MATERIALS AND MOLDING METHODS 4-9 



The disadvantages of cloth reinforcements are: 



A resin rich bond may exist between heavier cloth plies which will cause 

 weakness in interlaminar shear. As a result, a cloth laminate under 

 edgewise compressive loading may fail by delamination. 



Many layers of cloth are required for thick sections and, as a consequence, 

 labor and time are increased. 



Combinations 



Fiberglass reinforcements are also manufactured in several combined forms. Two 

 types of combined reinforcement are, mat stitched to cloth and mat stitched to woven roving. 



The use of combined reinforcements has a number of advantages in boat hull construc- 

 tion. Mat stitched to cloth or woven roving improves the interlaminar bond of successive 

 layers and reduces porosity. Combination reinforcements allow several plies to be laid up 

 at one time. 



Preimpregnated 



Preimpregnated reinforcements, commonly called pre-preg materials, are reinforce- 

 ments preloaded with resins. The resins are essentially the same as those used in normal 

 operations where the resin is added to the reinforcement during molding (1). 



The usual method for preloading is to pull the reinforcement through polyester or other 

 molding resins and remove the excess resins by scrapper bars or squeeze rollers to control 

 the glass to resin ratio. After resin impregnation, proper storage at low temperature is 

 required to prevent the polyester resin from curing. 



Depending on the application and cost, pre-pregs can be made by the fabricator or can 

 be obtained to any desired specification from material suppliers. 



The advantages of pre-pregs are: 



Greater control of glass -resin ratios can be obtained. 



Increased wetting of the glass fibers occurs. 



Polyester resins of high viscosity can be used. 



Resin wastage is reduced. 

 The disadvantages of pre-pregs are: 



Additional equipment and storage facilities are required. 



Storage life is reduced. 



Tackiness may cause handling difficulties. 



