It has been almost a year since we were approached by OBZOR Zlín to see if we could produce a concrete frame for a drawer from their Decente range. This range is very popular and is made from a range of other materials such as glass, stainless steel, aluminium, plastic, or even mahogany, oak or walnut wood. We were approached to see if we could do it in concrete. Concrete had been on offer until recently, but the previous manufacturer was no longer able to meet the dimensional and aesthetic criteria for unspecified reasons. Other manufacturers have been approached, but as far as we know no one has succeeded so far.
We soon understood why no one had yet succeeded in casting the frame. A number of pitfalls had to be dealt with, from strict dimensions, to colour to texture. Yet a small change in one area could have an unforeseen impact on another.
For example, if you prepare a more fluid mixture to pour into all the folds, you lose the desired caverns on the surface. If you prepare a thick mix, you may get caverns, but you won't get the concrete into all the nooks and crannies, and what's more, the concrete will ripen to a different color. If you moisten the casting too much, you get a cement texture, if you moisten it too little, the product dries out and loses strength, and so on.
So it was necessary to watch everything carefully, measure, compare, record, record and be very patient. To write down point by point exactly every single step. After almost a year of work, we can now proudly declare that we have succeeded in our task and can produce Decente concrete frames.
In this article we will share with you some of the interesting problems we had to solve.
Dimensions and their stability
The main problem is the dimensions, the concrete frame must not deviate from the technical drawing by more than 0.2mm. This is a huge problem because the concrete needs to be cast and shrinkage occurs as it hydrates and matures. So you need to be able to estimate how much the product will shrink, and in different thicknesses of concrete mass. Then adjust the mould dimensions accordingly. This process and recalculation must then be taken into account whenever you make changes to the concrete recipe, particularly the proportion of water used in the mix.
Colour shade and texture
You also need to have the hydration crust set and adhered to accurately. If the concrete matures too quickly and is not hydrated enough, it will change in size, but also in colour shade. The colour shade is also related to the durability and use of micro-armature. If too many micro-armature fibres are added to the mix, pigment grains will build up and streams of pigment 'floes' will form on the casting, spoiling the overall impression. It is therefore important to set the fibre ratio correctly and to grind the cement mixture thoroughly in small ball mills.
The distribution of bubbles (caverns)
In order to preserve the specific character of the concrete, it is important to maintain the correct distribution of the caverns, i.e. the air bubbles on the face. These must not appear in any of the edges, either functional, i.e. internal, or external, which have an aesthetic function. The whole thing is made even more complicated by the fact that, once the mould is assembled, you have almost no access to the inner edge and very little access to the outer edge. At the same time, you have to avoid touching the spatula with the mould when concreting. Because the contact would be visible on the casting. Casting thus has a precise procedure to follow and is a bit like defusing a bomb.
It's an incredible experience and we wouldn't have thought that a shape that is so simple on the surface could be so difficult to make. We would also like to thank the great communication and patience on the part of Zlin. You have broadened our horizons!
More about the frames from Obzor can be found at this link