Definition: (in RP industry)
A technique used to arrange chopped glass fiber in the exact shape of a part to be moulded by the matched die moulding methods.
The chopped fiber (1.5” long usually) are deposited uniformly over the screen surface having the shape of the final moulded part. Then a small amount of binder resin is applied. Deposition proceeds until sufficient preform thickness is obtained to provide the optimum glass: resin ratio in the moulded part. Then the preform is heated, on the screen, until the binder resin cures, so that the preform is sufficiently sound and can be stripped from the screen and transported to the press for final moulding.
The preform is also applied to the use of fully impregnated SMC material which is precut or cut and stapled to fit a mould prior press curing.
Non SMC prefroms:
There are two methods for producing the non SMC preforms:
a. Directed fiber preforming (or open preforming)
b. Plenum chamber preforming
1. Directed fiber preforming (or open preforming):
Here the cut fibers are sprayed manually on a rotating screen through which air is being drawn to create a pressure drop at the screen interface. This suction holds the fiber in place on the screen while the operator directs the airborne screen of fiber to all areas of the screen. The operator also intermittently sprays the proper amount of an aqueous emulsion having 5% concentration of polyester or acrylic liquid binder. When the fiber build up is complete, the preform is oven cured and removed from the screen. The percentage of binder varies between 2-10% of the finished weight.
Several type of machines are available depending on the size and desired production rate. On average size preforms, a common type is a four station rotary mount for the screens where:
1st screen is being sprayed
2nd screen is in the oven curing position
3rd screen is being stripped
4th screen in in the ready position
Extremely large preform machine have only one screen.
After preforming, a hood lowers over the screen to form an oven, and the air is redirected through a heat exchanger to provide curing temperature.
Preforms with varying wall thickness, buildup areas can be produced because a wide range of machine sizes are available.
The uniformity of product depends almost entirely on the operator’s skill.
2. Plenum chamber preforming:
It is similar to the directed fiber method and only differs in the method of applying the fiber to the screen. It is an adaptation of the old felt-hat forming machine.
Here the rotating screen is positioned in a plenum chamber and air is drawn through the screen. The roving cutter is mounted at the top of the plenum and the cut fiber falls on a rotating spinner which distributes the fiber uniformly in the plenum chamber. The fibers then “snow” down into the screen. The liquid resin binder is usually applied by spray gun, but in some machines, powder binder is sifted on from a vibrator feeder.
The product is uniform after the initial set up. The process can be completely automated, thus reducing the labour cost.
Restriction to uniform wall thickness, difficulty of initial set up on some types of parts and the compounding of the fiber distribution problem as size increases.
2. FABRIC AND MATERIAL TAILORING:When only small number of parts are required, tailored sections can be cut from impregnated raw stock by the use of hand shears or knives. For production of large numbers of parts inexpensive steel rule dies can be used or the material may be piled up in number of layers and cut, from a pattern, using a special textile cutting machine. If very simple shapes are required e.g. discs, then commercially available dies may be procured at normal costs from machine shop.