7 DFM Mistakes That Double Your Injection Molding Cost

90 % of expensive tool changes happen because the CAD model was never reviewed for manufacturability. I see these 7 mistakes in every third project. Each one can double the tooling cost – or delay the project by weeks.

Mistake 1: Undercuts Without Slides

Undercuts are geometries that prevent the part from ejecting. Without planned slides or lifters, the tool maker is forced into expensive afterthought solutions.

• Added cost: €2,000 – €8,000 per slide
• Solution: Identify undercuts early and either redesign the part or plan slides from the start.

Mistake 2: Too Many Cavities From the Start

A 4-cavity mold sounds efficient, but it costs 2.5× more than a single-cavity mold. For prototypes and small series, it rarely makes financial sense.

• Added cost: 150 % more tooling investment for 4-cavity vs. 1-cavity
• Solution: Start with a 1-cavity mold and scale when demand justifies it. This preserves capital and reduces risk.

Mistake 3: Wrong Material for the Application

Choosing material by price instead of function almost always backfires:

• PA6 – absorbs moisture, warps in uncontrolled storage
• PP – prone to warping with uneven wall thickness
• ABS – becomes brittle in cold, cracks in outdoor use

Solution: Base material selection on function, not price. A material review before tooling costs little and prevents expensive tool modifications.

Mistake 4: No Ejector Pin Planning

Without proper ejector pin placement, the part sticks in the mold. The result: scratches, deformation, scrap.

• Risk: Damaged parts, higher scrap rate, tool modifications
• Solution: Plan ejector locations early in the CAD stage. At least 3–4 evenly distributed pins per flat surface.

Mistake 5: Geometry Too Complex

Every undercut, free surface, and loose core drives up tooling costs. Multi-part molds are expensive and error-prone.

• Added cost: Multi-part molds, additional slides, manual extraction systems
• Solution: Simplify geometry where possible. Often a small design change eliminates an entire slide.

Mistake 6: Surface Finish Not Specified

If you do not specify a surface class, the tool maker defaults to the cheapest option – usually SPI-D2 (rough/matte). That does not always match your product requirements.

• Risk: Unexpected finish, post-polishing costs, delays
• Solution: Define the SPI class in the CAD model. For visible surfaces at least SPI-B1 (fine), for functional surfaces SPI-D1/D2 is sufficient.

Mistake 7: Tolerances Too Tight Without Function

Specifying ±0.05 mm everywhere sounds thorough, but adds 30–50 % to tooling cost. The tool maker must use harder steel, more grinding and polishing passes.

• Added cost: 30–50 % on the tool price with blanket tight tolerances
• Solution: Use standard tolerances (±0.1 – 0.2 mm) everywhere except at functional interfaces (fits, sealing surfaces).

What a DFM Review Delivers

A DFM audit identifies all seven mistakes before the tool is ordered. That saves time, money, and frustration:

• Catch all 7 mistakes early – before they get built into the tool
• Save 30–60 % on tool modification costs – consistently in practice
• DFM audit: €1,200 one-time – often saves €5,000 – €20,000 in post-hoc changes

The DFM review pays for itself on the very first tool. You receive a structured report with prioritized action items – not vague hints, but concrete measures with cost estimates.

Anton Steenken

B.Eng. · Hardware R&D Engineer · Founder of engineer your idea