mfg selecting frp composite for projects

7/29/2019 MFG Selecting FRP Composite for Projects

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Evaluation Guide for Selecting

the Best FRP Composite Process

for Your Project

Liquid Composite Molding (LCM) vs. SMC

MOLDED FIBER GLASS COMPANIES


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In this report, we compare and contrast the properties of the two majormaterial forms used in compression molding of modern FiberReinforced Plastic (FRP) products. Compression molding of FRP inmatched metal dies is the most cost effective method of producing highvalue composites in production volumes from thousands to millions ofpieces annually. This process has the best combination of cost, qualityand properties of all composite forming methods. The two majormaterial forms are Liquid Composite Molding (LCM) and Sheet MoldingCompound (SMC). Both forms take advantage of the benefits of FRP

including; corrosion resistance, high strength to weight ratio, dimensionalstability, parts consolidation, dielectric strength, minimal finishing, highrepeatability, low tooling costs, and design flexibility.

The principal difference between LCM and SMC compression molding iswhether or not the fiber reinforcement moves, or flows, as the moldcloses to form the part. LCM uses a variety of preformed fiberreinforcements that are positively placed in the mold exactly where theyneed to be in the final molded part. A measured charge of liquidresin paste is placed on a portion of the reinforcement and a controlledclosure of the press flows the paste throughout the stationaryreinforcement to fill the mold. In contrast, SMC is a leather-like,

pre-manufactured sheet combining fiber reinforcement and paste thatis cut and stacked to cover a portion of the mold. The controlled closureof the press flows the compound (fiber reinforcement as well as paste) tofill the mold cavity. In other words, the LCM process flows the resinthrough stationary reinforcing fibers to fill the mold cavity and the SMCprocess flows both resin and reinforcing fibers to fill the mold cavity.

Table 1 (page 2) summarizes the material and processing propertiesthat are similar in LCM and SMC.

Table 2(page 3) summarizes the material and processing propertiesthat are significantly different in LCM compared to SMC.

ABSTRACT


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Therawmaterials of LCM and

SMC are primarily thermosettingresin, fiber reinforcement andinorganic fillers such as calciumcarbonate or clay. They typicallyaccount for 95% of the weight ofthe composite, the balance beingrelease agents, curatives, pigmentsand other functional additives. Therelative amounts of the ingredientscan be the same for SMC and LCMresulting in identical physical propertiessuch as density, hardness andflammability. A useful feature ofthese composites is the ability to

tailor the properties to specificuses. For example, flammabilityand smoke characteristics can beadjusted to meet strict codes formass transit interiors; electricalresistance can be adjusted from ohms(dissipative) to 1012 ohms (insulating);and mechanical stiffness is adjustablefrom flexible to rigid.

The thermosetting resin gives thesematerials good stiffness and hightemperature retention of mechanicalproperties. Thus, much of the growth

in use of SMC and LCM is in metalsreplacement. A useful design analogyto metal forming is that LCM ismore like sheet metal and SMC ismore like a casting. One of theunique features of both SMC andLCM is low profile technology,which allows the molded part tohave exactly the same dimensionsas the room temperature steel molds.

This amazing property not onlyyields excellent dimensional control,it results in large panels as smooth

as the best sheet metals.Fiber content is easily measuredand indicative of strength andtoughness. Therefore it is commonlyspecified. Although usually reported asweight percent, it is the volumefraction of fiber that provides strength.The orientation of the fiber is alsocritical to strength. As compared to

a composite with isotropic fiberorientation, a composite with all itsfibers aligned in one direction willhave over three times the strength

in the direction of orientation butonly one third the strength at 90to that direction. From a designstandpoint, a common misapplicationis to assume isotropic laminates invectorless stress fields. Thissimplification is appropriate formetals design but leads to bothnon-optimized composite designsand to unexpected failures. Compositesare optimally used in designs thatorient reinforcement strengths tothe loads.

Regardless of the choice betweenLCM and SMC, both materialsprovide extreme design flexibility.The designer can specify combinationsof physical and chemical propertiesas well as part configurations thatare not possible with any othermaterial system.

Table 1. Properties that are similar in LCM and SMC

PROPERTY

Raw Materials

Fiber Content (by weight)

Strength (Average tensile and flexural)

Dimensional Control

Hardness

Surface Quality

Orange Peel

Paintability

Bond/IMC Adhesion

Part Details

Thickness

Process

Cycle Time

Net Shape

Tooling

LIQUID COMPOSITE

MOLDING (LCM)

SHEET MOLDING

COMPOUND (SMC)

Tool Steel Core and Cavity

Stops and Heel Plates

Integral Heating

Telescopic Shear Edges

Ejectors

Resin, Reinforcing Fibers, Fillers, Additives

10-75%

Varies the same with formulation

Excellent

40 Barcol

Class A

Very Good

Excellent

0.03 inch up to several inches

Nominal less than 3 minutes but depends on part geometry

Yes

Yes

Yes

Yes

Yes

Yes

Texture Yes


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Fiber Rein forcement.LCM laminate fiber content can be

routinely varied from about 15% to50% by weight with random fibersand up to 75% by weight withoriented fibers. The fiber contentand orientation throughout the

LCM part can vary as required bythe application because the fiberreinforcement is pre-placed in themold and does not move during themolding operation. Standard SMCwill contain between about 10%and 50% fiber by weight withspecial formulations containing upto 75% fiber by weight. Fibercontent throughout the SMC part isvery consistent as a result of thecompounding process and thecoupled flow of resin and fiberduring molding. For the same reason,

fiber content throughout the SMCpart cannot be tailored or optimized tomeet application requirements.Fiber orientation in SMC is greatlyinfluenced by compound flow asthe mold closes and is therefore moredifficult to control.

Fiber Length.

The most common fibers are E-glassrovings chopped to a length (orblend of lengths) from 1/8 to 4.There is a relationship betweenfiber length and strength longer isstronger. For polyester resins thestrength curve is asymptotic atabout one inch. Since longer fibers

do not flow as well, most SMCfibers are one inch long. LCMfibers are often longer because itimproves the integrity and thepermeability of the mat. Sometypes of impact performance andtoughness are improved by thelonger fibers. Both SMC and LCMcan incorporate continuous fibers ifrequired. Because the fibers do notflow, LCM can also incorporatefabrics and non-woven reinforcements.

Fiber Orientation.

The orientation of reinforcing fiberin the LCM laminate is determinedby its positive placement in the moldcavity. Since the LCM reinforcementdoes not move during mold closure,the fiber orientation is highly tailoredand predictable. The fiber orientation

in SMC laminate is more difficult topredict and control. Because the fiberflows with the resin, the fibers tend toorient themselves in the direction offlow. This phenomenon can becontrolled to some extent by chargesize and location but this practicegenerally causes performance trade-offswith other laminate properties.

Ribs and Bosses.Because of the difficulty ofpre-forming glass fibers to detailssuch as ribs and bosses, LCM islimited to generally uniform crosssection shapes. The pre-combinedfibers in SMC on the other handflow easily to conform to manycomplex shapes.

Material Density.In order to properly flow during themolding process, SMC must have at

least a given proportion of mineralfiller. This restricts the density orspecific gravity of SMC on the lowend. LCM does not have such arestriction and therefore given identicalpart thickness, LCM can produce alower density part.

Table 2. Properties that are different in LCM and SMC

PROPERTY

Strength

MinimumValue

Standard Deviation

Local Tailoring

Density

LIQUID COMPOSITE

MOLDING (LCM)

SHEET MOLDING

COMPOUND (SMC)

Higher

Lower

Can be Lower Low-side limited by minimumrequired filler content.

Yes Limited

Toughness (Impact and Hot Strength)

Surface Quality (Waviness)

Part Details

Ribs

Bosses

Molded - in hardware

Molded - in studs

Molded - in cores

Thickness

Process (Required Molding Pressure)

Better Good

Better Good

No Yes

Limited Yes

Yes Limited

No Yes

Yes Limited

Limited Yes

100-500 psig 700-1,000 psig


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Part Weight.Under the same design stressconditions, LCM can produce alighterweight part compared to SMC.

This is the result of more consistentstrength, higher impact toughness andthe flexibility to design with lower

density laminates. See MaterialDensity and Mechanical Propertiessections for further discussion.

Mechanical P roper ties.LCM creates composite structuresthat provide higher design strengthallowables than similar laminatesmade from SMC. Since 1974, theMolded Fiber Glass Companies(MFG) has periodically comparedits modern LCM processes withstate-of-the-art SMC formulations.

Recently, MFG completed its latesttest runs on nearly 500 samplestaken from truck hoods that werecompression molded using the twoforms. The current samples weretaken from two current model truckhoods one a three-piece hood ofSMC, the other a one-piece hoodof LCM. The LCM hood had anaverage glass content of about 20%by weight and the SMC hood wasslightly higher at about 25% glasscontent by weight.

Similar sample maps were devel-

oped for each hood with specimensoriented in various importantdirections (longitudinal, cross-car,vertical and horizontal). ASTMD638 tensile and D790 flexuralstrength tests were performed onthe samples. Results for samplesfrom each area of the hoods were

averaged and plotted in boxplotsand frequency histograms asFigures 1 through 6.

Flexural Strength.Figure 1 is a pair of boxplotscomparing the flexural strength of

LCM and SMC samples. FlexuralStrength describes the amount offorce required to bend and breakthe material when a specific thicknesstest piece is bent. A test piece issupported at both ends, a force isapplied to a small, concentratedarea in the center and the force andamount of bending is measured.Results are reported in pounds persquare inch (psi). See ASTM D790for specifics.

The horizontal line near the centerof each plot shows the samplemean, or average flexural strength.The box areas above and below themean line each represent onequartile or 25% of the datasamples. The whiskers above andbelow the boxed area each representthe remaining quartiles. The figureclearly shows that although theaverage flexural strength of the twoforms is about the same, the LCMform produces much less variationthan the SMC form. A closer lookshows that as many as 35% of theSMC samples have flexural strengthbelow the weakest LCM samples.

Figures 3 and 4 are the flexuralstrength frequency histograms forthe LCM and SMC samples. Thesehistograms show that although theaverage strength of both the LCMand SMC samples are about the

same, the variation in flexuralstrength is much greater in SMCthan in LCM. In fact, 6 of 14 SMCsample areas had average flexuralstrength below the lowest performingareas in the LCM part.

Tensile Strength.Figures 2, 5 and 6 show results ofthe tensile strength testing on theLCM and SMC parts. Tensilestrength describes the amount offorce (tensile stress) required tobreak a sample of specific dimensionswhen the material is stretched to itsbreaking point. The tensile stress atfailure is divided by the cross-sectionalarea of the sample and results arereported in pounds per square inch(psi). See ASTM D638 for details.The tensile strength comparisonresults are similar to the flexuralstrength in that the average tensilestrength of the two forms is aboutthe same but the variation in theLCM parts is much lower than theSMC parts. One particularlyimportant result in the tensilestrength test is that the highestfrequency (or mode) of SMCstrength samples is at the lowestend of the scale whereas the modeof LCM strength samples is abovethe average strength.

Although the current testing showsthat some areas of SMC can havevery high strength compared toLCM, it is the low end of thevariation that causes parts to fail.In both the design process and enduse, when any area of laminate issubjected to stress, it is the weaker


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locations that fail, similar to theweak links in a chain. The moreconsistent, controllable and predictableproperties of the resulting laminatemake LCM the process of choicewhen flexural and tensile strengthsare important.

Impact Strength.Figure 7 is a comparison of impactstrength, or toughness, of LCM andSMC laminates as measured usingthe Dynatup testing method. Thismethod measures the reactive loadproduced when a weighted dart isdropped onto the laminate surfacefrom a controlled height. Theimportant performance measurementsare the energy absorbed (Max Load

x Drop Height) and the heightwhere penetration of the dart intothe material occurs.

The LCM laminate performs about25% better than SMC in the energyabsorbed in this test. Also, penetrationoccurs in the SMC at the 24 inchdrop height while the LCM requireda drop height of 45 inches forpenetration to start.

Molding Pressure.The molding pressure for LCM isusually a fraction of that requiredfor SMC. Therefore, the presstonnage required is less for LCMthan for SMC for a given part area.This is because the viscosity of the

LCM paste is very low comparedwith that of SMC and no reinforc-ing fibers are displaced in LCM, asthey are in SMC. In practice, thisallows compression molding oflarger parts in LCM than in SMCfor a given press tonnage and withinthe platen size limits of the press.

SMC Manufacturing Equipment Robotic Preform Equipment


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Liquid Composite Molding (LCM) and Sheet Molding Compound (SMC) produce laminateswith the same average flexural strength. However, the variation in strength at random orientationsthroughout the part is much less in LCM than in SMC. Glass content is 20% by weightfor LCM samples and 25% for SMC samples.

Liquid Composite Molding (LCM) and Sheet Molding Compound (SMC) produce laminateswith the same average tensile strength. However, the variation in strength at random orientationsthroughout the part is much less in LCM than in SMC. Glass content is 20% by weight

for LCM samples and 25% for SMC samples

Figure 2 Tensile Strength (TS) Comparison Boxplots

Figure 1 Flexural Strength (FS) Comparison Boxplots


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Distribution of Flexural Strength test results for samples taken from a truck hood moldedin the Liquid Composite Molding (LCM) form. Variation is low with a minimum sampleflexural strength of about 21,000 psi. Glass content is about 20% by weight.

Figure 3 Liquid Composite Molding (LCM) Flexural Strength (FS) Histogram

Figure 4 Sheet Molding Compound (SMC) Flexural Strength (FS) Histogram

Distribution of Flexural Strength test results for samples taken from a truck hood moldedin the Sheet Molding Compound (SMC) form. Variation is higher with a minimum sample flexuralstrength down to about 15,000 psi. Glass content is about 25% by weight.


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Figure 5 Liquid Composite Molding (LCM) Tensile Strength (TS) Histogram

Distribution of Tensile Strength test results for samples taken from a truck hood moldedin the Liquid Composite Molding (LCM) form. Variation is low with a minimum sampletensile strength of about 6000 psi. Glass content is about 20% by weight.

Distribution of Tensile Strength test results for samples taken from a truck hood moldedin the Sheet Molding Compound (SMC) form. Variation is higher with a minimumsample tensile strength of about 6000 psi. Glass content is about 25% by weight.

Figure 6 Sheet Molding Compound (SMC) Tensile Strength (TS) Histogram


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The current study compares andcontrasts the properties of FiberReinforced Plastic (FRP) compositesproduced from Liquid CompositeMolding (LCM) and Sheet MoldingCompound (SMC). Careful selectionand application of either form yields

Liquid Composite Molding (LCM) laminate demonstrates higher impact strength than sheetMolding Compound (SMC) laminate. Dart penetration begins at nearly 50% higher level forLCM compared to SMC.

Figure 7 Impact Strength Comparison of Liquid Composite Molding (LCM) and

Sheet Molding Compound (SMC)

high quality products and providesextreme design flexibility. The designercan specify combinations of physicaland chemical properties as well as partconfigurations that are not possiblewith any other material systems.

Because of the versatility of FRP/Composites, the designer is encouragedto collaborate with a molder and/ormaterial supplier to optimizethe application.


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MOLDED FIBER GLASS COMPANIES

MFG Is a Full Spectrum Composite Molding Supplier

Molding Process Capability

Open Molding (Hand Lay-up, Spray-up)

Vacuum Infusion Processing (VIP) or RTM-lite

Resin Transfer Molding (RTM) DCPD RIM (Reaction Injection Molding)

MFG PRiME Process Compression Molding with Preform Reinforcement

Compression Molding

Direct Long Fiber Thermoplastic Molding (D-LFT)

Filament Winding

Other Value-Added Processes

Robotic Gel Coating

In-Mold Coating (IMC)

SMC/BMC Compounding

Directed Fiber Preforming, APP

Bonding and Assembly

Robotic Routing and Water Jet

Prime and Topcoat Paint Capabilities

In Line Sequencing

The best value supplier is strategically located for your needs perhaps near your operations, supply

source, market or a specic labor market. MFG has established the largest full-service network of moldingfactories in North America. This factory network is supported by a shared foundation of engineering,

design, R&D, procurement and program management that ensures consistent quality and service

company-wide. This structure allows us to provide the most competitive cost structure possible.

10 Strategically Located Factories in North America Provides Nimble and Cost-Ecient Fabrication

Corporate/International Headquarters

Molded Fiber Glass Companies

2925 MFG Place, P.O. Box 675

Ashtabula, OH 44005-0675

Toll-Free: (800) 860-0196 Phone: (440) 997-5851

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