Design Review Checklist

Introduction

A successful injection molded part begins long before the mold is built.

Many manufacturing problems can be traced back to decisions made during product design. Wall thickness, draft, ribs, bosses, tolerances, material selection, gate location, parting lines, and ejection all influence whether a component can be molded consistently and economically.

A structured design review helps engineers identify these risks before tooling begins.

The objective is not simply to determine whether a part can be molded. A good review asks whether the design can be manufactured reliably, inspected effectively, assembled correctly, and produced repeatedly at the required volume.

This design review checklist summarizes the key areas engineers should evaluate before releasing an injection molded plastic component for tooling.

1. Confirm Product Requirements

Before reviewing individual CAD features, confirm the functional requirements of the part.

Review:

• Intended application
• Mechanical loads
• Operating temperature
• Chemical exposure
• Environmental conditions
• Expected product life
• Cosmetic requirements
• Assembly method
• Annual production volume
• Regulatory or industry requirements

These requirements affect nearly every downstream design decision.

For example, a material suitable for an indoor electronics housing may not be appropriate for a component exposed to chemicals, elevated temperatures, repeated mechanical loading, or outdoor environments.

The engineering team should understand what the part must accomplish before deciding how it should be molded.

2. Review Material Selection

Material selection should be evaluated together with geometry rather than treated as a separate purchasing decision.

Check whether the selected polymer provides the required:

• Strength
• Stiffness
• Impact resistance
• Temperature resistance
• Chemical resistance
• Dimensional stability
• Electrical properties
• Wear performance
• Surface appearance

Engineers should also consider how the resin behaves during injection molding.

Different thermoplastics have different flow characteristics, shrinkage rates, moisture sensitivity, processing temperatures, and reinforcement systems.

Glass-filled materials, for example, may provide additional stiffness but can influence shrinkage, surface appearance, wear on tooling, and dimensional behavior.

Material selection should therefore balance product performance with manufacturability.

3. Check Wall Thickness

Wall thickness is one of the most important features to review.

Whenever possible, maintain relatively uniform wall thickness throughout the component.

Large thickness transitions may create different cooling and shrinkage rates, increasing the risk of:

• Sink marks
• Voids
• Warpage
• Internal stress
• Long cooling cycles
• Dimensional variation

If a thicker structural region is required, consider using ribs, gussets, or cored geometry rather than creating a large solid section.

Transitions between different wall thicknesses should generally be gradual rather than abrupt.

The appropriate wall thickness depends on the material, part size, flow length, structural requirements, and molding process.

4. Verify Draft Angles

Surfaces parallel to the mold-opening direction typically require draft so the molded part can release from the tool.

During design review, check:

• External walls
• Internal walls
• Ribs
• Bosses
• Deep cavities
• Textured surfaces
• Shutoff areas

Insufficient draft can increase ejection force and may lead to drag marks, scratches, distortion, or difficult mold release.

Deep features and textured surfaces may require additional draft.

Draft should be considered early because adding it after the product geometry has been finalized may change dimensions, assembly interfaces, or cosmetic surfaces.

5. Review Ribs and Gussets

Ribs are commonly used to improve stiffness without increasing the thickness of the entire component.

Check whether ribs are:

• Appropriately proportioned
• Properly spaced
• Drafted for ejection
• Connected smoothly to surrounding walls
• Positioned to support actual loading conditions

Very thick ribs can create material buildup where they intersect the primary wall, increasing the possibility of sink marks on the opposite surface.

Ribs should provide strength through geometry rather than excessive material.

For highly loaded areas, engineers should also review whether gussets or additional structural features can distribute loads more efficiently.

6. Review Boss Design

Bosses are frequently used for screws, inserts, alignment features, and assembly points.

A boss should not simply be designed as a thick solid cylinder.

Review:

• Boss wall thickness
• Core geometry
• Draft
• Connection to the primary wall
• Supporting ribs or gussets
• Screw or insert requirements
• Distance from adjacent walls

Poor boss geometry can create sink marks, stress concentrations, cracking, or excessive material accumulation.

When possible, bosses should be supported by controlled rib geometry rather than thick sections connecting directly to surrounding walls.

7. Check Corner Radii

Sharp internal corners can concentrate stress and interfere with smooth polymer flow.

Review internal and external corners throughout the design and add appropriate radii where practical.

Proper radii can help:

• Reduce stress concentration
• Improve material flow
• Improve part durability
• Reduce localized molding pressure
• Improve tool manufacturability

Corner geometry should also be coordinated with wall thickness so that adding a radius does not unintentionally create a significantly thicker section.

8. Identify Undercuts

Review the component relative to the intended mold-opening direction.

Features such as:

• Side holes
• Snap hooks
• Internal grooves
• External grooves
• Latches
• Windows
• Recesses

may create undercuts that prevent straight ejection.

Some undercuts are necessary for product function, but they may require slides, lifters, collapsible cores, or other tooling mechanisms.

Where possible, adjusting the geometry or parting line may eliminate an undercut and simplify the mold.

Every unnecessary side action increases tooling complexity and should therefore be justified by a functional requirement.

9. Review the Parting Line

Parting-line location affects both tooling construction and the finished component.

Check whether the proposed parting line crosses:

• Cosmetic surfaces
• Sealing surfaces
• Datum surfaces
• Precision interfaces
• Snap-fit features
• Sliding surfaces
• Assembly locations

Parting lines may produce a visible seam or small amount of mismatch or flash.

Critical functional surfaces should therefore be protected whenever possible.

The parting line should also be coordinated with draft direction, undercuts, and mold construction.

10. Evaluate Gate Location and Material Flow

The gate determines where molten polymer enters the mold cavity.

Its location can influence:

• Filling pattern
• Weld-line formation
• Air trapping
• Packing
• Pressure distribution
• Shrinkage
• Warpage
• Gate vestige
• Cosmetic appearance

Engineers do not always need to define the final gate during early product design, but the geometry should provide practical gating options.

Large flat parts, thin-wall components, long flow paths, and complex geometries may require additional flow analysis before tooling.

Potential weld-line locations are particularly important around holes, structural features, sealing areas, and highly loaded regions.

11. Review Ejection Strategy

A molded part must not only fill correctly; it must also leave the mold reliably.

Consider where ejector pins or other ejection features can contact the component.

Avoid placing ejection marks on:

• Important cosmetic surfaces
• Sealing surfaces
• Precision datums
• Optical areas
• Critical assembly interfaces

The design should also provide sufficient structural support during ejection.

Thin unsupported walls or delicate features may deform if ejection forces are concentrated in the wrong location.

12. Review Tolerances

Not every dimension requires the tightest possible tolerance.

During design review, separate dimensions into categories such as:

• Function-critical
• Assembly-critical
• Inspection-critical
• Cosmetic
• Non-critical

Tight tolerances may increase mold complexity, process-control requirements, inspection effort, and manufacturing cost.

Dimensions influenced by shrinkage, long flow distances, mold-half alignment, or complex tooling mechanisms may also be more difficult to control consistently.

Apply tighter tolerances where product function requires them and allow practical manufacturing tolerance elsewhere.

13. Check Assembly Interfaces

Injection molded components rarely operate independently.

Review how the part interacts with:

• Mating plastic components
• Metal inserts
• Screws
• Bearings
• Gaskets
• Electronics
• Adhesive joints
• Snap fits
• Press fits

Verify that tolerance accumulation does not create assembly interference or excessive looseness.

Also consider assembly direction, access for tools, alignment features, and the forces applied during installation.

Designing the molded component together with the surrounding assembly helps prevent problems that may not be visible when reviewing the part alone.

14. Review Surface and Cosmetic Requirements

Identify which areas of the component are visible to the end user and which are hidden after assembly.

Define requirements for:

• Surface texture
• Gloss
• Color
• Mold finish
• Gate vestige
• Ejector marks
• Weld lines
• Parting lines
• Sink marks

Cosmetic requirements should be communicated before mold design begins because they may influence gate placement, parting-line location, tooling finish, and processing strategy.

Avoid applying premium cosmetic requirements to surfaces that do not require them.

15. Evaluate Tooling Feasibility

The final CAD model should be reviewed from the mold maker's perspective.

Questions should include:

• Can the mold open and close without interference?
• Are shutoff surfaces practical?
• Is there sufficient steel around critical features?
• Can inserts be manufactured and maintained?
• Are thin steel conditions present?
• Are slides or lifters truly necessary?
• Can cooling channels reach important areas?
• Can the tool be serviced efficiently?

A component that looks simple in CAD may require complicated tooling.

Early DFM review helps identify these issues before they become expensive tooling changes.

16. Consider Production Volume

The intended production volume should influence both part and mold design.

A prototype or low-volume tool may prioritize flexibility and speed, while high-volume production may require greater attention to:

• Mold durability
• Automation
• Cycle time
• Cooling
• Multi-cavity layouts
• Maintenance
• Process consistency

Engineers should therefore communicate expected annual volume and future scaling requirements before tooling begins.

The lowest initial tooling cost is not always the lowest total manufacturing cost.

17. Plan for Inspection

Every critical requirement should have a practical method of verification.

Review:

• Critical dimensions
• Datum strategy
• Measurement access
• Gauging requirements
• Cosmetic inspection criteria
• Functional testing
• Assembly checks

If a dimension cannot be measured consistently, controlling it during production becomes more difficult.

Inspection planning should therefore begin during design review rather than after the mold has already been manufactured.

Final Design Review Checklist

Before releasing a plastic component for injection mold tooling, confirm:

• Product requirements are clearly defined
• Material is appropriate for both function and molding
• Wall thickness is reasonably uniform
• Draft is provided where required
• Ribs and bosses avoid excessive material buildup
• Sharp corners are minimized
• Undercuts are identified
• Parting-line location is acceptable
• Practical gate locations are available
• Ejection can occur without damaging the part
• Critical tolerances are clearly identified
• Assembly interfaces have been checked
• Cosmetic requirements are defined
• Tooling feasibility has been reviewed
• Production volume has been considered
• Critical characteristics can be inspected

A design does not need to be perfect before DFM review. In fact, involving manufacturing engineers before the geometry becomes fully locked often provides more opportunity to improve the component without costly redesign.

Need Engineering Support?

A thorough design review can identify manufacturing risks before they become tooling changes, molding defects, assembly problems, or production delays.

AccuMolds can review your CAD files, material requirements, wall thickness, draft, ribs, bosses, tolerances, parting lines, gating considerations, tooling requirements, and expected production volume before mold manufacturing begins.

Our engineering support can help move your project from design review and DFM analysis through tooling, validation, and injection molding production.

For a more accurate project evaluation, provide your 2D or 3D CAD files together with material, quantity, tolerance, surface finish, and project requirements.

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