Last updated: 2026-09-01 · 5 min read

Core vs Cavity in Injection Molding: How Do You Decide Which Side of the Mold a Feature Belongs To?

Every injection mold has two halves — the cavity side (stationary, typically the “A” side attached to the injection nozzle) and the core side (moving, typically the “B” side attached to the ejector mechanism). The decision of which features go on which side is not arbitrary: it determines whether the part stays on the ejector half after opening (where it must be, for automated ejection), whether the parting line falls on a cosmetic or non-cosmetic surface, and whether the mold can be manufactured efficiently.
Getting the core-cavity assignment wrong produces a mold where the part sticks to the wrong half, the parting line is visible on the cosmetic face, or the tooling cost increases unnecessarily.

Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.

Key Facts

  • The part must stay on the core (moving, B) side when the mold opens. This is achieved by designing more surface area, more undercuts, and more texture on the core side — the side with more grip retains the part. If the part sticks to the cavity (A) side, it cannot be ejected automatically and the cycle requires manual intervention.
  • The parting line is the boundary between the cavity and the core. It falls at the widest cross-section of the part (the maximum-draft diameter). The parting line is a witness line on the finished part — it should be placed on a non-cosmetic surface whenever possible.
  • The cavity side (A side) forms the exterior surface of the part; the core side (B side) forms the interior surface. For a cosmetic part, the exterior surface must be flawless — meaning the cavity inserts must be polished to the specified SPI finish, while the core inserts can have a rougher (machined) finish on non-cosmetic interior surfaces.

    • Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.

    The Grip-Balance Rule

    When the mold opens, the part must remain on the core side. This is ensured by designing the core side to have more grip than the cavity side:

    Grip Factor Core Side (B) Cavity Side (A)
    Surface contact area more (ribs, bosses, interior walls) less (exterior shell)
    Undercuts core-side undercuts retain part cavity-side undercuts pull part away
    Texture textured interior = more grip polished exterior = less grip
    Draft angle more draft = easier release from core less draft = part may stick to cavity

    If a part has a deep box shape with the exterior surface on the cavity side and the interior (ribs, bosses) on the core side, the core side naturally has more contact area and the part stays on the core.
    If the part is a shallow dish with most of the surface area on the cavity side, the part may stick to the cavity — the fix is to add retention features on the core side (texture, undercuts, or a textured ejector sleeve) or to use cavity-side ejection (a secondary ejector plate on the A side, which adds mold cost).

    Parting-Line Placement

  • Place the parting line on a non-cosmetic edge — typically the bottom edge of a housing, the back edge of a bezel, or the mating surface of an assembly. The parting line is a witness line (a thin ridge or step) that is visible on the finished part. On a cosmetic surface, the parting line is a defect; on a hidden or functional surface, it is acceptable.
  • Avoid step parting lines on cosmetic surfaces. A flat parting line (both halves meet on the same plane) produces a clean witness line. A step parting line (one half steps down to meet the other) produces a visible step that is difficult to hide, even with texture.
  • For cylindrical or round parts, the parting line is a circle at the maximum diameter. If the part is a tube or a cap, the parting line falls at the equator — which is visible. The only way to eliminate the parting-line witness on a round part is to use a collapsible core or a split cavity (both add significant mold cost).
  • For parts with multiple shut-off surfaces (where the core and cavity meet at different heights), each shut-off must be designed with a 3–5° angle to prevent metal-on-metal wear. A flat shut-off (0° angle) wears rapidly and produces flash after 10,000–50,000 shots.

    • Core Inserts vs Cavity Inserts

    Mold cavities are typically built as removable inserts rather than machined directly into the mold plate. This allows the inserts to be replaced, repaired, or modified without rebuilding the entire mold:

  • Core inserts (B side) form the interior features — ribs, bosses, screw posts, snap-fit hooks. Core inserts are typically machined from hardened tool steel (H13, S136, HRC 48–52) because interior features have thin cross-sections that are susceptible to wear and damage during ejection.
  • Cavity inserts (A side) form the exterior surface — the cosmetic face. Cavity inserts are polished to the specified SPI finish (A1 mirror to D3 matte). The steel grade is selected for polishability: S136 (stainless, HRC 50–52) for mirror finishes, NAK80 (pre-hardened, HRC 38–42) for texture and moderate polish.
  • Interchangeable inserts allow mold families — a single mold base with interchangeable cavity inserts that produce different parts. A family mold with 3 interchangeable core inserts and 3 interchangeable cavity inserts can produce 3 different parts on the same mold base, reducing the per-part tooling cost for low-volume programs.
  • Insert maintenance: core inserts wear faster than cavity inserts because they are subjected to ejection forces, abrasion from the resin (especially glass-filled grades), and thermal cycling. Budgeting for core-insert replacement every 100,000–200,000 shots (for glass-filled resins) or every 500,000–1,000,000 shots (for unfilled resins) is a standard maintenance practice.

    • Frequently Asked Questions

    What happens if the part sticks to the cavity side?

    The mold requires manual intervention — an operator or robot must into the mold and pull the part off the cavity side before the next shot. This adds 3–10 seconds to the cycle time and introduces a safety risk (operator hand inside a closing mold).
    The mold fix is to add retention features on the core side (deeper texture, a small undercut, or an ejector sleeve with grip) or to install cavity-side ejection (a secondary ejector plate on the A side, which adds $2,000–5,000 to the mold cost).

    Can the parting line be eliminated entirely?

    Not on a standard two-plate mold — the parting line is where the two mold halves meet. However, the parting line can be hidden by placing it on a non-cosmetic surface, blending it with texture (a textured parting line is less visible than a polished one), or using a secondary operation (vapor polishing, painting, or pad-printing) to cover it.
    For parts that require zero visible parting line (optical lenses, medical device housings), a collapsible core or a multi-piece cavity eliminates the witness line but adds significant mold complexity and cost.

    How does gate location affect the core-cavity decision?

    The gate is always on the cavity (A) side because the injection nozzle is on the stationary side. The resin flows from the gate into the cavity and fills from the A side toward the B side. As the resin cools and shrinks, it pulls away from the cavity walls and shrinks onto the core — which naturally helps the part stay on the core side.
    The gate vestige (the small mark left after gate removal) is always on the A-side surface, which should be the non-cosmetic side if the gate type leaves a visible mark (sub-gate, tunnel gate).

    Key Takeaways

    1. The part must stay on the core (B) side for automated ejection. Design more surface area, more texture, and more features on the core side to ensure retention. If the part sticks to the cavity side, the fix is either core-side retention features or cavity-side ejection (at additional cost).
    2. The parting line falls at the maximum-draft cross-section and should be placed on a non-cosmetic surface. A flat parting line produces a cleaner witness than a step parting line. Shut-off surfaces must have 3–5° angles to prevent wear-induced flash.
    3. Core and cavity inserts are removable for maintenance and modification. Core inserts wear faster (ejection forces, abrasion) and are typically hardened steel; cavity inserts are polished to the specified surface finish and are selected for polishability.

    A DFM review that addresses the core-cavity assignment before the mold is designed prevents the two most common post-T1 complaints: the part sticks to the wrong half, and the parting line is visible on the cosmetic surface. Both issues are fixable after the mold is built — but the fix is more expensive and more disruptive than getting the assignment right in the DFM stage.

    Sources
    Plastics Technology — mold design fundamentals
    Beaumont Technologies — mold engineering
    SPE — injection mold design handbook

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet