The Triumph Dolomite Club - Discussion Forum

The Number One Club for owners of Triumph's range of small saloons from the 1960s and 1970s.
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 Post subject: plastic welding
PostPosted:Mon Jun 21, 2021 11:40 am 
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Location:Harrow Middlesex
Has any one tried plastic welding,is the weak point the joint where youve welded ?

Dave


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 Post subject: Re: plastic welding
PostPosted:Mon Jun 21, 2021 1:28 pm 
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Can be depends what techniques you are using, how well you do it and the type of plastic. Remember only thermal plastics can be welded, thermoset types cannot.

We used to Ultrasonic weld ABS parts together where I used to work. Basically the two parts are vibrated together and heat is generated between the two surfaces which melts the plastic together - one part usually has a small sacrificial peak which melts to fill the joint. In those cases the joint was stronger than the surrounding case but only I suspect because the joint was thicker.

On another product we "solvent welded" parts together using granules of raw plastic "melted" or "dissolved" in a 3M's solvent. This was painted into the joint to fill/stick/weld/seal the joint. Similar to the solvent cement used on plastic waste pipes except we dissolved plastic the same colour as the mouldings.

Finally we also joined very large polypropylene mouldings together. These were heat welded via a conductive metal braid laid in the plastic rim, mouldings were clamped together and large current passed through the braid to heat the joint and melt the plastic together. The same mouldings sometimes came in for repair and in that case we used a filler rod of the same plastic and melted it into the repair using a hot gun with a fine nozzle.

As with normal welding of steel, cleanliness is important.

In all the above cases I would say the join was stronger than the surrounding moulding, but only because of increased local thickness. At the end of the day it is the same material so unless it has been mistreated - over heated, contaminated with the wrong type of solvent or stressed in a way that breaks the length of the polymer chains then it will be as strong as the "base" plastic part.

What are you wanting to join? And how are you proposing to do it?

Roger

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 Post subject: Re: plastic welding
PostPosted:Mon Jun 21, 2021 9:30 pm 
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Roger

Thanks for feed back,im tring to modify a inner plastic front arch to fit the Dolomite,So far i tiried closing off gaps with fibre glass but have found it to rigid,so now thought about plastic welding to close of gaps

Dave


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 Post subject: Re: plastic welding
PostPosted:Tue Jun 22, 2021 6:59 pm 
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Quote:

On another product we "solvent welded" parts together using granules of raw plastic "melted" or "dissolved" in a 3M's solvent. This was painted into the joint to fill/stick/weld/seal the joint. Similar to the solvent cement used on plastic waste pipes except we dissolved plastic the same colour as the mouldings.


Roger
The raw granules are sold as product know as 'polymorph' and can be bought on ebay etc although it is sold as a moulding plastic it can with lots of practice , or luck as a bonding agent the main problem with using this method is that the polymorph has to be heated to an exact temp and kept at that temp whilst moulding so its not a great product for home use.
Best method for DIY would be a a plastic cement as said similar as used for pipe work. as with any type of welding the type of joint (Lapping/ butt/ folding) when done properly will dictate the strength.

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