194
7
7.1.7 Thermoforming
5 Processing Technology—Thermoforming in composites processing is a secondary process where a thermoplastic composite
sheet initially is fabricated using an extrusion, matched die
moulding, or compression moulding process and then is
thermoformed on a mould with or without using the vacuum.
The main constraint in the thermoforming of thermoplastic
composites is that only short fibre composites can be thermoformed.
5 Processability Parameters—The processability in thermoforming
is only for short fibre-reinforced semicrystalline or amorphous
thermoplastic sheets because they have a softening temperature—not a sharp melting point as in the case of crystalline
polymers. The processability of fabric-reinforced sheets is not
feasible due to the restriction in stretchability caused by the
fabric.
7.1.8 Open Casting Process
5 Processing Technology—In this casting process, reinforcement
and resin are mixed and cast in a open mould. The disadavantage of the open casting process is that the volume fraction of
reinforcement is quite low and a true composite is never
achieved. The viscosity plays an important role in open casting,
and this process is most suited for thermosetting resin systems,
such as epoxy or polyester.
5 Processability Parameters—All of the processability parameters
discussed in spray layup technology are operable here. The
open casting process also supports particulate reinforcement.
7.1.9 Prepreg Processing
The term prepreg is actually a short form of pre-impregnation. A
prepreg is a reinforcement pre-impregnated with resin. Prepregging
is the process in which the reinforcement is impregnated with the
resin and a curing agent mixture. The curing agent is mixed with a
retarder that, at a low or ambient temperature, does not cause crosslinking of the matrix. The cross-linking of the matrix in presence of
the retarder only starts at a high temperature. Dicyanamide (DICY)
is commonly used as a retardant to the chemical cross-linking of
epoxy resin in preparation of prepregs. These pre-impregnated
reinforcements (also known as prepregs) are made using reinforcements in the form of woven fabrics, roving, and unidirectional tape.
Pre-impregnated short fibres in the form of a sheet are called sheet
moulding compounds (SMC), and in the form of dough, they are
called dough moulding compounds (DMC). SMC and DMC are
also classified as prepregs. In prepregs, fabrics and tapes are
provided as continuous rolls in widths up to 2 m and lengths up to
several hundred metres. The thickness of the fabric or tape
Chapter 7 · Processability in Open Mould Processing of Polymeric Composites
7
7.1.7 Thermoforming
5 Processing Technology—Thermoforming in composites processing is a secondary process where a thermoplastic composite
sheet initially is fabricated using an extrusion, matched die
moulding, or compression moulding process and then is
thermoformed on a mould with or without using the vacuum.
The main constraint in the thermoforming of thermoplastic
composites is that only short fibre composites can be thermoformed.
5 Processability Parameters—The processability in thermoforming
is only for short fibre-reinforced semicrystalline or amorphous
thermoplastic sheets because they have a softening temperature—not a sharp melting point as in the case of crystalline
polymers. The processability of fabric-reinforced sheets is not
feasible due to the restriction in stretchability caused by the
fabric.
7.1.8 Open Casting Process
5 Processing Technology—In this casting process, reinforcement
and resin are mixed and cast in a open mould. The disadavantage of the open casting process is that the volume fraction of
reinforcement is quite low and a true composite is never
achieved. The viscosity plays an important role in open casting,
and this process is most suited for thermosetting resin systems,
such as epoxy or polyester.
5 Processability Parameters—All of the processability parameters
discussed in spray layup technology are operable here. The
open casting process also supports particulate reinforcement.
7.1.9 Prepreg Processing
The term prepreg is actually a short form of pre-impregnation. A
prepreg is a reinforcement pre-impregnated with resin. Prepregging
is the process in which the reinforcement is impregnated with the
resin and a curing agent mixture. The curing agent is mixed with a
retarder that, at a low or ambient temperature, does not cause crosslinking of the matrix. The cross-linking of the matrix in presence of
the retarder only starts at a high temperature. Dicyanamide (DICY)
is commonly used as a retardant to the chemical cross-linking of
epoxy resin in preparation of prepregs. These pre-impregnated
reinforcements (also known as prepregs) are made using reinforcements in the form of woven fabrics, roving, and unidirectional tape.
Pre-impregnated short fibres in the form of a sheet are called sheet
moulding compounds (SMC), and in the form of dough, they are
called dough moulding compounds (DMC). SMC and DMC are
also classified as prepregs. In prepregs, fabrics and tapes are
provided as continuous rolls in widths up to 2 m and lengths up to
several hundred metres. The thickness of the fabric or tape
Chapter 7 · Processability in Open Mould Processing of Polymeric Composites