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Go Back   SailNet Community > On Board > Gear & Maintenance > Glue/Weld Plastic?
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Thread: Glue/Weld Plastic? Reply to Thread
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Topic Review (Newest First)
08-09-2008 08:52 AM
bhcva Thanks Pappy...I'm off the the boat to dismantle the fixture and get a photo...back in a few days.
08-08-2008 03:45 PM
poopdeckpappy Almost any plastic can be welded, one of the many tools I had while racing offroad was a plastic welder, I used for many things but mostly cracked fuel tanks or body parts ( not mine, the bikes )

PP can be welded, ( DIY ) the question is it worth investing a little money into the welder ( which isn't that much )

to help you figure out what plastic you have and what's needed to repair

Identify Plastic

and here's the supply house

Urethane Supply Company - Plastic Repair and Plastic Welders
08-08-2008 01:18 PM
sailingdog But you'd have to get the facing part of the body to be fairly flat to get a good spinweld IIRC. I've done a bit of work with spinwelding, mostly HDPE.
Quote:
Originally Posted by TheFrog View Post
SD, I agree with you but I am concerned that it would be very difficult to get epoxy to adhere well enough to the Polypro to glue a nipple in place. PP is just too slippery to glue easily.

Another idea. Find a polypropylene nipple, threaded on one end only. Cut off the old nipple, clean the stub of the old nipple and the unthreaded end of the nipple well. Chuck the nipple up in a drill (preferably a drill press, or make sure you have very steady hands and great alignment). Put the drill on high speed and spin weld the new nipple onto the fixture. Forward Technology | Plastic Assembly - Spin Welders
This is really done best with a drill press and the fixture well supported, you spin the nipple while apply light pressure of it against the fixture. When the plastic starts to melt you stop the spinning and maintain pressure until the plastic hardens. It does result in pretty solid bonds.
McMaster (McMaster-Carr) has a decent selection of polypropylene pipe nipples.

Good luck.
08-08-2008 01:08 PM
TheFrog
Spin welding

Quote:
Originally Posted by sailingdog View Post
True, but it would be worse with a metal fitting. If you used an NPS fitting, the hoop stress would be lower, since it isn't tapered. No need to use a threaded nipple if you're epoxying it in place though.
SD, I agree with you but I am concerned that it would be very difficult to get epoxy to adhere well enough to the Polypro to glue a nipple in place. PP is just too slippery to glue easily.

Another idea. Find a polypropylene nipple, threaded on one end only. Cut off the old nipple, clean the stub of the old nipple and the unthreaded end of the nipple well. Chuck the nipple up in a drill (preferably a drill press, or make sure you have very steady hands and great alignment). Put the drill on high speed and spin weld the new nipple onto the fixture. Forward Technology | Plastic Assembly - Spin Welders
This is really done best with a drill press and the fixture well supported, you spin the nipple while apply light pressure of it against the fixture. When the plastic starts to melt you stop the spinning and maintain pressure until the plastic hardens. It does result in pretty solid bonds.
McMaster (McMaster-Carr) has a decent selection of polypropylene pipe nipples.

Good luck.
08-08-2008 12:55 PM
sailingdog True, but it would be worse with a metal fitting. If you used an NPS fitting, the hoop stress would be lower, since it isn't tapered. No need to use a threaded nipple if you're epoxying it in place though.
08-08-2008 12:44 PM
TheFrog
Quote:
Originally Posted by sailingdog View Post
The problem with tapping a new hole is that you generally shouldn't use metal fittings in plastic openings... since the metal will generally expand more than the plastic and lead to stress cracks in the plastic.
I would actually say that while the metal expansion is part of the problem even if a plastic nipple were used there is a high chance of stress cracks due to the plastic fitting not being designed with sufficient wall thickness to deal with the tensile (hoop) stress it will encounter when a NPT thread (tapered) is screwed in. The reason when the NPT threads are effective at sealing well is due to the taper applying a force radially outward on the female thread. For this to work well in plastic there needs to be a fairly robust wall thickness to not crack. I doubt that the existing plastic has the required thickness.
08-08-2008 11:18 AM
sailingdog The problem with tapping a new hole is that you generally shouldn't use metal fittings in plastic openings... since the metal will generally expand more than the plastic and lead to stress cracks in the plastic.

GBurton-

You really should start a new thread instead of hijacking this one.
08-08-2008 10:59 AM
GBurton Not to hijack the thread but I have a related question.

I need to drill a 3/4" hole through my mast step (tabernacle) into my boat for the mast wiring. This hole will go through 1/2" of aluminum then 4" of deck to the interior of the boat. I want to use hose to form a drip loop above the hole but I'm worried that epoxy will not stick to the hose. Any suggestion?

Frog - do you have any experience with hose and epoxy?

Thanks for any help
08-08-2008 10:43 AM
TheFrog I have been working in and around injection molded plastics for 15+ years. To the best of my knowledge there is no adhesive that will reliably bond with polypropylene. There are ways to weld polypro using scraps of the same material and a low heat soldering iron but the strength never equals the original.

I think your best bet is follow the previous suggestion of cutting off the broken part and fitting in a new nipple. However, this may also lead to problems as the plastic may crack due to the hoop stress associated with NPT fittings...

You are in a tough spot.
08-08-2008 10:02 AM
sailingdog Polypropylene is what I was afraid of... I've dealt with it a fair bit..and the stuff just doesn't have any good adhesives for it... Don't believe there are any solvent-type cements for it either.

Can you post a photo of the pieces. That might make it easier to suggest an alternative... since it is hard to know what exactly you're dealing with from a verbal description.
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