Bending plastic for trim curves

[Len Woodruff]

I need to bend some plastic for trim curves over a roof. My solvent based glue has dried and become too thick. 

I think MEK would work but I don;t have any of that.

Any other suggestions?

 

[stitchdup]

If it evergreen strips ordinary poly cement bons pretty quick so you wouldn;t need to worry too much about bending it if its thin strips, Just hold it in place for a few minutes with clamps and within ten mins you can take them off. It doesn't seem to work with plastruct though so I would use super glue there if its not too tight and rought up both bonding surfaces first so it bonds better. Be sparing with both cos it can be a pain it it squishes out and wastes a lot of time

[peteski]

Len, unless you moved to the moon, why not just get some more of solvent based liquid cement or MEK?  Or have those become so scarce that you can't find any to buy?

[Len Woodruff]

1 hour ago, stitchdup said:

If it evergreen strips ordinary poly cement bons pretty quick so you wouldn;t need to worry too much about bending it if its thin strips, Just hold it in place for a few minutes with clamps and within ten mins you can take them off. It doesn't seem to work with plastruct though so I would use super glue there if its not too tight and rought up both bonding surfaces first so it bonds better. Be sparing with both cos it can be a pain it it squishes out and wastes a lot of time

I am attaching to a resin body. I tried it with the thin CA glue but the plastic strips broke when putting the glue on them. Not sure what caused that. I had the strip held in place with tape.

[Len Woodruff]

58 minutes ago, peteski said:

Len, unless you moved to the moon, why not just get some more of solvent based liquid cement or MEK?  Or have those become so scarce that you can't find any to buy?

My liquid cement dried up and I didn't have any MEK at the house. So I think I will make a trip to the Hobby Shop tomorrow.

[NOBLNG]

9 minutes ago, Len Woodruff said:

I am attaching to a resin body. I tried it with the thin CA glue but the plastic strips broke when putting the glue on them. Not sure what caused that. I had the strip held in place with tape.

I have had curved styrene strips break the instant that I applied a solvent cement to them. I wouldn’t think CA would do that If the strip is already preformed to the finished shape. I have also used 5 minute epoxy which allows you time to play with the positioning, but it does squish out a bit and requires cleanup.

Edited April 29, 2021 by NOBLNG

[peteski]

Now that I  think about it, you do have a point Greg.  I have also experienced thin strip disintegration when treated with solvent cement. Pre-forming, then gluing might be the way to go. Thin strips are usually flexible enough to be shaped by hand without breaking.

[Oldcarfan27]

I've had Evergreen plastic strips break at the bends upon contact with plastic cement. I can't recall that happening years ago when I bought it.

Then again, maybe the strips get brittle as they age and the glue causes it to break.

Not sure though.

[NOBLNG]

13 hours ago, Len Woodruff said:

I am attaching to a resin body. I tried it with the thin CA glue but the plastic strips broke when putting the glue on them. Not sure what caused that. I had the strip held in place with tape.

Solvent glues will only work for styrene to styrene joints I believe. To glue anything to resin you would need CA or epoxy. I’ve never tried a PVA (white glue) or canopy cement, so I don’t know about those? Best to shape the strips to the contour needed beforehand so the glue is not tasked with holding the shape, especially if it’s a sharp curve. 
 

[Len Woodruff]

6 minutes ago, NOBLNG said:

Solvent glues will only work for styrene to styrene joints I believe. To glue anything to resin you would need CA or epoxy. I’ve never tried a PVA (white glue) or canopy cement, so I don’t know about those? Best to shape the strips to the contour needed beforehand so the glue is not tasked with holding the shape, especially if it’s a sharp curve. 
 

Its a near 90 degree curve. I may try taping it again then use a hair dryer to break the tension. I tried some direct heat from my torch but it was too hard to control the amount of heat on the area.

[NOBLNG]

Just now, Len Woodruff said:

Its a near 90 degree curve. I may try taping it again then use a hair dryer to break the tension. I tried some direct heat from my torch but it was too hard to control the amount of heat on the area.

I have also heard that coating the strip with solvent and letting it “soak” for a while to soften the styrene before bending may work. I still think it may snap, but you could try a test?

[Len Woodruff]

2 minutes ago, NOBLNG said:

I have also heard that coating the strip with solvent and letting it “soak” for a while to soften the styrene before bending may work. I still think it may snap, but you could try a test?

That is my next step. I just have to go to the Hobby Shop to get the solvent glue.

[Rick L]

Can you draw out the curve on sheet styrene and cut the strips with a radius gauge and straight edge? Or build a template to butt up the strips with heat? MEK is pretty hot and will definitely loosen the plastic but you have to be careful not to use too much or will disintegrate.

[Len Woodruff]

2 minutes ago, Rick L said:

Can you draw out the curve on sheet styrene and cut the strips with a radius gauge and straight edge? Or build a template to butt up the strips with heat? MEK is pretty hot and will definitely loosen the plastic but you have to be careful not to use too much or will disintegrate.

The problem is I need to use 1/2 round and the curve has to follow the body. What I might so is tape it to the resin body then use the solvent glue to break the bend tension. 

[Rick L]

That will be a challenge.

[Paul Payne]

Try soaking the strip in hot water then bending near the shape, then heating again and bending to the final contour. 

[Len Woodruff]

25 minutes ago, Paul Payne said:

Try soaking the strip in hot water then bending near the shape, then heating again and bending to the final contour. 

That sounds like a good idea. I will try that and see what happens.

[Mark]

The solvent cement won't work on a resin body, and I don't think MEK will work either.  CA glue is your best option.  If you have to make it conform to any kind of curve (or not, for that matter) then tack the strip down at one end, force it into the desired curve and tape it down (or pull it taut for straight lines).  Then tack it down every so often over the full length of the strip.  Then pull the tape off if you used any. 

Once you are sure the strip is right where you want it, then finish tacking it along the entire length.  You will have to sand either side of the strip to clean off any excess CA, but that shouldn't be a problem.

Edited April 30, 2021 by Mark
Info

[Len Woodruff]

34 minutes ago, Mark said:

The solvent cement won't work on a resin body, and I don't think MEK will work either.  CA glue is your best option.  If you have to make it conform to any kind of curve (or not, for that matter) then tack the strip down at one end, force it into the desired curve and tape it down (or pull it taut for straight lines).  Then tack it down every so often over the full length of the strip.  Then pull the tape off if you used any. 

Once you are sure the strip is right where you want it, then finish tacking it along the entire length.  You will have to sand either side of the strip to clean off any excess CA, but that shouldn't be a problem.

That's what I did with the 1st strip. I had it fully taped down when I added the super thin CA glue. It broke at the curves. I was just going to use the solvent to soften the 1/2 round to make it conform then use the CA to glue it in place.

[Rick L]

If you have no luck, I can cast half round radius doughnuts for you in polyurethane. That way you can cut it to any tangent angle with a razor blade place it on the corner and attach straight strips with CA. A little filler putty on the joint and your done. Send me a PM and I’ll guide you on the numbers I need. 

[Len Woodruff]

2 hours ago, Rick L said:

If you have no luck, I can cast half round radius doughnuts for you in polyurethane. That way you can cut it to any tangent angle with a razor blade place it on the corner and attach straight strips with CA. A little filler putty on the joint and your done. Send me a PM and I’ll guide you on the numbers I need. 

Thanks I will let you know. I just got back home from the hobby shop. I will be trying the solvent glue in the next couple of hours.

[peteski]

You could also try using a pencil type soldering iron to heat and bend the strip (since a torch doesn't give you enough control).  Hold the strip over the iron (since heat raises), without actually touching the iron. That will likely let the strip heat enough to bend.  It might take several seconds.  The radius can be controlled by where you heat the strip.  If you hold it over the iron's tip, the radius will be small. If you hold it over the irons thick barrel, the radius should be larger.

Edited April 30, 2021 by peteski

[Rick L]

Half round stock will roll over when bending. Even when heated. The challenge to this is keeping it hot while bending and keeping the flat of the curve on plane with the straight. It cools too quickly to control. 

[Rick L]

19 hours ago, Rick L said:

If you have no luck, I can cast half round radius doughnuts for you in polyurethane. That way you can cut it to any tangent angle with a razor blade place it on the corner and attach straight strips with CA. A little filler putty on the joint and your done. Send me a PM and I’ll guide you on the numbers I need. 

Let me know how your solvent glue works. If you’re not satisfied my offer still stands. All I need to know is the radius of the half round rod and radius of the curve you want to bend and I can cast that shape with the flat in the back for you and send you as many rings  as you need.

[NOBLNG]

3 hours ago, Rick L said:

Half round stock will roll over when bending. Even when heated. The challenge to this is keeping it hot while bending and keeping the flat of the curve on plane with the straight. It cools too quickly to control. 

The OP could possibly bend round stock of the same diameter, sand the bottom half off and use the curved section? Not ideal as it makes for joints in the trim, but a possibility?