How to injection mold a groove without a parting line?

In injection molding, creating a groove (especially a circumferential/external groove like an O-ring groove, snap-fit channel, or sealing groove) without any visible parting line in the functional/sealing surface is challenging because parting lines are inherent where mold halves (or inserts) separate.However, several practical methods can achieve a parting-line-free groove or at least move the parting line completely out of the critical groove area. Here are the main approaches, ordered roughly from most desirable to more complex/expensive:

1. Use a collapsible core or split core (most common for true 360° groove without any parting line in groove)

• The core that forms the inside diameter + groove is made of multiple segments (typically 6–12 pieces) that collapse radially inward after cooling → before ejection.

• This allows the entire groove to be formed by a single, collapsing core piece set → no split line / parting line exists in the groove at all.

• Very common for:

            1.O-ring grooves on cylindrical/tubular parts

            2.Bottle necks with tamper-evident rings

            3.Caps with internal snap grooves

• Advantages: Clean 360° groove surface, excellent for sealing

• Disadvantages: High mold cost, more maintenance, limited to certain groove depths/shapes

2. Unscrewing mold (for helical/continuous grooves)

• If the groove is actually a thread-like helical feature (or can be redesigned as one), use an unscrewing core.

• The core rotates while retracting → forms perfect groove/thread without longitudinal splits.

• Sometimes used even for non-thread grooves when the geometry allows.

3. Strategic parting line placement + post-machining (cheapest way to get “functionally” no parting line in groove)

• Intentionally place the mold parting line above or below the groove area.

• Mold the groove with an intentional “lump/step” or flashing area.

• After molding, machine (lathe/turning center) the groove to final dimension → completely removes any witness line/flash from the functional surface.

• Widely used for:

       1.High-precision O-ring grooves

       2.Medical fluid connectors

       3.Hydraulic/pneumatic fittings

• Advantages: Mold is relatively simple & low-cost

• Disadvantages: Adds secondary operation (cost + lead time), cannot be used if part must be 100% as-moldel.

4. Side actions / sliding cams arranged in full circle (rare, very expensive)

• Multiple small side pulls completely surround the part (like orange segments).

• Each slide forms a portion of the groove → they all retract radially.

• Effectively mimics a collapsible core but with conventional slide mechanics.

• AImost never economical unless volume is extremely high.

Most realistic choices for most projects

• If sealing surface quality is critical → collapsible core or post-machining.

• If appearance is important but sealing less so → hide the groove parting line in a non-functional radius or texture the surrounding area heavily.

• If cost is the highest priority → accept faint parting lines and optimize mold fit/polishing (very tight shut-off, high mold clamping force, excellent PL matching/polishing) to make them almost invisible.