Have you ever wanted to draw really small pictures? Here’s a trick – you can just draw normal sized ones, and then use some special plastic to shrink them!
Safety: This activity uses an oven. Ask an adult to supervise. Use oven mitts and avoid touching hot surfaces.
Recycling number 6 is the symbol for polystyrene. It’s used for some disposable cups and some yoghurt containers. Don’t use foam, such as cups for hot drinks. Instead select a rigid plastic carrying the number 6.
It’s very important that you get the right plastic for this activity – other plastics won’t work at all, and may melt or emit unpleasant or toxic fumes.
Polystyrene is a very useful and common plastic. One reason it’s so popular is because it’s easy to mould into different shapes.
Polystyrene melts into a liquid at about 240° C, but it’s easier to work with at about 90° C. This is called the glass transition temperature, and it’s where the plastic becomes softer and more rubbery. Below this temperature the plastic is hard and relatively brittle, so it’s in a glassy (or glass-like) state.
To make a plastic cup, polystyrene starts as a flat sheet. It’s heated until it’s soft and then stretched into a cup shape. When it’s cooled, it becomes rigid again and won’t change shape until heated back up.
In this activity, the polystyrene is heated back to its glass transition temperature. A plastic cup has a lot of stress in it from the stretching process, so when it’s heated it will remove that stress by shrinking back down to the sheet it was originally made from.
You might notice that your pictures don’t shrink evenly in every direction when heated. This is due to the unevenness in manufacturing. When cups are made, the plastic sheets are stretched down a long way, but they are not really stretched wider at all. Maybe you can find interesting ways to use this uneven shrinking in your pictures!
Polystyrene is also used to make foam. Foam is made of the same plastic, but it has a huge amount of tiny air bubbles in it. The foam ends up being about 97% air and only 3% polystyrene!
There are lots of uses for polystyrene foam. Rigid white packing foam is very light, strong and soft enough to absorb shocks. And foam cups are good insulators, keeping hot drinks warmer for longer.
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15 June, 2018 at 2:11 pm
Plastic Sushi packaging is good for this
15 June, 2018 at 2:50 pm
Hi Lisa,
Thanks for the tip! If you can find a sushi container with a recycling 6, definitely go for it! If I’m imagining the right containers, they’re flat and rectangular with a clear lid? they probably shrink down more evenly and don’t get as distorted as cup shrinkies do.
Just be careful that you’re using the right plastic. Some sushi containers are made of PLA, and that melts at a much lower temperature than polystyrene. if you’re not careful, your oven could get very sticky indeed!
19 December, 2021 at 2:08 pm
I can’t find #6 plastic anywhere. It must be going away….
20 December, 2021 at 9:32 am
Hi Terri,
Yeah, it can be a bit tricky to find the right plastic. Other than plastic cups, it sometimes comes as the lids for disposable coffee cups? And as Lisa says, sushi packaging and bento box lids can be #6 too.
As many governments are banning single use plastics, it’s going to get harder and harder to find the right kind of plastic. Which is sad news for shrinky-dink fans, but good news for the environment.