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I always keep and reuse the plastic sacks which contain 25kg of water softener salt. Normally, I leave the bags open when I use them, but today I filled about 10 bags with gravel which will probably not be used for a number of years. I'd like to seal the tops of the bags properly just like they are when I buy them, probably involving some form of melting. Any suggestions?

EDIT: I don't want to have to buy anything to achieve this so am looking for a hacky solution. You can assume I have a very good selection of workshop tools and hot-glue gun etc. For example, my first thought is that I can probably do it by clamping lengths of metal either side of the bag and using a blowtorch to heat one or both sides to melt the plastic together.

  • Why not seal them with hot glue? I can foresee a potential problem with heated metal. Ensuring that the plastic sticks to plastic and not the metal could be tricky. – Dave Jul 8 '15 at 9:08
  • I have a feeling the hot glue won't work very well. Apart from anything else, the bags are pretty dirty. I'd have to apply the glue to the entire bag width (c. 12 inches) without it curing at the start before I get to the end! If I get no better ideas I will try this though. I agree about the plastic sticking to the metal too, that does sound quite likely. – Lefty Jul 8 '15 at 9:28
  • I would use duct tape, but that doesn't fulfill your requirement for sealing them like they are when you buy them. Commercial operations obviously know how to weld bags without the plastig sticking to the clamps. If you can find out how they do it then you may be able to find a way to recreate it. My gues is that it is very temperature dependant and there is a point where the plastic becomes highly malleable and can be made to stick to itself with enough pressure, but below the temperatue where it becomes liquid and will stick to the clamps. – Dave Jul 8 '15 at 9:39
  • @Dave I've used duct tape in the past for this sort of thing but it never really worked very well, plus, because I taped the open end down around the face of the bag, it made for a small package that was very heavy. I like the idea of keeping the "bag shape" so I can handle them from the top much easier. I suppose I could duct tape them across the top so long as the top is straight. Also, they will be stored outside and the duct tape never survived well last time. – Lefty Jul 8 '15 at 9:48
  • ...and I think you're right about the temperature/pressure being critical if I intend to weld them. I have a feeling they use a pair if wheels which are accurately heated, the bag moves between them and gets welded as it goes. – Lefty Jul 8 '15 at 9:54
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In the youtube-video How to Seal Plastic Bag at Home #Ludvic #Maker, demonstrates a rather hackish way of sealing plastic bags. In essence what he does is lay the plastic bag flat, put a piece of paper over the top of the bag and then use a soldering iron to transfer heat through the paper and melt close the plastic bag.

Variation over this method could be to use other heat sources:

  • soldering iron (as suggested in video)
  • edge of clothes iron
  • hot glue gun (possibly without glue?) (that is use the heated tip of the glue gun, not the actual gluing capabilities :-) )
  • a hair iron (the ones for straightening hair)
  • some other heated metal-strip

The point of using the paper between the bag and the heat source, I contemplate is to avoid getting melted plastic on your heat source.

I would also consider double sealing them for additional strength. That is don't run the sealing strip only once, but twice across the top of the bag.

  • Nice answer, +1. This is another technique I will try when I get the chance. I'll report back the results when I do. – Lefty Jul 8 '15 at 18:58
  • I've used a soldering iron to melt plastics together with great success before. Clothes irons never worked for me as they didn't get hot enough to properly bond the plastic. – UnhandledExcepSean Jul 9 '15 at 20:45
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    I tried the soldering iron technique and it worked quite well. I ended-up WRAPPING the tip of the iron in thick paper. The weight of the bags (filled with gravel, approx 20Kg) makes it impossible to lie them down like the video but I held a piece of wood on the other side to press the sides together. Timing is critical and I am getting better at it. Too long and it melts right through. Too short and nothing happens. The margin of error is about 1 second for a "spot weld", or about 5 seconds/inch for continuous seal. – Lefty Jul 16 '15 at 16:03
  • Remember to accept answers if they are a good match for your question – holroy Jul 16 '15 at 21:24
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If the bags are of reasonable width, and were originally heat-sealed, then there are many commercial heat sealers which will do the trick. Search for "heat sealer"; the wider the sealing bar, the more expensive the sealer. (You probably wouldn't get far with a consumer heat sealer; not enough power for the probably-thick plastic.)

  • Thanks for the answer - I didn't know such a think existed. I can't justify buying a specific machine since I only have 10 to do and, as you say, they are VERY thick plastic that I'm certain won't be sealed by one of these domestic ones anyway. – Lefty Jul 8 '15 at 8:22

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