Many molded thermoplastic products are not actually based on a single plastic material, but rather a compound substance with fiber additives that improve the product’s strength and durability. Manufacturing a plastic preform is the process of forming sliced fiber threads, usually made of glass, into mats that will serve as reinforcements for a plastic molding procedure. While conventional mats may wrinkle or provide uneven glass distribution due to their flat shape, preforms remain reliable when fabricating complex shapes or three-dimensional designs.
Most preforms are composed of glass fibers because they are beneficial for deep moldings or products with intricate design characteristics. Continuous fiber patterns also impart directional properties, adding to the material usefulness of glass fiber reinforcement. There are a number of different methods used for manufacturing plastic preforms. Some of the most common techniques are the directed fiber process, the plenum chamber process, and the use of a water slurry system.
The Directed Fiber Process
The directed fiber method involves blowing glass fiber strands through a preform screen that collects the material into its desired form. First, glass stock is directed toward a blower, which uses air flow to move it through a chopping mechanism that reduces the glass into small fibers. These fibers are then blown through a flexible hose into a perforated preform screen set against a rotating turntable that moves the screen in either a vertical or horizontal direction. The directed fiber process can be a challenging fabrication method, as it requires an operator to handle the hose with relative precision. Automated computer controls are sometimes used to increase the accuracy at this stage.
The Plenum Chamber Process
A plenum chamber system consists of a continuous stream of glass stock fed into a chopper that cuts the glass to specified lengths and allows the pieces to drop directly into a chamber. A preform screen lies at the bottom of the chamber, with a fan blowing exhaust air underneath it. A plastic binding agent is sprayed on the fibers, and as the glass drops into the chamber, it is directed by an airflow pattern that controls the arrangement of how it falls. The preform screen in the chamber usually rotates and can often be tilted to help provide a uniform distribution of glass fibers along its surface.
The Water Slurry Process
In the water slurry process, a slurry of chopped glass fibers is fed into a water tank. A fan or other rotating mechanism agitates the water to keep the fibers moving, while the preform screen is suspended over a water exhaust pump and telescoping pipe at the bottom of the tank. The preform screen is gradually raised over the course of the process, collecting more glass fibers as it moves upward. Water slurry preforms tend to be intricately shaped and durable, as the glass fibers are usually attached together with cellulose fibers or bonding resins. In addition, the fibers can be dyed during this process.
Preform Screen Design
The configuration of the preform screen is one of the most important criteria in preform manufacturing. Although screen shapes usually vary according to the application’s design, cylindrical preform screens tend to be less expensive and easier to create than rectangular models. If the cylindrical screen rotates at a uniform linear pace, the glass fibers will pass through the screen at a uniform rate as well, while a rectangular screen may have more difficulty maintaining uniform movement due to the airflow patterns at each corner. Having slight curves or contours at a rectangular screen’s corners can help alleviate this problem.
The perforation patterns on the surface of the preform screen can also influence the process. A typical screen has holes measuring approximately one-eighth of an inch with a three-sixteenths inch center, and in most cases, the outside edge of the screen complements the mating part of the mold. However, some applications may need more open space on the screen, requiring a different perforation pattern or outer contour. A screen that is too large may cause the preform to wirnkle or the fibers to overlap, while a screen that is too small can cause the preform to tear or crack during molding.
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