JohnD Posted November 15 Share Posted November 15 (edited) My other project needs headlamp covers. The lamps are recessed into the front wheel arches, and need a Perspex/acrylic cover, that follows the curve of the arch, which curves in three dimensions. I plan to heat the Perspex sheet in the oven until it's soft enough to form by gravity over a mould. I've never done this before. Any advice? So far, I've begun to make a mould foundation, by nearly filling the recess with a cardboard box and newspaper, stiffened by soaking in a solution of PVA. This has worked well, the mould foundation lifts in and out and holds its shape well. I know, it looks a mess! I then covered this with Plaster of Paris, in the form of bandages: It still lifts in and out easily and stays in shape. But the surface you see is very lumpy. Needs another coat of plaster, and then my plan was to smooth and shape it with a Surform. This turns out to be almost impossible as the fabric net in the plaster that gives it tension strength is very resistant to the plane, and shaping the mould would take forever. Of course, a skilled sculptor could do that while the plaster was still wet and pliable, but I doubt I have that skill! When I have a smooth surface of the right shape, I then planned to cover that with thin stretchy fabric, so as not to mark the hot Perspex, and then use a paint stripper heat gun to help it droop into shape. Instead of another layer of plaster, I thought of using polyurethane foam, the sort used by builders to fill gaps around window frames (!). That may be cut with a knife, and a Surform might do it. But I'm concerned about the effect of heat on polyU foam - it releases CYANIDE! Else, body filler and rub it down with sandpaper, another long task. Has anyone else made moulds for headlamp covers? EG Spitfire 'Le Mans' type bonnets? How did you do it, please? John Edited November 15 by JohnD Link to comment Share on other sites More sharing options...
Hamish Posted November 15 Share Posted November 15 John why not just build up the top layer in plain old plaster of Paris this will then sand back and I guess you’ll be heat proof for moulding? And if not too hot will shape without the Perspex taking on texture impression or https://www.aksportscars.co.uk/ak-40/ Headlamp Covers – Pair – £55.00 + VAT Link to comment Share on other sites More sharing options...
Escadrille Ecosse Posted November 16 Share Posted November 16 Agree with Hamish on the plain plaster of paris top layer. I've done that is the past to make moulds for various things. Including casting lead. It is pretty easy to shape with a surform and sandpaper. The perspex needs to be heated to 160 deg C, so if you can fit the item in your oven then it is much easier and reliable than with a hot air gun which lacks control. This is about as comprehensive a guide as I could find https://www.theplasticshop.co.uk/plastic_technical_data_sheets/perspex-design-guide.pdf But.... 14 hours ago, Hamish said: Headlamp Covers – Pair – £55.00 + VAT I'd go with Hamish's second suggestion Link to comment Share on other sites More sharing options...
Gt64fun Posted November 16 Share Posted November 16 John You might find this helpful. Not sure how successful he was, but shows some options. Ian Link to comment Share on other sites More sharing options...
JohnD Posted November 16 Author Share Posted November 16 Thanks to all! Most helpful! Hamish, at the price quote by the 40builder I'm tempted! But "built not bought!" is the watchword! And your advice with Colin to complete the mould with plain plaster is such a "DoH!" for me - of course! Hamish that mini-textbook from the Plastic Shop is invaluable. Ian, the Jag builder shows how I had intended to proceed, although with much more panel beating skill than I have. But his method amplifies my plan. Thanks again, back to the workshop! John Link to comment Share on other sites More sharing options...
JohnD Posted November 24 Author Share Posted November 24 (edited) Reporting back to m'tutors, rather than a bag of powder I bought a tub of ready mixed plaster and, er, plastered it on! Without the bandage, a Surform does nicely to shaped the hardened plaster, and after several coats I have this. A mould! Above, the mould, removed from the bonnet and drying out on the radiator. The black is a guide coat, sprayed before the final skim coat. From the back the cardboard and newspaper core. Very glad I did this as the plaster weighs a ton, just as a shell! I've reinforced the rim with more plaster bandage, as A/ I have a boxfull and B/ it's a bit thin. There is some aluminium mesh in there too, but reinforcement behind the rim won't need fettling. John Edited November 24 by JohnD Link to comment Share on other sites More sharing options...
John I Posted November 25 Share Posted November 25 11 hours ago, JohnD said: Reporting back to m'tutors, rather than a bag of powder I bought a tub of ready mixed plaster and, er, plastered it on! Without the bandage, a Surform does nicely to shaped the hardened plaster, and after several coats I have this. A mould! Above, the mould, removed from the bonnet and drying out on the radiator. The black is a guide coat, sprayed before the final skim coat. From the back the cardboard and newspaper core. Very glad I did this as the plaster weighs a ton, just as a shell! I've reinforced the rim with more plaster bandage, as A/ I have a boxfull and B/ it's a bit thin. There is some aluminium mesh in there too, but reinforcement behind the rim won't need fettling. John Looking good John Link to comment Share on other sites More sharing options...
Hamish Posted November 25 Share Posted November 25 Good job John. Don’t walk the dusty shoes through the house ! will you be able to size wise / plan to use the oven to relax the Perspex over the mould or are you going for a hot air gun ? keep us up dated Link to comment Share on other sites More sharing options...
JohnD Posted November 25 Author Share Posted November 25 Good point (the dust), Hamish! One long learnt, so that in the garage I wear the clogs seen in the top pic. Easily kicked off on the door mat. Haven't tried the mould in the oven yet, but that's the plan. Considering how to centre the Perspex on the mould while in there. Possibly a cardboard box around it, as first thought, guide wires through holes in the blank would probably hang up on the soft plastic. John Link to comment Share on other sites More sharing options...
JohnD Posted November 25 Author Share Posted November 25 (edited) OK, progress of a sort. I have some opaque white Perspex, so I thought to have a trial run. I covered the mould with oven foil, because there were some imperfections in the surface as well as a couple of places where the plaster cracked despite reinforcement. I supported the mould in the oven, with the Perspex blank on top, and turned it up to Gas Mk4 (170C). Checking the temp with my IR 'gun' I found that Mk4 only got the plastic to 158C so upped to Mk5 for a few mins and got 183C. Out with the mould, onto a folding bench in the kitchen, and press and smooth the Perspex down onto the mould with a folded cloth (an ironed handkerchief!). And rest, when the plastic was clearly no longer pliable. When cool, I took it off the mould, marked out the edges of the lamp cover with a Sharpie and took it out to the workshop to cut off the excess. Tried with a padsaw - Perspex is a lot harder than I thought! So resorted to my electric fretsaw. And found that Perspex is the only material I've known that has to be cut TWICE! The fretsaw blade cuts, but the friction melts the plastic, which cools, hardens and glues the cut together again! Also, that it is much more brittle than I thought, as can be seen (top edge, right). While going round the curve of the corner, it shattered. OK, no problem this is a trial to learn from. And when cut out, it fits, more or less! A bit more Surform/file work and it would fit perfectly! The apparent dent in the top is just reflections from my strip lights. I had marked the edges wrong as the mould left minimal marks of the edge, so cut too deep along one edge (below top right corner). I'll run a Sharpie around it before taking it off the mould next time. And, the edge of the blank nearest the gas flame, overheated and started to bubble. All part of the learning curve! Enough for tonight, it's getting really cold out there! I still have to make a mould for the other side, and for the side lights. I'm still surprized by how brittle Perspex is, but I think I have a sheet of polycarbonate. Let's try that next. John Edited November 25 by JohnD 3 Link to comment Share on other sites More sharing options...
Nick Jones Posted November 25 Share Posted November 25 Seems like you are getting there John Link to comment Share on other sites More sharing options...
Hamish Posted November 25 Share Posted November 25 Good work John. wonder if an elec fan oven would give better more even heat ? keep the updates coming Link to comment Share on other sites More sharing options...
PeteStupps Posted November 25 Share Posted November 25 Excellent stuff John, good trial run! Does polycarbonate mould as easily though? Watching with interest Link to comment Share on other sites More sharing options...
RogerH Posted November 26 Share Posted November 26 Hi John, When cutting with the fret saw run cold water over the cutting area. This should stop the second cut requirement. Yes - it is brittle Roger Link to comment Share on other sites More sharing options...
JohnD Posted November 26 Author Share Posted November 26 (edited) Thanks for the comments! Yes, a fan oven would be nice, see below, but I have what I have. I'm sure you're right, Roger, but using a hand held electric tool, under a running tap sounds a bit, well, risky! As for polycarbonate, I did have a sheet left over from fitting windows, so I cut a blank and started up the oven. I heated it to 120C for a while as online accounts say that PC is even more likely to contain water than acrylic. Then I upped the temp until it started to sag. But by this time, the back and side edges were going cloudy. No matter, this is a trial, but by now it must be hot enough! Out of the oven, straight onto the bench, pressing it down on the mould as before. And it won't bend! I had the paint stripper gun to hand and applied that. No better - get closer! And the plastic surface starts to bubble, while it still won't bend. So by this experiment, polycarbonate doesn't seem suitable, at least with my equipment. Perhaps some oven foil on the edge nearest the heat would avoid the bubbling that the Perspex suffered? Onwards and upwards! John Edited November 26 by JohnD 1 Link to comment Share on other sites More sharing options...
John I Posted November 26 Share Posted November 26 Hi John polycarbonate sheet needs a higher temperature of 180/210C to achieve the flexibility to mould. Time at that temperature about 5 minutes, might be a challenge in a gas oven not to suffer any crazing damage. Best of luck. Link to comment Share on other sites More sharing options...
Mark Posted November 27 Share Posted November 27 I made a new back window for my Eunos hard top about 10 years ago out of 4mm Polycarbonate. Roof needs a respray but window has held up well, I just make sure I soak the outside before wiping it to help prevent scratches. Its not quite as clear as it was when new. Its a large compound curve, and I was advised by the plastics company that the only way to make it would be to have it vacuum formed, a one off ££££. The original glass window was smashed and gone so I carefully made a paper template from the hard top, transferring it to the polycarbonate and cut out with a fine bladed jigsaw. I used a heat gun, short piece of scaffold tube to create the curves I wore welding gloves. Need to be very careful not to get things to hot. Came out well. Link to comment Share on other sites More sharing options...
JohnD Posted November 27 Author Share Posted November 27 Very nice, Mark! (in the true sense - "Exact" and "Finely done"!) I have done similar on the Vitesse rear window, but the curvature of that is much less and can be just bent and fixed in place. No heat required. John Link to comment Share on other sites More sharing options...
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