Heat bending polycarbonate for a windscreen

[Guest ratchethack]

Gents! I broke the flyscreen on my Stucchi 1/4 fairing and just picked up some 1/8" LEXAN polycarbonate. My understanding is that this is NOT an acrylic. Should be much tougher stuff.

 

According to the mfgr tech brief I picked up, working temperature for heat-bending this stuff is 280-300 degrees F. Since the finished dimensions (flat) are about 9" x 18", I could use an oven.

 

I'm wondering if using a heat gun would be a simpler, easier way to get a curve with about a 5" radius?

 

In my "perfect world", I use cotton gloves to handle it, and wave it in front of the heat gun, covering the center area as evenly as possible until it yields slightly, then flex it to shape and hold it up against the inside of the fairing until it cools, and voila!

 

Any voices of experience out there?

 

How about a preferred tool to cut it to shape without loading up? I know this can be a problem with some plastics.

 

Can the finished edges of this stuff be heat polished, or would a belt sander be preferred?

 

TIA

[txrider]

Ratchet, we used to produce curved instrument panels like used in throttle and flap quadrants and we used ovens to first heat the acrylic (not polycarbonate) to 260f, removed it from the oven using thick gloves then laid it down over forms and cinched tight using band clamps. When cooled we had the correct curvature and dimension. Once the yield temp is reached the stuff is very flexible, almost like spaghetti and requires some kind of support during cooling.

 

My best judgement from doing this kind of work is that a heat gun is too localized, that is it does not maintain desired yield temp over a large enough area to give predictable results.

 

As for cutting, I've used a saber saw with the finest metal cutting blade I could find and fed the saw slowly to avoid too much heat. Excessive heat can cause stress crazing which appear as tiny cracks in the cut edge. Face the material with 2" masking tape to protect the surface.

 

Finishing edges is best done with a palm sander beginning with 120-150 grit paper then 220, then 400. Last paper grit of 600 will give a nice edge. If you want smoother finish MicroTech paper goes all the way up to 8000 grit but the sample packs come in 2" x 4" squares. I've used 1500 grit then Novus #2 polishing compound to finish out scratched acrylic/polycarbonate windshields with good results.

[Guest ratchethack]

Many thanks, Txrider. Just wot I need, the voice of experience.

 

My take is that heat forming polycarbonate and acrylic must be very similar.

 

Your post leads me to think I should patch the broken windscreen pieces back into its former shape, make a fixture to secure it on my workbench so I can use it as a form, and quickly clamp the prepped & heated new piece to it around the edges where it bolts up to the 1/4 fairing before it cools.

 

I read somewhere that a good material to use to separate polycarbonate from a form used this way is flannel.

 

Thanks again.

[guzziownr]

Ratchet, I need a chunk of that stuff for my bikini shield. (car knocked over bike). Got any to spare?

 

DW

 

Before picture!

[Guest ratchethack]

DW, I went to a local commercial plastics supplier & fab shop and chatted 'em up. Nice guys gave me two pieces gratis , and even ran off a copy of a tech brief for me.

 

Cutoffs are next to worthless to commercial plastic fabrication shops. The one I went to sells it by the pound to anyone who wants to paw through the stuff in their cutoff bins. They probably take a loss on it just talking to guys like me , but it brings lots o' do-it-y'erselfers around, who I reckon turn into pretty good ambassadors for the company.

 

If I don't mangle both of the ones I've got (they're clear, not tinted like yours in the photo), I'll gladly ship any surviving piece to you after all the carnage is over with (no tellin' when that'll be ), but you might better find a tinted one that's just wot you want in y'er own neck o' the woods. . . . The place I went to is a fair distance from where I live. That's why I picked up 2. You might find one lots closer to you than the opposite corner of the US.

 

From y'er photo (Nice retro look, BTW), it looks like you could use a 1/16" piece and flex it to shape, and not have to heat bend it at all.

 

Good luck.

[guzziownr]

DW, I went to a local commercial plastics supplier & fab shop and chatted 'em up. Nice guys gave me two pieces gratis , and even ran off a copy of a tech brief for me.

 

Hmm, I guess I will try Canal Plastics.

 

From y'er photo (Nice retro look, BTW), it looks like you could use a 1/16" piece and flex it to shape, and not have to heat bend it at all.

Good luck.

 

If all else fails I will walk across Canal Street to Aerospace metals and overpay for a piece of aluminum to replace the screen.

 

Bend with care there Ratchet

 

DW

[dlaing]

DW, I went to a local commercial plastics supplier & fab shop and chatted 'em up. Nice guys gave me two pieces gratis , and even ran off a copy of a tech brief for me.

Where did you go?

If you want to learn from experience, I am your man

Just don't do what I did

For my V65 SP fairing I made a template by wrapping around fairing with card board,

cut the acrylic (from some place on Canal Street) to match template,

put in oven on top of a wax paper covered cookie sheet (I think I preheated to 250F, but it was long ago.)

After it started to shrink I carried it out to the bike wearing oven mitts and wrapped it around the fairing.

As I wrapped it around it cooled quickly and cracked a little from my pushing too much when it had firmed up too much. I drilled a stop crack and it then lasted a long time, but always had a two inch crack at the base.

It looked OK but nowhere near factory perfect. A second person to help hand mold it would have yielded better results.

But if I were to do it again, I would create a form, as txrider suggested.

[Guest ratchethack]

We make polycarbonate as well as plexi-acrylic parts. . . (etc.)

YYYEEEEEEEEFFFFTTTT!!!

 

Looks like my education here could get a little harder to come by than I'd imagined.....

With polycarb it is very important ot pre-dry the material to avoid bubbles . Polycarb absorbs moiture quickly and when heated too quickly bubbles will form. The drying is dome at 250 gegrees F .

Gary, this seemingly explains wot happened to something I tried to do long ago on another project that didn't work 'cause bubbles ruined it. . . .

 

How long do you recommend drying at 250 F before raising the heat for forming?

Let me know if vac forming sounds better for you It is a real simple process but distortion can be a problem

This is a simple curve, not compound. Vac forming sounds like overkill & an unneeded process.

 

Many thanks for the professional advice!

 

Where did you go?

. . . Just don't do what I did

Dave, I got my LEXAN at Ridout Plastics. I've been going there for decades for plastics to do small stuff, this is my first real attempt at a LEXAN project.

 

Thanks for the voice of experience.

[dhansen]

Hey Ratch. I did a quick Google search and came up with a neat site:

 

http://www.auf.asn.au/scratchbuilder/makro...fabrication.pdf

 

Lots of good info on forming polycarbonate.

[Guest ratchethack]

Hey Ratch. I did a quick Google search and came up with a neat site:

 

http://www.auf.asn.au/scratchbuilder/makro...fabrication.pdf

 

Lots of good info on forming polycarbonate.

Good stuff, dhansen, thanks. But now we have a conflict. Makrolon calls for heat forming at a temp of 350-360 F. The LEXAN guys said 280-300.

 

I note that no one yet has posted 100% successful results doin' this. . . .

 

I reckon I should've picked up more'n 2 pieces. . . .

[docc]

Ratch,

 

No luck contacting the Guzzi rider in NC who built my screen, I suppose?

[mike wilson]

Good stuff, dhansen, thanks. But now we have a conflict. Makrolon calls for heat forming at a temp of 350-360 F. The LEXAN guys said 280-300.

 

I note that no one yet has posted 100% successful results doin' this. . . .

 

I reckon I should've picked up more'n 2 pieces. . . .

 

Definitely. Note the statement on the same page about it "setting" quicker than other thermoplastics, too. Despite its lower strength, I think I would prefer acrylic for home working. Isn't polycarbonate also more susceptible to scratching?

 

Good luck (may you acquire the requisite skills with the minimum of mistakes) anyway.

[dhansen]

Good stuff, dhansen, thanks. But now we have a conflict. Makrolon calls for heat forming at a temp of 350-360 F. The LEXAN guys said 280-300.

 

Typical askin' for opinions situation. Everyone has a different formula for skinning the cat.

 

I bet 325 F would be just right.

[Guest ratchethack]

Ratch,

 

No luck contacting the Guzzi rider in NC who built my screen, I suppose?

No response to my email, Docc, but it was worth a try. Thanks.

[Guest ratchethack]

We dry the .12" material for a shift before going DIRECTLY to heating and forming. The thicker stuff takes exponentially longer, .25 material would dry a full day or longer.

Thanks again, Gary. This squares with the drying time chart on the Makrolon PDF from dhansen.

Once again consider just rolling your simple curve in a sheet metal slip roller. The kind used by furnace duct people and generally used for all sheet metal work. Since you are working with fairly thin material and a larger sweep results should be good. Start with a generously oversize blank, make some radius templates that you can hold up against the work to measure your progress. The slip roll will also easily roll a truncated conical form so the part can taper between different radii.

We do a lot of cold forming polycarb and it works fine. Thin material is also worked on the press brake and leaf or pan brake with good results.

Seems the plot thickens now. . . . .

 

Gary, the simple curve I'm trying to duplicate is conical with a 5" radius being the tightest end of a truncated cone. The other end is about 7" radius or thereabouts.

 

Are you suggesting here that It'd be possible to achieve this kind of a curve without cracking and WITHOUT HEAT?

 

If so, and this can be done in a sheet metal slip roller, wouldn't it also be possible for me to simply bend it around a suitable curve, cold, by hand?