How to Bend and Form PVC
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 a good inexpensive solution 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 gasses, 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 polyvinylchloride (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."
Making the Pipe Larger
My first experience with how much PVC can be moved was from my swimming pool man. He has been a friend for many years and one day was looking over the mess I had getting from my 4" dust collector pipe to my two heavy 12mm (about 4 3/4") European woodworking tools. He said he would help me out. Not long after he showed up with some 4" Schedule 40 PVC that he said he had lying around. He told me to unscrew the metal dust port off my big saw and follow him.
He took me outside and cautioned that heating PVC lets off gasses 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. He slowly heated that piece of PVC with a propane torch keeping it carefully far enough away to not burn the pipe. He said a good heat gun works far better keeping you from burning the pipe and it will let you work larger pipe. He said you can even buy commercial heating blankets made for this purpose that wrap around a length of pipe to be bent. 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. Anyhow nothing happened at all. He just slowly kept heating the last 3" to 4" of the pipe evenly while constantly turning. He kept testing the pipe with his gloved finger until suddenly, instead of PVC, that pipe had become flexible like rubber.
Wearing heavy work gloves he simply stretched and pulled a little then squirmed the end of that pipe slick as could be over the end of my collector fitting. He then sprayed the result with a water hose to cool it into a perfect fit. After much experimenting and lots of ruined pieces I've found that I can comfortably stretch normal schedule 40 PVC about one pipe size. Some say the maximum is about 25%. My own experience says the maximum with lots of care and patience is maybe 33%, but I am going to throw about every other piece out because it will tear.
Since he showed me his technique, I have found quite a few other ways to heat the pipe enough to work. Trying to get there with a single heat gun heating the outside of a 6" diameter pipe is akin to the one armed one legged paper hanger, might get there eventually working a little at a time, but not fun to watch. Two heat guns work a little better, but it is still better to use a better heat source. I went from heat guns to a hot box similar to what is used to bend wood can do the same thing. I made a good hot box out of a wallpaper steamer and wooden box. Besides being bulky hot boxes can deform the ends, so I will leave the male end out of the box and expand the heated end to make the female portion of the fitting at the same time as I make the bend. I eventually shifted to using a PVC heating blanket that I bought off eBay. That blanket was too expensive for most, but I was helping lots of people with their ducting so justified its cost. I now use my heating blanket to wrap pipe about 4â€� from the end for about 4' long. It brings the pipe up to a temperature where it can easily be moved into long sweeping bends. Others have written in that they have had good success heating with a glycerin bath, hot oil, and even just boiling water.
John Koster shared the following note: "It is quite common around here in the electrical trades and plumbing trades to use a vehicle exhaust to heat a length of larger ABS or PVC pipe and then bend or shape it in whatever configuration you want. The exhaust pipe is easier to use than torches or heat guns. Just slip the pipe over the tail pipe of a running engine wait for it to soften then shape it followed by cooling it with water to lock it in the position you want."
Anyhow, he then showed me that making long sweeping bends in large sized PVC is a piece of cake, if you know how and are patient. He again heated that same length of 4" diameter pipe that he had modified with a female end. He heated about 12" at a time turning constantly while warming it. That whole area got soft but held its shape turning into something like a flexible piece of rubber. He gently bent it until it collapsed then backed off a little. After locking in that curve with cold water he then heated the adjacent area and bent that the same way eventually getting a full 90 degree bend in my 4â€� pipe. He said there are springs that can be slipped on to keep the heated pipe from collapsing, but he 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 put in a near perfect 90 degree turn in 4" PVC with no hose collapse at all. He said this technique requires making gradual bends over quite a bit of pipe. It would take a much more gradual bend to do the same in 6" diameter pipe. Regardless, with a little work he had fixed that piece of PVC to fit perfectly on my DC hose and my machine 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 vacuum pressures will cause the pipe to collapse.
Making shorter bends in large sized PVC is a piece of cake, if you have the right tools and a nightmare if you do not because the pipe will kink as you try to make a tighter bend. Iâ€™ve personally had good success with one approach and have heard of a second.
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.
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. Iâ€™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, â€œEach 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.â€�
Making Mating Flanges
Most connect two pieces of PVC that do not have a flared flange end using connectors. It turns out that with very little heat gun work you can make your own tight flange and avoid that extra expense, plus the result has less resistance. My pool friend said he had a wood turner friend make him a stepped tapered mandrel of the normal pipe sizes he used in installing pools. He could quickly heat up the end of a pipe, slip in the mandrel that was tapered at each step to make that easier, and instantly have 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.
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. I found that by leaving about 4" of cool handle and warming up roughly 12" above that, you can slowly stretch the pipe as well, making for a diameter again about one pipe size smaller. 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. Another trick was to slip a big wooden dowel or plug into the pipe that was the right size and use a wide band clamp (I used my piston ring compressor) to neck it to just the right size.
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
Making a female pipe joint is really easy. I found that by heating up some cooking oil and dipping about 1.5" of pipe into that oil, it gets soft and easily slips right over another pipe. If you use a large wide rubber band or clamp, when the plastic sets, the resulting joint is air tight and a good fit.
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 gasses 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 the hot oil 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.
Another approach is to simply make joints as we would do in woodworking. One of the guys sent me an email saying he put a piece of 6" PVC on his lathe by making a couple of wooden plugs, then put sandpaper on that. The result let him sand the pipe before welding into perfectly fitting joints. You rough cut using a template, then dress on that sanding drum. You can either glue or weld the PVC. 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.
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).