acrylic strengthening

[clonus]

Trying to make an "*******" costume from "Spaceballs" out of an acrylic light sphere. However, the corners of the piece I cut out for the face is causing cracks (see photo).

Wondering if there is some kind of spray or similar that could strengthen it? Would need to be paintable or gloss white if on the outside (or could just spray the inside). Would Plasti-dip, mod podge or similar work? Other thought was some fiberglass and resin around the inside of the helmet on the edge of the cut out to give it added strength but wonder if that would just cause cracking further up where the fiberglass would end?

Thanks!

 

[JPH]

I would get another acrylic sphere (or try to patch this one), drill a small hole in it, spray and swish release agent in it, the fill it with rotocasting resin like smooth 325 with white pigment.

****OR***

If you have a soldering iron, wear a mask for fumes, and you can try melting the acrylic instead of cutting it. You still need to smooth the edges, but it also leaves you with a weak, brittle helmet

If the acrylic has thin walls, the vibration of cutting will crack it.

Lexan and Acrylic ( I believe) are both polycarbonates but Lexan is less brittle.

 

[chibobber]

Any square corner creates a stressor. Put a small radius at any 90 degree. Drill a small hole at the end of the crack to stop further travel. You can use a silk type fabric ( thinner than fiber glass cloth) with resin to strengthen the inner edge.

 

[TazMan2000]

At the end of the crack, drill a hole to ensure that the crack stops propagating. Then just use a strip of styrene and CA glue to patch the inside. Fill and sand as needed. Probably a good idea to do that to the other side as well.

TazMan2000

 

[clonus]

Thanks for the tips! For the next one, I plan to round the edges and reinforce. I also going with a different brand of sphere and see if its thicker stuff...

The square corner stressor thing reminded me of the issue with the de Havilland Comet airliner and square windows, etc....

 

[lmgill]

Thanks for the tips! For the next one, I plan to round the edges and reinforce. I also going with a different brand of sphere and see if its thicker stuff...

The square corner stressor thing reminded me of the issue with the de Havilland Comet airliner and square windows, etc....

A radius in the corner, is just like a diagonal brace on a right-angle corner. (like a brace on a bench) It transfers the forces to the adjoining edge, instead of focusing them into the corner.

I would also avoid Cyno on styrene and acrylic, some cyanoacrylate glues will imbrittle the styrene. "Weld-On" solvent cement will bond the two plastics together much better. You will want to make the strengthening piece as close a fit to the globe as you can make it. Heating the styrene in an oven or with a heat gun until it is flexible like a thick leather belt, will allow you to press it into the interior of the globe, without the piece being too floppy to manage easily. Once glued in place, you can sand it to the desired edge profile.

Also, to keep a crack from propagating, drill your hole (a 1/32" - 1/16" hole is adequate) slightly ahead of the crack. Many times the crack goes farther than it appears, so drilling the hole ahead of the crack will help insure you stop it. This by the way ias the same stress redirection like the brace, or radius in a corner.

I have purchased polycarbonate spheres before, these would be much better than acrylic. They are sold as light globes for street lights.

 

[lmgill]

I would get another acrylic sphere (or try to patch this one), drill a small hole in it, spray and swish release agent in it, the fill it with rotocasting resin like smooth 325 with white pigment.

****OR***

If you have a soldering iron, wear a mask for fumes, and you can try melting the acrylic instead of cutting it. You still need to smooth the edges, but it also leaves you with a weak, brittle helmet

If the acrylic has thin walls, the vibration of cutting will crack it.

Lexan and Acrylic ( I believe) are both polycarbonates but Lexan is less brittle.

Lexan is a trade name for Polycarbonate
Plexi-glas is a trade name for acrylic.
Polycarbonate has a much higher impact and shear strength than acrylic.

The best way to keep cut acrylic from cracking is to sand the edges, parallel or in line with your cut edge. Typically when you sand perpendicular to the surface, you leave small grooves across the thickness of the plastic. The tiny grooves, can be the perfect starting point for a crack. All it takes sometimes is an abrupt temperature change or a minor amount of flexing to start the crack.
But, if you do a final sanding in line with the edge, this will eliminate the cross groves, and will keep the plastic from cracking.

 

[JPH]

Lexan is a trade name for Polycarbonate
Plexi-glas is a trade name for acrylic.
Polycarbonate has a much higher impact and shear strength than acrylic.

The best way to keep cut acrylic from cracking is to sand the edges, parallel or in line with your cut edge. Typically when you sand perpendicular to the surface, you leave small grooves across the thickness of the plastic. The tiny grooves, can be the perfect starting point for a crack. All it takes sometimes is an abrupt temperature change or a minor amount of flexing to start the crack.
But, if you do a final sanding in line with the edge, this will eliminate the cross groves, and will keep the plastic from cracking.

Thanks Imgill,

I am making lexan micronaut wings, and 1/4 lexan cuts like wood.

Meanwhile, acrylic is very brittle. Also, it strings and clumps on the cutting disc under the heat of dremelling. Ugh!

That's why I'd try to tape down the cracked dome and slush cast from the inside.

Hmmm, also one could use the dome to make a mold. Use Sorta clear or Rebound 25 if you want to make multiple helmets. Then slush cast in smooth-on 325.

 

[Zinger]

What Chibobber and TaxMan said. Notice that the original doesn't have a square corner. It's lines are all nice and rounded.

Probably for aesthetics and because the prop maker knew a square corner would fail.

 

[clonus]

much better improved version...thanks for the tips!!!
.

 

[ID10T]

Looks good!

The crack comments are on point. But I wanted to mention that if you look at the reference photo posted at the beginning of the thread, you notice the corners are not square, but have a fairly large radius to them. Quite possibly for the same reason, but either way, a detail worth mentioning.

But otherwise, it looks awesome! I just watched this again the other night (hard to believe it’s from 1987!!) and I still enjoy it. Mel is a genius.