Common DFM Mistakes and How to Avoid Them

In product development, a design that looks perfect on screen or even works in prototype form can quickly fall apart when it reaches the factory floor. That’s where Design for Manufacturing (DFM) plays a critical role.

When DFM is overlooked, projects often suffer from spiralling costs, missed deadlines, and last-minute redesigns. The good news? Most of these issues are preventable with early awareness of common pitfalls.

In this article, we’ll explore the most frequent mistakes made in DFM — and more importantly, how to avoid them.

1. Overcomplicated Geometry

It’s easy to get carried away in CAD. Intricate curves, sharp internal corners, or ultra-thin walls might look appealing and even function in a prototype, but they can make production unnecessarily complex.

For example:

• Sharp internal corners require special tools or EDM processes, driving up machining costs.

• Thin walls in injection moulding can cause uneven cooling, warpage, or sink marks.

• Complex shapes may demand multi-part tooling, increasing both cost and lead time.

The fix: Aim for design simplicity wherever possible. Fewer features and cleaner geometry usually mean faster machining, simpler moulds, and higher yield.

2. Wrong Tolerances for the Process

Perhaps the most common — and costly — DFM mistake is specifying tolerances that don’t match the manufacturing process.

Every process has its own accepted tolerance ranges:

• CNC machining (metals): very tight tolerances, often ±0.01 mm or better.

• Casting: much looser, often ±0.5 mm or more depending on the part size.

• Injection moulding (plastics): moderate tolerances, typically ±0.1–0.2 mm.

• Rubber extrusion: highly variable tolerances due to material flexibility.

When tolerances are tighter than the process allows, two problems arise:

1. The part becomes impossible to manufacture as designed.

2. Or, it can only be produced with excessive cost due to extra finishing or special tooling.

This issue often links back to geometry and material choice. A feature achievable in machined aluminium may not translate into moulded ABS plastic, or into a rubber extrusion, because the tolerances and process limits simply don’t align.

The fix: Understand the capabilities and limits of each process early in design. Work with manufacturers or engineers who can advise what’s realistic. Set tolerances based on functional needs, not just arbitrary numbers.

3. Poor Material Choices

 

Material selection is another area where projects often go wrong. Choosing a material that doesn’t match the intended process or application can add unnecessary cost or complexity.

Common pitfalls include:

• Selecting stainless steel where aluminium would provide adequate strength at lower cost.

• Designing a part for injection moulding with a material prone to warping, making it unsuitable for the chosen geometry.

• Using high-end composites when a simpler plastic would achieve the same result.

The wrong material doesn’t just increase unit cost — it can slow down supply chains, require specialised tooling, or make scaling up impossible.

The fix: Always evaluate material options in parallel with the intended manufacturing method and performance requirements. A material that is ideal for one process may be impractical for another.

4. Designing Without Considering Assembly

Even if individual components are manufacturable, the assembly process itself can become a bottleneck if not considered early.

Examples of poor assembly design include:

• Too many separate fasteners when snap-fit or integrated features could work.

• Designs that require awkward manual handling or multiple tools to assemble.

• Over-engineered sub-assemblies that increase labour time.

These issues drive up labour costs, slow production, and increase the risk of errors.

The fix: Apply DFM principles to assembly as well. Look for opportunities to reduce part count, simplify connections, and design for ease of handling. A few small changes can dramatically improve production efficiency.

5. Forgetting Process Limitations

Each manufacturing process has built-in constraints. Ignoring them can result in designs that look fine on screen but are unbuildable in practice.

Some common examples:

• Injection moulding requires draft angles to allow parts to release from the mould.

• Sheet metal forming has bending limits based on material thickness and tooling.

• CNC machining is restricted by tool access; features hidden behind other geometry may be impossible to cut.

When these constraints are overlooked, redesign is inevitable.

The fix: Engage with the realities of the process from the start. Use design guidelines for each method, or consult suppliers and engineers who understand the limitations.

How to Avoid These Mistakes

The underlying solution is simple: involve DFM early.

• Validate geometry and tolerances against the intended process.

• Select materials in parallel with manufacturing considerations.

• Think about assembly efficiency, not just component function.

• Work closely with suppliers to understand process limits before locking in designs.

By embedding these steps into the design phase, you reduce risk, control cost, and create a smoother path from prototype to production.

The Takeaway

Most DFM mistakes stem from the same root cause: designing in isolation from manufacturing reality.

The consequences — redesigns, wasted investment, and missed deadlines — are serious, but they’re also preventable. With the right mindset and early application of DFM, you can ensure that your design doesn’t just look good, but works in the real world of production.

👉 This is the second post in our Design for Manufacturing series. Next, we’ll look at how material choices influence not only product performance but also manufacturing outcomes.