Mastering Undercuts in Injection Molding: Draft Angles & Best Practices

In Part 3 of our series, we explored solutions for internal undercuts, covering lifters, unscrewing molds, and collapsible cores. Those tooling strategies are critical when undercuts are unavoidable.

But what if you could prevent many undercut problems before they ever reach the mold?

This post is the final installment in our 4-part series, Mastering Undercuts in Injection Molding:

Part 1: What Are Undercuts in Injection Molding?

Part 2: Solving External Undercuts (Parting Lines & Cam/Slide Systems)

Part 3: Solving Internal Undercuts (Lifters, Unscrewing Molds & Collapsible Cores)

Part 4: Draft Angles & Best Practices for Complex Part Design (you're here)

Here, we'll focus on draft angles and proactive design practices. By applying these principles early in the design process, molders and engineers can reduce tooling complexity, improve part ejection, and lower overall production costs.

Why Draft Angles Matter in Injection Molding

A draft angle is a slight taper applied to the vertical walls of a molded part, allowing for easier removal of the part from the mold. It may look insignificant in CAD, but it plays a significant role during ejection.

Without enough draft:

• Parts stick to the mold cavity, requiring extra force to eject.
• Features like threads, snap fits, or textures can deform, warp, or tear during removal.
• Excessive stress is placed on the ejection system, shortening mold life.
• Cycle times increase as operators struggle with sticking parts.

In short, the right draft angle is one of the simplest ways to avoid undercut issues and improve production efficiency.

Recommended Draft Angles for Injection Molding

While exact recommendations vary by resin, texture, and part geometry, these are widely accepted guidelines:

• Exterior Walls: Minimum of 2° draft
• Interior Walls/Undercuts: Minimum of 3° draft
• Textured Surfaces: Add 1–2° for every 0.001" (0.025 mm) of texture depth
• Deep Cavities or Tall Features: Increase draft as depth increases, since the contact area with the mold is larger

Tip: Always check resin shrinkage data. Some materials (like polycarbonate or nylon) shrink differently, which can affect ejection behavior.

Additional Best Practices for Complex Part Design

While draft angles are the foundation, they're only part of the equation. Here are five other best practices to minimize undercut challenges:

Evaluate the Necessity of Features

Ask: Does this part truly require a slot, clip, or hook?

Removing unnecessary features can eliminate tooling complexity and save thousands in mold costs.

Plan Parting Line Placement Early

Strategic parting line placement can sometimes eliminate external undercuts entirely.

Involving toolmakers during the design stage ensures part geometry aligns with efficient mold construction.

Balance Wall Thickness

Uneven thickness can cause warpage, which makes ejection harder—even if draft angles are correct.

Consistent wall thickness improves mold filling, cooling, and release.

Use Standardized Features Where Possible

For threads, consider standard profiles that align with common unscrewing or collapsible core solutions.

For snap fits, follow industry-tested geometries that minimize stress during ejection.

Collaborate Early and Often

Moldmakers, designers, and processors should communicate early in the project to ensure a smooth workflow.

Early collaboration helps avoid last-minute redesigns or expensive tooling changes.

Balancing Design and Manufacturing Priorities

It's essential to recognize that not all undercuts can be eliminated. End users often demand features that enhance functionality, such as snap assemblies for rapid assembly, textured surfaces for improved grip, or threads for secure closures.

When that happens, your role as a designer or processor is to:

1. Apply the draft wherever possible to ease ejection.
2. Select the right tooling solution (slides, lifters, unscrewing molds, collapsible cores).
3. Balance cost vs. performance—sometimes investing in more complex tooling pays off in faster cycle times and lower scrap.

Real-World Example: Draft Angles in Action

Consider a consumer product housing with snap-fit tabs on the inside. If the tabs are designed with no draft, they'll grip the core tightly, leading to part sticking and broken features.

By adding just 3° of draft to the interior tabs:

• The part releases smoothly from the core.
• Cycle time is reduced by eliminating sticking.
• Scrap rate decreases, since clips don't break during ejection.

This small design change improves both part quality and production efficiency—without increasing the cost of the mold build.

Best Practice Checklist for Draft & Design

• Apply ≥2° draft on exterior walls

• Apply ≥3° draft on interior walls and undercuts
• Increase draft for textures (1–2° per 0.001" depth)
• Maintain uniform wall thickness to avoid warpage
• Align parting lines early to reduce external undercuts
• Use standard thread and snap-fit designs when possible
• Collaborate with toolmakers during the design stage

Conclusion

Undercuts are a natural part of modern injection molding, but they don't have to derail production. By incorporating the right draft angles and following best practices for part design, processors can reduce tooling complexity, improve cycle times, and extend mold life.

Key Takeaway: Proper draft angles are one of the simplest and most cost-effective ways to reduce undercut challenges. Combine them with wise design choices and early collaboration to prevent many problems before they occur.

This concludes our Mastering Undercuts in Injection Molding series. If you missed the earlier parts, you can catch up here:

Part 1: What Are Undercuts in Injection Molding?

Part 2: Solving External UndercutsPart 3: Solving Internal Undercuts

Part 3: Solving Internal Undercuts

Together, these four posts provide a complete toolkit for understanding, designing, and solving undercuts in injection molding—helping processors and engineers save time, cut costs, and deliver high-quality parts.

Next Step for Processors

Even the best-designed molds need clean machines to perform at their peak. That's where Asaclean® Purging Compounds come in.

Are undercuts giving you trouble in production? Our team has helped hundreds of processors improve cycle times and reduce scrap. Contact us with your questions—we’re here to help.