sandwich or two-component moulding, is the technique of injecting
two materials, one solid and one with chemical foaming agent, into
a mould in a programmed manner through the same injection nozzle
in order to produce a thick moulding with a solid outer skin of
regular plastic and an inner core of structural foam. This technique
substantially overcome warping, cracking, shrinkage and much of
the built-in-stress found in conventionally moulded thick components.
Coinjection or bi-component mouldin represents the best system of
producing thermoplastic items having a section of 5 mm. or more
(0,2 inch or more), but requiring a smooth glossy surface.
Considering that first quality material is needed only on the outside,
and that second quality material can serve as a filler, there can
be important savings. Furthermore, in making coloured items, colour
concentrate is needed only for the outer component thus reducing
also colouring costs. Coinjection is performed with the "Presma"
patented injection head by engulfing first quality material with
less costly material.