Size matters

Consequently, my laboratory press is similar to the majority that you will find around the world.

We have a press with 800 x 800mm platens and we normally make panels with mattresses measuring 600 x 600mm as this gives us the opportunity to cut standard-sized bending test specimens for panels up to 25mm thick.

It is almost impossible to recreate the conditions found inside a commercial mattress during hot pressing using a laboratory press.

You can use the same particles, glue mix, press temperatures and moisture contents and so on, but the particles inside will not experience the same temperature and moisture history found in a commercial press. This is for two main reasons: Laboratory presses rarely close as quickly as commercial presses; and, more importantly in my opinion, the size of the mattress does not allow the generation of the same steam pressures inside the mattress.

Much higher steam pressures are possible in a large mattress compared to a small one because the steam escape path is longer. The steam pressure determines the temperature inside a mattress which, in turn, will determine the temperature and moisture content of the particles or fibres in the mattress.

The rheology of the particles/fibres is very sensitive to temperature and moisture content and so, clearly, the compressibility of the wood, and the resultant panel density profile, will be different in a laboratory press. In addition, the polymerisation of the adhesive is very temperature-dependent and so this factor is also likely to be different in a laboratory press.

I apply a 3mm thick frame to the top face of my mattresses. This causes the borders to be compressed more than the centre and this restricts the escape of steam from the interior.

The resultant higher internal vapour pressure helps me to more closely simulate commercial mattress conditions. Unfortunately, my press closes rather slowly, but without a major investment, there is not much I can do about it.

In a way, the inadequacies of my press do not really matter because all laboratory presses have some inadequacies and so everyone is obliged to start by making a 'standard' panel which mimics the starting point, ie the typical panel of the manufacturer concerned.

Any changes applied to the standard panel - which might be a new adhesive or an additive or an under-utilised lingo-cellulosic material - will then become apparent as a trend; the absolute results, eg a bending strength, might not be realised once applied to a full-size press.

So size does matter, but there are ways around it.