1. Field of the Invention
This invention relates to apparatus and methods for pultruding reinforced plastic shapes, and more particularly, to an apparatus and method using a cooling postforming die after a hot preforming die.
2. Description of the Prior Art
In a typical prior art pultrusion process fo making reinforced plastic shaped objects has been accomplished by pulling fibers through a resin bath, through a preheater, and then through a hot shape forming die. The final shape consolidation then took place in the hot die. Such a process is disclosed in the prior art discussion of U.S. Pat. No. 4,462,946 to Goldsworthy.
A similar process, described in more detail herein, adds the step of passing the shaped objects through a cooling die after the objects are formed in the hot die. The cooling die is used merely to cool the objects which are already dimensionally in their final form.
A major problem with this prior pultrusion process is that "ball-ups" tend to occur in the hot die during consolidation into the final form. The term "ball-ups" refers to a problem which occurs when the fiber picks up too much resin. As the mass of resin is pulled through the die, fibers can be broken off along the surface of the pultruded article and such a damaged surface is undesirable. Sometimes, the entire string of material can break which is obviously also undesirable. When either occurs, the production line must be shut down, and the hot die disassembled for removal of the ball-ups. Restarting the production after disassembling the hot die is very difficult because the bundle of fibers impregnated with resin entering the hot die tends to spread out when the die is opened, thus making it difficult to close without catching fibers between the die halves.
The present invention solves this problem by using the hot die to form an oversized, shaped part, and then using the cooling die to form the final shape. The final consolidation is what usually causes ball-ups, and because the final consolidation takes place in the cooling die of the present invention, the ball-ups will tend to occur there, rather than the hot die. When the cooling die is opened to remove the ball-ups, there is less spreading of the fibers across the open die interface since the fibers are already partially consolidated in the plastic and thus do not spread easily. Further, ball-ups are less likely in the cooling die, since during cooling, the consolidated shape tends to shrink and pull away from the surface of the cool die such that the frictional forces contributing to most ball-ups is not present.
The reduced friction also allows the entire system to run more smoothly and for longer periods of time.
A die with two temperature zones is disclosed in U.S. Pat. No. 2,702,408 to Hartland. In this die, the second zone can be cooler than the first zone. The first zone is disclosed to have a slightly sharper taper than the second zone.
The device of Hartland does not accomplish the same purpose as the apparatus and method of the present invention. In the present invention, partial consolidation takes place in the hot die which has a long tapered first portion and a second portion of substantially constant cross-sectional thickness. If a short taper were used in the hot die, too much friction would be encountered in the correspondingly longer constant cross-sectional portion and, as already indicated, such friction causes ball-ups. In the present invention, the material exiting the hot die is sized larger than the final form. This oversized article then enters the cooling, final forming die as a separate step in the process. The cooling die has a shorter taper than the first die which allows fast consolidation. The fast consolidation is made possible in the cooling die because the cooling effect tends to pull the shape away from the surface of the die, reducing friction. If a longer taper were used in the cooling die, the material would cool too quickly before it reached the constant cross-section portion
Unlike the apparatus of Hartland, it is not necessary to handle the hot die in the event of a ball-up because the ball-ups will normally occur in the cooling die. It is a simple matter to open the cooling die, remove the ball-up, squeeze or massage the material so that the cooling die halves can be closed back together, and the system restarted.