Following on from the 2013 Blog discussing different materials and their applications in CNC machining, this edition goes into a bit more detail on foam. One of the things we’re often asked is what type of foam would be the most appropriate to use, to which the answer always tends to become quite involved with discussions surrounding the good – and not so good features of each material.Summarised below are the four main types of foam that are available, what their strengths are and where they are not quite as effective.
At the moment we’re using polystyrene for most of our ‘foam’ work.
Polystyrene is an excellent material that is easy to cut (machine), offers good strength to weight and is cost effective. Typically, 24kg/m3 polystyrene is priced around $300/m3 (for example) with lower densities costing less while more dense material will cost more.
Polystyrene's strength is determined by how dense it is (in other words, how many ‘beads’ of foam there are per m3). It’s commonly available in densities ranging from 12kg/m3 to 24kg/m3, but it is possible to get more or less dense polystyrene. Typically, we’re using 24kg/m3 foam for most polystyrene work as it is an excellent balance between strength, quality of finish and weight.
As density decreases, the material gets lighter (and vice versa), but its properties become more difficult to work with (i.e. its strength, and how well the foam will make and hold definitive shapes, like an edge or groove). Essentially lightweight foam becomes more fragile… but it will reduce the weight of the object if that's what you're after. For example, in the late 1990's, as part of an ongoing (and never-ending!) effort to reduce the weight of windsurf boards, Styrotech built some using 8kg/m3 polystyrene foam. Other than the foam being difficult to work with, the board itself was indeed really light, and it performed well when it was new. However, it was fragile and, as time went on, the board started to demonstrate that it was built using 8kg foam. An obvious solution was to use a stronger laminate (to protect the foam), however this pretty much defeated the purpose of using lightweight foam as the stronger laminate (featuring more carbon) would increase weight and cost more than just using 12kg/m3 foam in the first place. The reality was, we were only really delaying the inevitable - but it was a very good board!
24kg/m3 polystyrene is ideal for making the structure of a mould, for example a yachts mast or superstructure, a canopy mould for a ute, or basic shape features of a campervan. For applications where the finished weight is important (e.g. windsurf or surfboards), a lighter density can be used (typically 12kg/m3) which gives the overall shape and structure, whilst maintaining as light a weight as possible. As in the example above, 8kg/m3 is probably too light, but 12kg/m3 is about right.
The consistency and quality of polystyrene can be enhanced by kiln drying. This is a process whereby the foam is dried out in a large oven; the purpose of which is to remove virtually all moisture from the block before it’s used. By removing the moisture, the block is far less susceptible to environmental changes (for example increases or reductions in humidity) that will make any moisture contained in the block expand or contract. What this means is that, once an object is CNC machined using kiln dried material, changes in weather (humidity) will not change the dimensions of the material, meaning the object cut from the polystyrene will remain true to the original design. Styrotech CNC exclusively uses kiln dried polystyrene foam for this reason.
Urethane foam is denser than polystyrene (it ranges from 40kg/m3 through to 100kg/m3, while the densest polystyrene is 30kg/m3), and once machined gives a high quality of finish. It’s fine (dense), holds a shape and detail well and is relatively easy to work with.
Not quite as good news when it comes to urethane is that, without taking the appropriate precautions and safety measures, it’s quite toxic. In particular the dust is harmful to breathe and is a skin irritant – it’s going to bring you out in a rash if you don’t wear protective clothing. On top of this, you can't hot wire urethane foam as it gives off poisonous chemicals when being cut, however it is resistant to fuel and solvents.
Urethane is an expensive material – 100kg/m3 Urethane foam is approximately $2000/m3 – that’s about seven times the cost of polystyrene!
But, urethane foam can be applied and used for tasks that polystyrene is unable to be used for. For example, urethane foam will hold more small detail and machines to a high standard, easily. And, given that the material is so dense, it will require less finishing work than a polystyrene object, meaning that, although it is more expensive there are savings in terms of the amount of work needed to finish the object.Urethane foams were until quite recently the material of choice for the surfboard industry (that is until 2005, when Clark Foam, the largest supplier of surfboard blanks in the world, closed down), however, due to its environmental footprint, is now typically replaced with polystyrene.
Closed-cell PVC foamboard is a light-weight material used primarily in the manufacture of signs and displays. It is considered robust for outdoor use, being immune to rain and resistant to wind and sunlight. This material is used extensively to make centre board, rudder and wing cores due to its stability (its ability to maintain shape), lightweight and the ease that it can be used for this application.
However, it cannot be cut using any sort of heat based process as this will result in the material giving off toxic chemicals.
EVA foam has numerous applications including deck pads on surfboards, handle grips, flotation devices, bumper guards (i.e. to stop abrasion/impact with other materials or your head) and sports equipment. EVA foam is also used extensively to make packaging for goods (left).
EVA foam is available in a wide range of densities ranging from approximately 30kg/m3 through to 400kg/m3. Similar to polystyrene, the higher the density, the heavier it is, but the final surface finish will be better.