Compression moulding - A302

 

Compression moulding
Document Type	Article
Document Identifier	302
Themes	Materials deposition and consolidation management Thermal management
Tags	Deposition Fibre-bed compaction Forming Thermal transformation Cure and crystallization
Prerequisites	[[A259|]] Materials deposition and consolidation management (MDCM) [[A201|]] Viscosity (resin) Prepreg

Introduction

Compression moulding (aka, matched-die moulding) uses two mould halves that close to form a cavity that is the shape of the desired composite part. A press (typically hydraulic) is usually used to close the two mould halves and provide the consolidation pressure. A wide range of thermoset and thermoplastic composite materials can be compression moulded. This close-mould process can provide excellent (class A) surface finishes on both sides and good dimensional control. It is also capable of manufacturing parts with complex geometry and is commonly used in high volume (100,000 parts per year), short cycle production environments.

Significance

Compression moulding is one of the fastest processing method and has manufactured the vast majority of composite components. Thanks to its similarities to sheet metal stamping, compression moulding has had great success in the automotive industry producing cost effective parts such as body panels.

Scope

This page provides an overview of the compression moulding process. The process is explained from the material, shape, tooling and consumables, and equipment perspective to include all the important variables for compression moulding. Key considerations from Thermal and cure/crystallization management (TM), Materials deposition and consolidation management (MDCM) and Residual stress and dimensional control management (RSDM) are also discussed.

Process description

In a compression moulding process, the moulding material can be sheet moulding compound (SMC), bulk moulding compound (BMC), dry fibre combined with resin (similar to wet layup) or pre-preg. The moulding material is first taken out of storage, prepared to the carefully designed volume and weight. Depending on the moulding material and process, moulds are usually pre-heated. The prepared moulding material, which is called the charge, is then deposited in the lower mould half at strategically determined locations determined by a manufacturing engineer. For complex parts, pre-forms can be made ahead of time and placed directly into the mould. The mould halves are then closed at prescribed pressure and speed by a press. Once the part cures, pressure is released and the mould is opened to remove the part. Some key considerations for compression moulding are discussed below.

Schematic of moulding charge into a part with hot press

Moulding temperature

For SMC and BMC, typically the mould halves are pre-heated to 120 °C to 170 °C. The processing temperature for epoxy based moulding compound is 145 °C to 200 °C. This pre-heat temperature can change depending on the moulding material formulation, part size and geometry. To avoid thermal gradients, the mould halves should be heated uniformly, however, the female side is sometimes heated to a slightly higher temperature to assist demoulding.

Moulding speed

Once the charge is deposited, the press closes the mould halves in two steps: a faster initial step to bring the halves together and a slower second step to fully close the mould and allow the charge to flow. It is also common practice to open the halves briefly to let volatiles escape when working with condensation curing resin. A typical working speed of the press is 40 mm/s with SMC.

Moulding pressure

Typically, the moulding pressure ranges from 2 to 30 MPa (290 to 4350 psi) depending on part shape and complexity. Higher pressure is required when parts have deep drawing features, complex shapes, and higher fibre content.

Material coverage and flow

SMC charge usually covers 30 to 90% of the mould surfaces. The remaining mould cavity is filled by the forced flow when the mould halves are pressed close. The weight of the charge can be estimated by the part weight plus some excess material for edge trimming. The charge can be placed at different locations in the mould to accommodate thickness and geometry variations. However, charge placement should avoid the formation of knitlines (weld lines). Knitlines form when the charge flow fronts meet inside the mould. Because little (to no) fibre bridges across the line, knitlines have poor mechanical properties. When higher mechanical properties are required, a larger portion of the mould is covered to minimize flow-induced fibre mis-orientation.

Surface finish

For parts that need Class A surfaces, the fibre volume fraction should be below 30%. To compensate for the decrease in mechanical properties, short ribs are commonly used to stiffen the part. When utilizing ribs, design precautions should be taken to avoid sink marks forming on the surfaces. The flow of the charge also promotes good surface finishes. Hence, when good surface finish is required, usually a smaller portion of the mould is covered.

Materials used with compression moulding

Moulding compound, a material composed of short fibre and resin, is the most common material used for compression moulding. It can be in the form of sheet moulding compound, bulk moulding compound and thick moulding compound. Wet layup and prepreg can also be used with compression moulding.

Moulding compound formulation

Moulding compounds are typically formulated with reinforcements, resin, curing agent, chemical thickener, release agent, filler, low profile additives and other additives at different proportions. Ideally the moulding compound should flow easily into details of the mould while remaining homogenous.

Resin

moulding compound resins typically have low viscosities (from 1 to 250 Pa s) for easy mixing and can be modified for different applications. Among others, unsaturated polyester resin is the most used resin in moulding compounds. Phenolic, epoxy resin or vinyl ester can also be used for better chemical resistance and mechanical properties, however, cost and cure rates limit their broad use. The resin content in moulding compound can range from 18% to 50% depending on the absorbing characteristics of other ingredients. A typical resin content is around 30%. . By using high temperature cure resin systems, the material is stored at B-stage (without advancement in cure). To prolong the shelf life, moulding compound can be stored in air-conditioned rooms at 18 °C.

Reinforcement

Fibre content for moulding compound usually ranges from 5% to 50%. As fibre content increases from low to high, the moulding compound's behavior transitions from wet and sticky to dry and fluffy. In extreme cases, compound with up to 65% short fibre or 75% continuous fibre can be made. Typically, compound with around 20% fibre content provide good handleability. Long or continuous fibre reinforcements can limit part complexity. In general, compression moulded parts with low content of short and random fibre have much lower mechanical properties comparing to the continually reinforced high fibre content counterparts.

Chemical thickener

Aging (advancing the degree of cure) or chemical thickening is used to increase the compound viscosity so that it is tack free and "leather hard" for easy handling. Thickener can also prevents the segregation or separation of the ingredients. The viscosity of moulding compound is typically around 0.75-1.5 x 10⁵ Pa s. The moulding compound viscosity can be increased by aging (advancing cure) at 29.4 °C to 32.2 °C (85 °F to 90 °F) for 1 to 7 days. Chemical thickening is achieved by adding oxide or hydroxide of magnesium or calcium. Common thickener such as MgO and Mg(OH)₂ are used at 1%–1.5% and 3%–5% of the resin content.

Release agent

Internal release agent can be added to the moulding compound to assist demoulding. However, release agent content should be kept to a minimum since they can intrinsically affect properties of the composite part. Zinc or calcium stearates for example are used at 1% to 3%.

Filler

Filler plays many roles in moulding compound including increased handleability, enhanced electrical resistance and fire retardance as well as reducing cost and material shrinkage. The filler content is inversely proportional to the fibre content. A typical composition would include 25% resin, 25% reinforcement and close to 50% filler. Some desirable filler properties include low specific gravity, cost and oil absorption; nontoxic and nonabrasive, chemical purity etc. Fillers are typically silicate (silica, carbonates, sulfates and oxides) particles with size 0.5μm to 50 μm. Among which, calcium carbonate and clays are most commonly used.

Low profile additives

Low profile additives are used to reduce dimensional changes caused by resin cure shrinkage of compression moulding parts. Some low shrinkage formulation shrinks around 0.05% to 0.3% while compound with low profile additives shrinks even less (<0.05%). In comparison polyester base compound shrinks 0.3%. Low profile additives are typically pre-dissolved in styrene with solid content of around 30% to 40%; the styrene solution is then added to the moulding compound at around 50% of the basic resin.

Sheet moulding compound (SMC)

SMC is typically available in 4 mm to 6 mm (0.16 inch to 0.25 inch) thickness and can be stored in either roll forms or stacked sheets. They typically have a fibre content of 10% to 35% and fibre length between 25 to 50 mm . Up to 50% fillers can be used in SMC. A typical SMC can cost $0.70 to $1.50/lb. SMC has a limited shelf life and should be used within 2 weeks of manufacturing the moulding compound.

SMC can come in forms of SMC-R (random), SMC-C (continuous) or SMC-R/C (random/continuous). SMC-R has randomly oriented short fibre while SMC-C is reinforced with unidirectional fibres. SMC-R/C contains both randomly oriented and unidirectional fibres. With continuous unidirectional fibres, SMC-C and SMC-R/C provide better mechanical properties but are harder to mould.

Bulk moulding compound (BMC)

Bulk moulding compound (aka. "premix" or dough moulding compound) typically has a fibre content of 10% to 25% and fibre length between 6 to 12 mm (shorter than those of SMC). Fibre length longer than 12 mm are not used in BMC because they tend to entangle and degrade in the mixer. BMC can come in bulk form or extruded log or rope shapes for easy handling.

Thick moulding compound (TMC)

Thick moulding compound is SMC but in thicker form (up to 50 mm). TMC is used to eliminate multiple layers of SMC. Due to the thickness, TMC is generally less pliable.

Chopped pre-preg moulding compound

Chopped pre-preg moulding compound (CPMC) has been developed in recent years. The motivation is to offer more complex structural components with high properties at lower cost compared to traditional pre-preg/autoclave components. CPMC can come in bulk form as well as sheet form. To achieve high performance with complex geometries, CPMC components can be pre-formed before being compression moulded to minimize the charge flow. Ribs and gussets can also be easily moulded in to enhance the structural performance.

Charge flow design is one of the most important aspects for designing and manufacturing CMPC components. Within which, chip (fibre) length and width, resin viscosity and part geometry as well as the interaction of the parameters can have significant impact on the flow behavior. Chip length can range from 6.4 mm to 50.8 mm (0.25 inch to 2.0 inch). Shorter fibre can flow easier into very intricate details while longer fibre provides better structural performance. Typically 25.4 mm (1.0 inch) long chips provide a good balance. A common chip width is 3.2 mm (0.125 inch). Thermal thickening (aging) or chemical thickening can help resin achieve the optimum viscosity during moulding so that the resin and fibre move as a unit when pressure is applied. Gradual transitions (thickness change), generous radii are good practice when designing the mould/part. The loading, flowing of the charge as well as the demoulding process should also be taken into consideration.

Shape of compression moulding

Compression moulding can be used to manufacture parts with a wide range of size and shape complexity. As a general rule of thumb, parts with deep drawing features and complex shapes typically use lower moulding temperatures since it takes more time to fill the moulds. Parts with simpler shapes and uniform thickness can be moulded at higher temperatures. For thick parts, lower temperature and longer time should be used to achieve uniform cure.

Typical wall thickness for SMC parts is 2 mm to 6 mm. However, wall thickness ranges from 1.5 mm to 25 mm is possible. Special initiators may be needed when compression moulding thick sections. BMC can achieve wall thickness of 1.3 mm to 50 mm. When the thickness is non-uniform, the variation should not exceed 2:1 (thickest to thinnest portion). However, variation as much as 6:1 can be tolerated by BMC.

Flanges and shoulders can also be compression moulded to avoid secondary operations such as machining . One of the advantages of compression moulding over sheet metal stamping is the capability to mould in vertical holes and inserts. While it is possible, the charge flow has to diverge before the hole or insert and converge after it, thus creating a knitline. Hence in some cases, it is better to use a drill and install inserts after the part has been moulded. If the hole is large, the charge can be continuously deposited around the hole to avoid the forming of knitlines.

Draft angles, which facilitates easy demoulding, must be taken into account when designing compression moulded parts, 0.5 degree per side of the part is at minimum.

Tool of compression moulding

Compression moulding tooling material

Tools for compression moulding are usually made of steel and are nickel or chrome plated for better surface finish and wear resistance. AISI 4140 alloy steel or equivalent pre-hardened to Rockwell C hardness of 30-32 is used for high volume production parts that require good surface finish. AISI 1045 steel or equivalent can be used for low volume production and low surface requirement parts. Aluminum is increasingly being used with the recent advancements in high-speed aluminum machining, making compression moulding more economical.

Tool heating

Compression moulding tooling is typically heated with circulating steam, oil or electrical cartridges. Steam or oil usually provide more uniform heating because the heating source is kept at a constant temperature.

Tool features

Common features in compression moulding tools includes:

• Guide pins: guide and align the tool/mould halves
• Venting holes: allow volatiles or excess resin to escape
• Stop blocks/lands: prevent mould damage and control part depth .
• Shear edges: ensure mould is completely filled and provide additional pressure in the mould, allow excess resin to escape from the parting line and control flash. Shear edges are typically a 0.5 mm to 1 mm gap between the mould halves when closed. Excess resin in the shear edge is usually removed with sanding or waterjet when the part is processed..

Demoulding

Mechanical or pneumatic ejector pins are often used to demould the compression moulded parts. Compressed air entrance can also be fitted in the tool to demould the parts.

Equipment of compression moulding

A hot press (typically hydraulic) is commonly used to apply the required pressure and temperature cycle for compression moulded parts. The returning force (usually 25% to 30% of the press capacity) is an important parameter for separating the mould halves. Daylight, the maximum distance between the upper and lower platens of the press, should be more than three times the maximum part depth. The press stroke (travel distance) should also be more than twice of the maximum part depth.

Thermal management of compression moulding

Compression moulding usually produces parts that are thinner compared to other manufacturing processes such as RTM, or pre-preg autoclave process. However, from a thermal management perspective, achieving uniform through thickness temperature during curing can still be challenging. Upon compression, the charge that is adjacent to the mould heats up faster than the rest of the charge. Variation in cross-sectional geometries, mould temperature non-uniformity during heating/cooling, resin exotherm can all cause the part to experience a non-uniform thermal history. Avoiding sudden and large thickness changes, using steam or oil heating instead of an electrical heating source and lower heating/cooling rates typically help to achieve more uniform part temperature. If applicable, using aluminum instead of steel (mould material) can largely improve the mould heating/cooling behavior. Please refer to thermal management for more information.

Residual stress and deformation management of compression moulding

Residual stresses and deformation of compression moulded parts are mainly caused by the part experiencing a non-uniform thermal history . Please refer to thermal management of compression moulding section above.

Quality/inspection management of compression moulding

Defects: surface finish defects - fibre read through, porosity, blister, weldline (knitline)

Advantages

• Short cycle, high volume production. Cycle time is around 1 to 4 minutes
• Low material cost and low variable cost comparing to other processes
• Excellent surface finishes on both sides of the part
• Lots of similarities to metal stamping process
• Not labor intensive
• Requires little post processing and finish operations

Disadvantages

• High initial investment for equipment and moulds
• Not suitable for small volume production
• Not suitable for very large products
• SMC and BMC usually provide non-structural parts

Applications

• Automotive body panels, doors, hoods and decklids
• Components with complex geometries
• Mass transportation
• Domestic Appliances (high volume)
• Medical (ergonomic geometries)

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents	How to measure gel time - M101 How to measure reinforcement content (and corresponding matrix content) - M109
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents	Practice for Developing a Deposition Step - P157
Case Studies	Cost Comparison Study of a GFRP Leisure Boat Hull Manufacturing Method; Spray Up vs Resin Infusion - C115 Optimizing Cost vs Weight for Low Production Volume Parts - C112
Perspectives Articles	Composite materials engineering webinar session 5 - Manufacturing processes - Introduction - A124 Composite materials engineering webinar session 6 - Manufacturing processes - Prepreg processing - A125 Composite materials engineering webinar session 7 - Manufacturing processes - Liquid composite moulding - A126 Fabric Forming: how it affects design and processing, and how simulation can address this - A310 Fibre Architecture: Availability, pros and cons, and selection for my application - A309 Pultrusion of Thermoplastic Composites - A334

References

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Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

A class of polymer, some common examples include polypropylene and polyethylene.

They soften and melt upon heating (i.e. potentially recyclable), high viscosity when melted, therefore difficult to saturate fibres. Usually needs a lot of pressure and heat to process.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

Resin transfer moulding (RTM) involves loading a preform into a two (or more) piece, matched tool, closing it, and injecting resin under pressure (~15-100 psi, or ~1-7 bar).

Well suited to small to medium sized parts, limited to large sizes due to injection pressure loads and tool cost.

A central processing theme in the manufacturing cycle. This theme is concerned with managing the thermal response of materials during storage and handling or parts/tools when they are subsequently heated.