17-Matched Metal Compression Molding of Polymer Composites

MATCHED METAL COMPRESSION MOLDING OF POLYMER COMPOSITES

17

Enarnul Haque and Burr (Bud) L. Leach

17.1 INTRODUCTION

In today’s highly competitive global economy, the need for materials with the right properties to meet the demands of design, environment, durability and economics is growing. Composite materials, with their high strength and stiffness-to-weight ratios, have many advantages and are a desirable engineering material.

There is no universal definition of composites. In general, a composite material is a heterogeneous material system consisting of two or more physically distinct materials. In a composite material system, the individual materials exhibit their unique properties and the composite as a whole shows properties that are different from its constituents. In addition to the constituents’ unique properties, the properties of composites are also dependent on the form and structural arrangements of the constituents and the interaction between the constituents.

Broadly speaking, composites consist of two components, a binder or matrix and a reinforcement. The matrix functions as the body constituent, serving to bind the reinforcement together and giving the composite its bulk form. The reinforcements are the structural constituents, providing high strength to the internal structure of the composite.

Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7

Reinforcement may take the form of fibers, particles, laminate, flakes and fillers. Depending upon the type and orientation of the reinforcement and the manufacturing technology required to produce them, composites with various properties and cost can be fabri- cated. Polymer composites are composites in which the binder or matrix is a polymeric material and the reinforcement is usually a thin fibrous material. Polymer composites can have either a thermosetting or thermoplastic matrix. In this chapter we will discuss thermosetting matrix based composites. Reinforcing fibers may also be of various kinds, with glass (E-type), carbon, or organic fibers (e.g. aramid) being the most common. Glass fibers are the most widely used type of reinforcement since they offer good strength and moderately high temperature resistance (about 260°C) at a cost effective price. Glass fibers also come in various forms. They can be continuous filaments, cut or chopped strands, roving and yarns, or in the form of cloth, mats or tapes. Thus allows glass fibers to be used in a variety of applications such as lay-up, fila- ment winding, matched die-molding, etc.

In this chapter we discuss the matched metal compression molding of thermoset based polymer composites. Matched metal compression molding is a molding process in whch the cure is obtained while the material is restricted between two mold surfaces and the loading and closing of the mold causes the material to conform to the desired configuration. This

Background 379

process enables large scale production of large surface area parts with contour problems and tight tolerances. Matched metal compression molding employs a ’mold’ or match dies. The male mold is matched to the female mold so that when the dies are closed, a controlled space results. A preform charge is placed on the core and the cavity is pressed against it, applying direct pressure on the material. The pressure in this type of molding varies from 1.38 to 6.895 MPa (200 to 1000 psi) and curing temperatures from 125°C to 160°C (260°F to 320°F).

17.2 BACKGROUND

Advanced polymer composites are now being applied extensively for all types of applications in the industrial and automotive markets. Table 17.1’ shows the usage of composites in various markets during 1991-1993. This section deals primarily with thermoset polymer composites used in matched metal compression molding. The two most popular reinforced molding compounds used in the plastics industry are Premix or BMC and SMC (also referred to in modified versions as HMC and XMC). Low Pressure Molding Compounds (LPMC), ZMC and TMC are also becoming popular.

Table 17.1 US Composites shipments: 1991-1993”

17.2.1 BULK MOLDING COMPOUND (BMC)

BMC has been defined as ‘a fiber reinforced thermoset molding compound not requiring advancement of cure, drying of volatile, or other processing after mixing to make it ready for use at the molding press’*. BMC can be molded without reaction byproducts under only enough pressure to flow and compact the material. BMC is usually manufactured by combining all the ingredients in an intensive mixing process. Recent advances in BMC technology dictate that both the dry ingredients and wet ingredients be batch mixed separately and then combined together in an intensive mixer. The BMC is usually in a fibrous putty form when it comes out of the mixer and resembles ’sauerkraut’. It is usually compacted and extruded into bars or ’logs’ of simple cross section.

The earliest BMCs were probably made about 1950, employing a process of impregnat- ing roving strands with blend of resin, filler, etc. and chopping them to a length in the wet stage. Since wetting glass fibers with a resin containing much filler is difficult and slow, these premixes had a high glass content. The first high volume commercial BMC was made with sisal fibers and used in molding automobile

Millions of pounds

Markets 1992-1 993 1993 1991-1 992 % change (projected) % change

1991 1992

Aircraft/ aerospace /military Appliance/business equipment Construction Consumer products Corrosion-resistant equipment Electrical/electronic Marine Transportation Other Total

38.7 135.2 420.0 148.7 355.0 231.1 275.0 682.2 73.8

2359.7

32.3 143.2 483.0 162.2 332.3 260.0 304.4 750.0 83.4

2550.8

-16.5 +5.9

+15.0 +9.1

+12.5 +10.7 +9.9

+13.0 +8.1

-6.4

26.0 146.7 522.0 164.1 336.8 273.0 317.2 810.0 88.0

2683.8

-1.95 +2.4 +8.l +1.2 +1.4 +5.0 +4.2 +8.0 +5.5 +5.2

a Includes reinforced thermoset and thermoplastic resin composites, reinforcements and fillers. Source: SPI Composites Institute

380 Matched metal compression molding of polymer composites

heater housings. Improvement in the binder chemistry of glass fibers, development of a chemical thickening system and thermoplastic low profile additives help BMC to attain strength, chemical resistance and to overcome surface irregularities. Consequently, BMC was accepted for use in the electrical, chemical and appliance industries.

Today, BMCs are accepted as high performance engineering thermoset molding compounds and used extensively in the electrical, automotive and consumer goods industries. BMC is increasingly injection molded to take advantage of the automation and reproducibility afforded by the process, although it is also both transfer molded and compression molded.

17.2.2 Z MOLDING COMPOUND (ZMC)

ZMC was developed in 1979 in France to improve BMC performance. BMC suffers from glass fiber degradation during injection molding and ZMC was developed to keep shear forces as low as possible during mold- in$. A special type of injection molding machine developed by Billion in France com- bines the advantages of both a screw machine and a plunger machine. The ZMC injection machine uses a screw to homogenize and pre- cisely measure the shot. The injection is made like a plunger by the displacement of the screw and inner barrel inside the main barrel. In a ZMC, the different components are mixed in conventional mixers like BMC. The compound viscosity is usually low and adapted to injection machine characteristics. The design of the mold plays a key role in the processing phase and ZMC parts cannot be successfully made unless part design and mold design are combined upfront. Compared to SMC, ZMC parts have lower mechanical properties, but higher performance when compared to conventional injection molded BMC.

17.2.3 THICK MOLDING COMPOUND (TMC)

TMC was developed by Takeda Chemical Industries, Ltd (Osaka, Japan). TMC is suited to compression, injection and transfer molding and is usually processed on the same equipment as SMC and BMC materials. TMC composites are usually produced up to 51 mm (2in) thick and glass fiber length can vary from 6.4 to 50.8 mm (0.25 to 2 in). In TMC, continuous impregnation and high sheet weight result in complete wet-out of resins, fillers and reinforcing fibers. Better wet-out results in improved mechanical properties and reduced porosity. TMC is usually used in business machine housings, appliance components and other consumer related industries.

17.2.4 SHEET MOLDING COMPOUND (SMC)

SMC is a type of fiber reinforced plastic which primarily consists of a thermosetting resin, glass fiber reinforcement and filler. Additional ingredients such as low-profile additives, cure initiators, thickeners and mold release agents are used to enhance the performance or processing of the material4.

The development of SMC started in the early 1950s after the finding that the viscosity of unsaturated polyester resins increases with the addition of Group IIA metallic oxides, hydroxides, or carbonates5. The first published report on SMC was presented at the Cleveland Section of the Society of Plastics Engineers meeting. The report involved work done in Germany using fiberglass mat impregnated with a resin mixture containing magnesium oxide6. At the same time a number of US patent^^,^ were published on the use of Group I1 metal oxides, hydroxides, or carbonates for use on adhesives. The early applications of SMC materials were in electrical and industrial goods. During the next two decades, growth in commercial usage of SMC followed the evolution of continuously improving equipment, low profile additive,

Background 381

catalyst, etc. The automobile industry started using SMC in the early 1970s for producing

17.2.5 LOW PRESSURE MOLDING COMPOUND (LPMC)

exterior body components, such as hoods or grille opening panels. With the introduction of high strength SMCs in the mid-l970s, usage of SMC increased to structural components.

SMC is currently used extensively in transportation, construction (door panels), appliances (washing machine door, refrigera- tor housing), furniture (chair, tabletop) and business machines (computer housings). The transportation industry has the highest level of consumption of SMC. For instance, in the North American market alone, the annual rate of consumption exceeds 100 million kg9.

Details of SMC manufacturing are available in the literature5. SMC offers many advantages which include variety, part consolidation, lightweight and dimensional stability.

With the evolution of flexible backbone polyester resin systems and development of special additives, flexible SMC is becoming very popular and is now competing with thermoplastics for vertical body applications. Special applications SMC is also becoming popular. With the addition of hollow micros- phere glass bubbles in a standard SMC formulation, lower density (1.3-1.4) is obtained for weight reduction. High strength molding compound (HMC) is a SMC containing 65% chopped glass fiber instead of the usual 25-35%. HMC uses little or no filler and can be compounded on a standard SMC machine. Directionally reinforced molding compound (XMC) is a directionally oriented moldable resin-glass fiber sheet containing 65-75% continuous reinforcement. XMC is also usually compounded on standard fila- ment winding equipment and has strength five times greater than SMC. Unidirectional molding compound (UMC) is a system of chopped and continuous fibers produced on a modified SMC machine. An advantage of UMC is that different varieties of fibers can be used.

Low Pressure Molding Compound (LPMC) is an SMC type material which can be molded at 1.38-2.07 MPa (200 to 300 psi) instead of 5.52 -6.90 MPa (800-1000 psi) required for standard SMC.

LPMC is made by replacing the chemical thickening mechanism of alkaline earth oxides (Group 11) with a physical thickening mechanism utilizing a crystic polyester. The material is heated to melt the crystic and then the other ingredients are added, mixed together and run through a modified SMC machine maintaining the elevated temperature.

The thickening occurs as the material cools to ambient temperature and the compound is ready to mold at that time. Cooling rolls speed the cooling process and thus the material can be molded right off the SMC machine without waiting for the 2448 h thickening of standard SMC. LPMC allows the molder to use lower tonnage presses to mold larger parts and use less steel in building the tools as they do not have to deal with high pressures and corre- sponding forces. The shelf life of LPMC is much longer than SMC and the physical properties are comparable.

17.2.6 CONTINUOUS IMPREGNATED COMPOUND (CIC)

In 1986, continuous impregnated compound (CIC) was developed in Germany. This is similar to TMC. Like TMC, the impregnation is made between two rolls but the compound is removed by doctor blades and carried by a screw or plunger to boxes or drums. CIC is usually injection molded, but can also be injection/compression molded. Properties are comparable to BMC but processing is easier than BMC. Modified CIC is also known as KMC (Kneaded Molding Compound).


17.3 FORMULATIONS

Polymer composites have the unique and distinct advantage in that their properties can be tailored to meet different applications by designing the formulations. The major components of polymer composites used in matched metal compression molding are resin, low profile additive, fiber, filler, initiator, inhibitor, internal mold release agent and other additives (e.g. viscosity reducer, toughness enhancer, etc.).

17.3.1 RESIN

Unsaturated polyesters and vinyl esters are the principal resins used in polymer composites for compression molding. Epoxies are also used for specialty products which require longer cure cycles and higher strength. Phenolics are being used for formulating composites, especially SMC, in applications wluch require lower flam- mability, reduced smoke generation and higher thermal stability’O. Details of resin chemistry are available in the literaturel’-l4. Styrene is commonly used for cross-linking of both polyester and vinyl ester resins. Low styrene polye~ter’~ is becoming popular due to stringent EPA requirements on styrene vapors. New resin technology is also being considered for compression molding. They include hybrids of unsaturated polyester and urethane16, acryles- terol resin with polyi~ocyanate’~, etc.

17.3.2 LOW PROFILE ADDITIVES

Low profile additives are thermoplastics that are added to the formulation in 2-5% (by weight) of the final product or 10-20% (by weight) of the organic portion of the formulation to control the shrinkage of the cured composites. Typical thermoplastics include polyvinyl acetates, poly methyl methacrylate and copolymers with other acrylate, vinyl chlo- ride-vinyl acetate copolymers, polyurethane, polystyrene, polycaprolactone, cellulose acetate butyrate, saturated polyester and styrene-

butadiene copolymers. Extensive details of LPA mechanism are published in the literature9J8J9.

17.3.3 INITIATORS Ah-D INHIBITORS

Initiators are used to initiate the curing reaction at elevated temperatures. Composites are polymerized or crosslinked by a free rad- ical mechanism in which the double bond of the polyester chain reacts with the vinyl monomer (usually styrene). This copolymerization reaction provides a three-dimensional network that converts the viscous liquid resin paste into a hard thermoset solid. Initiators decompose at elevated temperature and provide a source of free radicals to initiate the copolymerization reaction. Peroxyesters and peroxyketals are the most common classes of peroxides.

Inhibitors are added in small quantities to prolong shelf life, modify cure rate and mag- nitude of exotherm to prevent cracking of thick molded sections. Inhibitors are also used to improve resin stability. Two general classes of inhibitors are commonly used, sub- stituted phenolic derivatives and the quaternary ammonium salts (e.g. hydro- quinone, p-benzoquinone, etc). An excellent review on initiator and inhibitor chemistry is available elsewhere y.

17.3.4 FILLERS

Fillers are used to improve physical properties, reduce volumetric shrinkage of the resin and to reduce costs. Fillers are typically divided into functional and non-functional categories. Examples of functional fillers include alumina trihydrate for flame retardancy, hollow glass bubbles for lower weights, mica and wollas- tonite for reinforcement. Non-functional fillers are used for cost reduction and are mineral based. Ground limestone (CaCO,) is the most common type of filler. An excellent review is available on filler use in composite^^^^^^

Molding 383

17.3.5 FIBERS

Glass fiber reinforcement is used to achieve necessary dimensional stability and mechanical properties. E-glass is the most common fiber reinforcement for composites. Depending on the binder chemistry and amount, glass fibers are classified as hard or soft type. Other types of fibers include carbon, aramid (Kevlar), S-2 glass, etc. Glass loading normally averages 30% by weight in compression molded composites, but can vary from 18-65?” by weight. Table 17.2” shows typical fibers used in polymer composites.

17.3.8 OTHER ADDITIVES

Pigments are added to produce color in the molded part. Common pigments include cad- mium salts, carbon black, titanium dioxide, iron oxides, organic dyes and pigments, etc. Various types of viscosity reducers are used to lower the viscosity of the paste to increase filler loading and glass wet-out. Other additives include various elastomeric additives (e.g. Hycar, Kraton, etc.) to increase toughness.

Tables 17.3 to 17.6 show typical formulations for BMC, SMC, ZMC and LPMC.

17.3.6 INTERNAL MOLD RELEASE

Internal mold release agents are added to facilitate part ejection from the mold. Major types of release agents include metallic stearates, fatty acids, fatty acid amides and esters and hydrocarbon waxes. Zinc stearate and calcium stearate are the most widely used internal mold release agents in SMC and BMC.

17.3.7 THICKENERS

The addition of Group I1 oxides and hydroxides to carboxy-terminated unsaturated polyester/vinyl ester resin increases its viscosity. Magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide or combina- tions of those materials are the most popular thickeners.

Table 17.2 Glass fibers used in compositesz1

17.4 MOLDING

Matched metal compression molding is one of the oldest manufacturing techniques in the plastics/composites industry. The recent development of high strength, fast cure,

Table 17.3 BMC formulation

Polyester resin Low profile additive Styrene Initiator Inhibitor Mold release Pigment Thickening agent Filler Compound Glass fiber Paste

PHR

60.0 40.0 5.0 1.5

Trace amount 4.0 0.25 1.0

50-200

10-25% 75-90%

Fiber Specific Tensile Tensile Tensile Coeficient

(GPa) (GPa) strain (%) (x 1 @6/OC) gravity strength modulus failure of thermal expansion

E-glass 2.54 3.45 72.4 4.8 5 S2-glass 2.48 4.30 86.9 5.0 2.9 Carbon 1.76-2.15 1.5-5.6 220-690 0.3-1.2 -0.1 to -1.2 (longitudinal) (graphite) 7-12 (radial) Kevlar 49 1.45 3.62 131 2.8 -2 (longitudinal)

59 (radial)


Table 17.4 SMC formulation

PHR Polyester resin Low profile additive Styrene Initiator Inhibitor Mold release Pigment Thickening agent Filler Compound Glass fiber (25.4 mm) Paste

55.0 40.0 5.0 1.5

250 ppm 4.0 1.0 2.0

150-250

25-30% 70-75%

Table 17.5 LPMC formulation

PHR

Table 17.6 ZMC formulation

physical and mechanical properties can be obtained in compression molded parts. Figure 17.1 shows a schematic of a compression molding process.

This section addresses the compression molding of composite parts using SMC. BMC molding is similar except for the charge preparation step. The compression molding process can be divided into four distinct steps.

Heat and pressure

+ Cavity

Polyester resin Crystic Styrene Initiator Mold release Pigment Filler Compound Glass fiber Paste

65.0 15.0 5.0 1.2 5.0 1.2 220

25-30% 70-75%

PHR Polyester resin Low profile additive Styrene Initiator Inhibitor Mold release Filler Compound Glass fiber Paste

65.0 40.0 5.0 1.5 100 ppm 4.0 220

SMC/BMC and advancement in press technology is making the compression molding process very popular for mass production of composite parts. In comparison with the injection molding process, in general, better

4

I Heat and pressure

Fig. 17.1 Schematic of a compression molding process.

17.4.1 CHARGE PREPARATION AND PLACEMENT

When the SMC has reached its desired molding viscosity, pieces of SMC are cut to pre-specified size after removing the carrier films. The SMC is cut using slitters, pizza type or guillotine type cutters. Several pieces of SMC plied together form the 'charge'. The charge pattern/ply dimensions are chosen so as to cover 20-80% of the mold surface area. The charge pattern and placement on the mold determines the quality of the molded parts, since it influences the length of flow in the mold, fiber orientation,

Properties 385

flowline and other surface defects. In order to entire surface. After IMC injection, the press is reduce cycle time, sometimes the charge is pre- closed and the curing operation is repeated at heated to a temperature below gel point using or above the SMC molding pressure. infra-red or dielectric heaters. Sometimes the IMC is injected at high pressure

without mold opening and closing prior to 17.4.2 MOLD CLOSING AND FILLING complete cure of the SMC charge.

After proper placement of the charge in the core of the mold, the cavity is quickly closed to 17.4.4 PART EJECTION AND POST-CURE contact the top surface of the charge. The cavity is then closed at a slower rate, usually 4-12 mm/s. In most cases the mold is heated to (for example) 150°C, which causes the charge viscosity to be reduced. With increasing mold pressure as the mold is closed, the charge flows towards the cavity extremities, forcing air out of the cavity. The mold closing speed is very important as it induces gelation of the top charge surface if the closing speed is slow or it causes trapped air if closing speed is fast. The filling stage is usually completed in 0.5-20 s5.

Vacuum molding is increasingly being used during charge flow to reduce surface porosity and air entrapment in the part. Vacuum level is usually in the range of 7-9 x lo4 Pa (21-27 in Hg). The molding pressure based on projected part area ranges from 1 to 10MPa (100 to 1200 psi). Higher molding pressure causes sink marks, while lower pressure cause scum- ming of the mold and porosity.

17.4.3 CURING

After filling, the charge remains in the hot mold for the crosslinking reaction to be completed. The curing time is usually between 25 s to 3 min, but depends on several factors, including resin-initiator-inhibitor reactivity, part thickness and mold temperature.

Sometimes in class A or appearance grade parts, in-mold coating (IMC) is used to enhance the surface of a molded part. The most common method of IMC injection requires opening the mold by a small amount (0.2-0.5 mm) after the curing cycle. IMC is usually a coating of polyester or polyester-urethane hybrid which covers the

At the end of the molding cycle, the mold is opened and the part is ejected from the core with the use of integral ejector pins and allowed to cool to ambient temperature. Hot parts are handled carefully and are usually placed on a support racks to cool to ambient temperature. As the part cools outside the mold, it continues to cure and shrink which creates residual stresses due to differential cooling at various sections in the part. After the part is placed on support rack, it is deflashed while still hot and stored in racks for secondary operations like punching, drilling, bonding, etc.

The compression molding process is complex and there are several important variables that influence molding. Compression molding may also produce a variety of surface and internal defects which can be eliminated by proper material selection, part design and molding technique. Details of the molding variables and the source and remedies of major molding defects are available in the lit- erature5,'.

17.5 PROPERTIES

The properties of a polymer composite can be tailored, within limitations, to meet different applications by designing its formulation. This unique characteristic of polymer composites makes definition of detailed properties difficult. The properties are usually used for information and guidelines for preliminary part design, material selection and to under- stand the effect of formulation variables on mechanical properties.


17.5.1 STATIC PROPERTIES

Table 17.7 shows the static and impact properties of SMC, BMC, ZMC and LPMC. In general, tensile and flexural properties are routinely measured and are presented here. Compressive and shear properties are measured only for use in special applications. The static properties of SMC and BMC are highly dependent on the fiber content, length, type and orientation. Tensile strength increases sig- nificantly with increasing fiber content; however, the tensile modulus is affected only moderately. Increasing the length of chopped fiber increases the tensile strength, but has no effect on the modulus. Glass fiber type (E-glass or S-glass) has a significant effect on both the tensile strength and modulus. The resin chemistry also influences tensile properties at low fiber content. In general, flexural and compressive properties follow the same trend as the tensile properties. Flexural strength is always higher than tensile strength, though the modulus may be comparable.

17.5.2 FATIGUE PROPERTIES

The fatigue properties of SMC and BMC are usually based on tensile cyclic loading of unnotched specimen. A typical S-N diagram is shown in Fig. 17.221. In general, the fatigue strength increases with increasing fiber content and there is no fatigue limit, unlike low carbon steel. Details of such testing are published elsewherez1,".

Table 17.7 Static and impact properties"

17.5.3 OTHER PROPERTIES

Several other tests are now being performed to correlate properties with operation conditions. The dynamic mechanical analyzer (DMA) is used to measure complex and storage modulus at various temperatures and frequency ranges. The effect of environmental conditions on various properties is tested to simulate end-use environment. Creep and stress relaxation tests are also done on SMC/BMC for use in structural applications. Electrical properties are also important, permitting BMC to be used in electrical applications. Arc resistance is important and dielectric strength, dielectric constant, dis- sipation factor, etc. are also measured.

17.6 APPLICATIONS

Reinforced composites materials offer the maximum design versatility and capability of any material. With the excellent cost/performance characteristics of reinforced composites, the variety and quantity of products being produced with these materials grow annually around the world. Matched metal molded reinforced composites should be considered when the finished product can be enhanced by one or more of the following characteristics.

Part consolidation Reinforced composites can be molded in three dimensions in one operation. Complex shapes that require multi-piece assembly using materials, such as wood or steel, may be molded in one step with the use of ribs, bosses and varying wall thickness.

Property Tensile Tensile Flexural Flexural IZOD Specific Coeficient strength modulus strength modulus impact gravity of thermal (MPa) (GPa) (MPa) (GPa) (unnotched) expansion

(x 1PPC)

SMC 65-100 9.5-14 130-200 8-14 600-1200 1.3-2.0 8-14 BMC 30-70 8-12 50-150 9-1 7 100-700 1.7-2.1 15-20 ZMC 30-70 8.5-12.5 50-150 7-12 200-500 1.8-2.0 11-27 LPMC 65-100 9.5-14 120-200 8-14 600-1200 1.8-2.0 7-10

a Published industry data

Applications 387

100

80

a" z 6 6 0 - v)

cn - E 2 40- E .- X

r" 20

-

-

-

R = 0.05

01 I I I I I I I 0.1 1 10 1 02 103 104 105 106

Number of cycles, N

Fig. 17.2 Typical fatigue !+N diagram for SMC (21) A: at 40°C; 0: at 23°C; Cl: at 93°C. [Reproduced from Composite Materials Technology: Processes and Properties (ed P.K. Mallick and 5. Newman) by permission of the publisher.]

Light weight Reinforced composites offer a greater strength-to-weight ratio than most non-reinforced plastics and many metals.

Dimensional stability Reinforced composites can maintain the critical tolerances required of the most demanding applications. Composites meet the most stringent material stiffness, dimensional tolerance, weight and cost criteria in many diverse applications.

High strength Reinforced composites have excellent strength-to-weight properties. By weight, reinforced composites surpass the tensile strength of iron, carbon and stainless steels. Many glass reinforced compounds equal or exceed the flexural strength and impact resistance of metals23.

Corrosion resistance Reinforced composites do not rust or corrode, are resistant to attack

from most organic chemicals and can be for- mulated to resist acidic and basic solutions.

Electrical resistance Reinforced composites are very poor conductors of electricity. As such, they have a lvgh dielectric strength for application in the electrical and electronic industriesz3.

Resistance to minor impact Reinforced composite components have an excellent memory characteristic. Instead of yielding or deforming under minor impact as with steel, a reinforced composite panel will deflect and spring back to its original surface form4 (Fig. 17.3).

Surface quality Reinforced composites can achieve a variety of surface textures, from very smooth and glossy to a rough texture. Insignias and alphanumeric characters can be molded as raised or indented characters.


- Steel

///// Composite

Fig. 17.3 Minor impact.

Molded-in color Color can be added to the reinforced composite compound, often elimi- nating the need for a secondary painting process.

Recycling Most reinforced composites can be recycled either by regrinding or pyrolysis. Reground material can be used as filler or reinforcing material. Pyrolysis reduces the composite into its basic components by heat- ing the material in the absence of oxygen. The process yields gas, oil and solid by-products that can be recycled back into composites, or used in building and agriculture materials 4.

Thousands of products are molded each year utilizing reinforced composites: aerospace, automotive parts, sports and recreational equipment, boats and business machines to name a few. This wide variety of applications is indicative of the versatility, capability and cost effectiveness of reinforced composites.

17.7 DESIGN CONSIDERATIONS

Given the wide range of options provided by reinforced composites, it is imperative that the designer accurately establish the functional

and performance requirements of the product or component. The designer must:

1. establish size and shape limitations based on:

0 basic end use function; 0 aesthetics and marketing; 0 shipping limitations; 0 weight requirements; 0 strength and stiffness requirements; 0 flexibility requirements or limits; 0 process limitations.

2. establish the structural requirements based on:

various loads that will be impacted to the part including weight, pressure and dynamic loads; duration of the loads on the part; temperature variations on the part and surface; number of cycles of temperature change; liquid, moisture and vapor resistance requirements; relative significance of strength-to- weight ratios.

3. establish the non-structural requirements

0 corrosion, weathering, moisture and

0 moisture and vapor penetration for con-

0 fire safety relative to combustibility; 0 flame-spread rate requirements; 0 light transmission (transparency, translu-

cency and opaqueness); 0 surface textures, both aesthetic and func-

tional; 0 surface coatings for protection or aesthet-

ics; 0 thermal insulation;

noise and sound control; 0 dielectric requirements for electrical

With the establishment of the functional and performance requirements the product design can be developed.

based on:

temperature resistance;

densation protection;

insulation24.

Design considerations 389

There are some general design principles which can assist in the development of struc- turally efficient configurations for reinforced composite components.

17.7.1 SHELL AND PLATE CONSTRUCTION

These are the most common configurations of reinforced composite parts. A reinforced composite component is constructed from layers of reinforced composite materials molded into the shape desired creating a geometric 'shell'. It is good practice to design so that only forces that place a part in tension or compression are applied to any component. Compound curve shapes provide good transmission of uniform loads into tensile and compressive forces within a part.

Ribbed configurations are often used to achieve required strength and stiffness in structural components (Fig. 17.4).

Corrugated or open ribbed configurations are used to achieve needed structural depth while efficiently using materials and fabrica- tion processes (Fig. 17.5).

17.7.2 DRAFT

Draft is a slight angle introduced relative to the direction of the opening and closing of the mold. It is necessary to design the part so that all side walls, both interior and exterior, have draft. This enables the part to be removed from the mold without hanging or rubbing, which

Circular jf Pitch )t '

Rectangular

Fig. 17.5 Corrugated configuration.

can degrade 'appearance' surfaces (Fig. 17.6). Minimum draft angle of 1" for the first

76.2 mm (3 in) of depth, 2" for 76.2-101.6 mm (34 in) of depth, 3" for 101.6-127 mm (4-6 in) of depth and 1" for every additional 50.8 mm (2in) thereafter is recommended on all surfaces parallel to the mold movement. This pertains to all part details, such as ribs, bosses, elevation changes and holes. A draft angle of 1" on standing ribs and bosses will yield a thickness change of 0.43 mm (0.017 in) per inch per sidez4.

'Zero draft' may be obtained by designing the mold in such a way so the draft-free surface lies at an angle to the mold direction. This will affect the positioning of bosses, ribs and other details of the part.

Fig. 17.4 Rib configuration.


Fi

Draft angle (1’ recommended)

mold movement

ig. 17.6 Draft.

17.7.3 RADIUS

In mold making, the radius defines the curva- ture established between two intersecting surfaces. The more generous the radius, the better the flow of molding material for a stronger part (Fig. 17.7).

A minimum radius of 1.59 mm (1/16 in) is recommended for all radii for both interior and exterior plane intersections. Radii should be designed to maintain relatively uniform part thickness (Fig. 17.8).

Ribs and bosses opposite an appearance surface should have the radii eliminated to reduce the likelihood of warpage or ’sink’ (surface depression).

Fig. 17.7 Outside radii.

1Ae” Minimum Recommended

Radius Determined ‘By Part Thickness

Fig. 17.8 Minimum draft.

17.7.4 NOMINAL THICKNESS

The nominal thickness is the overall design thickness of most of the part. It is desirable to establish uniform thickness throughout a part, to achieve minimum cure time, uniform cooling and minimize warpage and shrinkage (Fig. 17.9).

Nominal thickness for reinforced composites is 2.544.57 mm (0.100-0.180 in). Recommended minimum thickness is 1.53 mm (0.060 in). Recommended maximum thickness 25.4 mm (1.00 in).

By designing hollow ribs, bosses and elevation changes can achieve intricate part geometry while maintaining nominal thickness throughout the part.

17.7.5 EDGE STIFFENING

Edge stiffening is a design characteristic applied to unsupported edges to prevent warping or bowing. Edge turning is preferable to edge thickening due to the possibility of porosity at the edge of part caused by the lack of molding pressure on the thick area (Fig. 17.10).

Design considerations 391

Fig. 17.9 Nominal thickness. Uniform thickness promotes uniform flow and curing and minimizes the risk of warpage, distortion and telegraphing at thickness changes through the surface.

17.7.6 RIBS

Linear projections 90" from the plane surface of a part are called ribs. The use of ribs will allow the part to meet strength and rigidity requirements, preventing warpage and bowing in large plane surfaces while reducing the bulk and mass of a part.

Ribs should be designed to maintain the nominal thickness and follow the guidelines

Fig. 17.10 Edge stiffening (a) Preferred edge flange designs to increase panel stiffness; (b) Thickening the edge flange may increase cycle times.

for draft angles. They should be dimensioned so that their thickness at the juncture of the rib with its plane surface is between 75 and 90% of nominal (Figs 17.11 and 17.12).

4- 0.5"draft

Fig. 17.11 Rib geometry for class 'A' surfaces.

(0.06'') radius

+ 1.0" draft

Fig. 17.12 Rib geometry for non-appearance surfaces.


17.7.7 BOSSES

Projections from a plane surface of a part, called bosses, provide attachment and support for related components. They may be solid, hollow or have molded in inserts. They should also follow the guidelines for draft angles and nominal thickness (Fig. 17.13).

Nominal thickness should be maintained throughout part I

Fig. 17.13 Boss design.

17.7.8 INSERTS

Inserts are objects (usually metal) which are molded into a part to facilitate repetitive fastening and unfastening of associated parts and can be provided with male or female threads. Inserts can provide bearing or bushing surfaces, electrical or other mechanical connections. Inserts should have knurls, grooves or shoulders to lock them in place and should be located parallel to the direction of mold travel.

17.7.9 MOLDED-IN THREADS

It is difficult to mold a thread into reinforced composites and requires highly sophisticated and costly molds and molding procedures. Molded threads should be rounded rather than sharp. Rounded threads will resist chipping and cracking and will also facilitate flow of molding material into all areas of the thread. Molded threads are usually preferred over inserts if the threaded hole diameter is over 12.7 mrn (0.5 in), unless the thread is to be sub- jected to continual fastening and unfasteningz4.

17.7.10 MOLDED SURFACES

Surfaces exhibited by the part as it comes from the mold have not been subject to any post- molding operation other than the removal of flash. The surface of the mold will reproduce itself as the surface of the part. Many different textures can be produced on the surface of a part. High gloss surfaces can be produced by highly polished molds. Draft is critical when parts are to be textured on a vertical wall. For every 0.254 mm (0.001 in) of texture depth, draft must be increased by 1".

Raised or indented characters can be molded into the part. The characters should be rounded and smooth and positioned on the surfaces parallel to the parting line of the mold.

17.8 TOOLING

A good set of matched metal chromed steel tools is required for the optimum conditions when one is molding reinforced composites. Anticipated production quantities expected from the mold or the product end use, or both, should dictate the choice of steel as shown in Table 17.B5.

17.8.1 MOLD STRESSES

It is important to consider the stresses created by the flow of material at typical molding

Tooling 393

pressures from 4.13 to 8.37 MPa (600 to 1200 psi). Due to unbalanced flow, narrow mold sections that project from the mold surface could bend or break under such stresses. To ensure sufficient strength in the mold, rein-

forced composite parts should be designed so the height of any projecting mold section does not exceed two times the width of its base. Angular sections must not be less than 30°4 (Figs 17.14 and 17.15).

Table 17.8 Mold steel selection5

A. Production planning volumes Type of steel

Planning volumes Core Cavity

5000-20 000 parts/y

20 000-30 000 parts/y

Over 30 000 parts/y

100 000 parts or less

100 000-200 000 parts for mold life

Over 200 000 parts during mold life

AISIa-1045 steel

AIS14140 forged steel prehardened to Rockwell C


AISI-1045 steel


AIS14140 forged steel prehardened Rockwell C

of 28-32

of 28-32

of 28-32

of 28-32

AISI-1045 steel


P-20 forged steel prehardened to Rockwell C of 28-32

of 28-32

AISI-1045 steel

AIS14140 forged steel prehardened to Rockwell C of

P-20 forged steel prehardened to Rockwell C of 2&32

28-32

B. Product end use

Structural items where AISI-1045 AISI-1045 steel surface appearance is not critical, such as reinforcing panels, truck front ends, etc., where molded surface quality is of secondary importance. High-quality surface appearance decorative items, such as grille opening panels, head lamp surroundings, quarter wheel opening covers, etc. where a high degree of polish is required on the outer part of cavity surface.

AIS14140 P-20 forged steel

a American Iron and Steel Institute


B

Fig. 17.14 Projecting mold section; any projecting mold section should not exceed two times the width of its base.

17.8.2 SHEARS

All matched metal compression molds use telescoping shear edges around the perimeter of the part. The shear of the mold halves is never in contact but bypass each other as the mold closes, leaving a thin amount of flash (0.154-0.254 mm, 0.006-0.01 in). The bypass- ing feature allows the mold cavity to be fully filled regardless of small variations in charge weight.

A minimum of 3" of draft is preferred for return flanges. A minimum of 1.0 mm (0.04 in) nominal flat clearance should be provided to keep the cavity and core halves of the mold from contacting each other. A surface normal to die draw should be provided at the edge of the part. A nominal angular tolerance of QO" and a 1.5 mm radius should be allowed for mold

Fig. 17.15 Angular mold section: should not be <30°.

building and finishing allowances Fig. 17.16). Knife edge shears are to be avoided as they

create a thin mold section which can bend or break under molding pressures (Fig. 17.17).

Shear edges should be flame hardened to a Rockwell C of 55-60.

17.8.3 HEELBLOCKS

Compression molds should have heel blocks and wear plates suitable to withstand all lat- eral forces at 12.41 MPa (1800 psi) molding pressure. The heel blocks should be an integral part of the mold and flame hardened to a Rockwell C of 50-55. Bronze wear plates should be bolted opposite the heel blocks and have a minimum of 3.18 mm (1/8 in) chamfer lead in to avoid shearing off the wear plate.

Acknowledgements 395

l.0mm (0.04") min flat

t Normal to die draw +/- 200 - + 0.15mm (0.006") 1 I flash

Fig. 17.16 Shear edge design; part design requirements to maintain sufficient tool strength at the shear edges.

Fig. 17.17 Knife-edge shear should be avoided. It can be eliminated by adding a minimum flange of 5 mm (0.2 in) to the part edge.

17.8.4 GUIDE PINS

Compression molds should have leader or guide pins, the diameter of such to be a minimum of 2% of the width plus length of the mold. All guide pins should be the same height and be a minimum of 6.35 mm (0.25 in) longer than the highest point of the core. They should be chamfered for lead in and guide pin retention should be a minimum of 1.5 times the pin diameter. One guide pin should be off- set to prevent misalignment of core and cavity.

17.8.5 MOLD STOPS

Mold stops should be provided to control the mold's vertical travel. They should be flat and oil-hardened steel to a Rockwell C of 55-60. The minimum part thickness can be controlled by the mold stops.

17.8.6 SURFACE POLISH

The mold surface should be polished to the degree that is required on the surface of the part. The final 'stoning' or polishing on vertical walls should be done in draw direction.

ACKNOWLEDGEMENTS

The authors would like to express their grati- tude to Mr. M. Kilpinen and Mr. E. Kleese of GenCorp Automotive for their support and permission to write this manuscript. Thanks are due to Ms. Marialyce Orr for editing and proof-reading the manuscript.


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17-Matched Metal Compression Molding of Polymer Composites