Polyether-based Polyurethane (PU) is the cheapest of its type, has low density, good elastic recovery, heat retention and it can be cut into any thickness.
These features make polyurethane foam cups widely used in the molded-cup production. (PU) foam is soft and flexible but is easily glazed and discoloured by the high temperature, time and pressure variables used in the molding processes. These production problems are now well documented but trial and error methods are common with less experienced firms.
Recent research at Xi’an Polytechnic University has shown that polyurethane foam with density of 35kg/m3, a thickness of 1cm, at a molding temperature of 170dC, and a dwell time between 130s and 140s, gives the best cup properties. (This is rather thick, 10mm, so be aware)
In recent years, knitted ‘Spacer fabrics’, designed to replace PU foam was developed. It has great advantages of surface decoration but it has a limited range of thickness, the internal spacer yarn is easily damaged, affecting the shape of the molded cup, and its cost is high.
The Molding Process
Moulding is a process which transfers a piece of fabric or sheet foam into a three dimensional shape of lasting quality. This process involves an exact combination of heat, pressure and dwell time to suit each individual fabric and mould form.
Although molded bras are thought of as a modern creation they were available in the UK retail market as early as 1947. They were made with foam cups and were produced by ‘cold molding’, the foam sheet was heated and formed using cold wood molds. Currently molded bras represent the largest selling type of wired bra in the world.
The molding process involves an exact combination of heat, pressure and time to suit each individual fabric and mold form.
There are two molding operations,the first clamps the fabric to the bed, the second compresses the fabric between the male and female mould.
The cut fabric is often laid in 2 plies, face up and down to produce a pair of cups.
Some fabrics require more clamping pressure to prevent the total cut piece from sliding into the mould.