How Density Has Such a Big Impact on Foam Conversion
by Duncan Geddes
Flexible polyurethane open-cell foam is manufactured in a wide range of densities. This is due to the huge number of different applications that foam is used within, ranging from washing up sponges and bath sponges, up to heavy-duty packaging and mattresses.
Within Europe, the lowest density is 10kg/m3, which in reality is so light that one cubic metre can be picked up by one person. The highest density of regular foam production is 110kg/m3, so eleven times heavier! At this weight, heavy lifting equipment is required to pick up and move the foam block, such as specialist fork-lift trucks with ‘claws’ which grip both sides of the foam block.
Low to mid-density foams
The very light-weight foams, with the densities between 10 to 20kg/m3, feel very soft and flexible. The foam manufacturers produce blocks that can be two metres by two metres, with a one metre height, so a four cubic metre block of 10kg/m3 foam, will weigh only 40kgs.
Therefore, one person can move this around on a foam band-knife, which makes it sound easy to cut, but this is where the misconception of foam cutting starts. Think of the expression ‘it’s like a knife cutting through butter’, which can apply to a blade going through low density foam. Then take into consideration another expression ‘it’s like cutting fresh air’, which is just as applicable.
When the cutting blade enters the block, due to the lightness of the foam, often the blade can deflect the foam, or if it is a serrated blade the foam can get dragged down into the scrap area.
Mid to high-density foams
Now let’s compare it to mid-range density foams, those around 30 to 40kg/m3. At this level, the foams are still compressible by hand, but not entirely soft. This means the cutting blade moves smoothly through the foam, at a reasonable speed.
Higher density foam, so 50 to 110kg/m3, is an entirely different proposition. The increased density tends to correlate with higher hardness, and we are now heading towards the similarity of cutting a soft wood.
The attraction at this stage is the ability to control cutting dimensions, because the denser foams are less likely to move around when cutting. However, as any skilled joiner will tell you, cutting and milling wood has to be done at a calm pace, and this concept is exactly the same for dense foam.
The foam manufacturers tend to produce at least 60 cubic metres of each grade and colour at a time, which after the foaming process is then cut into smaller blocks of around two cubic metres. For the foam convertor, the first step is to cut the smaller block by blade, either vertically, horizontally, or CNC to profile.
What is then the next step? For two dimensional items, such as sheets, pads etc, that’s the end of the conversion process, but of course there are many other products manufactured from foam which require greater detail. For example, bath sponges, paint rollers, automotive polishing pads etc.
These involve grinding a foam blank at high speed, to a curved profile, which gives a rounded edge. With the same previous logic in mind, higher density foams can be ground at a faster pace than low density foams, because of the extra weight of the material in the higher density foams. Ironic really, but the hardest type of foam to grind is actually the softest grade of foam.
Within a foam converting company, the skills needed to cut foam are often learnt over a period of many years. Similar to the woodworking or sheet metal industry, often it is the long-standing production employees who bring the most value to production processes, because they have the precise knowledge of how to cut each grade, at what speed, taking into consideration the foam thickness, along with the surface finish and dimensional tolerances of the finished part. Cutting foam is a skilled job, experience is necessary.