Material:
To
facilitate the mass production of reinforced plastic parts, a correct
combination of temperature, catalyst concentration, and resin formulation must
be used which allows gelation to occur in a time long enough for complete resin
flow and short enough for complete resin flow and short enough to be
commercially feasible.
Equipment:
(the mould)
a.
The
metal moulds may be made of Aluminum, Zinc, or Kirksite for short runs and
steel for long runs.
b.
The
mould should be designed to trap resin and shear the fibers during last 1/16 to
1/8 inch movement. The matching (or shearing) surface should be accurately
machined and hardened. There should be a small (0.002 to 0.004 inch) clearance
between the shearing surfaces.
c.
The
mould is heated usually by hot oil, steam or electrical heaters which are
thermostatically controlled.
d.
Guide
pins are used for location and it is well out of the resin area to prevent
fouling.
e.
Ejector
pins are usually not used in these moulds to prevent fouling. The parts are
removed by compressed air or sometimes air passage is included in the mould to
eject the part. This passage is closed with tape during moulding operation.
f.
Pinch
off are used as a means of cutting fibers. If it were used for location, rapid
tool wear will result.
Process:
a.
The
moulds are coated with mould release compound.
b.
The
male or punch is on the bottom of the mould and the heated preform (or cold
form where only the resin exotherm is used for cure) are placed over the punch.
This tend to minimize dragging or creasing of the preform when the mould
closes.
c.
The
preform and resin loading should be consistent to obtain the optimum in
reproducible parts. The pressure applied may range from 50 to 300 psi or high
up to 3000 psi or higher and the processes are accordingly known as:
i.
Low
pressure moulding
ii.
High
pressure moulding.
Ø Low pressure
moulding: It is used when preforms are combined with the resin at the mould
just prior to moulding or SMC is used.
The moulding pressure distributes the resin
throughout the fibers and trims the preform to the required dimensions.
The press has dual closing rate. (1) Fast: To bring
the two parts of the mould together rapidly and (2) Infinitely variable slow
rate: To effect the final closing.
Manual control are used but consistent quality can
be obtained with presses having automatically controlled closing and
automatically monitored temperature, pressure and time.
Ø High pressure
moulding: It uses pre impregnated SMC material or fabric either chopped to
small pieces or cut to specific configuration. Moulding at pressure up to 3000
psi is very similar to the normal compression moulding.
Advantages of matched die moulding:
a.
Reproducible
parts with relatively high speed.
b.
Good
finish on all surfaces.
c.
Very
little trimming is required.
d.
Process
is independent of the operator’s skill.
e.
Low
cost reinforcement can be used.
Ø Rubber plunger
moulding (variation of matched die moulding):
a.
Preform
is placed in the heated female cavity.
b.
The
rubber male plunger (made of RTV Silicone) is then forced in to the cavity
touching first at bottom, then deforming to put pressure all over the part.
Advantages:
a.
Low
tool cost because only one machined tool is required.
b.
Slight
undercuts can be produced, depending on the part.
Disadvantages:
a.
Has
only one good surface i.e. the side contacting the punch and it is similar to a
pressure bag surface.
Limit exists for the size of
parts that can be produced, because large rubber punches are impractical.
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