Injection Moulding Design for Manufacturability

Understanding Injection Moulding Design for Manufacturability

Injection moulding remains one of the most versatile and cost-effective manufacturing processes for producing plastic components at scale. However, the success of any injection moulded part hinges critically on decisions made during the design phase. Design for Manufacturability (DFM) in injection moulding is not merely a best practice—it is the foundation upon which product quality, production efficiency, and cost-effectiveness are built.

For manufacturers across Atlantic Canada, from automotive suppliers in Nova Scotia to marine equipment producers throughout the Maritimes, understanding DFM principles can mean the difference between a profitable product line and a costly manufacturing nightmare. At its core, DFM for injection moulding involves designing parts that can be efficiently produced while meeting all functional requirements, minimizing defects, and optimizing cycle times.

The injection moulding process involves injecting molten plastic material into a mould cavity under high pressure, typically ranging from 10,000 to 30,000 psi. The material cools and solidifies to form the final part. While this process sounds straightforward, the interplay between part geometry, material selection, mould design, and processing parameters creates a complex system where small design oversights can lead to significant manufacturing challenges.

Wall Thickness Optimization and Uniformity

Perhaps no single design element affects injection moulding success more than wall thickness. The general rule for wall thickness uniformity cannot be overstated: maintain consistent wall thickness throughout the part whenever possible. Variations in wall thickness lead to differential cooling rates, which in turn cause warpage, sink marks, and internal stresses.

For most common thermoplastics, recommended wall thicknesses fall within specific ranges:

• ABS (Acrylonitrile Butadiene Styrene): 1.2 mm to 3.5 mm

• Polypropylene: 0.8 mm to 3.8 mm

• Polycarbonate: 1.0 mm to 4.0 mm

• Nylon (PA6, PA66): 0.8 mm to 3.0 mm

• HDPE (High-Density Polyethylene): 0.8 mm to 4.0 mm

When wall thickness variations are unavoidable due to functional requirements, transitions should be gradual rather than abrupt. A transition ratio of 3:1 (length to thickness change) helps maintain proper material flow and reduces stress concentrations. For example, if transitioning from a 3.0 mm wall to a 2.0 mm wall, the transition zone should extend at least 3.0 mm along the flow path.

Thick sections pose particular challenges in injection moulding. They require longer cooling times, extending cycle times and reducing production efficiency. A wall thickness increase from 2.0 mm to 4.0 mm can increase cooling time by approximately 300%, as cooling time scales with the square of the wall thickness. For Maritime manufacturers competing in global markets, these efficiency losses can significantly impact competitiveness.

Addressing Thick Section Requirements

When structural requirements demand thicker sections, coring out material and adding ribs provides equivalent strength with reduced material usage and improved manufacturability. This approach maintains uniform wall thickness while achieving the necessary mechanical performance. The general guideline for rib design suggests rib thickness should be 50-60% of the adjacent wall thickness to prevent sink marks on the opposite surface.

Draft Angles and Part Release

Draft angles are the slight tapers applied to vertical surfaces of a moulded part to facilitate ejection from the mould. Without adequate draft, parts can stick to mould surfaces, requiring excessive ejection force that may damage the part or mould. The friction between the cooling plastic and the steel mould surface creates significant resistance to part removal.

Standard draft angle recommendations vary based on surface finish and material:

• Smooth or polished surfaces: Minimum 0.5° to 1.0° per side

• Light texture (SPI B-2 or similar): 1.5° to 2.0° per side

• Medium texture: 2.0° to 3.0° per side

• Heavy texture or deep patterns: 3.0° to 5.0° or more per side

A practical rule for textured surfaces is to add 1.0° to 1.5° of draft for every 0.025 mm (0.001 inch) of texture depth. This ensures the texture pattern releases cleanly without dragging or surface damage. For parts with logos, text, or decorative patterns, this consideration becomes critical to maintaining aesthetic quality.

Deep draws present particular challenges for draft angles. A part with 100 mm of draw depth at 1.0° draft will have a dimensional difference of approximately 1.75 mm between the top and bottom of the drafted surface. Designers must account for this variation in mating part interfaces and functional features.

Ribs, Bosses, and Structural Features

Ribs and bosses are essential structural features in injection moulded parts, providing strength and stiffness without requiring excessive wall thickness. However, improper design of these features is among the most common causes of cosmetic defects and structural weaknesses in moulded components.

Rib Design Guidelines

Effective rib design follows established proportional relationships:

• Rib thickness: 50-60% of adjacent nominal wall thickness

• Rib height: Maximum 3 times the nominal wall thickness

• Rib spacing: Minimum 2 times the nominal wall thickness between ribs

• Draft angle: 0.5° to 1.5° per side (in addition to standard draft)

• Base radius: 25-50% of nominal wall thickness

For a part with 2.5 mm nominal wall thickness, these guidelines yield ribs approximately 1.25-1.5 mm thick, up to 7.5 mm tall, spaced at least 5.0 mm apart, with base radii of 0.6-1.25 mm. These proportions minimize sink marks on the opposite surface while providing effective structural reinforcement.

Boss Design Considerations

Bosses—the cylindrical features used for fastener attachment, alignment, or assembly—require careful attention to wall thickness and support. The outer diameter of a boss should typically be 2.0 to 2.5 times the inner diameter (hole size). For a boss accepting a No. 6 self-tapping screw (pilot hole approximately 3.0 mm), the outer diameter should be 6.0-7.5 mm.

Standing bosses connected only at their base are prone to weld line weakness and may not withstand assembly forces. Connecting bosses to nearby walls with ribs or gussets significantly improves strength. These supporting ribs should follow standard rib design guidelines to avoid creating sink-inducing thick sections.

Gate Location and Flow Analysis

The gate—the point where molten plastic enters the mould cavity—profoundly influences part quality. Gate location affects fill pattern, weld line placement, cosmetic appearance, and residual stress distribution. Modern flow simulation software allows engineers to analyse various gate configurations before committing to expensive mould steel.

Key considerations for gate placement include:

• Flow length: Position gates to minimize the maximum flow distance, keeping flow length-to-thickness ratios within material limits (typically 150:1 to 300:1 depending on material)

• Weld line control: Place gates to position weld lines in non-critical, non-visible, or structurally reinforced areas

• Gate vestige: Locate gates on non-cosmetic surfaces where gate marks are acceptable

• Balanced filling: For multi-cavity moulds or parts with complex geometry, ensure all areas fill simultaneously to prevent overpacking

Common gate types each offer distinct advantages. Edge gates provide easy removal and good flow but leave a visible vestige. Submarine (tunnel) gates automatically trim during ejection but are limited to smaller cross-sections. Hot runner systems eliminate runner waste and enable larger gate diameters but increase mould cost and complexity.

For manufacturers in Nova Scotia serving industries from aerospace to aquaculture, proper gate design ensures parts meet demanding quality requirements while maintaining production efficiency. Flow analysis during the design phase can prevent costly mould modifications after initial sampling.

Undercuts and Complex Features

Undercuts—features that prevent straight-line ejection from the mould—significantly increase mould complexity and cost. While modern mould-making techniques can accommodate virtually any geometry, each undercut requires additional mould actions such as side cores, lifters, or collapsing cores. These mechanisms add to mould cost, maintenance requirements, and cycle time.

Design strategies for managing undercuts include:

• Redesign to eliminate: Often, creative redesign can achieve the same functional result without undercuts. Through-holes can sometimes replace side holes; snap fits can be redesigned to release in the direction of draw.

• Pass-through cores: Internal undercuts on opposite sides of a part can sometimes be formed by a single core that passes completely through the part.

• Stripping undercuts: Flexible materials (PP, PE, TPE) can sometimes be stripped over shallow undercuts. The general limit is undercut depth less than 5% of the undercut diameter.

• Split moulds: Some complex parts justify moulds that split along multiple parting lines.

The cost impact of undercuts varies significantly based on complexity. Simple side actions may add $5,000-15,000 to mould cost, while complex lifter systems or hydraulic cores can add $20,000-50,000 or more. For production runs common in Atlantic Canadian manufacturing—often ranging from 10,000 to 500,000 parts—these additional mould costs must be carefully weighed against piece-part savings from design simplification.

Material Selection and Processing Considerations

DFM extends beyond geometry to encompass material selection and its implications for part design and mould construction. Different polymer families exhibit distinct behaviours that influence design parameters.

Shrinkage varies significantly between materials, affecting dimensional accuracy and tolerance capability. Semi-crystalline materials (PP, PE, nylon, POM) typically shrink 1.5-3.0%, while amorphous materials (ABS, PC, PS) shrink 0.4-0.8%. This shrinkage must be compensated in mould dimensions and can affect tolerancing strategy.

Flow characteristics determine achievable wall thickness, flow length, and feature detail. High-flow materials allow thinner walls and longer flow paths, while high-viscosity engineering resins may require thicker sections and additional gates.

Temperature sensitivity affects processing windows and part quality consistency. Materials with narrow processing windows require tighter machine controls and more sophisticated moulds with enhanced cooling systems. For Maritime manufacturers where temperature and humidity can vary significantly across seasons, material selection must account for environmental processing considerations.

Canadian manufacturers increasingly consider sustainability in material selection. Recycled content materials, bio-based polymers, and design-for-recycling principles are becoming competitive differentiators. However, recycled materials may exhibit greater property variability, requiring more robust part designs with increased safety factors.

Tolerance Specification and Quality Considerations

Appropriate tolerance specification balances functional requirements against manufacturing capability and cost. Over-tolerancing increases piece-part cost through higher reject rates, more frequent mould maintenance, and extended cycle times for improved dimensional stability.

Standard commercial tolerances for injection moulded parts typically follow material-specific guidelines. For a 100 mm dimension in ABS, a commercial tolerance might be ±0.25 mm, while a fine tolerance would be ±0.10 mm. Achieving fine tolerances requires optimized mould design, precise process control, and often post-mould conditioning.

Features critical to function should be toleranced carefully; non-critical features should use standard or coarse tolerances. Geometric tolerances (flatness, perpendicularity, concentricity) often prove more important than dimensional tolerances for assembly fit and function.

Partner with Experienced Engineering Professionals

Successful injection moulding begins long before plastic ever enters a mould. The design decisions made during product development determine manufacturing efficiency, part quality, and ultimate commercial success. By applying DFM principles systematically—optimizing wall thickness, incorporating proper draft angles, designing effective structural features, locating gates strategically, managing undercuts intelligently, and specifying appropriate tolerances—engineers can develop products that excel in both performance and manufacturability.

For companies throughout Nova Scotia, New Brunswick, Prince Edward Island, and across Atlantic Canada, access to experienced engineering support can transform product development outcomes. Whether you are developing new products for the marine industry, designing components for agricultural equipment, or creating consumer goods for national distribution, professional engineering guidance ensures your designs are optimized for injection moulding success.

Sangster Engineering Ltd., based in Amherst, Nova Scotia, provides comprehensive engineering services to manufacturers across the Maritimes and beyond. Our team brings decades of experience in product design, manufacturing process optimization, and design for manufacturability analysis. We work with clients from initial concept through production launch, ensuring designs meet functional requirements while remaining practical and cost-effective to manufacture. Contact Sangster Engineering Ltd. today to discuss how our engineering expertise can support your next injection moulding project and help bring your product vision to successful commercial reality.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.