LysmachiaMoon's blog: Seam in plastic sheeting

I use 6 mil clear plastic sheeting 10 x 25 feet to cover my greenhouse. I buy it at Wal-Mart for around $25. Because my 10 x 10 foot greenhouse has a peaked roof, there is no good way to completely drape the plastic over the roof and still have enough to cover the gable ends (front and back). I've been using heavy duty clear plastic packaging tape (from Wal-Mart) to add pieces to the ends and to close up any open seams/gaps. It works fairly well, but after a few months of weather, the tape begins to lose its adhesiveness and become brittle. Then what usually happens is the wind gets under a small gap in the tape and begins to lift the whole seam loose. I've also tried to staple the seams (using an ordinary desk stapler), but it leaves gaps that the wind can pry apart. Also staples rust fairly quickly.

I've been researching how to "weld" plastic sheeting together. In the YouTube video that I found, the man used a heat gun and two pieces of wood. He put one piece of wood under the overlapped pieces of plastic sheeting, then heated the plastic with the heat gun and used the second piece of wood to press the two softened pieces together.

I've found another way using a regular household clothing iron and parchment paper.
First, the plastic must be dry and clean. If you're working with big sheets of plastic, overlap the seam by at least 3 inches, 4 is better. Smaller pieces can use smaller overlap; it will depend on the weight/strain placed on the seam. Tack the overlapped seam in place with clear plastic packaging tape (you won't need to remove the tape as you weld the seam; it will melt right into the plastic). Use just enough tack pieces to hold everything together while you work. Because I was working with a very big sheet of plastic, I put a strip, vertically across the seam, about 3 inches long and spaced the tacking pieces about 2 feet apart. If you're working with small pieces, you may not need to tack the seam together.

I used a regular wooden household ironing board, but you can use any smooth, heat-proof surface. I padded the ironing board with an old towel, then placed a piece of parchment paper (the kind used for baking) over the towel. I cut a second piece of parchment paper the same length to cover the plastic (between the plastic and the iron).

Set the iron to medium-medium high. NO STEAM. You may want to experiment with some scraps until you learn what temperature, time, and pressure works best.
Place the plastic sheeting seam on the ironing board, on top of the parchment paper. Cover the seam with the second piece of parchment paper. Gently press with the hot iron, moving it back and forth, as if you're ironing a shirt. It only takes a few seconds to melt the plastic sheeting enough to form the weld. The nice thing about the parchment paper is that you can see if the plastic has melted: it will get a sort of slightly puckered look.
Keep the iron moving. You don't need to press; simply let the weight of the iron do the work. When you've sufficiently welded the plastic, gently and carefully peel the parchment paper off the weld. If it sticks, you've over-melted the plastic; use lower temperature or don't leave the iron on the seam as long. It takes a bit of fine-tuning but once you get the feel of it, the work goes quickly.

Carefully peel the plastic from the bottom parchment paper as well. Then reposition the plastic sheeting and repeat.

You have four goals: 1. Don't get burned by the iron or the hot plastic. 2. Don't burn anything (you'll be putting down and picking up the iron a lot and plastic sheeting is stiff and heavy. Be careful not to tip over or drop the iron) 3. Don't melt the plastic: play around with heat settings and time on seam to find the sweet spot. 4. Don't get plastic melted onto the iron. This is where the parchment paper is vital. Never touch the iron directly to the plastic. If you get melted plastic on the iron's plate, immediately turn it off, let it cool down to where you can safely touch it and then try to peel the plastic off.

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My project turned out well. The weather turned on me just as I was finished, so I did not get to actually install the plastic sheeting on the greenhouse today. I'm really eager to see how these heat sealed seams hold up. I would like to try to do a vertical seam, in place, on the greenhouse wall, but not sure if I can get an extension cord long enough. I think in that situation I will use a narrow strip of wood in place of the ironing board.

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