How to injection mold a groove without a parting line?

Grooves that run perpendicular to the mold opening direction are undercuts — they can't be demolded with a simple two-plate mold. Here are the main solutions:

First — Understand Why It's a Problem

Mold opens this way →

        ↑

   [TOP MOLD]

   ___________

  |     ↕     |   ← groove here traps the part

  |___________|

   [BOTTOM MOLD]

The groove locks the part inside the mold — it can't eject straight out.

Solution 1: Side Actions (Slides)

Most common professional solution

• A steel slide moves horizontally to form the groove

• Before ejection, the slide pulls back (actuated by an angle pin)

• Part then ejects freely upward

How it works:

CLOSED:          OPENING:         EJECTING:

[Top]            [Top lifts]      [Part free]

[Slide→][Part][←Slide]  [Slides retract]   ↑ eject

[Bottom]         [Bottom]         [Bottom]

Best for:

• External grooves around perimeter

• High volume production

• Deep, precise grooves

Cost: Adds $500 – $3,000+ to mold cost per slide

Solution 2: Lifters

Best for internal grooves

• Steel lifter pins sit at an angle inside the mold

• As part ejects upward, lifters simultaneously move sideways

• Releases internal groove features

Best for:

• Internal grooves or undercuts

• Snap fit features inside part

• Ribs or slots on inner walls

Cost: Adds $300 – $1,500 per lifter

Solution 3: Collapsible Core

Best for internal circumferential grooves

• Specially designed core collapses inward after molding

• Allows part with internal groove to eject cleanly

• Common for bottle caps and threaded parts

Best for:

• 360° internal grooves

• Bottle caps, pipe fittings

• Circular internal features

Cost: High — $2,000 – $8,000+ per collapsible core

Solution 4: Rotating / Unscrewing Core

Best for helical or threaded grooves

• Core rotates as part ejects — unscrews from groove

• Motor or rack-and-pinion drives rotation

• Very precise thread or helical groove reproduction

Best for:

• Threaded grooves

• Helical features

• Bottle necks and caps

Solution 5: Hand-Loaded Inserts

Best for prototypes or very low volume

• Metal insert is manually placed in mold before each shot

• Insert forms the groove geometry

• Operator removes insert after ejection

• No complex mold mechanisms needed

Best for:

• Prototype molds

• Very low volume (under 1,000 parts)

• Complex one-off groove shapes

Cost: Low tooling cost — high labor cost per part

Solution 6: Flexible / Forced Demold

Best for shallow grooves in flexible materials

• Works only with flexible materials (PP, PE, TPU, soft PLA)

• Groove is shallow enough that part flexes and pops off core

• No special mechanism needed

• Rule of thumb: groove depth under 5% of diameter

Best for:

• Shallow snap grooves

• Flexible material parts

• Lid snap fits, living hinges

⚠️ Not suitable for rigid materials like ABS, PC, nylon

Solution 7: Split Cavity (Multiple Parting Lines)

Reposition parting line to hide it in groove

• Design parting line to fall inside the groove itself

• Parting line is hidden and non-functional in groove geometry

• No extra mechanisms needed

         Parting line hidden here

                   ↓

    ___________[groove]___________

   |           ← P.L. →          |

   |___________________________|

Best for:

• Grooves that run along natural parting direction

• O-ring grooves on cylindrical parts

• Cost-sensitive projects

Solution 8: Two-Shot / Overmolding

• First shot creates base part

• Second shot fills or forms groove area

• Eliminates need for complex single-mold solutions

Comparison Summary

Design Tips to Minimize Cost

• ✅ Reorient the part — can the groove face the parting direction?

• ✅ Add draft angle to groove walls (1–3°) for easier release

• ✅ Make groove shallower — may allow forced demold

• ✅ Use flexible material if application allows

• ✅ Hide parting line in groove — cheapest solution

• ✅ Discuss with mold maker early — before CAD is finalized