So you have a project coming up and you have got pretty much everything squared away in your mind about how to build the moulds, but there is one sticking point. Round edges or cutouts.
This could be for a bowl sink or for a round BBQ such as an Egg or Kamado Joe. Or it could even be rounded corners so the kids don’t whack their heads on a sharp corner of concrete.
Square corners are easy aren’t they. You can butt two bits of mould material together, even mitre them to make them super neat, but round profiles….?
So you have a few options but there are a few factors which will determine your route to success.
Firstly are then edges “seen” edges or “unseen”.
The tools you have available is another consideration. I’m going to assume if you are building moulds then you should have a decent array of tools at your disposal.
The material you use will also be a consideration in terms of how you prepare it for use. You don’t want a material that sucks moisture out of the concrete as it’s poured into the mould because that will ultimately weaken the concrete. So some materials will need to be primed/sealed prior to use.
The materials we generally use are;
- moisture resistant MDF
- thin plastic sheet
- rigid foam insulation
- type or project - Precast or Cast-in-Place
A “seen” edge you will require a bit more attention than an “unseen” edge so you will probably want to use thin plastic. At anything from 1.5mm-3mm in thickness this will go around most curves. Using a glue gun and melamine blocks will allow you to firmly glue the plastic in place.
A “seen” edge you will require a bit more attention than an “unseen” edge so you will probably want to use thin plastic. At anything from 1.5mm-3mm in thickness this will go around most curves. Using a glue gun and melamine blocks will allow you to firmly glue the plastic in place.
This works well with Precast projects. For Cast-In-Place projects the plastic will need to be set lower in the mould for “seen” edges to hide the substrate beneath.
Another option would be moisture resistant MDF, but this will suck moisture out of the mix if left untreated so it will need a 3 or 4 coats of varnish first. The benefit of MDF is that its easy to sand and can be finished quite nicely.
If you do go down this route, it’s worth making a ring as opposed to a full circle so it can be cut out of the mould more easily when the time comes.
Finally rigid foam is a useful get out of jail option! Easy to cut with the right tools and very easy to sand. Be warned though, it is highly absorbent so will require considerable sealing to stop moisture loss from the concrete. It can be a bit of an unknown in terms of finish unless you are very fastidious with your prep!
That said, we have used it many times with great success but any of those cut lines you can see in the photo above will show up in the concrete so get it perfectly smooth prior to casting up against it.
I hope that demystifies the conundrum of getting a nice curved cut out in your mould.
As ever if you have any questions feel free to contact us for further guidance.