Shrinking Plastic: How Much Does It Reduce?

Plastic material shrinkage is a critical aspect of the plastic manufacturing process. The rate of shrinkage varies depending on the type of plastic and the conditions in which it is produced. For example, shrink plastic, when baked in the oven, shrinks to about 40% of its original size, while polystyrene sheets shrink to about a third of their original size. The shrinkage of plastic occurs due to the cooling and solidification processes, with most shrinkage happening during the cooling process.

The rate of shrinkage depends on the type of plastic and production conditions

The rate at which plastic shrinks depends on several factors, including the type of plastic, the production conditions, and the cooling process.

Different types of plastics have distinct molecular structures, which can cause varying shrinkage rates. For example, polystyrene, a common polymer, shrinks dramatically when heated, but its mass stays the same. This is due to the polystyrene chains returning to their most stable, random configuration. Other factors that influence the shrinkage rate include the heat applied during moulding, the design of the mould, and the environment in which the moulding takes place.

The production conditions under which plastic is manufactured can also impact the rate of shrinkage. For instance, the amount of pressure applied when injecting plastic into a mould can affect the material's distribution and subsequent shrinkage. Additionally, the cycle times, temperatures, and wall thickness of the mould can influence the shrinkage rate.

The cooling process is another critical factor in determining the rate of shrinkage. Most shrinkage occurs during the initial cooling phase inside the mould, but some shrinkage continues after ejection as the part continues to cool. Rapid cooling can cause more shrinkage than slower, controlled cooling. The part may continue to shrink slightly for several hours or even days until the temperature and moisture content stabilise.

Overall, understanding the rate of shrinkage is crucial in the plastic industry, as it directly impacts the final size, shape, and quality of the product. Manufacturers can minimise waste and save resources by predicting and managing shrinkage, ensuring that the end product meets the specified dimensions and functions as intended.

Plastic moulding shrinkage occurs during and after cooling

Plastic moulding shrinkage is an important consideration in the manufacturing process. It occurs during and after cooling, and the rate of shrinkage can be influenced by several factors. Firstly, the type of plastic used plays a significant role. Different plastics have distinct shrinkage rates, and these rates are provided by the material manufacturer. For example, high-density polyethylene exhibits higher shrinkage due to its crystalline structure, which causes polymer chains to contract during cooling.

The temperature of the plastic resin during heating and cooling also impacts shrinkage. As a general rule, higher temperatures result in greater expansion and subsequent shrinkage, while lower temperatures lead to reduced expansion and shrinkage. This relationship between temperature and shrinkage rate is approximately linear, with a 10% change in temperature corresponding to a 10% change in the shrinkage rate.

Injection pressure is another critical factor in controlling shrinkage rates. Higher injection pressure results in tighter packing of plastic molecules, reducing their movement during cooling and leading to lower shrinkage. Maintaining pressure until the plastic solidifies is crucial to managing this process effectively.

The use of moulds can also influence shrinkage. Cold moulds allow the outer edges of the plastic to dry first, while hot moulds create less shrinkage by permitting freer molecular movement. Additionally, mould dimensions and processing details, such as the time and pressure at which the plastic is stressed, can introduce variations in shrinkage rates.

Optimising the cooling process is essential to minimising shrinkage. Uniform cooling throughout the mould cavity can be achieved by incorporating cooling channels or plates in the mould design, thus preventing hot spots that cause uneven shrinkage. Post-mold cooling techniques, such as submerging the plastic in cold water, can also be employed to rapidly stabilise the material and halt the shrinkage process.

The polystyrene chains return to their remembered stable configuration when heated

When polystyrene is heated, its chains return to their "remembered" stable configuration. Polystyrene is a thermoplastic polymer, which means it is solid at room temperature but flows when heated above about 100 °C, its glass transition temperature. This property allows it to be easily moulded into various shapes.

The process of heating polystyrene sheets causes them to contract and shrink dramatically, reducing to about 40% to a third of their original size while retaining their mass. This occurs because the polymer chains are no longer locked in a high-strain orientation and relax into a low-energy orientation, resulting in a curled and bent shape that shrinks the material.

The behaviour of polystyrene when heated is unusual compared to most other materials. Typically, objects soften, melt, or decompose when exposed to high temperatures. However, polystyrene exhibits a unique shrinking behaviour while its mass remains unchanged. This decrease in area is offset by an increase in thickness, resulting in a more compact and rigid structure.

The ability of polystyrene to return to its stable configuration upon heating is due to the nature of its polymer chains. Polystyrene is composed of long-chain hydrocarbons with alternating carbon centres attached to phenyl groups. The chains can slide past each other and take on different conformations due to intermolecular weakness, allowing the material to be deformed and moulded.

The specific temperature and duration of heating polystyrene in an oven may vary depending on the desired outcome. For example, instructions for Shrinky Dinks®, an art/craft toy made of polystyrene, recommend heating at 325°F (163°C) for 1 to 3 minutes. It is important to note that the size of the hole in the plastic, if required, should be made larger than needed as it will shrink along with the rest of the material.

The thickness of the finished design will be around 1.25mm

To achieve this thickness, you can bake shrink plastic at 130 degrees Celsius in a regular oven for about 3 to 4 minutes. It is important not to use the fan setting, as this may cause the lightweight plastic to move during baking. Instead, bake your designs on a baking sheet lined with parchment paper.

At first, the plastic will curl up, but after a few minutes, it will smooth out and flatten. Once the plastic becomes flat, it is ready. You can test this by lightly pressing down on the plastic to ensure it is completely flattened.

The amount of shrinkage can vary depending on the type of plastic and the manufacturer. Therefore, it is important to keep a close eye on the plastic while it is in the oven to ensure it does not shrink too much. The plastic should be removed from the oven once it has flattened out.

Additionally, it is worth noting that the hole punch, if required, should be made before baking. The hole will shrink along with the plastic, so it needs to be made large enough to accommodate the desired application after shrinkage.

You can make your own Shrinky Dinks from recycled plastic

Shrinky Dinks are an art/craft toy invented in 1973 by Betty Morris of Wisconsin. They are made of thin, flexible sheets of polystyrene, a common polymer. When heated in the oven, the plastic shrinks to about a third of its original size, and becomes nine times thicker and more rigid, while retaining the coloured design.

To make your own Shrinky Dinks, first, cut the plastic into your desired shapes. You can also punch a hole in your design if you plan to make a charm. Next, decorate the plastic sheets as desired. If you are using recycled #6 plastic, you will likely only find smooth surfaces, so you will need permanent markers for this project. You can also use coloured pencils on the frosted (pre-sanded) product. Once you are done decorating, preheat your oven to 350 degrees Fahrenheit (approximately 176 degrees Celsius) and place your plastic pieces on a sheet pan with a piece of parchment paper underneath them to prevent sticking. Place your baking sheet on the middle rack of your oven and watch your DIY Shrinky Dinks closely. After a minute or so, they will suddenly curl up tight and then they will gradually uncurl. In another minute or so, they are done.

It is important to note that the hole will shrink, so make it large enough for your application. Also, remember that Shrinky Dinks do not work with microwave ovens. You can use a toaster oven or a conventional oven and heat at 325°F (163°C) for 1 to 3 minutes. Once the pieces lay flat, allow an additional 30 seconds of baking time to complete the process.

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