A Curious Experiment

Today was Mr Thrifty’s birthday and in our typical style we indulged in what some may call “odd behaviour”. There was no fancy dinner out, we didn’t go and see a film- instead we bought a bucket and played with expanding foam.  We must have been the type of children that were cheap to entertain because we were both excited about this experiment for at least a couple of hours. 

You’re probably curious about why we wanted to play with expanding foam and a bucket. The short explanation is because I want to make another “custom” dressmakers model and we didn’t believe the manufactures claims about the expanding foam. Mr Thrifty helped me to make a bust form about 18 months ago and while it turned out very well it wasn’t a full dress form, it stopped just under my bust. When we made the first one we blindly chose a can of expanding foam (I think it was the cheapest) and stuck to it for the whole project. By the end of it we’d used five cans… just for my boobs!

For the next doppelgänger I’ve done far more research into expanding foam. For a long time I’ve contemplated two-part polyurethane foam (aka AB foam) but it’s not exactly cheap, nor is it readily available at the local hardware store. Yesterday, Mr Thrifty and I toddled off to our local Bunnings and took some photos of all the expanding foam they have on their shelves. When we came home I looked them all up online. It seems that not all foams are created equal! Most of them have a 2:1 or 3:1 expansion ratio- meaning one 500ml can expands to somewhere between 1-1.5L, some claim a 10:1 expansion ratio meaning one 500ml can expands to about 5L. None of them seemed to come anywhere near the two-part pouring foam expansion ratio of 25:1. 

BUT!!! Joy, oh Joy! One of the 500ml cans claims a whopping 52:1 expansion ratio which means one 500ml can expands to roughly 26 litres! *insert excited happy dance here*. Here’s a link to a PDF about the stuff we bought- Sika Boom AP Expanding Foam.

We were cautious about it though. After the first form we knew that sometimes it doesn’t work like you’d hope. It was time to experiment *mwha-ah-ah-ahh* (that’s my best evil laugh). We bought the expanding foam and a 26L bucket and came home. We had an ok day for it- high humidity and hovering in the high 20’s (celcius, that is). The ideal temperature is 25c, so it was close enough. The date stamp on the bottom said it was fine until November 2012  so there was no fear about the shelf life affecting the outcome.

We worked in layers, spiralling the foam from the outside in on the first layer. The PDF I linked to earlier said that it would be cuttable in 30 minutes so we assumed that meant it would have been completely expanded by that time but not fully cured/hard. We managed to get two full spiral layers out of the can but the third and final layer only traced the outer edge. Under todays conditions I can see that the 52:1 ratio is a bit too hopeful but it’s definitely close to a 40:1 ratio which is even more impressive than the two-part PU foam specifications of 25:1.

How does it compare for price to the two-part foam? One 500ml can of the Sika Boom AP expanding foam cost $11 and expanded to 20L. I would need to spend $32 on the two-part polyurethane foam to get the same end volume. In summary- this was a very worthwhile experiment that will keep the cost of the next custom dress form down.

The lump of foam we made today won’t go to waste. I plan on chopping it up and burying it in the proper dress form when we get to that stage in a week or two. But here is a parting pic of the foam lump to commemorate the afternoon spent bonding with Mr Thrifty on his birthday. It’s not a normal birthday cake but I don’t think he’ll forget it. 😉

Miss Thrifty


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