Last updated: 2026-09-04 · 5 min read

Undercut Design Solutions — How Do You Eliminate or Minimize Undercuts in Your Part Design, and What Are the Cost Trade-Offs of Each Side-Action Mechanism?

An undercut is any feature that prevents straight ejection — it requires a side-action (slider, lifter or hand-loaded insert). Each side-action adds $1,500–5,000 to mold cost and 1–3 seconds to cycle time. The DFM goal: eliminate undercuts through design changes first (5 methods), then use the simplest side-action when elimination is not possible.

Key Facts

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  • Five elimination methods: add draft angle (1–2°), replace side holes with pass-through holes, snap-fit redesign (shutoff surface), flexible collapse (under 0.5mm depth), two-part split (assembly cost $0.02–0.10 per part vs. $1,500–5,000 side-action).
  • Four side-action mechanisms: angle pin slider ($1,500–3,000, +1–2s), hydraulic slider ($3,000–5,000, +2–3s), lifter for internal undercuts ($1,500–3,000, +1–2s), hand-loaded insert ($500–1,000, +10–30s, low volume only).
  • Draft analysis in CAD (Creo, SolidWorks, NX) identifies undercuts in 5–10 minutes — color-coded map: green = OK, red = undercut (needs side-action), yellow = near-vertical (may need draft). First step in DFM review.

    • Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    What Are the Five Methods to Eliminate Undercuts Through Design Changes?

    Five methods, from simplest to most invasive:

    1. Add draft angle to vertical walls. A vertical wall (0° draft) is not an undercut but can create a vacuum that prevents ejection. Adding 1–2° draft eliminates the vacuum and allows straight ejection. The dimensional change is 0.1–0.3mm at the wall base — usually acceptable for enclosures and housings.
    2. Replace side holes with pass-through holes. A hole perpendicular to the draw direction requires a side-action. If redesigned as a pass-through hole (parallel to draw direction), the side-action is eliminated — the hole is formed by a core pin (no mold cost add). The functional impact on the mating part must be verified.
    3. Use snap-fit hooks instead of threaded fasteners. If the undercut is a thread or assembly hook, redesign as a snap-fit formed by a shutoff surface (no side-action). The hook is formed by a B-side core and A-side shutoff — it ejects straight out because the shutoff provides clearance.
    4. Collapse the undercut with a flexible feature. If the undercut is small (less than 0.5mm deep), design a flexible section (thin wall, living hinge) that collapses during ejection. Material must have elongation above 50%: PP (100–600%), PE (200–1,000%), TPE suitable; PC, PMMA not suitable. Validate by flex test (1,000 cycles at maximum deflection).
    5. Split the part into two halves. If the undercut cannot be eliminated, split into two simpler parts (each without undercuts) assembled after molding. Assembly cost: $0.02–0.10 per part (ultrasonic welding or adhesive). Eliminates side-action cost ($1,500–5,000). Payback at volumes above 100,000 pieces.

    The rule: always try elimination first. The pass-through hole method is the most effective — it costs nothing in the mold and adds nothing to the cycle time. The two-part split is the last resort before adding a side-action.

    What Are the Side-Action Mechanisms When the Undercut Cannot Be Eliminated?

    Four mechanisms, from simplest to most complex:

    Mechanism Mold Cost Add Cycle Time Add When to Use
    Angle pin slider $1,500–3,000 +1–2 seconds External side holes, simple geometry
    Hydraulic slider $3,000–5,000 +2–3 seconds Multiple undercuts, complex geometry
    Lifter (internal) $1,500–3,000 +1–2 seconds Internal undercuts (snap-fit hooks, threads)
    Hand-loaded insert $500–1,000 +10–30 seconds Low volume (under 10,000 pcs), complex undercut

    The angle pin slider is the simplest — an angled guide pin moves the slider block in and out as the mold opens and closes. The hydraulic slider is used when the undercut requires a longer stroke or multiple slider movements. The lifter rides on an angle pin and lifts the internal feature as the part ejects. The hand-loaded insert is used only for low-volume programs where the cycle time increase is acceptable.

    Each side-action introduces a parting line mismatch risk — the slider parting line must be matched to the main parting line within 0.05mm. The mismatch is visible on the part surface as a step or flash line at the slider interface.

    What Is the Cost Comparison — Elimination vs. Side-Action?

    Five options compared:

    Option Mold Cost Cycle Time Part Cost Impact
    Design elimination (pass-through hole) $0 $0 Requires design approval
    Angle pin slider $1,500–3,000 +1–2s No design change
    Hydraulic slider $3,000–5,000 +2–3s Complex geometry only
    Lifter (internal) $1,500–3,000 +1–2s Internal undercuts
    Two-part split $0 (simpler mold) $0 +$0.02–0.10/part assembly

    At 500,000 pieces per year with a 20-second cycle, a +2 second cycle time from a side-action adds $0.03 per part × 500,000 = $15,000 per year in cycle time cost. Over a 3-year program: $45,000. The elimination method (pass-through hole) costs $0 in mold and $0 in cycle time — the payback is immediate.

    Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. The related guide covers adjacent topics.

    Each side-action adds $1,500–5,000 to the mold and 1–3 seconds to the cycle. Over a multi-year program at high volume, the cycle time cost dwarfs the mold cost. Eliminate the undercut first — the side-action is the last resort.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    How do you identify undercuts on a 3D model?

    Undercuts are identified by the draft analysis function in CAD software (Creo, SolidWorks, NX). The analysis highlights any surface not parallel to the draw direction — negative draft or surfaces facing the wrong direction. The analysis takes 5–10 minutes and produces a color-coded map: green = ejection OK, red = undercut (needs side-action), yellow = near-vertical (may need draft).

    The draft analysis is the first step in the DFM review — it identifies the side-action requirements before the mold design begins.

    Can you mold a full circle thread without a side-action?

    A full circle thread (like a bottle cap thread) requires a lifter (internal threads) or slider (external threads) — there is no way to form a full circle thread without a side-action because the thread wraps around the part circumference.

    However, a partial thread (less than 180° of the circumference) can sometimes be formed by a shutoff surface without a side-action. The DFM review checks whether the thread can be redesigned as a partial thread (functionally acceptable) or whether a full thread with a lifter/slider is necessary.

    What is the maximum undercut depth that can be collapsed by a flexible feature?

    The maximum collapsible undercut depth is 0.5mm for semi-crystalline materials (PP, PE, PA) and 0.3mm for amorphous materials (PC, ABS, PMMA). The flexible section must have a wall thickness of 0.5–1.0mm — thin enough to flex, thick enough to not break.

    The material must have elongation at break above 50%. PP: 100–600%, PE: 200–1,000%, PA66: 60–300%. PC: 60–100% but not suitable for repeated flexing. The design must be validated by a flex test (1,000 cycles at maximum deflection) to confirm no cracking.

    Key Takeaways

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  • Each side-action: $1,500–5,000 mold cost + 1–3s cycle time + parting line mismatch risk.
  • Five elimination methods: add draft, pass-through holes, snap-fit redesign, flexible collapse, two-part split.
  • Side-actions: angle pin slider ($1,500–3,000), hydraulic ($3,000–5,000), lifter ($1,500–3,000), hand-loaded ($500–1,000).
  • Always eliminate first — pass-through hole costs $0 and adds $0 to cycle time.

    • If your part has undercuts, send the 3D model and drawing for a DFM review.
    The review identifies each undercut, recommends the elimination method or simplest side-action mechanism, and estimates the mold cost impact.

    Sources: SolidWorks — Draft Analysis and DFM Tools · Omnexus — Design Guidelines for Injection Molding · Plastics Technology — Undercut Design Solutions · EngNet — Side-Action Mechanism Design Guide · Protolabs — Side-Action Design Reference