Posts Tagged ‘plaster of paris’

flexi mould not so flexi.

September 26, 2009

I tried to make another model out of plaster of paris. I made sure I sealed the bottom of the mould completely this time. I poured the plaster into the mould fine and left it for a 2 days to set. The trouble started when I tried to take the Aluminium mould off.

There were 3 problems-

  1. The plaster mould wasn’t thick enough. The object im casting needs to be done on a bigger scale (at least 1:2) with a wall thickness of at least 60mm. The walls I made were way too thin. At some points the wall thickness was less than 5mm. A slight knock and it crumbled.
  2. The 0.3mm Aluminium wasn’t flexible enough so I had to pull it off with force causing the plaster to crumble.  Aluminium has the strength to hold its self upright, but it’s not flexible enough to fold back on itself like a sheet of paper or fabric. A rubber sheet would be ideal for a mould. See the video below titled “bending concrete”  and you will see what I mean with the flexible mould.
  3. Too much water in the plaster. It was slightly soft to touch after 2 days of drying. Maybe the cold weather slowed down the drying process too.

I’m still confident that this process can work but I think it just needs to be done on a larger scale. Stronger materials and thicker walls.

1 minute after being poured

1 minute after being poured

Some parts came out of the mould alright. They were about 10mm thick and had very smooth surfaces. They are the 2 larger chunks you can see below).

The remaining parts are scattered on the lawn.

blog plaster2

I had a feeling this wouldn’t work on a small scale from the start. But I had to try anyways. And at least I know what to do if I ever try this on a large scale. I will be making my 1:1 scale prototype out of different materials anyways because it needs to be transportable. Concrete would be great for the street sculpture but it won’t be suitable for the graduate exhibition.


Plaster of paris & flexible moulds

September 4, 2009

After seeing pro’s making sand castles and learning about the Pound-up process. I thought it would be good to use the same process to cast my concrete product. I stumbled across a picture of a guy using a vinyl sheet to build sand up vertically. I have never found that image again. And I cant find anything else on the net where they use this vinyl sheet. Although you can’t make any undercuts… I think it’s still the best way to achieve complex curves with concrete. Because it’s a liquid, you’re normally limited to the shape of your mould because of gravity. So why not turn the mould on its side and pour it in from the top. The rebar can still be placed in the centre of the mould. Once the concrete dries you can remove the mould and flip the finished product on its feet.

I had some plaster of paris and some aluminium left over from my previous models. I couldn’t help myself. I had to try it out.

This shape comes from a clay model I made a few weeks ago- the shape was modified by a skater (see skate participant #1 blog post).

Aluminium mould

Aluminium mould- mock up

clay was used to seal the bottom of the mould

clay was used to seal the bottom of the mould

Note to self…Clay will not seal the bottom of the mould! Next time use gladwrap and sitcky tape.

Pouring Plaster of paris in to the mould

Pouring Plaster of paris in to the mould

Here is a clip I found on youtube that uses a flexible mould. Its interesting to see what is possible with a liquid material. Notice the foam-like flexilbe mould being used on a horizontal surface. Non-stick!