Bending and welding acrylic sheet?

Is it possible to bend acrylic sheet (3mm thickness) around a tubular former with the application of heat? Also could one then "weld" the sheet to form a tube?

The weld is important, glue would not be acceptable in this application. I know that you can buy acrylic tubing, but not in the required diameters,12.5 " to 13".

The application is to prevent strikes on the secondary winding of a large Tesla coil. The strikes will go through glued joints with ease.

Perspex tubing of the required diameter can be had but at around £700 it is a route that I don't want to go!

Andrew.

I've bent acrylic sheet back in my school days - can't remember the temperature we used, but did it using an old domestic oven to heat the sheet.

The cement used for Acrylic typically does create a 'weld' - it is a solution of monomer in solvent.

Bending acrylic well is a skilled task. I needed some doing earlier in the year and prices for the whole job ranged from ~£300 to more than that just for the raw materials to $illy.

Avoid anyone offering lasercutting - they add a premium to cover the capital costs of the lasers, traditional cutters are cheaper and you only want simple shapes.

The way would to be to make a hexagonal or octagonal shroud by heating the acrylic/perspex (very) gently and slowly with a hot air gun over a form and allow it to adopt the shape under gravity.

I found these discs cut perspex very well. I recently made four 300mm cuts along 100mmtube without any cock-ups using one.

The usual way of joining acrylic/perspex is solvent welding, done well with a fluid that contains some dissolved acrylic there should be a 100% join. The fabrication I had made was 100% watertight.

Don't try cleaning cut edges with solvent...

Neil

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Thanks guys, so it does seem possible to do the job, but very difficult in execution! I was going to try a hot air gun and place the sheet over a cylindrical former. It is obvious that some experimenting needs to be done, but well worth a try.

Thanks again,

Andrew.

I use fuming with conc ammonia for wood in long acrylic tubes, They have to have a totally leak proof joint on one end . I tried various glue etc... over months and each failed after a while with conc. ammonia leaking out which isnt very pleasant.

I then made my own glue from powdered acrylic from an offcut and dissloved it in dichloromethane , it has held fine for around the last 12 months..

I remember when i was serving an apprenticeship at Sellafield nuclear plant having to weld up 300 metres of square acrylic ducting lengths together for laboratory extraction system. We just used filler rods of the same material and a hot air gun with a fancy nozzle (seemed very easy at the time).

It doesnt need much heat to bend, i have a sheet of 3mm perpex in the greenhouse which was a temporary window for some glass that had blown out . It has now all drooped over the bench due to the heat  in the greenhouse.

Edited By mark smith 20 on 12/08/2017 13:10:59

Edited By mark smith 20 on 12/08/2017 13:14:34

Quite a while back I was in a factory that made covers for hi-fi turntables, the covers were bent at the front to almost 90 degrees.

The method used was quite simple. Imagine a flat surface with a slot in it, the slot was about 25mm wide from memory, the slot was insulated or was it water cooled? in some way i cant remember. In the middle of the slot was a shielded electrical element maybe 8mm thick that glowed a dull red.

The method was to place a sheet of Perspex flat on the table over the slot in the right place for a certain length of time then place it over a former and manually bend it. Due to the concentrated heat line the bends were perfect!

Oh prior to bending the carbide saw cut edges of the sheets were polished. this was done with a flame, a hydrogen flame no oxygen, directed over the edge. It was a woodworking shop, they made cabinets so the sheets were fed using a feed head as used on saws and spindle moulders, The feed had to be constant the polished edge was very good, I still have a turntable housed in one of the cabinets. the edge shows no sign of burning the material is about 5mm thick.

Regards
John

I've always understood that perspex can be solvent welded with chloroform, but I've never tried it. I suggest it is done in a VERY well ventilated workplace! I have bent perspex by heating small pieces in a domestic grill on a low setting, and would warn that it retains heat for a long time after it has apparently set solid.

When I worked for a research lab we tried to make a tesla type step up transformer, first attempt strikes cut through 4" PVC soil pipe so we ended up using glass tube for the secondary former. Good luck keep well clear with hands in pockets!

Paul

Andrew Tinsley:

I think your hot air gun with cylindrical former is well worth trying. By a modest coincidence, I tested this method of bending acrylic just the other day using the high setting on a cheap B & D hot air gun.

I should perhaps point out that the material is some decades old and its brittleness is apparent from the holes which were probably made with a hole saw. I didn't use a former and just continued heating until the sheet was willing to bend. From previous attempts I know that overheating causes undesirable bubbles.

When I was involved in high voltage induction heating we used white ptfe between the flate plate busbars, and UHMWP for enclosures. The enclosure around the heating coil had to hold pressure, imagine the fun I had getting a polythene pressure vessel past the works safety committee. To prevent any moisture we had to pressure test it with air, but with adequate procedures we got approval for that as well. Sensible H&S pays off!

3mm UHMWP would bend into a tube without heat, no idea if you can weld it. You can't weld PTFE

As far as I am aware Acrylic is not Perspex. There must be a difference as the Acrylic is much cheaper than Perspex! Unless it is a case of "Branding" puts up the price!

Obviously I want to use Acrylic sheet as it is much cheaper!

Regards,

Andrew.

Sorry! I double posted,

Andrew.

Edited By Andrew Tinsley on 12/08/2017 15:52:30

Andrew Tinsley:

As you say, Perspex is (or was) a brand name. I have photo'd the label from some similar material (about 2 sq ft priced at 35p!)

As you will no doubt see, I have had to put two torn labels together to make substantially one.

So Acrylic and Perspex are one and the same! Interesting, you live and learn!

Thanks,

Andrew.

When I worked at the Mullard Research Labs in the 1970s we used to use Perspex Cement for quick and dirty assemblies. More accurate joining was done in the technology workshops by accurately machining and polishing the mating surfaces and just wetting them with chloroform for an invisible joint.

Russell

There can be significant variation in material properties depending on the details of the manufacturing process and how well things are controlled. The big division is between cast and extruded material. Generally extruded sheets are thinner than cast ones. Extrusion is faster so weight for weight usually cheaper. More variables to keep under control tho' so "El Cheap Plastics Inc" products may not be as good or as consistent as premium brands.

Ageing and stress cracking properties are where premium products usually justify their cost. I suspect the differences are due to details in the polymerisation process and in particular the mean length of polymer chains. Certainly in the long ago days when I had a bit to do with perspex and acrylic fabrications it was accepted that the cheap stuff was fine for flat windows and simple bends. But if you wanted to do serious fabrication with reliable solvent welded joints ponying up for nice new sheets of cast Perspex was the sane way to go. Solvent welded joints in the cheaper types tending to be of unpredictable reliability. Even with the good stuff any sheets or offcuts that had been hanging around for a year or so could be iffy. I'm convinced that bending properties were different between fresh new and been on the rack for a while material. But I was always pretty rubbish with the stuff. Never did enough to nail the knack.

Clive

Edited By Clive Foster on 12/08/2017 17:14:29

There are methyl methacrylate monomer adhesives out there. Have you tried those?

Clive is right about there being a difference between extruded and cast Perspex. The big diy stores typically stock the extruded type, and I have always found it very difficult to work. Often the problems appear a day or two after you think you have finished! Stress fractures appearing on edges that have been sanded are common, usually at 90° to the edge. Stress cracks are also common along the bend line. It has has a very narrow working temperature range - too cold and it isn't plastic enough to bend, and too hot and bubbles appear within the material along with burning on the surface.

In contrast, cast acrylic seems to be less brittle. A bandsaw is fine for cutting, provided the feed rate is fairly moderate - 6 tpi is ok, though I have also used a 3 tpi blade and a slow feed rate successfully. Edges can be smoothed with a finely set smoothing plane, or on a disc sander, with none of the stress cracks that appear on extruded acrylic. I have always machined it at high tool speed and low feed rate, with small to moderate depth of cut - a compressed air blast can be used as a coolant. Excellent finish can be obtained with conventional metal cutting tools, such as on a lathe. Wet and dry followed by Brasso will give a mirror finish.

Clarke are one of the manufacturer of strip heaters (extensively used in schools), and with a former it is possible to achieve good quality bends. For glueing I always used Tensol cement - available as a liquid which relies on tight joints and capillary action, or as a thicker, almost gel, consistency which can fill smallish gaps (<1mm). Both are solvents, so the addition of acrylic dust from the sander can be used when no-one is looking!

I have used a hot air gun for bending - fine on small parts but absolutely useless on large parts because it is impossible to get the complete sheet at the correct uniform temperature, so any bending becomes an ugly distorted mess in no time. An oven is the only way, which may then lead to problems in handling the hot sheet without introducing finger marks or glove prints. My preference would be to use a strip bender and go for a hexagon or octagon shape rather than aim for a circular tube. Because you mention the possibility of glue joints being a problem, I wonder if a large lap joint would help, bonded with Tensol, so still glued of course, but maybe more suitable for what you have in mind (about which I know nothing!).

Educational sellers might be a good place to look for supplies - e.g. Technology Supplies