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.
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.
a. Low tool cost because only one machined tool is required.
b. Slight undercuts can be produced, depending on the part.
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.