6: Differences in 3D Models and Detail Prints for Various Manufacturing Methods

Last updated

Mar 18, 2025
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- 5: Cost Analysis of Traditional Manufacturing vs. Additive Manufacturing Methods
- 7: Justify Your Design Decision

Bryan Guns (Northeast Wisconsin Technical College)
Northeast Wisconsin Technical College

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Figure $\PageIndex{1}$ : Picture of 3D Printed Wheel and Assembly Drawing of Wheel

Introduction

In mechanical design, the design process hinges on two critical deliverables: the 3D model, which defines the part’s geometry in a digital environment, and the detail print (or engineering drawing), which communicates precise specifications for manufacturing. These deliverables are not static; they must adapt to the chosen manufacturing method, as each process—whether injection molding, casting, or additive manufacturing—imposes unique constraints and opportunities. Tooling differences further shape the final design, influencing tolerances, features, and material use. This chapter explores how 3D models and detail prints differ across seven common manufacturing methods, how tooling impacts design, and how process shifts necessitate design adjustments, supported by case studies of real-world examples.

Overview of Manufacturing Methods and Their Design Implications

Each manufacturing method has distinct characteristics that affect the 3D model’s geometry and the detail print’s annotations. Below, we outline these methods and their implications.

Injection Molding

3D Model: Requires draft angles (1–2°) on vertical surfaces for mold release, uniform wall thickness (typically 1–3 mm) to prevent sink marks, and rounded corners to reduce stress concentrations. Features like ribs or bosses may be added for strength.
Detail Print: Specifies parting line location, gate positions, and ejection pin marks. Tolerances are tight (e.g., ±0.05 mm), reflecting mold precision.
Tooling Influence: Designs must accommodate a two-part mold (core and cavity), often requiring complex slides or inserts for undercuts, increasing tooling costs.

Casting (e.g., Sand or Die Casting)

3D Model: Incorporates draft angles (2–5° for sand, 1–3° for die) and generous fillets to aid molten metal flow. Sand casting allows thicker sections; die casting favors thinner, uniform walls.
Detail Print: Includes allowances for shrinkage (e.g., 1–2% for steel) and machining stock (2–5 mm). Sand casting prints note rougher surface finishes (Ra 12–25 µm).
Tooling Influence: Sand casting uses disposable patterns, permitting complex shapes, while die casting’s steel molds limit geometry but enhance repeatability.

Sheet Metal Fabrication

3D Model: Designs flat patterns that fold into 3D shapes, requiring bend radii (e.g., 1x material thickness) and relief cuts to prevent tearing. Features like hems or flanges are common.
Detail Print: Details bend lines, K-factors (neutral axis shift), and hole-to-edge distances (min. 2x thickness). Tolerances are moderate (e.g., ±0.1 mm).
Tooling Influence: Press brakes and punches dictate minimum bend radii and hole sizes, constraining design flexibility.

Welding

3D Model: Comprises assemblies of simpler parts, with chamfers or grooves for weld preparation. Avoids tight fits to accommodate thermal distortion.
Detail Print: Specifies weld symbols (e.g., fillet, butt), joint types, and post-weld machining needs. Tolerances are looser (e.g., ±1 mm) due to variability.
Tooling Influence: Minimal tooling (e.g., jigs, fixtures) allows complex assemblies but requires design for accessibility.

EDM Machining (Electrical Discharge Machining)

3D Model: Focuses on intricate features (e.g., sharp corners, deep cavities) unachievable by traditional cutting. No draft angles needed.
Detail Print: Notes electrode geometry and surface finish (Ra 0.1–1.6 µm). Tolerances are extremely tight (e.g., ±0.005 mm).
Tooling Influence: Custom electrodes shape the design, adding cost but enabling precision in hard materials.

CNC Machining

3D Model: Avoids undercuts unless multi-axis machines are used. Includes larger radii (e.g., 0.5 mm min.) for tool access and chamfers to reduce burrs.
Detail Print: Specifies tool paths, fixturing points, and tight tolerances (e.g., ±0.01 mm). Surface finish (Ra 0.8–3.2 µm) is detailed.
Tooling Influence: Tool size and reach limit internal features, often requiring multiple setups.

Additive Manufacturing (3D Printing)

3D Model: Permits complex, organic geometries (e.g., lattices, internal channels) without draft angles. Support structures may be modeled for overhangs (>45°).
Detail Print: Notes build orientation, layer thickness (e.g., 0.1–0.3 mm), and post-processing (e.g., support removal). Tolerances vary (e.g., ±0.1–0.5 mm).
Tooling Influence: No traditional tooling; design freedom is high, but surface finish and strength depend on process (e.g., FDM vs. SLM).

Tooling Differences and Their Impact on Final Design

Tooling—the molds, dies, fixtures, or electrodes used in manufacturing—directly influences part design. For injection molding, a split mold necessitates draft angles and restricts undercuts, whereas CNC machining’s rotating tools limit internal feature depth. Additive manufacturing eliminates tooling, enabling intricate designs but requiring support structures that affect surface quality. These constraints shape the 3D model’s geometry (e.g., adding fillets in casting) and the detail print’s specifications (e.g., noting mold parting lines).

Process Effects on 3D Models and Detail Prints

Manufacturing processes dictate design adjustments:

Material Flow: Injection molding and casting require smooth transitions (fillets, tapers) in the 3D model to ensure fill, reflected in the print’s draft annotations.
Subtractive vs. Additive: CNC machining simplifies geometry for tool access, while additive manufacturing embraces complexity, altering model topology and print callouts.
Assembly: Welding’s 3D model splits a part into sub-components, with the print detailing weld joints, unlike sheet metal’s single-part approach with bend notations.

Case Studies: Design Transitions Acrosee Manufacturing Methods

Case Study 1: Automotive Bracket (Casting to CNC Machining)

Original Method: Sand casting produced a steel bracket with a 3D model featuring 3° draft angles and 5 mm machining allowance. The detail print specified shrinkage (1.5%) and a rough finish (Ra 25 µm).
New Method: Switching to CNC machining for prototyping eliminated draft angles and reduced wall thickness from 8 mm to 6 mm for weight savings. The 3D model added chamfers for tool clearance, and the print tightened tolerances (±0.01 mm) and specified a smoother finish (Ra 1.6 µm).
Reason for Change: Faster prototyping and tighter tolerances were needed for testing.
Impact: Simplified tooling (no mold) but increased machining time and cost.

Case Study 2: Pump Housing (Injection Molding to Additive Manufacturing)

Original Method: Injection-molded ABS housing had a 3D model with 2° draft angles, uniform 2 mm walls, and ribs for stiffness. The detail print noted gate locations and ±0.05 mm tolerances.
New Method: Transition to 3D printing (SLA) for low-volume production removed draft angles and integrated internal channels for fluid flow. The 3D model included support structures, and the print specified build orientation and ±0.2 mm tolerances.
Reason for Change: Small batch size (50 units) made molding uneconomical.
Impact: Greater design freedom but coarser finish and higher per-unit cost.

Best Practices for Adapting Designs

Start with Process in Mind: Design the 3D model with the intended method’s constraints (e.g., draft for molding, tool access for CNC).
Iterate Across Methods: Use parametric modeling to adjust features (e.g., wall thickness) when switching processes.
Update Prints Rigorously: Reflect process-specific details (e.g., weld symbols, support removal) to avoid miscommunication.

Conclusion

The interplay between manufacturing methods, tooling, and design deliverables is a cornerstone of mechanical design. Injection molding demands draft and uniformity, while additive manufacturing offers geometric freedom; CNC machining excels in precision, but welding suits assemblies. These differences ripple through 3D models and detail prints, requiring engineers to tailor their approach. By understanding process constraints and tooling impacts—illustrated through case studies—students can design adaptable, manufacturable parts that meet performance and cost goals.

Exercises

Create a 3D model sketch and detail print excerpt for a simple flange using injection molding, then adapt them for CNC machining. Note changes.
Research a part made by sheet metal fabrication and propose how its design would change for additive manufacturing.
Analyze the Case Study 1 transition. Suggest a third manufacturing method and predict design adjustments.

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Case Studies: Design Transitions Acrosee Manufacturing Methods

Case Study 1: Automotive Bracket (Casting to CNC Machining)

Case Study 2: Pump Housing (Injection Molding to Additive Manufacturing)

Exercises

Introduction

Overview of Manufacturing Methods and Their Design Implications

Injection Molding

Casting (e.g., Sand or Die Casting)

Sheet Metal Fabrication

Welding

EDM Machining (Electrical Discharge Machining)

CNC Machining

Additive Manufacturing (3D Printing)

Tooling Differences and Their Impact on Final Design

Process Effects on 3D Models and Detail Prints

Case Studies: Design Transitions Acrosee Manufacturing Methods

Case Study 1: Automotive Bracket (Casting to CNC Machining)

Case Study 2: Pump Housing (Injection Molding to Additive Manufacturing)

Best Practices for Adapting Designs

Conclusion

Exercises

Further Reading

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