PDA

View Full Version : Soldering plastic using SMD hot air gun



Rotate
10-07-2002, 05:44 PM
Ever need to mend small plastic parts?

Our office had a desktop label making machine which they threw out today because the small plastic lever inside the unit which presses the label to the thermal printer head had snapped. I took the brocken piece and realized that glue would never hold it and it was small and complicated enough that I would not attempt to machine one.

Our lab has SMD hot air gun which I thought was worth a try. I've heard about plastic soldering but never tried it. This air gun has a variable temperature and flow rate control. The hot air comes out of small nozzle (2mm) which you can direct very accurately to the work. I was mazed at how accurately I was able to "reflow" the plastic. Using ceramic screw drivers (used for adjusting RF parts), I was able to push the plastic around to create a joint which was thicker and stronger than before. What's more impressive was the finished joint which looked nice and smooth and because of the temperature control no burning plastic odour http://bbs.homeshopmachinist.net//smile.gif

I'm a happy camper today because I just got myself a label maker for free and save the dump from getting filled.

I thought I passed the tip along.

Albert

bdarin
10-07-2002, 08:14 PM
Amazing what you can do with a little hot air, ain't it?

chip's
10-07-2002, 09:44 PM
I used to work with some people who used alot of hot air.

nheng
10-07-2002, 10:57 PM
Rotate,

Here's some more hot air ...

I've used a SMT hot air pencil to "weld" cracked zoom cam followers on Bausch & Lomb Stereozoom 7 microscopes. Replacement parts were not to be found at the time. As a bonus, used the pencil to relieve stress on similar parts before they broke. It was a poor design with thin plastic walls holding two tight fitting steel cams. Heating the steel cam and surrounding plastic relaxed the plastic without destroying alignment.

You can even scrape plastic in a skin graft manner from an area that can afford it in order to fill a crack elsewhere.

Also handy for sealing parts into plastic bags for storage. Avoids a heat sealing tool. Just set temp high enough to melt the double thickness as you drag the hot air jet a short distance from where you want to create the seal line.

Den

CompositeEngr
10-10-2002, 04:20 AM
We have a commercial setup for plastic welding at work. It takes a spool of some type of thermoplastic and feeds it like a wire welder.
Works OK once you get the technique down. Didn't work terribly well on a water tank I mended with it. 300 lbs of water was probably the main factor in it leaking again.

docsteve66
10-10-2002, 01:31 PM
I heat welded the water tanks in a motor home back in 1991. The Gary water tank was welded using strips cut from the flashing. The balck water tank i made from a 4x8 sheet of pvc, using commercial rods.

Plastic welding is more like brazing than welding. the trick (in my opinion) is to get the three pieces (rod and pieces to be joined) to same temp and to press the rod into the soft plastic. Rented a welding gun- it had what looked like a soldering iron body, a hollow point and a a small air compressor. I made another welder from an old heat gun. worked better than the commercal job. Just put a nozzel of 1/4 steel tube and adjusted the air pressure. Haven't had any problems with leaks or cracks in the tanks over the years and I do travel some rough roads.
Steve

Sprocket
10-11-2002, 12:04 AM
I use a Seelye for thermoplastic welding at work. I never felt the self feeding tip worked very well, I get a better weld hand feeding the rod. We use mostly Polypropylene or a co-polymer polypro. Best joints though are when both parts are heated in the oven and pressed while they are at forming temp.

docsteve66
10-11-2002, 04:56 AM
Sprocket jogged my memory- for any one who contempltes repairing plastics. There are at least two basic types 1: thermoforming, 2: thermosetting.

if it gets soft when heated, its thermoforming and can be welded. MUST use a rod of simular material to get a good job.
If it dont get soft, its been "set" and you may (maybe) glue, use duct tape or what ever.

That about the extent of my knowledge of working plastics. I like to hear more from you sprocket cause you are doing it for real, I did it becasue I could not buy what you probably build http://bbs.homeshopmachinist.net//smile.gif
Steve

Sprocket
10-11-2002, 11:24 PM
Docsteve - I build braces and artificial limbs, and so use both thermoplastics and thermosets. The thermoplastics are mostly vacuum formed over a mold and extra pieces added for reinforcement or local extra thickness. We form the large piece,and while it is still hot (clear, where it is opaque when cool) press on the additions. It's all one piece then, no visible seam between the layers. Sometimes, we weld extra pieces on after the plastic cools, not as strong, but strong enough. Plastics are subject to oxidation as are metals, and a clean, slightly rough surface is is best to weld. Large joints are "v"'d out as metal would be and multiple layers of rod fill the "v". I like to hold the rod perpendicular with a fair bit of down pressure so the rod bends as it heats and pushes into the base material. I don't consider plastic welding as we do it to be structural, and I don't think we can adequately repair breaks, because they always break again (potential liability problem). There are some plastics that require gases other than hot air to make good welds. Thermosets are another topic.

docsteve66
10-12-2002, 12:04 AM
Sprocket- the stuff my gray water tank was made of was a milky tough plastic that turned clear when heated- just as you said. Its really funny to see. and It must have a high specific heat cause it took some time to become clear, and long time to regain its opaque milky color- I thought i had really messed up when i saw it happen. no one had told me. The pvc tank (black water never changed color. I bought sheet plastic and 1" plastic angle. I figuredthe joints would be weak- salesman agreed but said i could get near original strength. I laid it out so the inside was water tight, the angles(all on out side) were all in shear. never had a problem. Theplasiticcompany has bins of cut offs, sold by pound for less than price per sheet. Got some HMW (I think thats it- High molecular weight) plastic that acts like teflon, light stuff for trays for Motor home, plastic is almost as much fun as metal and a lot more forgiving. Gonna buy me a book and get smart one of these days!

Steve