Cutting smooth holes in foam

I chanced upon this method of making almost perfect holes in rigid foam after I was asked about the best way to do the same in cake .. making holes to take tube supports between storeys of a wedding cake! For this I very roughly carved ‘teeth’ in the end of a plastic plumbing pipe, which as it turned out could be twisted into the cake without a problem. So I wondered whether something similar would work for other materials. I was fairly certain it stood a chance of working if I could cut the teeth finely enough, and for this a slightly harder form of PVC pipe is needed .. MuPVC ..which stands for ‘modified unplasticised polyvinyl chloride’.

I’ll need to cover this briefly first. The various plastic waste/drainage pipes you’ll see in any building supplier or DIY store (whether white, black, grey or brown) are likely to be two or more different plastic types. They will either be polypropylene (PP), ABS (a form of styrene), uPVC or MuPVC. MuPVC is harder than the others, but actually easier to cut or file cleanly because it is slightly more brittle. The only sure way of identifying it is finding the letters ‘MuPVC’ as part of the printed info along the side of the pipe, but also if the pipe is referred to by the store as ‘Solvent Weld’ it’s very likely to be MuPVC.

hole-cutting tools for rigid foam

Obviously there are only a few available diameters when it comes to these pipes and it depends on the store which ones they actually have. Standard outside diameters start at 22mm and normally end at around 44mm .. before they become something else, i.e. drainage or guttering pipes which are either brown or black. At the moment in the UK Wickes has the best selection of MuPVC .. 22mm, 36mm, 40mm and 50mm .. but don’t search for ‘mupvc’ in their online catalogue because that will only bring up one of them. Instead find the sub-section ‘Plastic pipe’ in ‘Heating & Plumbing’ which should show all the others, just described as ‘solvent weld’.

Wherever and whatever you find, the supplier normally should list the pipe according to its outside diameter, not its internal one. For more technical details see ..

squaring end of Pvc pipe

After trialling a few ways of making the ‘cutting’ edge, I found this to be the best method. Firstly, the end needs to be cut and filed/sanded straight i.e. at 90 degrees to the pipe length, as above. Then, using either sandpaper wrapped around a dowel or a round file, the thickness of the cut edge needs to be reduced to almost a point, as below. This should be a gradual slope (over 5-10mm) and on the inside .. the outside diameter remains constant.

chamfering end of Pvc pipe

The next job is to file the ‘teeth’ and I would recommend using a small-size power drill and a diamond-coated bit .. if you have those things. If not you have to do it by hand using a round needle file, but again, I’ve found the diamond-coated needle files more effective on hard plastics. The box of diamond-coated bits below is from Rolson, found in Maplin, costing around £5.00 as I remember.

fine filing tools

For each of the ‘teeth’ shown below I held the drill bit or file at a slant outwards from the centre of the ring and also slanting forwards around the ring, as saw teeth normally are. This may look painstaking, but it wasn’t really .. it look a while, but the plastic sanded comfortably. I made the teeth on the inside of the pipe so that the hole cut might be smoother.

finished 'teeth' on pipe

These tools worked well on the three types of rigid foam I work with .. blue styrofoam (extruded polystyrene); Kapa-line foamboard (polyurethane foam), and ‘Recticel’ another polyurethane insulation foam sold in thick sheets i.e. from Wickes.  Below, the pipe is first positioned on the foam and then turned very carefully until it has some purchase. After that it needs only moderate pressure while turning, making sure to keep the path of the tube straight .. the angled teeth will draw the tube in. Recticel is very soft and fibrous, but the tool still managed to make quite a smooth-sided hole.

turning foam 'hole cutter'

cutting holes in Recticel PU foam

I made use of the technique for the sculptural pieces I’m working on at the moment .. i.e. this prototype base shape for casting, which is meant to take a number of peg forms.

base unit shaped from styrofoam


7 thoughts on “Cutting smooth holes in foam

  1. Genius! I’m just a self taught ‘crafter’ wannabe Grand Designer and you are truly a wonderful inspiration that gives me sleepless nights thinking how to incorporate your techniques into my tiny world! Thank you for being so generous in what you share. I think I might need to move to London so I can attend your classes!

  2. Hi David. Love your postings. Look forward to checking out your work when I check my emails. I am a member of Nativity Builders and you can checkout our work on Facebook. If I remember correctly it was where I found your site. Thanks for sharing your work.

    Sent from my iPad


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