v HAND LAY UP
TECHNIQUES: They are classified as:
1.
Contact
moulding
2.
Vacuum
bag moulding
3.
Pressure
bag moulding
4.
Autoclave
moulding
1.
CONTACT
MOULDING: (open mould fabrication)
It
is one of the oldest forming techniques and simplest of the reinforced plastic
processes.
Material
selection:
It
depends on the requirements of the part and the contours to be met with.
However general characteristic of the material are:
Resin:
a.
The
resin must be thin enough so that it is easily impregnated into the
reinforcement.
b.
It
should also be thixotropic enough to allow drainage on vertical sections before
gelation takes place.
c.
It
should be suitable for the contours of the part.
d.
The
catalyst system should allow sufficient time for fabrication of the part.
Catalyst:
a.
Heat
activated catalyst: Here the part can be worked on all day and oven cured at
night.
b.
Do
it yourself: Using a room temperature cure catalyst: Small parts can be made at
atime.
c.
Sun
shine cure; UV catalyst: Here the part can be worked on all day in the shade
and then placed in the Sun for cure.
Equipment:
It
includes following:
a.
Resin
mixers: It includes special stirrer to introduce a minimum of air.
b.
Scale
and graduated cylinder for adding catalyst into the resin.
c.
Containers
for mixing resin preferably expendable card board.
d.
Squeegees
and rollers: Implement with rubber blade or roller for scrapping, squeezing
away the moisture or air. This is used to rub out the resin.
e.
Brushes
to apply resin.
f.
Table
and scissors for cutting fabric and mat.
g.
Viscometer
to check the resin viscosity.
h.
Hot
bath and gel timer.
i.
Barcol
The
last three equipment are highly desirable but are not essential (compulsory)
Process:
a.
Apply
a mould release (parting agent) to the mould.
b.
Gel
coat application: Apply a coat of resin heavily filled with mineral fillers.
This pigmented gel coat insures an optimum surface.
c.
Fiber
glass mat application: After the gel coat becomes tacky, the reinforcement
layer is laid on the gel coat. This layer may be (i) A sheet of surfacing mat
when very fine surface is required. Or (ii) A layer of dry or impregnated
fabric or mat on to a resin layer. It is preferred because it results in least
air entrapment.
d.
Resin
application on mat: Catalyzed resin is either brushed or roller coated.
Impregnation of the resin is assisted by the use of a roller or squeegee, which
promotes wetting and pushes out air bubbles. Complete wetting is allowed and
then more resin is added for the next layer.
e.
Part
removal: The part is then allowed to remain un disturbed until gelation takes
place. After gelation, the assembly may be heated to accelerate the curing
process. The exposed surface may be covered with cellophane or PVA film to
prevent the evaporation of any volatile monomer and to minimize the possibility
of blocking of air for some polyester resins. The finished part has a smooth
inner surface (i.e. surface contacting the mould) due to the gel coat
application.
Limitation:
a.
Only
the surface contacting the mould has a good, smooth surface. Thus the choice of
whether a male or a female mould can be employed depend on the surface which
must be smooth (i.e. inner or outer surface) and not on the fabrication
condition or part contour.
b.
The
process is very slow.
c.
The
quality of the part depend on the skill of the operator.
d.
Physical
properties of the parts is lower than those made by almost all other processes.
Application:
a.
Well
suited for fabrication of very large articles and/or small production runs.
Used for production of
prototypes regardless of the eventual high production process.
No comments:
Post a Comment