How to Bend and Form PVC
When I first wrote these pages metal ducting was ridiculously expensive and PVC was inexpensive. PVC will not pass fire codes and fire marshal inspections, but many including me used PVC due to cost, ease of use, and ability to later make changes. Today costs have flipped where it is generally faster and less expensive to use metal ducting. You need to decide on what you want to use.
Meanwhile, with more and more hobbyist woodworkers turning to use of PVC for their dust collection systems, many kept asking for dust collector adapters, fittings, bent pipes, etc. I realized that I had many good inexpensive solutions that just needed to be shared. You can easily make your own fittings and adapters with very little work from PVC. PVC can easily be reshaped to make almost any kind of adapter and far more!
Before getting started you need to know that there is a potential of letting off some very dangerous dioxins with other toxic and potentially cancer causing gases, so you must work in a well ventilated area and should wear a good respirator. Moreover, the result will not be as strong as it was before you made your changes.
Rob Robertsen shared the following note: "The greatest danger comes from vinyl chloride, a primary component of polyvinyl chloride (PVC) and an odorless gas which could be released using this process. Vinyl Chloride is a toxic carcinogen which has been proven to cause angiosarcoma, a deadly primary liver cancer. Use a good respirator and adequate ventilation to protect yourself from this toxin."
My first experience with how much PVC can be bent came from my swimming pool plumber. He showed me how to easily make long sweeping bends in PVC pipe, if you know how and are patient. He first glued one fitting to the end of the pipe. He said this let him heat closer to the end without having to worry about the joint later not fitting because the pipe got out of round. Never heat the end of a pipe you later want to join. He first setup so we were not only outdoors, but also had a good breeze on his back to blow the toxic fumes away. He said without a good breeze, due to toxic fumes he would only do this while wearing a good respirator with organic vapor cartridges. He then heated the pipe using a propane torch turning the pipe constantly while warming it. I burned the pipe the first time I tried this until practice showed me how close to hold the flame and how quickly to turn the pipe. The whole area he heated did nothing for a long time and he just kept heating. It suddenly got rubbery soft holding its shape turning into something like a flexible piece of rubber tube. Because PVC holds the heat so well, he was actually able to shape the entire pipe that went from the bottom of the swimming pool drain, curved up a trench in the pool side then turned to go to the equipment pad. When forming he gently bent the pipe until without letting it collapse. After locking in the first big curves with cold water he then heated the adjacent area and bent that the same way eventually getting two long gradual full 90-degree bends.
He did the same thing for me with some 4" pipe for my first PVC ducting. Although he had to work in shorter lengths because of the pipe diameter, he managed to make me some really nice curves that went from my main to my down drops. He said there are springs that can be slipped on to keep the heated pipe from collapsing, and there are ways to fill the tube with sand or pressurize the air inside to avoid kinking. He still thinks if you are going to bend that tightly, plumbers should use a fitting because the weakened pipe can cause problems with water pressure. In dust collection we have so little pressure this is not a problem. Anyhow, he had me screw a couple of 2x6s onto a square of plywood which became our form to make near perfect 90 degree long radius turns in 4" PVC with no hose collapse at all. He said for longer pipe we need to make many gradual bends rather than try to do the whole pipe at once. I found it takes much more gradual bends to form 6" diameter pipe. Regardless, with a little work he had fixed that piece of PVC with a nice ninety-degree bend.
He also shared for long runs with lots of gradual bends and minimal pressure he sometimes uses a special version of PVC that comes flexible from the factory. It will not take tight bends, but does wonders in terms of reducing resistance over long runs by eliminating joints. He said he has to special order this pipe from irrigation supply houses because his plumbing and pool supply outlets were too pricey. I went to my local irrigation supply and found this flexible pipe is too light weight for dust collection as our pressures will cause the pipe to collapse.
Making shorter bends in large sized PVC is a piece of cake with the right tools but becomes a nightmare without the right tools because the pipe will kink with tighter bends. I've personally had good success with one approach and have heard of a second.
For tight bends some professionals use special springs that slide over the pipe to keep it round during the bend then heating the inside. I accomplished the same by substituting duct tape and the fiberglass reinforced tape for the outside spring to force the pipe to stay round during the bend. The glue on the tape creates a significant mess that required Goo-gone or lacquer thinner to clean up. That solvent also took off the obnoxious PVC printed lettering.
I found my heating blanket worked poorly for heating the pipe enough to make tight bends, so I went back to my heat guns. I made a couple of wooden donuts that fit snugly inside the pipe far enough to keep the ends from softening. The donut centers just fit my heat guns shrouds behind the hot metal. A couple of heat guns quickly warmed up the inside of the pipe amply to bend the pipe. I've also used my hot box made from a wallpaper steamer and wooden box. The hot box requires leaving the ends of the pipe outside to keep them from deforming.
Mike Watson emailed saying he works with the larger PVC pipe regularly and has excellent success using plugs and a hot box to make tighter bends. He says, "Each plug is a piece of rubber just smaller than the ID of the pipe sandwiched between two round pieces of metal with a bolt and wing nut through the middle. You slide a plug in each end of the pipe and tighten the wing nuts until the plugs make airtight seals. Then heat the pipe in a hot-box. The air that is in the pipe gets hot and expands with the pipe. That trapped air inside the pipe will let you bend a much tighter radius than any other way without collapsing the pipe. If you take the plugs out too soon the pipe will flatten out and kink unless you cool the pipe first with water or a cool wet rag. When you loosen the plugs, keep your hands out of the way because they will shoot across the room if the air is still real hot. If you are ready for it and your working buddy isn't...it can be pretty funny too. You can find these plugs in factory sets by Greenlee but they are pricey, so if you can find a suitable rubber it is not hard to make your own."
Making the Pipe Larger
My first experience with how much PVC can be stretched was with my friend who does swimming pool repairs. One day he was looking over the mess I had getting from my 4" dust collector pipe to my two heavy 100mm (about 3.91") European woodworking tools. He said he would help me out. He showed up with some 4" Schedule 40 PVC that he had lying around. He told me to unscrew the metal dust port off my big saw and follow him.
We went outside and like my pool plumber cautioned that heating PVC lets off potentially deadly gases you do not want to breathe. We went to the kid's sandbox. He said he prefers to work on a large piece of plywood as the wood acts as a better insulator to let the pipe stay warmer longer. Wearing heavy leather work gloves, he slowly heated that piece of PVC with a propane torch turning it constantly and keeping it carefully far enough away to not burn the pipe. Anyhow nothing happened at all as he slowly kept heating and constantly turning. While heating the pipe he said you can even buy commercial heating blankets made for this purpose that wrap around a length of pipe to be bent. He said a good heat gun works far better than a propane torch making it harder to burn the pipe and will let us work larger pipe. I offered to get my heat gun and he said not to worry, he got a lot of torch practice when he was a plumber. Plumbers always have their torches and rarely have a place to plug in a heat gun. He kept heating, turning and testing the pipe with his gloved finger until suddenly, instead of PVC, that pipe held it shape but became soft and flexible like a large cooked noodle. It was amazing how long that pipe stayed flexible. Anyhow in short order he had custom bent that pipe so I no longer needed to use flex hose to connect my tool to the down drop.
After getting the pipe bent properly, he then showed me how to make that pipe fit my oversized tool port. He reheated one end with the torch, but said if he had to do lots, he would simply use a pot of boiling water that you can dip the pipe into for a few minutes. He evenly heated the last 3" to 4" of the pipe, and constantly turned it. Still wearing his heavy work gloves, he simply stretched and with the help of a wide tipped screw driver pulled a little then squirmed the end of that pipe slick as could be over the end of my collector fitting. His technique reminded me of putting a tight bicycle tire on a rim. He then sprayed the result with a water hose to cool it into a perfect fit. He said if we had to work on the tool, he would skip the water and just let the PVC cool on its own, but that would be lots slower because PVC holds heat and takes a long time to fully cool. He warned me not do try this with my tools that had plastic dust ports as they would melt. After much experimenting and lots of ruined pieces my favorite heating technique is the pot of boiling water. I can consistently stretch normal schedule 40 PVC about one pipe size meaning increase the pipe circumference a maximum of about 7% which is about a 21% increase in diameter. Some say they can increase the diameter by as much 30% but I found the pipe tears when going much over 20%.
Making Mating Flanges
Most connect two pieces of PVC that do not have a flared flange end using connectors which get very expensive for larger diameter pipe. It seemed to me that with very work we could make our own flanges and avoid that extra expense, plus the result would have less resistance. Most smaller PVC pipe such as we get for our sprinklers has one expanded end to let us join long lengths of pipe without having to buy expensive fittings. My pool friend said he had a wood turner friend make him an HDPE plastic stepped tapered mandrel of the normal 2" pipe size he most often uses in installing pools. He could quickly heat up the end of a pipe, slip in the mandrel that was step tapered, wet the joint for quick cooking and instantly had a perfectly fitting flange joint. The result always gave him smooth pool piping runs with minimal joints as that mandrel and his heat gun made it possible to bend the pipe to turn all the corners with just a single joint on each end of the hose. He said in water flow a smooth path is every bit if not more important as it is with dust collection.
This inspired me to work out a way to put the same kind of female flared end on the larger pipe. First I had to make sure I put the female end down on the outbound air stream. This makes sure the air falls off the edge of the pipe instead of ramming into it which causes turbulence and causes little scraps to sometimes get wedged in our joints causing blockages. To heat the pipe end I dipped about 3" of pipe into boiling water. It is awkward to hold a long pipe vertical in the water, but it only takes a few minutes for the end to get rubbery. I found it really tough to slip over just another pipe even after using a file to taper the incoming pipe end. Knowing I had a lot of fittings to make in my shop and am constantly helping others, I took the time to turn a tapered mandrel on the lathe that opens to the same diameter as my pipe. A little lubrication on that mandrel makes it easy to slip into and expand to a perfect fit. Because I made mine from wood, I could not wet the pipe for cooling as that would ruin my mandrel. At first I used a large wide rubber band or clamp to force a perfect fit, but found that really is not needed if I am patient enough to just let the pipe cool with the mandrel inside.
Making the Pipe Smaller
When I asked if he could make it smaller, he said he did not know how. Thinking how my old chemistry professor taught me how to work with glass, I began doing some play of my own. With glass we can heat up a portion then simply pull and the glass diameter can be shrunk down to any size including near microscopic. I found that by leaving about 4" of cool handle on each end then warming up roughly 12" between, you can slowly stretch PVC pipe as well, making for a diameter again about one pipe size smaller meaning about an 7% maximum reduction in diameter. More than that tended to tear the PVC. Key to my success was that end piece, help from a wire loop that went through a hole in the cold piece, and a steady weight to help with the stretching while I heated. Like glass the pulled pipe has a nice taper that then goes to a fairly constant diameter. Getting just the right size required a lot of cutting up that taper to find just the right spot. To get more consistency I found putting a big wooden dowel or long plug into the pipe let it pull down to just the right size. Getting just the right sized inside that did not stick was a pain that required me to turn my own on a lathe. I found to get a consistent fit it needed wide band clamp on the softened pipe to neck it to just the right size. For smaller pipe I use a piston ring compressor and for larger pipe I just coiled some metal and used a screw type band clamp quickly tightened with a power drill.
Another technique that lets me make much larger reductions is to make cuts with a hacksaw at least 3" deep into the end of the pipe. Deeper cuts make for a longer and better looking taper. Small reductions are made with straight cuts but bigger reductions can be made with V shaped cuts. I cover a metal pipe of the size we want to reduce to with aluminum foil. Eventually enough material gets removed that the PVC pipe can be warmed then a band clamp used to pull it down onto the metal pipe it is to fit. After the pipe cools, I use many light coats of PVC cement to fill in the gaps and make it seal and be a strong transition.
Making Caps and Donuts
Example 1 - An 18" Donut
Start by making a large flat circle. With a 2.5" overhang on each side you need D + 5" diameter, or in this case 18"+5" = 23". Knowing that PI times diameter equals circumference and that this pipe only comes in 2" increments after 6", it was not advanced math to figure out you need to start with an 8" pipe 24" long to leave just a little extra to work with. Split it vertically using a jig saw then use a portable heat gun to open it up. My technique is to do about 3" at a time that I keep sliding into a sandwich between two 3/4" thick pieces of plywood. The plywood flattens it and holds it stable. Although you do have to work in small sections, the result in about 20 minutes is a flat near square piece of PVC. I've never found a place I can buy it direct. Generally, for this kind of bending most use Rovel plastic, but I had the PVC and Rovel is more expensive and for me about an hour and a half trip away. Regardless, that piece firmed back up into a nice flat square. A compass helped put four circles on it.
One circle was for the pipe it was to fit and another for the diameter of the pipe that was to fit in it. The other two were for cutting to make the unit a round and to make a center hole. Again using a jig saw cut the inner circle and the outer circle.
Next you would normally use a wooden mold and vacuum form system it you were doing many, but in a one of a kind as this was, you can just use your target pipes being careful to not let them get too hot. Carefully heat up 1/3 of the edge on that round, place it on your cool pipe and gently bend it over the side of the pipe using the circle as a guide. Remove and cool your pipe, then do another third the same way. It takes about three times around to get a workable fit and a lot of fussing that can be helped by having a large band to press it tight and hold it in place until cool. With a jig saw mounted so the blade is at 2.5" horizontally, you can then just lie the top flat and cut the outside edge to make a most professional looking finish. This process does work, just takes lots of patience and time, plus a heat gun.
Putting in the center hole was far easier. Simply warm the whole center area out about 2" then press the pipe it is to fit in down through the hole making sure it sit vertical as the top cools. You can see in the attached picture that there are two "donuts" and if you look closely you will also see a cap on the bottom of the cylinder. All were made just this way.
Making your own joints and fittings
To make a simple wye is also easy, but may not be worth the time. My local cost is only $8 versus about 4 hours time to make a nice one. Start with your larger pipe and the pipe to join to it. I made a template from paper that I wrap around the incoming pipe to make a cutting line. It turns out some of the free Internet sheet metal transition programs work great for making the needed shapes. After rough cutting the incoming pipe a little proud, I gig up the same sized pipe with a piece of sandpaper in my low speed drill. A couple of minutes makes a near perfect fit. Now I use a Sharpie marker inside that pipe to mark where to cut the other pipe. Instead of cutting on that line, I instead draw another line inside that ample to make a nice lip. Cut on that inside line, then using my heat gun on the hole until I can slip in the smaller pipe. I try and work the edges so they go outward. This makes a nice female joint if held tightly to the pipe while cooling. All it then takes is a little cement and we have a nice looking and workable wye.
Making your own more complex wyes like the 6" to 5" and 3.5" I recommend for my ducting requires starting with a 6" PVC pipe and cutting a long V that makes the pipe taper from 6" to 5" when closed. If you are connecting to flex hose, instead measure to just fit the hose. The pipe V cut edges can be glued or plastic welded while clamped with a metal band clamp. Next make 3.5" straight again using a V cut to reduce a 4" to a 3.5". After both pieces are made, then join them using the same technique just described for making a simple wye. After the pieces are joined dip each of the smaller ends in hot oil and use clamps to make the female connections. Clamping to flex hose makes a great joint that can be screwed right onto the hose without need for securing clamps.
There are a few different ways to make the joints.
One involves using a heat gun and short piece of the pipe to be joined on your larger pipe. Unfortunately, PVC when heated gives off dioxins, one of the most carcinogenic gases known, so you need a well-ventilated setup to do this kind of work. You heat the larger pipe pretty intensely where the joint is to be made then use the short piece to press from the inside out to form the fitting that you want to see. PVC is an interesting plastic in that it pretty much stays the same size as it was cast without too much ability to stretch or shrink, so this approach is a lot of work and takes time. The advantage of course is the result gives a good tight fit, lots of gluing area, and you can do any kind of customizations you want.
Another joint uses hot oil or boiling water and a jig that holds the pipe round but flares the end out at ninety degrees. Two pipes can be put together with a seal and screws or just glued together to make a pretty good joint.
Since writing this page a friend sent me a bunch of pictures of magic he did by carefully sawing then gluing together different fittings. He made his joints very similarly to how we would glue together boards in woodworking. For instance he carefully sawed two right angled fittings to make a nice long double radius Y fitting. He made all of the wye joints off his mains by carefully jointing two pieces of straight pipe. He glued in a couple of plugs into a piece of 6" PVC that was slit so it was just a sandpaper thickness smaller in diameter than a standard pipe. He put sandpaper on this pipe and mounted it into his lathe. He made a jig so he could bring a piece of pipe in at a 60 degree angle. Then with lots of patience he sanded the pipe to make a perfect fit on another 6" pipe. He then made a paper template so he can use a jig saw to precut most of the material to minimize the sanding. To make the hole in the main, he put the pipe on and drew the shape that needed to be cut from the main. He again used his jigsaw to remove most of the material. He then used a large sanding drum with bearing on his portable drill to sand a perfect flush fitting hole. He glued on the leg of the wye plus a little triangle of PVC for reinforcement. He initially used a PVC welder, but found that gluing not only is lots easier and looks better, it seems to hold better. I found at Harbor Freight for $29 (on sale) a PVC welder. This unit uses a stream of very hot air driven by your compressor to melt a little bit of plastic and plastic "welding" rod. With a lot of work you can make your own joints etc. Like my friend I found it far easier to just glue.
One additional thought from two different friends is that much larger pieces of PVC and other similar plastic are regularly formed using two other processes. One uses a hot bath of glycerin to make the plastic flexible. The other uses a heating grid and vacuum form system. I have no need or interest in working with pieces this big, but some might.
Anyhow, this will let you turn corners and do all kinds of pretty impressive magic in terms of working with PVC. Hope this helps. Bill
Welding plastic is not all that difficult. Since writing this article I purchased an inexpensive plastic welding gun. It is really a controlled temperature soldering iron that needs to be hooked up to an air compressor. I soon learned the hard was that without ample airflow, it can quickly burn up. The key I found to making mine work was a modest airflow, slow speed, and lots of filler rod. Here is a Wegener video on using a plastic welder (click here to load).