Plastic Part Design Differences

This course examines the design considerations that make plastic components different from comparable metal parts. It focuses on designing around time-dependent material behavior, molding requirements, geometry, stress concentration, shrinkage, warpage, dimensional capability, and the interaction between part design and the mold used to produce it.

Core Topics

Plastic-specific design guidelines
Wall thickness, coring, ribs, fillets, and draft
Tolerances, shrinkage, warpage, and molding variation
Stress concentration, notch sensitivity, and weld lines
Metal-to-plastic conversion and structural stiffness
Sink marks, snap fits, and thermal expansion

Detailed Course Outline

Fundamental Plastic Part Design Guidelines
- Designing with the material’s creep modulus
- Maintaining uniform wall thickness
- Coring out thick sections
- Using ribs instead of unnecessary wall thickness
- Maintaining straight-pull mold designs where possible
- Eliminating sharp corners
- Using fillets at transitions
- Providing draft for mold release
- Increasing draft for textured surfaces
Designing with Creep Modulus
- Initial stress caused by the applied load
- Duration of loading
- Working temperature
- Using time-dependent material stiffness for long-term loading
- Distinguishing short-term material properties from long-term design properties
- Applying creep modulus when determining required plastic section size
Wall Thickness and Thick Sections
- Importance of uniform wall thickness
- Problems created by unnecessary thick sections
- Coring thick geometry
- Using geometry rather than mass to create stiffness
- Tapered wall sections in cylindrical and cup-shaped parts
- Center-gated cylindrical parts
- Using tapered sections to influence structural loading
Ribs and Structural Stiffness
- Using ribs to increase stiffness without increasing nominal wall thickness
- Increasing section modulus through geometry
- Relationship between wall thickness and stiffness
- Relationship between rib geometry and equivalent stiffness
- Designing ribbed sections to achieve the required section modulus
- Using fillets at rib intersections
- Providing draft on rib and core features
Fillets, Corners, and Stress Concentration
- Eliminating sharp internal corners
- Using fillet radii at wall transitions
- Relationship between fillet radius and nominal wall thickness
- Importance of fillets under flexural loading
- Stress concentration factors
- Relationship between decreasing radius and increasing stress concentration
- Using published stress-concentration data for critical applications
Notch Sensitivity
- Effect of geometric discontinuities on plastic performance
- Notched versus unnotched impact behavior
- Material sensitivity to stress concentrations
- Recognizing notch sensitivity as both a material and design consideration
- Reducing local stresses through appropriate geometry
Draft and Mold Release
- Providing draft on molded features
- One-degree minimum draft guideline where practical
- Draft on cores
- Additional draft requirements for surface texture
- Relationship between texture depth and required draft
- Designing to reduce mold-release forces
- Considering mold direction during part design
Straight-Pull Tool Design
- Designing features in the mold-opening direction
- Avoiding unnecessary undercuts
- Reducing tooling complexity through part geometry
- Considering tooling requirements while developing the part
- Recognizing when part geometry drives additional mold features
Plastic Part Tolerances
- Standard versus fine molding tolerances
- Recognizing realistic injection-molded dimensional capability
- Relationship between part size and achievable tolerance
- Applying tighter tolerances only where function requires them
- Considering material and molding variation when establishing dimensions
- Avoiding metal-machining tolerance expectations on molded plastic parts
Weld Lines
- Formation where separate melt-flow fronts join
- Weld lines, knit lines, meld lines, and witness lines
- Structural-strength reduction at weld-line locations
- Potential weld-line strength approaching approximately half of base-material strength
- Cosmetic effects of weld lines
- Chain entanglement across the joining flow fronts
- Air entrapment between flow fronts
- Relationship between part design, material, tooling, equipment, and process in weld-line performance
- Considering weld-line location during component design
Metal-to-Plastic Conversion
- Recognizing that direct geometric substitution is generally inappropriate
- Using plastic material behavior rather than metal properties as the design basis
- Using creep modulus for sustained loads
- Accounting for moisture-conditioned nylon properties
- Avoiding dry-as-molded nylon data for service-load calculations
- Accounting for known application moisture levels
- Increasing section geometry rather than simply increasing material thickness
Equivalent Stiffness and Section Modulus
- Relating elastic modulus and section geometry to component stiffness
- Using equivalent stiffness when converting from metal to plastic
- Determining an initial plastic wall thickness from required stiffness
- Applying the same approach when comparing alternate plastic materials
- Increasing section modulus through geometry
- Using ribs to obtain equivalent stiffness with reduced material thickness
- Two-piece and alternate structural section approaches
Rib Design and Sink Marks
- Relationship between rib thickness and local material mass
- Sink formation opposite thick sections
- Designing ribs to reduce visible sink
- Reducing concentrated material at rib intersections
- Using geometry to control sink tendency
- Balancing stiffness requirements with molding-quality requirements
Reducing and Managing Sink Marks
- Reducing excessive local wall thickness
- Coring heavy sections
- Modifying rib and boss geometry
- Using surface contour to reduce visual sensitivity
- Using texture to conceal minor sink
- Recognizing increased cosmetic sensitivity of high-gloss surfaces
- Gas-assisted injection molding as an option for appropriate thick-section designs
Surface Finish and Texture
- Relationship between surface finish and visible molding imperfections
- High-polish surfaces
- Medium- and low-polish finishes
- Satin and dull finishes
- SPI mold-surface finish classifications
- Using texture to reduce visibility of sink marks
- Considering functional surfaces separately from cosmetic surfaces
- Relationship between surface texture and draft requirements
Shrinkage and Warpage
- Material shrinkage during molding
- Interaction between part geometry and shrinkage
- Natural warp tendencies
- Warpage toward hotter areas of mold steel
- Effects of mold inserts and thin steel sections on local temperature
- Considering likely shrink and warp direction during design
- Relationship between molding conditions and finished-part geometry
Snap-Fit Design
- Using elastic deflection to create integral assembly features
- Material-dependent allowable snap-fit strain
- Relationship between snap geometry and deflection
- Considering mold stripping requirements for undercut features
- Effect of shrinkage and warpage on snap-fit function
- Uniform-stress snap designs
- Snap-fit calculations
- Cantilever snap features
- Annular snap rings
- Designing for repeated or one-time assembly as required
Orientation and Assembly Features
- Designing molded features to control component orientation
- Preventing incorrect assembly
- Using geometry to establish position and alignment
- Integrating orientation features into molded components
Thermal Expansion
- Coefficient of thermal expansion of plastic materials
- Differences between filled and unfilled materials
- Dimensional changes with temperature
- Interaction between plastic and adjoining materials
- Designing clearances for temperature change
- Thermal expansion in assemblies containing materials with different expansion rates
- Potential stresses created by constrained thermal growth
Integrating Plastic-Specific Design Requirements
- Designing around material behavior rather than nominal geometry alone
- Balancing stiffness, creep, and allowable deflection
- Maintaining moldable wall sections
- Using ribs and geometry efficiently
- Managing stress concentrations
- Providing appropriate draft and tooling access
- Establishing realistic dimensional tolerances
- Anticipating weld lines, shrinkage, sink, and warpage
- Accounting for assembly and snap-fit requirements
- Accounting for temperature-driven dimensional change
- Considering part design, material, mold tooling, equipment, and process as an interacting system

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