4-6 



MATERIALS AND MOLDING METHODS 



The disadvantages of mat reinforcement are: 



Laminate thickness cannot be controlled accurately in contact molding. 



Glass content, because of movement of the individual fibers during molding, 

 becomes a variable and is somewhat difficult to control in contact molding. 



In a polyester laminate, a typical mat fiber surface results because of 

 polymerization shrinkage during cure of the resin, requiring surface 

 finishing or a surface gel coat (1). 



Contact molded mat laminates have a lower glass content than cloth or 

 woven roving laminates which results in a lower modulus of elasticity for 

 equal thicknesses. In order to overcome this deficiency, a thicker section 

 is required. 



In matched die molding uneven distribution of the chopped fibers and varying 

 density can occur with deep draws and sharp corners. 



Preforms: These are generally similar to mat except that they are slightly more ex- 

 pensive but are more practical for complex and deep draw matched die moldings. With 

 preforms greater utilization of the reinforcement is obtained and the possibility of tearing, 

 wrinkling and uneven glass distribution is reduced. 



Fig. 4-7. Preform of 15' boat hull 

 (Courtesy Molded Fiberglass Boat Company) 



Preforms are made by depositing chopped fibers of glass over a screen shaped in the 

 form of the object to be molded (1, 14). Continuous rovings are cut into loose fibers which 

 are randomly deposited on the screen by a stream of air drawn through it. Resin binder, 

 compatible with the molding resin, is sprayed over the preform and cured to hold it to- 

 gether, Fig. 4-7. The preform is then removed from the screen and placed in the mold. 



Chopped Strands: Some hulls and decks for small boats are presently being fabricated 

 with chopped strands and resin simultaneously deposited on the mold from a spray gun. 



