When it comes to plastic injection moulding, the difference between a good product and a great one often comes down to design. At PMG Engineering, we’ve spent over 30 years collaborating with Australian manufacturers to deliver custom plastic solutions that don’t just meet specifications—they exceed expectations.
Design for Manufacturing (DFM) isn’t just a buzzword; it’s the foundation of successful injection moulding. Whether you’re developing automotive components, packaging solutions, or specialty products, understanding these seven critical DFM principles will help you avoid costly mistakes, reduce production time, and achieve the quality you demand.
1. Draft Angles 101: The Key to Smooth Ejection
One of the most fundamental—yet often overlooked—aspects of mould design is the draft angle. Simply put, a draft angle is the slight taper applied to vertical walls of a moulded part to allow it to release easily from the mould cavity.
Why Draft Angles Matter
Without adequate draft angles, parts can stick to the mould during ejection, leading to:
- Surface scratches and cosmetic defects
- Increased cycle times as operators struggle with removal
- Potential mould damage from excessive ejection forces
- Higher rejection rates and wasted material
PMG’s Recommended Draft Angles
At PMG Engineering, we typically recommend:
- Minimum draft angle: 1-2 degrees for smooth surfaces
- Textured surfaces: 3-5 degrees or more, depending on texture depth
- Deep cavities: Increased draft proportional to cavity depth
Our in-house toolmaking capabilities allow us to precision-engineer draft angles that balance ease of ejection with your design intent, ensuring smooth production runs across our 25 world-class machines, ranging from 15 to 850 tonnes.
2. Preventing Sink Marks: The Art of Consistent Wall Thickness
Sink marks are depression defects that appear on the surface of injection-moulded parts, typically opposite thick sections or ribs. They occur when thicker areas cool more slowly than the surrounding material, creating visible surface imperfections that can ruin an otherwise perfect part.
Design Strategies to Eliminate Sink Marks
Maintain uniform wall thickness: The golden rule of injection moulding is to keep wall thickness as consistent as possible throughout your part. Aim for variations of no more than 25% between sections.
Optimal wall thickness ranges:
- Thin-walled parts: 1.0-2.5mm
- Standard parts: 2.5-4.0mm
- Thick-walled parts: Require special consideration and cooling strategies
Strategic rib design: When reinforcement is necessary, keep rib thickness at 50-60% of the nominal wall thickness and use generous fillet radii at rib-to-wall junctions.
At PMG, our product development team works with you from concept to completion, using our expertise to identify potential sink mark issues before tooling begins. This proactive approach saves time and money while ensuring your parts meet aesthetic and functional requirements.
3. Gate Selection: Your Gateway to Part Quality
The gate is where molten plastic enters the mould cavity, and its location and type have profound effects on part strength, surface finish, and overall quality. Choosing the right gate is a critical decision that impacts everything from flow patterns to cosmetic appearance.
Common Gate Types and Their Applications
Edge Gates
- Best for: Flat parts, simple geometries
- Benefits: Easy to trim, good for automated production
- Considerations: May leave visible gate marks
Submarine (Tunnel) Gates
- Best for: Parts requiring automatic degating
- Benefits: Gate mark on parting line, self-trimming
- Considerations: Limited material flow, smaller gate sizes
Hot Runner Gates
- Best for: High-volume production, multi-cavity moulds
- Benefits: No runner waste, faster cycles, better part quality
- Considerations: Higher initial tooling cost
Pin-Point Gates
- Best for: Multiple gates on single parts, balanced filling
- Benefits: Minimal gate vestige, excellent aesthetics
- Considerations: Requires trimming, can create flow lines
How Gate Selection Influences Part Strength
Gate placement determines the direction of molecular orientation in the finished part. At PMG, our toolmaking specialists analyze each design to:
- Minimize weld lines in high-stress areas
- Ensure balanced filling for dimensional stability
- Position gates where the vestige won’t compromise function or appearance
- Optimize flow to maximize part strength
Our experience with materials ranging from ABS and Polycarbonate to Nylon and TPE means we understand how different resins behave through different gate configurations.
4. Understanding Undercuts and How to Avoid Them
An undercut is any feature of a part that prevents it from being ejected straight out of a two-part mould. Undercuts complicate mould design, increase tooling costs, and can extend cycle times significantly.
Common Undercut Features
- Internal threads
- Side holes or windows
- Snap-fit features protruding outward
- Any protrusion or recess perpendicular to the draw direction
Strategies to Eliminate or Manage Undercuts
Design alternatives:
- Relocate features to align with the draw direction
- Split features across the parting line
- Use external threads instead of internal
- Design snap-fits that flex during ejection
When undercuts are unavoidable:
- Side actions/lifters: Mechanical slides that move perpendicular to the draw direction
- Hand-loaded inserts: For low-volume production
- Collapsible cores: For internal undercuts like threads
- Multi-action moulds: Complex solutions for intricate geometries
At PMG Engineering, our in-house toolmaking capabilities mean we can efficiently incorporate necessary undercut solutions while minimizing cost and cycle time impact. However, our design consultation process focuses on optimizing your part design to reduce or eliminate undercuts wherever possible.
5. Part Consolidation: Simplify Your Manufacturing
Part consolidation is the process of combining multiple components into a single injection-moulded part. When executed properly, this DFM strategy delivers significant benefits across manufacturing, assembly, and product performance.
Benefits of Part Consolidation at PMG
Reduced assembly time and labour costs: Eliminating fasteners, adhesives, and assembly steps streamlines production and reduces opportunities for assembly errors.
Fewer potential failure points: Every joint, fastener, or bonded interface represents a potential weak point. Consolidated designs improve reliability.
Lower total part cost: While the mould may be more complex, eliminating multiple parts, their individual moulds, and assembly operations typically delivers net savings.
Improved design aesthetics: Seamless, integrated designs often provide superior appearance and user experience.
Consolidation Considerations
Part consolidation isn’t always the answer. Work with PMG’s product development team to evaluate:
- Material compatibility for multi-functional parts
- Mould complexity and cost implications
- Serviceability and repair requirements
- Volume thresholds where consolidation makes economic sense
Our project management approach ensures we analyze your entire product lifecycle, not just the moulding process, to recommend the best design strategy for your specific needs.
6. Achieving Tight Tolerances: Precision Engineering at PMG
Injection moulding is capable of remarkable precision, but achieving tight tolerances requires careful attention to design, material selection, process control, and tooling quality.
Understanding Tolerance Capabilities
Standard commercial tolerances: ±0.13mm (±0.005″) for dimensions under 25mm
Achievable tight tolerances: ±0.05mm (±0.002″) with proper design and process control
Factors affecting dimensional accuracy:
- Material shrink rates (varies by resin type)
- Part geometry and wall thickness
- Tool wear over production life
- Processing parameters (temperature, pressure, cooling time)
- Environmental conditions (humidity, temperature)
PMG’s Approach to Precision Moulding
Our commitment to delivering the highest quality achievable is reflected in our precision injection moulding techniques:
High-quality tooling: Our in-house tool and die making ensures moulds are built to exacting standards with proper steel selection, surface finishes, and thermal management.
Material expertise: We work with a comprehensive range of resins, understanding the specific shrink characteristics and behaviours of each material from ABS and Acetal to Polypropylene and PBT.
Process validation: We maintain the highest quality standards through rigorous process control and validation, supported by our AspectPL MES Software system.
Design collaboration: During the product development phase, we identify dimensions requiring tight control and work with you to specify achievable tolerances that meet functional requirements without unnecessary cost.
Design Tips for Tight Tolerances
- Specify tight tolerances only where functionally necessary
- Allow wider tolerances on non-critical dimensions
- Understand that extremely tight tolerances may require secondary operations
- Design parts with minimal post-mould shrinkage and warpage potential
- Consider environmental conditions in end-use applications
7. Rib Design Tips: Strength Without Warping
Ribs are thin-wall projections used to increase part stiffness and strength without adding excessive material or weight. Proper rib design is essential to prevent warping, sink marks, and cosmetic defects while maximizing structural performance.
The Golden Rules of Rib Design
Thickness ratio: Keep rib thickness at 50-60% of the nominal wall thickness. Thicker ribs cause differential cooling that leads to sink marks and warpage.
Height considerations: Maximum rib height should be 3-5 times the wall thickness. Taller ribs require thicker walls at the base, which increases sink mark risk.
Draft angles: Apply 0.5-1.5 degrees of draft per side to facilitate mould release.
Fillet radii: Use generous radii where ribs meet the main wall (typically 25-40% of wall thickness) to reduce stress concentrations and improve material flow.
Spacing: Multiple ribs should be spaced at least 2-3 times the wall thickness apart to ensure proper material flow between them.
PMG’s Rib Design Optimization
Our product development process includes finite element analysis considerations to:
- Determine optimal rib placement for load-bearing requirements
- Balance structural performance with mouldability
- Identify alternative stiffening strategies when traditional ribs aren’t ideal
- Simulate filling and cooling to predict and prevent warpage
By combining structural engineering principles with deep injection moulding expertise, we help you design parts that deliver the strength you need with the quality finish you demand.
Bringing Your Project to Life: The PMG Advantage
Design for Manufacturing is more than following rules—it’s about understanding the intricate relationship between design intent, manufacturing capability, material behaviour, and end-use performance. At PMG Engineering, we’ve built our reputation on this understanding.
Why Australian Manufacturers Choose PMG
End-to-end capability: We’re one of the few manufacturing businesses in Australia providing full service from concept to finished goods, all using our own in-house processes.
Local expertise without the hassles: Based in Melbourne, we eliminate the frustrations of language barriers, quality issues, unpredictable delivery, and unexpected costs associated with overseas manufacturing.
Speed to market: With same-day delivery options and our commitment to delivering projects on time, every time, we help you stay ahead of the competition.
Collaborative partnership: You’ll be involved throughout the process, with easy communication and opportunities for feedback. No time zones, no language barriers—just straightforward collaboration.
Quality assurance: We maintain the highest quality standards in all aspects of our business, from client communication and product design to tooling and production.
Our Comprehensive Services Include
- Product development and prototyping
- In-house toolmaking and die manufacturing
- Injection moulding (15 to 850 tonne machines)
- Material selection expertise (virgin and sustainable options)
- Product packaging solutions
- Quality control and inspection
Whether you need 3 parts or 3,000, our team works with you to recommend the best process for your needs. We offer numerous prototype options to help validate the form, fit, and function of your future injection-moulded part design.
Start Your Next Project with Confidence
Great injection-moulded products start with great design. By implementing these seven DFM principles—proper draft angles, consistent wall thickness, strategic gate selection, undercut management, part consolidation, precision tolerancing, and optimized rib design—you’ll set your project up for manufacturing success.
At PMG Engineering, we don’t just manufacture parts; we partner with you to deliver smart plastic solutions that meet your unique requirements. Our 30 years of experience, combined with our comprehensive in-house capabilities, mean you get the expertise and service you need to bring your vision to fruition.
Ready to discuss your next injection moulding project? Contact the team at PMG Engineering today. Let’s work together to transform your product concept into a manufacturing reality.
About PMG Engineering
PMG Engineering Services Pty Ltd is a Melbourne-based, Australian-owned family business with over 30 years of experience in custom plastic injection moulding and toolmaking. We provide smart plastic solutions to manufacturers across Australia, delivering quality, innovation, and on-time performance with every project. From product development to production, we make life easier for our clients with comprehensive capabilities and an unwavering commitment to excellence.
Contact PMG Engineering
📍 Location: Braeside, Victoria
🌐 Website: https://pmgeng.com.au/
📧 Get in touch to discuss how we can help bring your next project to market