A Guide To The Compression Moulding Process

Compression moulding, also known as press moulding, is a plastic manufacturing process used to shape and form Plastic injection moulding and composite materials. In this process, a material is placed in an open mould cavity, after which heat and pressure are applied to force the material to conform to the mould’s shape. The pressure is maintained until the material cures and solidifies. 

Compression moulding is a versatile, cost-effective production method suitable for high-volume manufacturing of complex, high-strength, and lightweight parts.

This article provides an in-depth look at the compression moulding process. It covers the basics of how compression moulding works, the types of compression moulding, suitable materials, the process’s advantages and limitations, applications across different industries, and best practices. 

Whether you are an engineer considering manufacturing methods or simply curious to learn more about this technology, you will find all the key information in this guide.

The basics of compression moulding 

Compression moulding uses heat and pressure to mould plastics and composites into finished products and components. Unlike injection moulding, where the material is fed into a closed mould, compression moulding starts with an open mould. 

The material, commonly thermoset plastic compounds or composites, is placed directly into the bottom half of the mould cavity. The top half of the mould is then closed, and pressure is applied. This pressure forces the material to take the shape of the mould as heat from the mould walls causes the material to cure. 

Compression moulding techniques were first developed in the mid-1800s for rubber products. The process became popular for bakelite and phenolic materials in the early 20th century, establishing itself as an industry-standard production method. 

Today, compression moulding is commonly used in the Australian manufacturing industry for high-performance composite components thanks to its ability to produce consistent, high-quality parts with excellent strength, complexity, and dimensional stability. 

Unlike injection moulding, compression moulding relies on simpler tooling, making it ideal for short prototype runs. Lead times are also faster compared to machining or thermoforming.  

However, compression moulding does have longer cycle times and higher labour requirements than injection moulding, making productivity lower for high-volume generic part production. 

How compression moulding works?

The compression moulding process consists of several key stages:

1. Material preparation

The raw material, usually a thermoset composite or plastic compound, must first be prepared. Sheet moulding compound (SMC) and bulk moulding compound (BMC) are commonly used. These compounds consist of base resin, fillers, and fibre reinforcement in putty-like solid billets engineered for the compression moulding process. SMC offers higher strength with directed fibre orientation, while BMC has isotropic properties.

2. Mould loading

Once prepared, an amount of material is weighed out to match the volume of the mould cavity. This preform may be cut, stacked, or shingled inside the bottom half of the open, heated mould. High temperatures preheat and soften the material to aid mould flow.

3. Mould closing

The top half of the mould then closes over the material, sealing and capturing the preform inside the mould cavity. Just before closure, the mould may pause to allow volatiles to escape and avoid trapped gases.

4. Pressing

With the mould fully closed, the hydraulic press then applies tonnes of pressure. Typical pressures range from 3–100 MPa. This high pressure forces the viscous material to flow and conform to every mould contour, filling out the cavity.

5. Curing

While under pressure, heat between 120 and 200°C from the mould wall initiates cross-linking as the thermoset material cures. Cure times range from seconds to minutes based on material chemistry and part thickness.

6. Cooling

Once cured, cooling channels circulate cold water to solidify the hardened part faster before removal. This cooling time ranges from 30 seconds to a few minutes, depending on the part size.

7. Demoulding

Finally, the press opens the mould halves, ejector pins push the finished part out, and the moulded component is manually removed for any finishing steps before final inspection and testing. Trim, drill, paint, or assemble parts as needed.

Compression moulding presses range from smaller 600-tonne machines for prototyping to massive 10,000-tonne presses for automotive panels and aircraft components. Press types include vertical, angle, straight-side, multi-station, or rotary compression presses.

Types of compression moulding

While the fundamentals are similar across all compression moulding techniques, some variations help tailor the process to specific part requirements:

• Positive moulding: This conventional method uses male and female mould halves, where the top mould protrudes into the bottom mould cavity. It accounts for over 75% of production.
• Negative moulding: Here, both mould halves form cavities, leaving extra space for curing flat or near-nett-shaped preforms. No high-pressure mould filling occurs.
• Transfer moulding: Preforms are first partially cured in a chamber, allowing volatiles to escape. The semisolid preform is then quickly transferred into a closed mould for final curing under high pressure.
• Injection-Compression Hybrid: Combines aspects of both processes. A short shot of thermoplastic is injection moulded into an open cavity, then compressed, similar to compression moulding, to fill, pack, and cure the material using trapped heat.

Positive moulding sees the highest pressures for optimal strength and material flow. Negative moulding offers easier preform loading. 

Transfer moulding effectively removes volatiles. Injection-compression blends productivity and quality. Understanding these methods facilitates process optimisation for a given application.

Materials suitable for compression moulding

The most common materials moulded through the press moulding process are thermosetting plastics and composites. Thermosets cure through an irreversible chemical reaction to form a rigid, crosslinked polymer, unlike thermoplastics, which simply melt or freeze to change shape. Thermosets and composites offer high strength and heat resistance but cannot be remoulded or recycled.

Common thermoset plastics suitable for compression moulding include phenolics, epoxy, vinyl ester, and polyester resin compounds. Composite materials like sheet moulding compounds are glass or carbon fibre-reinforced. Fillers like calcium carbonate improve dimensional stability further.

Material properties determine moldability and applications. Low viscosity aids flow, while thixotropy provides shape retention. Cure shrinkage must be accounted for in mould design. Thermal and mechanical properties ultimately define part performance; high strength and heat resistance are often desired.

Ongoing composite innovations for compression moulding aim to improve toughness further through nanoparticles and hybrid reinforcement, while new reactive additives trim cycle times. Bio-resins from sustainable sources are also growing in use. Such material advances expand possibilities for producing resilient, lightweight, and green products.

Advantages and limitations of compression moulding

Advantages

Compression moulding offers several benefits that make it the ideal manufacturing technique for many applications:

• Cost efficiency: Mould costs are 50–70% cheaper than injection moulding as flow is achieved by pressure, not complexity. Simpler tools cut design time. Efficient, rapid cycles boost output.
• Material versatility: Works with any thermoset plastic or composite that flows, cures, and withstands pressure and heat. Glass and carbon reinforcements achieve exceptional strength.
• Strength: Parts boast high strength-to-weight ratios thanks to optimised materials and fibrous reinforcement along flow paths. Strength meets or exceeds metal in some cases.
• Part consolidation: Complex, deeply cored sections with ribs and bosses consolidate parts, saving assembly. Labour costs are lower.
• Appearance: Class A finishes with high-gloss, smooth surfaces replicate injection moulding quality; no secondary finishing is required. 
• Distortion control: Non-directional material flow controls part warpage. Dimensional stability is excellent.

Limitations

• Part size: Large surface area parts like auto body panels approach size limitations, constrained by maximum mould force. Sections cannot vary too widely in thickness.
• Low production volumes: Above 5000+ units, injection moulding offers better productivity through faster cycles. However, break-even volumes fall as automation improves.
• Material waste: Material preparation results in 10-15% scrap rates relative to injection moulding. Costs are offset by cheaper tooling for large parts.
• Design limitations: Deep undercuts and negative draft angles should be avoided. Limiting complex geometries to non-mould filling sections allows precision where needed most.

Through simulation and innovative tooling, these limitations can be mitigated to expand possibilities for large or complex compression-moulded parts.

Applications of compression moulding

Compression moulding produces a diverse range of products across many industries. High-strength requirements and improved quality standards drive adoption.

• Automotive: body panels, bumper beams, leaf springs, grill opening panels, and trunk spacers. High mechanical performance meets safety needs.
• Aerospace: aircraft ducts, access doors, engine nacelles, and interior components. Light yet strong parts aid fuel efficiency.
• Consumer goods: high-end appliance housings, handles, connectors, and keys. Sports gear components like golf club heads and cycling frames also benefit. Premium aesthetics and performance are possible.
• Electrical: Switch plates, plug housings, sensors, and connectors capable of insulation can be moulded. Dimensional precision is maintained.

Even products like manhole covers, work surfaces, and construction materials rely on compression moulding for the optimal balance of strength, appearance, and value. As capabilities progress, many more industries could embrace its advantages.

Best practices in compression moulding

Like any manufacturing technique, following essential design and process guidelines ensures compression-moulded components meet requirements reliably at the lowest cost.

1. Mould design

Draft angles above 2 degrees aid ejection, while generous radii minimise stress areas. Preconditioned heating systems and cooling channels optimise efficiency. Proper venting prevents trapped gases, which cause flaws. Modern CAE analysis identifies issues virtually before costly prototyping.

2. Process temperature management

Heating or cooling mould sections to different set temperatures create favourable flow fronts and curing patterns. This programme maximises fill while meeting targeted cure times to boost productivity.

3. Pressure profile settings

Programmed pressure ramps, pressure holds, and gradual releases tailor force precision over the entire cycle to fill the mould yet prevent flashing. Real-time closed-loop monitoring also compensates for variabilities.

4. Quality planning

Process qualification, control plans, measurement systems analysis, and validation protocols like CpK analysis verify capability, identify improvements, and ensure ongoing compliance well before production.

When best practices are diligently followed, compression moulding provides a repeatable production method for impeccable quality and performance. Continual optimisation through the latest techniques and data-driven insights can maximise its fullest potential.

Conclusion

With its unique ability to manufacture exceptionally strong, complex, and stable composite components at high volumes economically, compression moulding fulfils a vital niche across diverse industries. 

Now a refined plastic manufacturers Australia technique guided by science and robust quality methods, its many advantages and proven reliability will certainly drive above-average growth versus other plastics processing technologies in the years ahead.

This guide has covered all the essential elements, from compression moulding basics and suitable materials to equipment and best practices. 

Innovations may expand possibilities further, but understanding the principles outlined here provides a foundation to pursue compression moulding as a versatile, high-performance production solution for almost any application. The next step lies in consulting processing experts to translate conceptual designs into moulded reality.