Ciencia De Los Materiales

b.8.8

Compression Molding Process

C o m p r e s s i o n m o l d i n g is very p o p u l a r i n the automotive industrv because of its h i g h v o l u m e capabilities. This process is used íor m o l d i n g large automotive panels. Sheet m o l d i n g c o m p o u n d s (SMCs) a n d b u l k m o l d i n g c o m p o u n d s ( B M Cs ) are the more c o m m o n ravv materials íor compression m o l d i n g . C o m p r e s s i o n m o l d i n g is also used tor m a k i n g structural panels using prepregs a n d core materials; but because of the popularity of compression m o l d i n g of S M C , the m o l d i n g of S M C is discussed here. C o m p r e s s i o n m o ld i n g is p o p u l a r i n the auto i n d u s t r v because of its s i m ilarity to the s t a m p i n g process. The auto i n d u s t r v has been u s i n g the stampi n g process íor a l o n g time a n d has b u i l t g o o d k n o w - h o w for this process. S i m i l a r l y today, compression m o l d i n g has become quite amature process for the auto industrv. In compression m o l d i n g of S M C , the final component is p r o d u c e d i n one m o l d i n g operation steel whereas i n the s t a m p i n g operation, the steel sheet metal goes through a series of s t a m p i n g processes to get the final shape. Therefore, compression m o l d i n g p r o v i de s several a d v a n tages over the s t a m p i n g process a n d saves signiñeant cost i n terms of m o l d s a n d equipment. O n e of the advantages of S M C over steel lies i n its abilitv to include ribs a n d bosses i n a t h i r d dimensión. H o l e s , flanges, shoulders, a n d n o n u n i f o r m thicknesses can be created d ui i n g the m o l d i n g process, thus a v o i d i n g secondarv operations such as w e l d i n g , d r i l l i n g , a n d machining. C o m p r e s s i o n m o l d i n g is used for m a k i n g Class A surfaces. For Class A suríaces, the overall percentage of fiber content is l i m i t e d to 30% to o p t i m i z e smoothness of thesurface as w e l l as to reduce fíber read-through. Therefore, there is a trade-off i n getting mechanical property enhancement or Class A surface quality. Short ribs are used o n b o d y panels a n d other one- a n d twopiece components are used to increase the stiffness of these panels. Ribs o n Class A panels s h o u l d be usedcautiously because of their potential to cause sink marks. H o w e v e r , w i t h proper design, sink m a r k s f o r m e d o n the top side can be e l i m i n a t e d or m i n i m i z e d .
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6.8.8.1 Majar Appücations
C o m p r e s s i o n m o l d i n g of S M C is used for m a k i n g one- a n d two-piece panels for automotiveapplications. One-piece panels are u s e d where the p a n e l is fixed a n d cari be s u p p o r t e d a r o u n d the majority of its periphery. Roof panels, quarter panels, fenders, a n d add-ons such as spoilers, ground-eííects packages, a n d limited-access panels are examples of this category. Two-piece panels are used for closure panels su c h as doors, hoods, a n d d e c k l i d s . These paitéis are u s u a l l v s u p p o r t e d b v hinges w i t h b o d y strucrures a n d therefore s h o u l d have e n o u g h r i g i d i t y of their o w n . The stiífness of two-piece panels is obtained b y j o i n i n g inner and outer panels as s h o w n i n Figure 6.63.