Design for Moldability (DFM) in Medical Device Components: Best Practices and Common Pitfalls

In medical device design, performance, compliance, and patient safety all begin with one fundamental element: moldable design. While cutting-edge technology and advanced materials help innovate medical devices, if a part is not designed with moldability in mind, even the most promising ideas can stall during production.

Design for Moldability (DFM) bridges the gap between innovative product ideas and manufacturable, scalable components. This article dives deep into best practices and common pitfalls when designing medical device components for injection molding.

Why DFM Matters in Medical Devices

Medical devices are governed by strict regulatory requirements, and most of them require consistent, high-volume production with tight tolerances, cleanroom molding, and low defect rates. A poorly designed part leads to:

• Difficult or impossible molding
• High rejection and rework rates
• Cosmetic or structural defects
• Delayed design verification (DV), OQ/PQ, and regulatory submissions
• Unnecessary tool complexity and higher costs

By implementing DFM principles early, designers can eliminate unnecessary iterations and create components that work seamlessly—both in function and manufacturability.

Key Design Elements and Common Pitfalls

1. Undercuts

The Problem: Undercuts prevent straightforward ejection from the mold. In medical components, where features like snap-fits or locking tabs are common, undercuts can increase tool complexity and cost.

Best Practices:

• Avoid internal undercuts where possible.
• Use bumpoffs (flexible features that release during ejection) if tolerances and material flexibility allow.
• Consider replacing snap features with interference fits or latching geometries that don’t require undercuts.
• If undercuts are functionally necessary, integrate side actions or lifters—but only if cost and risk are justified.

Medical Example: In diagnostic cartridges, a redesign that removed internal snap hooks (which caused sticking in the mold) allowed the use of a simpler two-plate mold, reducing cycle time and scrap.

2. Wall Thickness Variations

The Problem: Uneven wall thickness leads to uneven cooling, causing warpage, shrinkage, and internal voids.

Best Practices:

• Maintain uniform wall thickness throughout the part.
• Transition between thick and thin walls using tapered or filleted transitions.
• Use core-outs and ribs to maintain strength while reducing mass.
• Target a wall thickness range appropriate for your resin (e.g., 1.5–3.5 mm for ABS, 0.5–1.5 mm for PC or PP).

Medical Insight: In IV set components, inconsistent wall thickness in Y-connectors caused unacceptable leakage due to warpage. Redesign with uniform sections stabilized performance.

3. Gating and Ejector Pin Placement

The Problem: Improper gate or ejector pin location can cause cosmetic blemishes, short shots, or part deformation during ejection.

Best Practices:

• Gate placement should allow uniform melt flow and avoid weld lines in critical areas.
• Use sub-gates or valve gates for cleaner gate vestiges on visible or functional areas.
• Ejector pins should act on flat, supported surfaces, ideally in non-visible areas or those not involved in sealing or alignment.
• Avoid pins on thin sections—they’ll deform or punch through.

Tip: Run mold flow analysis to determine optimal gate placement and identify pressure imbalances.

4. Draft Angles

The Problem: Without adequate draft, parts stick in the mold, leading to drag marks, cosmetic flaws, and damaged tools or parts.

Best Practices:

• Apply at least 1° of draft for vertical walls; increase up to 3–5° for textured or deep features.
• On deeper parts, increase draft gradually to maintain ejection ease.
• Add draft early in design—not as a last-minute adjustment.

Medical Example: In a catheter handle mold, increasing the draft from 0.5° to 2° eliminated galling marks and reduced tool maintenance by 60%.

5. Sink Marks and Rib Design

The Problem: Thick ribs, bosses, or intersections lead to sink marks, where the outer surface deforms due to internal shrinkage.

Best Practices:

• Rib thickness should be 60–70% of adjacent wall.
• Maintain rib height ≤ 3× wall thickness to avoid flow hesitation and sink.
• Use multiple smaller ribs instead of a few large ones.
• Place ribs opposite cosmetic surfaces or include design features to hide potential sinks.

Medical Application: In pump enclosures, reducing boss base thickness and optimizing rib placement significantly improved the surface quality and eliminated rework.

6. Weld Lines

The Problem: When two molten fronts meet during mold filling, they form weld lines that may be cosmetically visible or mechanically weak.

Best Practices:

• Adjust gate location to shift weld lines away from critical stress or sealing areas.
• Use flow leaders, ribs, or overflow wells to control flow paths.
• Consider increasing melt temperature or injection speed to improve knit line bonding.
• Use mold flow simulation to predict and mitigate weld line effects.

Medical Example: A housing cover for a diagnostic device failed drop tests due to a weld line across the latch. Repositioning the gate moved the weld line to a non-load area.

7. Stress Concentrations

The Problem: Sharp corners or abrupt transitions can cause cracks, especially in brittle or transparent materials like polycarbonate.

Best Practices:

• Use internal radii ≥ 0.5× wall thickness.
• Avoid zero-radius corners; they amplify stress.
• Use fillets and gradual transitions between features.
• For load-bearing parts, simulate stress distribution to identify hot spots.

Medical Impact: In pressure-control valves, cracking at sharp corners led to field complaints. Redesign with radii and better material transitions resolved the issue.

DFM Optimization Tools

• Mold Flow Simulation (e.g., Autodesk Moldflow, SolidWorks Plastics): Predict weld lines, air traps, fill patterns.
• DFM Software (DFMPro, Siemens NX DFM): Flags undercuts, draft issues, and wall inconsistencies.
• FEA (Finite Element Analysis): For stress, fatigue, or pressure-bearing component design.
• Material Data Sheets (UL Prospector): Helps match resin properties to your part geometry and function.

Final Takeaways: The DFM Mindset

Design Aspect

Best Practice Summary

• Undercuts: Avoid or use side actions sparingly
• Wall Thickness: Keep uniform, taper thick-to-thin
• Gating: Place for balanced flow and avoid cosmetic areas
• Draft Angles: Minimum 1–2°, increase for deeper parts or textured surfaces
• Ribs & Bosses: Thin and spaced, avoid thick intersections
• Weld Lines: Keep away from functional/cosmetic areas
• Sharp Corners: Replace with generous radii or chamfers

 Common Pitfalls Summary

• Designing parts with tight tolerances but ignoring tooling variability
• Over-relying on complex side actions without evaluating cost impact
• Neglecting early DFM reviews or mold flow studies
• Ignoring material-specific shrinkage or warpage behavior
• Using features (e.g., threads, snap fits) without standard molding guidelines

Closing Thoughts

Design for Moldability (DFM) goes beyond reducing costs—it plays a critical role in enhancing quality and minimizing regulatory risk in the medical device industry. In a field where precision and compliance are non-negotiable, moldability can be the deciding factor between success and setbacks.

By incorporating DFM best practices and steering clear of common design pitfalls, engineers can streamline manufacturing, boost product performance, and accelerate time to market. The key is to integrate DFM principles early in the design phase—well before tooling begins—to support First Time Right manufacturing in this highly regulated environment.

Prioritizing moldability from the outset delivers tangible benefits, including:

• Accelerated tooling timelines
• Fewer validation cycles
• Increased part yields
• Greater device reliability
• Easier regulatory compliance

A design that is moldable is a design that is reliable, cost-effective, and scalable—the trifecta for medical device success.