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  1. #1
    Moderator RX951's Avatar
    Join Date
    Apr 2005
    San Antonio, Texas

    Lightbulb Definition of SMC

    What is SMC ?

    Sheet Molding Compound (SMC)
    • SMC is the paste that is compression molded
      • 33% polyester resin and stryrene, which polymerizes and crosslinks
      • 33% glass fibers (1” fibers)
      • 33% Calcium Carbonate

    Definition: A fiber glass reinforced thermosetting compound in sheet form, usually rolled into coils interleaved with plastic film to prevent autoadhesion. Made by dispensing mixed resin, fillers, maturation agent, catalyst and mold release agent onto two moving sheets of polyethylene film. The lower one also contains chopped glass roving or glass mat. SMC can be molded into complex shapes with little scrap.
    Definition Copyright ©1989 CRC Press LLC. All rights reserved.

    Sheet Molding Compound:

    Sheet molding compound (SMC) is fiberglass reinforced composite material, produced in a “sheet” format.
    The process begins when continuous strand fiberglass (also called a “roving”) is chopped into desired lengths (usually under 2”). The strands are deposited onto a bottom layer of paste made from resin and filler. The paste and fiberglass strand travel through the processing machine, on a carrier film. A top layer of resin and filler paste sandwiches the fibers to the bottom layer of paste, and is covered by a top layer of carrier film. The “fiber and paste sandwich” is then compacted by a series of rollers to make a continuous sheet of molding compound.
    SMC is made of 3 basic components: the base resign system (polyester, vinylester, epoxy, phenolic or polyimide), the reinforcements (fiberglass, graphite, aramide), and additives which include inert fillers, pigments, UV stabilizers, catalysts, inhibitors, and thickeners.
    While there is basically no limit to the number and types of formulations that can be produced, it is important to prioritize the desired properties for the product. This helps to ensure that the optimum material system is achieved within the economic guidelines of the program.
    Compression Molding:

    Compression molding primarily uses thermoset composite resin systems, which use heat to form and permanently set the shape of a part. Once the resin is cured, the part cannot be reformed, making the process irreversible.
    Compression molding uses matched steel molds to compress a weighed “charge” of sheet molding compound (SMC) into a new and specific shape. The two halves of the mold, which are mounted in a hydraulic molding press, are closed around the charge. The combination of heat (300 degrees) and pressure (1000 PSI) caused the thermoset material to cure in the mold. Molding cycles will range from well under 1 minute to over 5 minutes, depending on the size and cross sectional thickness of the part. The mold is then openend, and the part is removed.

    Description: Sheet Molding Compound (SMC)

    Sheet molding compound (SMC) consists of a doughy material composed of chopped glass fiber rovings, 0.75-2” in length, fillers (e.g. calcium carbonate, low profile additives, catalyst, catalyst aids, and a thermoset resin, normally a polyester based thermoset, epoxy or vinyl ester resin. The long fibers present in this material provide good stiffness and impact resistance. The initial paste has a viscosity near the 40,000-100,000 cps range. SMC is then packaged and placed into storage for 1 to 2 days allowing the resin to advance to achieve viscosities in the range of 60,000 to 750,000 poise. The thickening during this time period is usually assisted with additives like magnesium oxide, magnesium hydroxide, or calcium hydroxide. The aged SMC is then weighed, and placed into a heated compression mold (250-350oF) which under pressure (500-2500 psi) forms a composite part. Typical compression molding cycle times of 30 seconds to several minutes are possible with thicker parts requiring more dwell/curing time.

    Last edited by RX951; 04-03-2007 at 03:13 PM.

  2. #2
    Nothin' runs like a Deere IowanGPR's Avatar
    Join Date
    Sep 2006
    Waterloo, IA
    wow, thanks good info here.

  3. #3


    What products do you use for repairs to SMC? I've heard that you could only use that correct?

  4. #4
    Hydrotoys's Avatar
    Join Date
    Jul 2005
    Mesa, AZ
    Correct. epoxy repairs from reputable places such as west systems.

    All of your Yamaha's (nanocell, smc) are smc with specific ingredients. SD are fiberglass, as well as some kawi's. Polaris used different types on different skis.

  5. #5
    cameron da river rat's Avatar
    Join Date
    Sep 2006
    college station, tx
    what is the other 1% missing in the smc formula?

  6. #6
    jp0013's Avatar
    Join Date
    Mar 2009
    Lake Lanier, GA
    the technical knowledge in this place amazes me. unfortunately not even the stealerships around here know what SMC is, lol. It's ALLLLLL Fiberglass.

  7. #7
    OLD # 7 IEGPR's Avatar
    Join Date
    Dec 2006
    Rialto CA
    I checked the west website and they offer 6 fillers. What would or should I use to fill a small gash ??

  8. #8

    Join Date
    Dec 2009

    Fantastic thread, thanks for the info.



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