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Old 2nd May 2021, 11:40 am   #1
pentoad
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Default Meter plastic repair advice needed

The Heathkit meter movement pictured below has significant cracks which go right through the plastic, is there known reliable technique to reinforce it so it doesn't eventually fall apart?
I am assuming it is polystyrene
I have seen a product called Plastruct EMA Weld which is supposed to work by capillary action, would this do the job?
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Old 2nd May 2021, 1:17 pm   #2
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Default Re: Meter plastic repair advice needed

I use EMA Plastic Weld, which is actually methylene chloride (dichloromethane), and it does indeed work well by capilliary action. I used it recently to repair the transparent window for the tuning scale of a transistor radio that had been shattered when something sharp fell on it. It works by dissolving the plastic it is in contact with, and is so volatile that a thin layer of solvent on the surface evaporates before it can affect the surface finish of the plastic, and is so thin that it readily wicks into cracks. I apply it using a small artists' paint brush. I get mine from a local shop that sells model railways and accessories.

If transparency is not required, you can reinforce by applying polystyrene sheet across the cracked area. Funnily enough I did this only yesterday with the hinge of my Epson printer that had snapped when I had inadvertently put something heavy on the top. I first joined the broken pieces and, because the hinge had to be resiiently deflected to engage the housing, plated over the join with a piece cut from 1mm styrene sheet. As the solvent is so volatile, this required flooding the surfaces of hnge and plate first to soften them up before pressing together and then applying more solvent to the exposed edges. Oce the solvent has diffused away, the resulting structure is very strong as the plasic layers unite into a soid mass.

Last edited by emeritus; 2nd May 2021 at 1:35 pm. Reason: typos
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Old 2nd May 2021, 1:33 pm   #3
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Default Re: Meter plastic repair advice needed

Does it need repairing?
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Old 2nd May 2021, 1:42 pm   #4
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Default Re: Meter plastic repair advice needed

Quote:
Originally Posted by emeritus View Post
I use EMA Plastic Weld, which is actually methylene chloride (dichloromethane), and it does indeed work well by capilliary action. I used it recently to repair the transparent window for the tuning scale of a transistor radio that had been shattered when something sharp fell on it. It works by dissolving the plastic it is in contact with, and is so volatile that a thin layer of solvent on the surface evaporates before it can affect the surface finish of the plastic, and is so thin that it readily wicks into cracks. I apply it using a small artists' paint brush. I get mine from a local shop that sells model railways and accessories.

If transparency is not required, you can reinforce by applying polystyrene sheet across the cracked area. Funnily enough I did this only yesterday with the hinge of my Epson printer that had snapped when I had inadvertently put something heavy on the top. I first joined the broken pieces and, because the hinge had to be resiiently deflected to engage the housing, plated over the join with a piece cut from 1mm styrene sheet. As the solvent is so volatile, this required flooding the surfaces of hnge and plate first to soften them up before pressing together and then applying more solvent to the exposed edges. Oce the solvent has diffused away, the resulting structure is very strong as the plasic layers unite into a soid mass.
Thanks - I will try it out
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Old 2nd May 2021, 2:04 pm   #5
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Default Re: Meter plastic repair advice needed

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Originally Posted by merlinmaxwell View Post
Does it need repairing?
Yes, "if ain't bust, don't fix it". I assume it's invisible to the outside world and it's functional. Why open Pandora's box?
I respect what Emeritus says, but there's an element of risk here, and no "dummy run" is possible.
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Old 2nd May 2021, 4:49 pm   #6
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Default Re: Meter plastic repair advice needed

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Originally Posted by Bazz4CQJ View Post
Quote:
Originally Posted by merlinmaxwell View Post
Does it need repairing?
Yes, "if ain't bust, don't fix it". I assume it's invisible to the outside world and it's functional. Why open Pandora's box?
I respect what Emeritus says, but there's an element of risk here, and no "dummy run" is possible.
B
It looks a lot worse than the photo as if it could easily crumble around the bolts.
My thinking was "a stitch in time" but on the other hand I wasn't aware of the problem until I took it off.
You maybe right as there is a risk of screwing up and no second chance as it appears that replacements are not available.
I might leave for the time being!
If it does break in the future I will address the problem then.
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Old 2nd May 2021, 5:53 pm   #7
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Default Re: Meter plastic repair advice needed

I wonder if you could reinforce it by cutting a panel of some sheet material the same shape and size as that back plate and bonding that with a contact adhesive?

I know that I'm a very risk-averse individual, and trickling in methylene chloride sounds "adventurous", even if it does work.

B
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Old 2nd May 2021, 7:44 pm   #8
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Default Re: Meter plastic repair advice needed

Methylene chloride is highly volatile, which is why it is used for assembling plastic model kits, in particular, glazed windows and windscreens, where adhesive oozing onto the surface is not desirable. Applying using a fine pointed camel hair brush limits how much you apply and there is minimal risk of it getting inside to mess things up. It is really only suitable for joining things that fit snugly together because, unlike glues or cements, it cannot fill gaps. It is however eminently suitable for mending cracks.
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Old 2nd May 2021, 8:55 pm   #9
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Default Re: Meter plastic repair advice needed

I think a thin strip (1/8th inch) of double-sided tape round the edge will secure it enough for use. Or bond a thin piece of polystyrene with polystyrene cement to the back. Available from model shops as stated before, maybe take it into one for advice.
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Old 3rd May 2021, 4:24 pm   #10
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Default Re: Meter plastic repair advice needed

Quote:
Originally Posted by emeritus View Post
Methylene chloride is highly volatile, which is why it is used for assembling plastic model kits, in particular, glazed windows and windscreens, where adhesive oozing onto the surface is not desirable. Applying using a fine pointed camel hair brush limits how much you apply and there is minimal risk of it getting inside to mess things up. It is really only suitable for joining things that fit snugly together because, unlike glues or cements, it cannot fill gaps. It is however eminently suitable for mending cracks.
Might be worth testing on a tiny area to see how it responds?
I'm assuming the plastic is actually polystyrene.
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Old 3rd May 2021, 4:38 pm   #11
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Default Re: Meter plastic repair advice needed

Do you know how did the meter got like that; the edges and corners look like it's had a rough life? Is the front OK?
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Old 3rd May 2021, 8:05 pm   #12
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Default Re: Meter plastic repair advice needed

Meter plastic repair.
I would endorse what Emeritus says. Methylene Chloride will soften the plastic and act as a cement, for Perspex, Melinex and most plastics of that sort.
It has a nasty heavy vapour. Now difficult to get in any quantity as it was discovered some years ago by the sniffing brigade.
If gap filling is required, then I suggest Tensol (from RS et al) as most suitable for perspex type plastics.
I used it last for a broken rear light cover on my trailer, where I had to build up a bit. Worked well.
Years ago, I used to use perspex dissolved in chloroform, but that was banned. then I used Tensol (made by ICI) of perspex dissolved in methylene chloride. It was very effective. Now it is a two part mix, Tensol 70. Works well. Shelf life of some years in practice. Seems much more effective than polystyrene cement.
I would hesitate to use contact adhesive (?Evostick). It is a brown colour and not good on perspex.
I am told by an expert china restorer locally that she uses methylene chloride to clean up the cracks where people have attempted to stick up broken china with odd adhesives. It cleans up almost everything. Then she uses araldite or similar and tints the seam. When finished the repair is invisible.
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Last edited by WME_bill; 3rd May 2021 at 8:12 pm.
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Old 3rd May 2021, 9:49 pm   #13
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Default Re: Meter plastic repair advice needed

Try PLASTI-ZAP PT-19 this has always served me well for this kind of repair and it cures clear.

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Old 4th May 2021, 8:44 am   #14
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Default Re: Meter plastic repair advice needed

I've repaired a few meters with cracked backs by having a 1mm thick perspex laser cut to fit the back with holes for the mounting screws and the meter, this way I could glue the whole back with one piece that kept all the cracked pieces in place. I used Acrifix 192.
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