Monday, February 26, 2018


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:
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.
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.
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)

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.
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.
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.

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