Designing with polyurethane memory foam (a.k.a viscoelastic foam, a.k.a tempurpedic foam) is different. It’s not as conventional or dimensionally stable as plastics or metals, but when it comes to comfort, it can’t be beat. So how do you do it? How much does it cost?
There are three practical options for manufacturing memory foam. You can Reaction Injection Mold (RIM) the material, cut it with a giant CNC bandsaw, or die-cut it.
Reaction Injection Molding (RIM)
If the shape has surfaces that curve in 3 dimensions, RIM is your only option.
When using RIM to mold memory foam, two chemicals used to make the foam (isocyanate and polyol) are heated, mixed together and shot into a CNC cut aluminum mold.
Once in the mold the reaction of the hot mixture causes it to expand until it reaches the wall of the mold. When the hot mixture touches the relatively cold wall of the mold a skin forms that is just like the skin on the old Nerf foam footballs.
RIM is a low-pressure process, so transitions from thick sections to thin sections must be VERY gradual.
Cost
- The aluminum molds used for RIM will run you from ~$3k for something the size of a cereal box to ~$7k for something the size of a bed pillow.
- The biggest cost driver for molded memory foam parts is the how long it takes. From prepping the mold to ejecting the part can take up to 15 minutes.
Typical applications
- Bed pillows
Giant CNC bandsaw
If you’re design is looking like an extrusion, you can get away with a CNC band saw (sometimes called “vertical contour saw”.
The way it works is that a block (up to a 5′ cube) of foam is placed on the platen, and a CNC computer cuts the necessary profile. Once that is done, the 5′ long “extrusion” is cut to length and your part is finished.
Check out a CNC saw in action (the process is shown from 0:40 to about 1:30, the rest is a sales pitch on the features of the machine)
Cost
The cost of a CNC saw is ~20% that of RIM, mostly because There is no tooling, just setup and machine time.
Typical applications
- Head pillows
Die cutting
If you’re design looks as though it could be made with a cookie cutter, it can be die cut.
To do this they take sheets of foam (that have been cut with a 2D saw), bring them under the tool, and stamp out the desired shape.
The only curve ball here is that cutting a piece of foam thicker than 0.5″, will get you little bit of a radius on the top edge.
Cost
- TThis is also the cheapest way to form foamas die cutting tools seem to run ~1/10th the cost of a RIM tool
Typical applications
- Padding for motorcycle helmets and wheelchair seats.
That’s it. Now you know more about designing memory foam than everyone who doesn’t work in the factory.
Related posts:
- Reaction Injection Molding (RIM) design guide
- injection molding plastic part design guide (GE)
- urethane casting guide: 5 things you need to know
Article by chris loughnane
Chris is Design Engineer + Sustainability Advocate at a Boston area design consultancy and has written 127 sweet posts for the Product Development Notebook.
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