Undercut Design Guide for Injection Molding

(Design Guidelines, Tooling Solutions & Manufacturing Services by Manufyn)

In injection molding, undercuts are one of the first features flagged during DFM reviews because they directly interfere with straight mold opening and part ejection. While undercuts are common in plastic parts, they introduce additional tooling complexity, moving components, and long-term reliability risks.

From a manufacturing standpoint, undercuts are not inherently “bad,” but they are never neutral. Every undercut adds cost, affects cycle time, and increases the likelihood of maintenance issues over the mold’s life. This is why tooling engineers evaluate undercuts early—before mold design is finalized.

Poorly planned undercut injection molding strategies often lead to mold redesigns, delayed tooling, or compromised part geometry. At Manufyn, undercuts are treated as commercial and tooling decisions, not just geometric features.

Undercut Design Guidelines for Injection Molding

Undercut design in injection molding must follow stricter rules than standard features because undercuts directly interact with mold movement and ejection. Poorly designed undercuts increase stress on tooling mechanisms and raise the risk of part damage during release.

From a manufacturing standpoint, undercuts should be:

• Limited in size and depth
• Located where tooling access is feasible
• Designed with controlled engagement and release

Design undercuts that are shallow, well-supported, and positioned close to the parting line are significantly easier to tool than deep or isolated features. Large undercuts far from the parting line almost always require complex side actions or collapsible tooling, increasing cost and maintenance.

At Manufyn, undercut guidelines are applied early during DFM to ensure undercuts are not only moldable, but economically sustainable over the life of the tool.

What Qualifies as an Undercut in Plastic Molding

An undercut in plastic molding is any feature that prevents a molded part from being ejected straight out of the mold along the primary opening direction. Undercuts physically lock the part onto the mold steel unless additional tooling mechanisms are used.

In injection molding undercuts typically fall into two categories:

• External undercuts, such as side holes, snap hooks, or protrusions that extend beyond the parting line.

• Internal undercuts, including grooves, threads, or internal retention features that trap the core.

In straight-pull molds, these features block ejection and cause part damage if not addressed through tooling solutions. Many undercuts are introduced unintentionally during CAD design, especially when functional features are added without tooling considerations.

Recognizing undercuts early in the design stage is critical. Once tooling begins, resolving undercuts becomes significantly more expensive.

Can the Undercut Be Eliminated?

The most important question engineers should ask is not how to tool an undercut, but whether the undercut is necessary at all.

In many cases, undercuts exist due to design convenience rather than functional necessity. Minor geometry changes—such as splitting a feature across the parting line, adjusting wall transitions, or re-orienting snap features—can completely eliminate undercuts.

Eliminating undercuts often results in:

• Simpler mold design
• Lower tooling cost
• Shorter lead times
• More reliable long-term production

For low-risk parts, redesigning to remove undercuts can reduce mold cost by 30–50%, especially when side actions or lifters are avoided.

At Manufyn, undercut evaluation begins with redesign feasibility. Only when removal compromises function or assembly do we recommend tooling mechanisms to support undercuts.

If the Undercut Stays: Available Tooling Solutions

When an undercut cannot be eliminated without compromising part function, the next step is selecting the right tooling mechanism. This decision directly affects mold cost, cycle time, reliability, and long-term maintenance.

In undercut injection molding, there is no universal solution. Each option comes with trade-offs that must be evaluated against production volume, tolerance requirements, and expected mold life.

The three most common solutions are side actions, lifters, and collapsible cores, with occasional use of manual or secondary operations for low-volume production.

At Manufyn, tooling solutions for undercuts are selected only after evaluating whether the added complexity is commercially justified.

Undercuts in Different Plastic Materials

Material selection plays a major role in how undercuts behave during molding and ejection. A design undercut that works well in one material may fail in another.

• Flexible plastics can sometimes deform enough to release minor undercuts without tooling mechanisms.
  
• Rigid plastics require precise tooling and sufficient draft to avoid cracking or stress whitening.
  
• Glass-filled plastics amplify undercut risks due to higher stiffness and abrasive behavior, increasing wear on lifters and side actions.
  

In injection molding undercuts, material shrinkage also affects release. Materials with higher shrinkage may pull away from tooling slightly, while low-shrink materials tend to grip undercut features more aggressively.

At Manufyn, undercut design is always evaluated alongside material behavior to avoid tooling solutions that work in theory but fail in production.

Tooling Mechanisms for Undercuts: How They Really Work

• Side Actions (Side Cores)

Side actions are moving mold components that slide in from the side to form undercut features and retract before ejection. They are the most widely used solution for external undercuts in injection molding.

Side actions are reliable when properly designed, but they increase mold size, tooling cost, and cycle time. Each side action adds moving components that require precise alignment and regular maintenance. Overuse of side actions is a common cause of long-term mold downtime.

Manufyn typically recommends side actions when undercuts are functionally critical and production volume justifies the added tooling investment.

• Lifters

Lifters are angled components that move as the mold opens, releasing undercut features while assisting ejection. They are often used for smaller or less aggressive undercuts.

While lifters reduce the need for separate side actions, they have geometry limitations. Improper lifter design can cause part scuffing, inconsistent release, or premature wear. Lifters also require careful draft and surface finish control to avoid part damage.

Lifters are best suited for moderate undercuts where space or mold size constraints make side actions impractical.

• Collapsible Cores

Collapsible cores are specialized tooling solutions used for internal undercuts such as threads or internal grooves. They collapse inward during mold opening, allowing the part to be ejected.

Although effective, collapsible cores are expensive and require tight tolerances. They also limit material choices and part geometry. For this reason, they are typically reserved for high-value or high-volume parts where alternative designs are not feasible.

Manufyn evaluates collapsible cores only when internal undercuts cannot be redesigned or replaced with secondary operations.

Common Cooling Channel Design Challenges and Root Causes

Even well-machined molds underperform when cooling channels are not designed for uniform heat removal. Most cooling-related issues only become visible during trials or production, when fixes are expensive and time-consuming.

The table below summarizes the most common cooling channel injection molding problems, their root causes, and how they affect manufacturing.

Cooling Channel Design Challenges (Manufacturing View)

Undercut Design and Mold Parting Line Strategy

Many undercuts exist simply because of poor parting line decisions, not because the feature itself requires complex tooling.

By adjusting the mold parting line:

• External undercuts can often be eliminated
• Features can be split between core and cavity
• Side actions can sometimes be removed entirely

Strategic parting line placement is one of the most effective ways to reduce undercut complexity without changing part functionality. However, parting line changes affect surface finish, cosmetic appearance, and tooling layout.

Manufyn reviews undercuts together with parting line strategy to find solutions that balance appearance, tooling simplicity, and production reliability.

How Undercuts Affect Mold Cost, Cycle Time & Maintenance

Undercuts have a measurable impact on the total cost of ownership of an injection mold. While the part geometry may look unchanged, the tooling required to support undercuts significantly alters mold economics.

From a cost perspective, every undercut that requires a side action, lifter, or collapsible core increases the initial tooling cost. Side actions add machining, assembly, and alignment complexity. Lifters add wear-prone components. Collapsible cores increase both tooling price and lead time due to tight tolerances and specialized manufacturing.

Undercuts also influence cycle time. Additional tooling movements require synchronization during mold opening and closing, which extends each cycle. While the increase per cycle may appear small, it compounds significantly in high-volume production.

Maintenance is another critical factor. Moving components used to release undercuts experience wear over time. This leads to periodic downtime for adjustment or replacement, especially in molds running high volumes or abrasive materials. In production environments, this downtime directly affects throughput and delivery schedules.

At Manufyn, undercut-related tooling decisions are evaluated not just for feasibility, but for long-term cost, uptime, and production stability.

When Undercuts Are a Bad Business Decision

Not every undercut is worth tooling.

In many cases, undercuts:

• Increase mold cost disproportionately
• Add failure points in long production runs
• Reduce overall equipment efficiency

Undercuts are often a bad business decision when:

• Production volumes are high and margins are tight
• The part does not require the undercut for function
• Simpler design alternatives exist

A manufacturing-led review helps identify when undercuts add real value versus when they silently inflate cost and risk.

Manufyn’s role is not to tool every undercut—but to recommend the most commercially sensible path for each part.

Undercuts in Prototype vs Production Tooling

The way undercuts are handled depends heavily on production volume and project maturity. Solutions that are acceptable in prototype tooling often become inefficient or risky in full-scale production.

In low-volume or prototype tooling, undercuts may be handled using simplified solutions such as manual inserts or limited side actions. These approaches reduce upfront tooling cost and allow design validation without committing to complex mechanisms.

However, in production tooling, the same undercut may require a different approach. Manual operations increase labor cost and variability, while overused side actions reduce mold reliability. For high-volume parts, undercuts must be designed with automation, repeatability, and maintenance in mind.

Manufyn evaluates undercuts differently for prototypes and production molds, ensuring that early design decisions do not limit scalability or inflate long-term costs.

Common Undercut Design Failures Seen in Production

Most undercut-related failures are not caused by the presence of undercuts themselves, but by how they were evaluated—or ignored—during design and DFM. These issues typically surface after tooling trials or during sustained production.

These failures reinforce why undercuts must be treated as manufacturing risks, not just CAD features. Once tooling is cut, fixing undercut issues becomes slow and expensive.

How Manufyn Handles Undercuts During Mold Design

At Manufyn, undercut handling is embedded directly into the DFM and mold design workflow.

Every undercut is reviewed using a structured decision process:

1. Identification – All external and internal undercuts are flagged early.
   
2. Elimination Review – Design alternatives are evaluated to remove undercuts wherever possible.
   
3. Tooling Trade-off Analysis – Side actions, lifters, and collapsible cores are compared for cost, reliability, and volume suitability.
   
4. Production Validation – Selected solutions are reviewed against cycle time, maintenance, and scalability requirements.

This approach ensures that undercut solutions are commercially viable, production-ready, and aligned with long-term manufacturing goals, not just technically feasible.

Conclusion: Designing Production-Ready Undercuts

Undercuts are one of the most impactful design decisions in injection molding. When handled correctly, they enable functionality without compromising manufacturability. When handled poorly, they inflate tooling cost, slow production, and reduce mold reliability.

A manufacturing-led approach—focused on early elimination, careful tooling selection, and long-term production impact—is essential for successful undercut design.

With Manufyn, undercuts are evaluated not just for feasibility, but for cost, reliability, and scalability—ensuring your parts are ready for real production, not just CAD.

FAQs: Undercut Design in Injection Molding