Many plastics come in flat sheet and one way to bend the sheet to desired shapes is to perform what is called heat forming or thermoforming process. In thermoforming, the plastic sheet is heated until it is softened to allow bending without breaking. Once the shape is formed the plastic is cooled and it will retain its shape without springing back to flat sheet.
So how do you know what temperature to heat up the plastic in order to thermoforming? One way to find out is to ask the plastic manufacturer. The plastic manufacturer typically would provide a recommended working temperature for the said plastic for thermal forming processes. Another way to find out the process temperature is to do a little bit of scientific investigation yourself. The rule of thumb for working temperature range is above its glass transition temperature where it softens significantly but below its melting temperature (if it has one) where it may flow too freely. Once you find out what kind of polymer your plastic sheet is, you can look up its Tg and Tm values and come up with a working temperature for the thermoforming process.
If you are dealing with specialty polymers and you do need hard data, you can perform a simple DMA analysis to help you determine the temperature range and to verify thermoforming process ability. A DMA thermograph will display the material’s storage modulus values with respect to temperature, as illustrated in the graph below.
The DMA thermograph shows that the polymer material softens during the Tg transition. Typically, the modulus drop is between one to three orders of magnitude. For example, a polymer that has a modulus drop of 2-order of magnitude during its Tg transition means its modulus value is reduced from ~ 100 MPa in the glassy state to ~ 1 MPa in the rubbery state.
The rule of thumb for working temperature range I mentioned several paragraphs before means that you should process your polymer while it is in the rubbery state, and at its rubbery plateau.
There are polymers such as high crystallinity thermoplastic polymers or highly crosslinked thermoset polymers, however, where this rule of thumb does not work well. The reason is because the modulus drop from glassy state to the rubbery plateau region is simply not significant enough to bend the plastic. The modulus changes in thermographs with respect to polymer structures are well documented in scientific literature and textbooks. The classic DMA thermograph (shown below as a cartoon drawing) you find in textbooks shows how Mc (molecular weight between crosslinks) of a thermoset polymer or crystallinity of a thermoplastic polymer affect the material modulus. In a thermoset polymer, the higher the crosslinking density (i.e., low Mc) the less modulus drop after Tg transition. In parallel, in a thermoplastic polymer, the higher the crystallinity the less modulus drop after Tg transition.
With little changes in modulus values even by taking up the temperature to rubbery plateau region, we can explain why those high crystallinity polymers or those high thermoset polymers are difficult to form shapes using thermoforming process.
In summary, thermoforming process works well for polymers at temperature range between its Tg and Tm except for polymers that are either highly crosslinked or crystallinity. When data is needed to assess thermoforming ability, a simple DMA analysis can be performed to obtain the needed answer.