For the current contest maze, UKMARS is using the black-dyed MDF sheet from Kitronic.
For future reference though, here is a brief consideration of some other material choices.
Practically speaking there are few choices of material for the base (prices have been taken from a single supplier and so are only a relative guide, not absolute). Note that layered designs are not absolutely required to use the same material for all layers though there is some risk of differential expansion.
- Plywood. Typically 5.5mm thick and avaialble usually with a very thin hardwood veneer. This is lighter than MDF and more resistant to moisture and impact on fine edges. It is also perhaps less likely to be flat across a whole sheet. Varies a lot by supplier, Relatively cheap hardwood external grade stuff is common. Guide prices around £25/sheet. High quality Marine plywood in 6mm is likely to be £80/sheet and 6mm Birch plywood is about £76/sheet. Unless a known quality, named brand with guaranteed thickness can be found, plywood is probably not a safe choice for the top surface of the maze.
- MDF. Quality varies widely from very poor to very good. Moisture resistant varieties are better but not moisture proof. All MDF, including Medite should be sealed. MDF is about 20-25% more dense than plywood. Typically 6mm thick. MR is about £25/sheet. Because of the variability, generic MDF is not considered as a candidate in these notes.
- Medite. This is a high grade MDF, but much better than the stuff from your local DIY Store. Most importantly, it is a known quality not subject to the vagaries of your local builders merchant. It is probably what you (should) buy when looking for MDF for a laser cutter.If you want MDF, choose Medite. Also avaliable in 3mm and 4mm thicknesses. 6mm thickness about £28/sheet. MR variety £33/sheet. For more details see Medite MDF in the Materials directory
- Valchromat is the most widely available dyed MDF. Its properties are similar to MDF but the fibres are dyed before manufacture producing a consistent colour throughout the sheet. The thinnest sheets of Valchromat are 8mm which makes it good for a top layer but too thick for a three-layer design on all three layers. Surface properties are similar to Medite but it is possible to use it without any further paint of finish. A surface sealer might be wise though. Valchromat if very expensive though and starts at £100 per sheet. For more details about various black MDF types look at Black MDF in the Materials directory.
- Kitronic Dyed MDF is a product that is dyed black throughout, very durable and availale in cut panels at a significantly better price than Valchromat. Cutting tests are needed for suitability with common laser cutters. The surface is very good and has high absorbtion of IR light. For more details about the Kitronic material see Kitronic Black MDF in the Materials directory.
Note that, in general, plywood is more stable in the long term than MDF and unlikely to suffer lasting damage from excess moisture.