Common Engineering Mistakes

Introduction

Many manufacturing problems begin long before production starts.

A part may look correct in CAD, meet its basic functional requirements, and even perform well during prototyping. However, if manufacturability, tooling, material behavior, tolerances, assembly, and production volume are not considered early, small engineering decisions can become expensive problems later.

For injection molded components, these mistakes can lead to:

• Complex tooling
• Mold modifications
• Longer cycle times
• Higher scrap
• Dimensional variation
• Difficult assembly
• Delayed production
• Increased manufacturing cost

AccuMolds' current engineering guidance consistently emphasizes reviewing product requirements, DFM, material selection, tolerances, mold design, process behavior, and production requirements before tooling begins.

The goal of engineering review is not to eliminate complexity completely.

It is to make sure that every design decision provides enough functional value to justify the manufacturing consequences.

Below are some of the most common engineering mistakes that can create avoidable production risk.

1. Designing for Function but Not Manufacturability

A component can function correctly and still be difficult to manufacture.

This often happens when product design focuses only on:

• Mechanical performance
• Appearance
• Packaging space
• Assembly function

without considering how the part will actually be molded.

Injection molding requires the part, mold, material, and process to work as one system. Small geometric decisions can affect mold complexity, filling behavior, cooling, ejection, and dimensional consistency.

A stronger approach is to review manufacturability while the design is still flexible.

Questions should include:

• Can the part release from the mold?
• Are the walls reasonably uniform?
• Are undercuts necessary?
• Can the parting line be simplified?
• Are ribs and bosses proportioned appropriately?
• Can the mold fill and cool predictably?

A successful engineering design must satisfy both product requirements and manufacturing requirements.

2. Waiting Too Long to Perform DFM

One of the most expensive engineering mistakes is delaying manufacturability review until tooling has already started.

At the CAD stage, a change may require only a model revision.

After mold manufacturing begins, the same change may require:

• Steel modification
• New inserts
• Mold welding
• Additional machining
• New sampling
• Reinspection
• Schedule changes

AccuMolds' existing DFM guidance specifically notes that manufacturing issues discovered after tooling begins can lead to mold modifications, design revisions, delays, and increased development costs.

DFM should therefore be part of design development rather than a final approval step.

Typical early review items include:

• Wall thickness
• Draft
• Ribs and bosses
• Corner radii
• Undercuts
• Gate locations
• Ejection
• Material
• Tolerances

The earlier these issues are identified, the more design freedom engineers usually have to solve them efficiently.

3. Using Uneven or Excessive Wall Thickness

Wall thickness strongly influences injection molding behavior.

Excessively thick areas can use more material and may cool more slowly than surrounding regions. Abrupt thickness transitions can also contribute to uneven shrinkage and dimensional instability. AccuMolds' DFM guidance links excessive or poorly distributed material with defects such as sink marks and warpage.

Common mistakes include:

• Thick solid sections
• Heavy wall-to-boss intersections
• Thick rib intersections
• Abrupt wall changes
• Adding mass instead of structural geometry

Rather than increasing material everywhere, engineers can often create stiffness through:

• Ribs
• Gussets
• Cored bosses
• Hollow sections
• Better load paths

The objective is not simply to make the part thinner.

It is to create more consistent and structurally efficient material distribution.

4. Ignoring Draft and Ejection

A molded part must be removed from the tool after every cycle.

Insufficient draft can increase ejection force and may contribute to:

• Drag marks
• Surface damage
• Distortion
• Difficult release
• More complex ejection systems

AccuMolds' design review guidance includes draft angle, mold opening direction, ejector pin locations, and cosmetic surface requirements as standard manufacturability considerations.

Engineers should consider ejection while designing features such as:

• Deep walls
• Textures
• Ribs
• Bosses
• Internal pockets
• Snap features

A part that cannot release predictably creates problems every production cycle.

5. Adding Unnecessary Undercuts

Undercuts are sometimes functionally necessary.

However, unnecessary undercuts can introduce:

• Slides
• Lifters
• Additional mold actions
• More moving components
• Increased maintenance
• More complicated tooling

AccuMolds' DFM content identifies unnecessary undercuts and additional mold actions as important drivers of mold complexity, manufacturing time, and maintenance requirements.

Before accepting an undercut, engineers should ask:

Can the same function be achieved through a different geometry or mold-opening direction?

Changing feature orientation, adjusting the parting line, or redesigning a retention feature can sometimes eliminate an additional mold action completely.

Complex tooling is not inherently wrong.

The mistake is adding complexity without a clear functional reason.

6. Over-Tolerancing Non-Critical Features

Tight tolerances are appropriate where function requires them.

The problem is applying precision indiscriminately.

AccuMolds' design review guidance separates critical dimensions such as assembly interfaces and sealing surfaces from non-critical dimensions such as cosmetic or non-functional features, because tolerance requirements affect manufacturing capability and production cost.

Unnecessarily tight tolerances may require:

• Higher mold precision
• More process control
• Additional inspection
• Tool adjustment
• Increased scrap risk

A better approach is to classify dimensions based on function.

Critical Dimensions

Examples include:

• Sealing interfaces
• Snap engagement
• Bearing locations
• Alignment features
• Assembly fits

Non-Critical Dimensions

These may allow greater manufacturing variation without affecting performance.

Engineering drawings should communicate where precision creates value rather than requiring maximum precision everywhere.

7. Selecting Material Too Late

Material selection affects much more than mechanical strength.

Engineers should also consider:

• Shrinkage
• Flow behavior
• Moisture sensitivity
• Mold temperature
• Chemical resistance
• Dimensional stability
• Impact performance
• Wear
• Operating temperature

AccuMolds' design review guidance specifically recommends considering both product-performance requirements and manufacturing characteristics when selecting material.

Changing resin after tooling has been developed can create unexpected consequences because different materials may require different shrinkage assumptions, processing conditions, and part geometry.

Material should therefore be evaluated early enough to influence both part and mold design.

8. Treating Prototype Success as Production Validation

A prototype proves certain things about a design, but it does not automatically prove that the part is ready for high-volume injection molding.

Production introduces additional requirements such as:

• Cycle time
• Tool durability
• Process stability
• Dimensional repeatability
• Automation
• Inspection
• Long-term consistency

AccuMolds' current DFM guidance specifically warns against optimizing only for prototype production, noting that mass production must also consider cycle time, tool life, and process stability.

A design that works at low volume may still need changes before production tooling.

Engineers should therefore distinguish between:

Can this part be made?

and

Can this part be made repeatedly, efficiently, and consistently?

9. Ignoring Production Volume

Production volume changes manufacturing strategy.

A prototype or low-volume program may prioritize:

• Lower tooling investment
• Flexibility
• Faster launch

Higher-volume production may justify:

• More durable tooling
• Additional cavities
• Automation
• Improved cooling
• More robust process control

AccuMolds' current design review content explicitly notes that prototype and mass-production programs require different approaches to tooling, process stability, and long-term reliability.

The mistake is selecting tooling or production strategy without considering expected annual and lifetime volume.

Engineering decisions should reflect the economics of the entire program.

10. Trying to Fix Design Problems with Process Settings

Injection molding machines provide many adjustable parameters.

However, process adjustments cannot permanently correct every design or tooling problem.

For example:

• Excessive wall thickness can contribute to sink
• Poor gate location can affect filling
• Inadequate venting can trap air
• Unbalanced geometry can contribute to warpage
• Difficult flow paths can contribute to short shots

AccuMolds' DFM guidance highlights these relationships between part geometry, molding defects, material flow, and cooling behavior.

A narrow process window may produce acceptable parts temporarily, but it can also make manufacturing more sensitive to normal variation.

When a defect appears repeatedly, engineers should investigate:

Part Design → Mold Design → Material → Process

rather than assuming the machine settings are always the root cause.

11. Ignoring Assembly Until the End

Manufacturing does not stop when the molded part leaves the machine.

Parts may still need to:

• Fit together
• Snap together
• Seal
• Accept fasteners
• Align with other components
• Support automated assembly

AccuMolds' design review guidance identifies assembly requirements as an important part of early engineering review because poor design decisions can increase labor, quality issues, and assembly difficulty.

Engineers should evaluate:

• Access for assembly
• Locating features
• Fastener direction
• Datum strategy
• Tolerance stack-up
• Mistake-proofing
• Serviceability

A component that molds easily but assembles poorly is not fully optimized.

12. Making Engineering Changes Without Evaluating Downstream Impact

A seemingly small CAD change can affect much more than the visible feature.

Changes may influence:

• Mold steel
• Gate location
• Cooling
• Ejection
• Tool balance
• Tolerances
• Inspection
• Assembly
• Process parameters

That is why engineering change management should include manufacturing review, not just design approval.

Before implementing a change, teams should ask:

• Does the mold need modification?
• Does the change affect critical dimensions?
• Does material distribution change?
• Will the part still eject correctly?
• Is new validation required?

Engineering changes should be evaluated as system changes.

A Better Engineering Approach

Avoiding common engineering mistakes requires a structured review process.

A practical sequence is:

1. Define functional and environmental requirements
2. Select candidate materials
3. Review the part through DFM
4. Optimize wall thickness, ribs, bosses, and draft
5. Identify critical tolerances
6. Review gate, cooling, ejection, and tooling complexity
7. Consider production volume and automation
8. Validate assembly requirements
9. Prototype and test
10. Confirm production readiness before tooling is finalized

This type of early collaboration helps reduce manufacturing risk and supports a smoother transition from design to production. AccuMolds' existing design review process similarly emphasizes collaboration between product designers, tooling engineers, material selection, DFM, and process review before production tooling is committed.

Good Engineering Prevents Problems Before Production

Many production problems are easier to prevent than to correct.

The strongest engineering teams do not wait for defects, tooling issues, or assembly failures before asking manufacturing questions.

They evaluate manufacturability while design decisions can still be changed efficiently.

For injection molded products, that means considering:

Function + Material + Part Design + Tooling + Process + Quality + Production Volume

as one connected manufacturing system.

Good engineering is not simply about designing a part that works.

It is about designing a part that can be manufactured reliably, repeatedly, and economically throughout the life of the program.

Need Engineering Support?

Early engineering review can help identify manufacturability risks before they become expensive tooling or production problems.

Whether you are developing a new injection molded component or reviewing an existing design, AccuMolds provides DFM analysis, material recommendations, mold design review, prototype validation, and production molding support. AccuMolds currently presents these engineering and production capabilities across its DFM and Engineering Insights resources.

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