Repair Burnt Out HF Plastic Welder?

[vtsteam]

Asking for a little help on this one, an old HF Plastic Welder. It's the early type with just a wand, a hose and a pressure gauge that you are supposed to connect to a compressor, and a electrical cord to the wall. The wand receives both air and current and contains a ceramic element wound with resistance wire.

I've had it for at least ten years and carefully heeded the warnings to always turn on air first before connecting the cord, and always wait ten minutes after unplugging to turn off the air. But last time I used it, the compressor lost power, I didn't notice and the air ran out before I figured out what had happened. The welder stopped working.

I don't think spare parts are available -- the welder (of this type without a fan) hasn't been offered by HF in years. So I'm hoping for a MadModder style fix, if possible. Any help would be appreciated.

Here are some pics. They are respectively, overview, first side near end, second side near end, far end and cord end:

[vtsteam]

I've checked with an ohmmeter, and narrowed down the location of a break. There are 7 coils of resistance wire in 7 holes in the ceramic shell. They are all connected in series.

One of the incoming line terminals is presently connected to 2 of the coils, and the other is connected to the remaining 5. The break is somewhere in that second coil, and it isn't at either of the ends. So it's somewhere inside the second hole in the ceramic element.

Can that be repaired?

[awemawson]

Steve, nichrome wire is available on the reel, The Scientific Wire Company carry it. Before it has been heated it can easily be formed into your coils round a suitable mandrel. Once heated it becomes very brittle so unwinding your coils is likely to end in tears, similarly rescuing wire from other appliances will be hard.

As a kid I made several attempts to scrounge nichrome wire from used 1 kw fire bars and was only successful when I was given a brand new one !

About the only practical way of terminating it is a mechanical joint such as a nut and bolt as the oxide layer prevents brazing. Soldering is obviously no use due to the running temperature. I did once manage to spot weld some to end tags using a lethal capacitor bank.

[vtsteam]

Thanks , Andrew. That was kind of what I imagined would be the problem(s), but wasn't sure if there was some way of getting around it. I mean other than winding new coils -- which I imagine is both expensive for a one off repair, and not so easy in itself.

Too bad HF's mfr in this case hadn't got the concept of a thermal switch.

[vtsteam]

I wonder if we might come up with a MadModder plastic welder of our own?

The requirements are pretty simple -- some focused hot air of not very great velocity, and not very high temperature.

--- soldering iron element?

I don't know how many watts this welder was, but I used to attach a router speed controller to the input to dial down the temperature. The "manual" said to adjust via airflow, but that was much too coarse an adjustment, and also changed the welding characteristics. Heat control was much better.

[vtsteam]

Here's a pic of all the pieces in the torch. This one is considerably simpler and more compact than more recent offerings in hot air plastic welders. New ones seem to have a built-in fan, Understandable because of the mistake/burn-out problem. But I prefer this simpler and more compact welder.

Its only need is a compressed air source. The actual air requirements are very low, and this unit came with an auxiliary 15 psi regulator (1 bar) for adjustment. If I recall correctly the actual welding pressure was around 2 psi, with the router controller turned down substantially for lower heat. Out of the box it was way too hot for clean welds.

[awemawson]

I have a Leister Triac Heat Gun that I bought at a boot fair at least 20 years ago  and it's still working. I first came across them when we had some industrial flooring laid at work and the contractor was welding the vinyl sheets together and it worked superbly.

Just a 'hair drier on steroids' - various diameter nozzles - variable speed air blower - quite controllable. Loads of used ones on ebay at very modest prices so not worth making one and messing with hand held mains devices in my view.

I also have a Chinese 'hot air gun' that is perfectly capable of melting plastics (including itself!) that I use for heat shrink tubing amongst other things.

Mending yours Steve: if you can isolate the broken coil and short it out and feed the rest with 6/7th of it's rated voltage (103 volts in your case) from a transformer you should be good to go.

[vtsteam]

Andrew, since I use a speed controller with it, anyway would it still require a transformer?

[awemawson]

Well that depends on all sorts of things Steve !

Dropping the volts means that you know that you're not going to expend too much power in it and melt things unintentionally. Your speed controller may achieve to same end if it's range is suitable

[John Rudd]

The speed controller could be modified to prevent too high a voltage being applied….

Usually these things are just a phase angle triggered thyristor circuit, so by adding a value to the charging resistor will effect the output voltage at max output….you can try by using a filament bulb as a load and measuring the max voltage developed across it….

[vtsteam]

Just a point -- it never worked well out of the box "full on" anyway. It was substantially overpowered to begin with and my added power reduction (via the router speed controller) was necessary to get good welds. Otherwise plastic would rapidly burn on the surface without heating underneath. Browned crust and no penetration.

If repaired and losing a coil, I'd start with the controller zeroed, set my desired air pressure and slowly run up the power until melting the plastic rod at the rate I want. Inj the past, that was substantially below 80% of power.

So, though I'd now be running additional (effective) current through the coils .... more  than before, for the same quality weld, it would still be less than the normal out-of-the-box current that the coils saw with no regulation.

Unless I'm missing something important.....?

[RussellT]

It might be easier to use 5 out of 7 coils, as that way the connections would be at one end - although running a bare wire down the broken coil would be fine too.

Russell