cast vs extruded

[George Elston]

I have always bought cast acrylic as I was told it is more consistent in thickness than extruded, on a recent critical job, pieces were not fitting and I mik'ed the thickness and it was all over the place. When I complained to the salesman he said I needed to but extruded. Can I get some real world advice? On a recent job with 3/4 inch cast acrylic the thickness went from .687"to .735", that's a lot.

Thanks

[Joe Pelonio]

That's normal. I can't cut that thick but find that % of variance in 1/4".

You cannot get an accurate thickness like that on either cast or extruded.

I have spoken to several suppliers about the variance. What they tell me is that when they buy the thickness is considered nominal. When a customer in the prototype engineering business ask me for something requiring a close tolerance I make them supply the material because otherwise you have to run around all over with calipers in hand to get it.

Often their materials are shipped to me from McMaster-Carr so perhaps they have more dependable thickness specifications than most suppliers.

[Larry Bratton]

I have read that it can vary as much as 10%. Makes no difference cast or extruded as far as that goes.

[Doug Griffith]

From my understanding and experience, extruded is far more consistent in thickness than cast. Cast is poured between 2 sheets of glass with seals around the perimeter and can vary a lot over the length of the sheet and between batches. Extruded is squeezed out of machine rollers and has tighter tolerances (especially across the length of the sheet).

If my job requires clear acrylic, I usually base my decision on whether it's etched (I use cast) or cut (I use extruded). If it needs high clarity optics, I go for cast. If it needs to notch together, I use extruded. Any other color, I use cast because that's about the only choice.

Cheers,
Doug

[Mike Null]

The variation in dimensions is not limited to thickness. When you buy pre-cut acrylic awards it's rare that they are the same size. This is particularly true of the ones coming out of Mexico.

[George Elston]

Thanks guys, I do one job regularly (see attached pic), it's a clear insert with logo on a baitwell cover for a boat. I have to cut out the hole for the insert and then fit the insert in so it's flush with the top. What I have been doing is cutting the female part .1" too deep and then filling with silicone and adjusting (banging) until the top is flush, (nobody looks at the bottom). I'll try ordering extruded next time I need a sheet of 3/4" and see if it's more accurate. I just like my work perfect.

Thanks again for the input

[Doug Griffith]

I would try etching some scrap extruded before committing to such a thick (and pricey) piece. It does not etch as well as cast.

Cheers,
Doug