Processing of bioresorbable and other polymers for medical applications - Durability and Reliability of Medical Polymers - page 63

sheet is heated in order to reach a temperature that is slightly above the glass
transition temperature for amorphous polymers and slightly below the melting
temperature for semicrystalline polymers. The process is most suitable for
amorphous materials because they have a wide rubber temperature range. at
these temperatures, the polymer can be easily shaped and it conserves enough
rigidity to avoid sheet sagging. The risk with semicrystalline polymers is
that the material rapidly loses its strength when the crystalline structure is
broken. Heating is achieved through radiative heaters. The temperature must
be high enough to allow sheet shaping and the forming temperature profile
must be uniform to obtain an optimal process.
One of the main problems in thermoforming is minimizing thickness
differences throughout the product. This can be achieved with a plug-assisted
system (Fig. 3.13). The plug pushes the sheet into the cavity. Because of this,
only the part of the sheet not touching the plug is subjected to stretching.
This is followed by use of a vacuum for the final shaping. Once cooled, the
product is removed. The sheet can also be prestretched at the beginning of
the process (Osswald and Hernández-Ortiz, 2006).

3.6 Calendering
In the calendering process, a polymer melt is transformed into film and sheet
shapes by squeezing it between co-rotating pairs of rollers. The number of
rollers depends on the material processed and the final product. Rubber can
be calendered with a two-roller system. Other thermoplastic polymers require
a four-roller system to ensure good surface quality. A typical calendering
system is composed of:
∑ a plasticating unit,

￿ ￿ ￿ ￿ ￿ ￿

Heated sheet

Plug-assist

Vacuum hole

Stretching Vacuum
3.13 Plug-assisted system (from Osswald and Hernández-Ortiz, 2006).