily and with a lot of precision. There are still little ridges and valleys, but they smooth out when I press the metal. I have several designs that I normally chase or raise with traditional chasing and repousee tools. I am just looking for a way to get the same design done quicker and with repeatability. In other words, I have gotten lazy. Thanksâ€¦Rob

Hi Tob,
what does your air turbine tool look like?

curious!

julie

Mine is from Steve Lindsay, but if you look at Nakamura non-lubricated air turbines, you will get a pretty good idea what it isâ€¦Rob

Hi,
oh, is it your Lindsay Airgraver? or something different?

Julie

Mine is a lubricated Lindsay Air Turbine. It is very similar to the Nakamura, which is available in both lubricated and non-lubricated versions, or it was prior to Covidâ€¦Rob

I have an air turbine that works with my gravermax. Judy H

A better choice for carving designs is delrin. Judy H.

1 Like

Judyâ€¦I have never worked with delrin, but will give it a try. Thanksâ€¦Rob

We use acrylic at school with the student to make dies. We never go over 6000 but their cutouts are often large the acrylic should not crack we use quarter inch.

6000 lbs at a quarter inch is 24000 psi, plus what ever has been removed as part of the design.