Warp is ‘the deviation from flatness as a result of stresses on and within the material itself’.
These stresses are most commonly caused by shrinkage and expansion one or more of the surfaces of the material, causing it to distort. The image below shows some common types of warp in wood:
With MDF the most common problems are those of bow, cup and twist… kink and crook are more typical of wood which has a grain within it (which MDF does not) but that is not to say that MDF cannot cup and twist – it can, it just happens less often.
If we think of a flat sheet of MDF we can see how stress is applied across the surfaces of that sheet by either the contraction of a substance on its surface, or an expansion (swelling) of the material itself (more on one side than the other) via it getting wet. (see image)
The result of the above is the same as indicated by the “Bow” in the next image where the two ends of the board pull upwards relative to the face of the sheet:
The exact result of the stress is not predictable, so we cannot say for certain if you will get a uniform bow, or a twist or cup. That depends on the material itself, the substance added to it, the shape of the section and a host of other details that go together to cause warp.
However there are some things which we can look out for:
The longer the length of a sheet is compared to its thickness, the easier it will be to bow it: imagine a flat MDF sheet that is 30cm long, 10 cm wide and 3mm thick. That sheet is more easily bowed (especially along its 30cm length) than a piece of MDF that is 30cm long, 10cm wide and 2mm thick… i.e. a greater stress is required to warp a thicker sheet and the stress builds-up the greater the surface area it has to work upon.
To put that another way: you require less thickness of paint (or glue or “substance x that contracts or expands as it dries”) to warp a 30cm length of MDF than you do a 20cm length *of the same thickness*.
That may seem counter-intuitive at first, but imagine that the stress that builds-up from the glue/paint contracting, builds-up over the entire surface area of the sheet, so given a greater surface area a ‘greater stress’ can be built-up.
To counter this as we increase the surface area of the wood/MDF we increase the thickness of the wood/MDF.… or, just be aware that the greater the size of any flat sheet, the “easier it is for it to warp” and take more care.
We can predict that in MDF, without a grain, a long thin section of MDF is more likely to bow than cup, however, large flat sheets are quite likely to do both (which generally ends up looking like a twist).
We know that the thicker the glue or paint, the more stress we create, and we also know that the wetter the MDF gets, the more stress it creates, so: a lot of glue, or paint, on a large-area thin sheet of wood/MDF is going to be asking for problems!
With materials that do not absorb moisture we can still induce a warp through the drying-contraction of paints and glues. All of the other factors remain the same; so long thin sections are easier to warp and the thicker our sheet material (and the denser it is) the less likely it is to pull out of shape.
Knowing this basic theory helps us to work out when any part of our scenery or table is most likely, or just more likely, to warp out of shape when we are either gluing it or painting it – it also helps us be aware of what substances we should be careful using; the “stronger” a glue or paint probably the greater stress it can generate so the more careful we should be.
It also makes us consider shape as being important – a long thin and narrow strip of material is far easier to warp out of shape than a short-length, short-wide and thick shape of material will be. We can now determine where on our table or scenery certain materials will be better than others for making certain elements.
We can know use this knowledge to look at how to prepare wood or MDF fro assembly and painting.