Shenzhen Alu Rapid Prototype Precision Co., Ltd.

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)

1.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.

2.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.

3.Very common for:

          O-ring grooves on cylindrical/tubular parts

          Bottle necks with tamper-evident rings

          Caps with internal snap grooves

4.Advantages: Clean 360° groove surface, excellent for sealing

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

2. Unscrewing mold (for helical/continuous grooves)

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

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

3.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)

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

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

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

4.Widely used for:

     High-precision O-ring grooves

     Medical fluid connectors

     Hydraulic/pneumatic fittings

5.Advantages: Mold is relatively simple & low-cost

6.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)

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

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

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

4.AImost never economical unless volume is extremely high.

Most realistic choices for most projects

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

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

3.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.