Views: 0 Author: Site Editor Publish Time: 2026-10-22 Origin: Site
If you work with custom injection molded plastic parts, you have almost certainly seen a thin seam running along the edge of a component. This is the parting line (PL) — one of the most fundamental, and most misunderstood, features of injection molding.
Many buyers mistakenly think a parting line means the product is two separate pieces glued together. This is not true. The finished plastic part is molded as one single piece in one shot. The line simply marks where the two halves of the injection mold close and seal together before plastic is injected.
An injection mold is built from two main halves:
Cavity (A-side): The concave half that forms the outer cosmetic surface of the product
Core (B-side): The convex inner half that shapes internal features, ribs and bosses, fitted with ejector pins to push the finished part out after cooling
When the mold closes, cavity and core lock tightly. Molten plastic fills the empty cavity. When the plastic cools and hardens, the mold opens along their contact boundary. That boundary trace left on the plastic surface is the parting line.
A parting line is a natural result of the molding process. It cannot be removed completely, but experienced mold engineers can control its position and visibility. At our shop, we normally keep parting lines subtle for cleaner appearance.
The golden rule for parting line selection: place it at the largest cross-section silhouette of the part. This avoids trapped undercuts and lets the part eject smoothly without scratches or deformation.
Our priority ranking for placement:
Hidden surfaces: Bottom, rear side, inner walls, or areas covered after assembly
Sharp edges, rims and corners: The line blends into the edge and becomes barely visible
Natural design breaks: Step lines, chamfers, texture transitions or decorative grooves
Avoid wide, smooth, glossy cosmetic surfaces whenever possible — a line here stands out immediately
Take the cylindrical parts we produce for outdoor use, both solid barrels and versions with cutout openings. Our preferred parting strategy is to run the circular parting line at the top rim or bottom edge of the cylinder. The full outer curved wall is formed entirely by the cavity side. That means no vertical parting line runs down the barrel surface, leaving the outer side clean and uniform.
For the variants with cutout notches, we need slide mechanisms to form the undercut opening. A minor slide witness mark will appear around the cutout area. This extra trace from side slides is separate from the main parting line and we will minimize it as much as possible.
Even with careful mold design, parting lines can become prominent under these common conditions:
The line crosses a large glossy or transparent surface: Smooth, high-gloss materials show tiny mismatches or flash far easier than matte textured surfaces.
Poor mold shut-off or worn mold steel: Gaps between core and cavity allow plastic to squeeze out, creating flash (burrs) along the seam.
Insufficient clamp force or high packing pressure: The mold can be slightly pried open during injection, worsening flash.
Glass-filled materials such as PP+10GF: Glass fiber reinforced grades are naturally matte. While minor lines are less eye-catching, flash and mismatches are harder to polish away compared with unfilled PP.
Slide and lifter locations for undercuts: Side actions always introduce extra witness marks that are harder to hide than a simple flat parting line.
We follow these standard practices on every mold project:
Select the parting split during DFM review before steel cutting, not as a last-minute decision
Keep parting surfaces simple flat shut-offs where possible; complex inclined shut-offs carry higher flash risk
Precision grinding and polishing on the cavity and core shut-off faces
Tune injection speed, pressure and clamp force during T1 trials to reduce flash
Choose matte texture for cosmetic surfaces when appearance tolerance is tight
Communicate all expected witness marks to customers upfront, especially for parts with slides or lifters
This is a frequent confusion for product buyers: ✅ Parting line: Mold split seam, created where core and cavity meet ✅ Weld line: Internal mark formed when two separate plastic melt fronts flow together inside the cavity. It comes from material flow, not mold splitting.
The parting line is not a defect. It is an inherent feature of injection molding. The difference between a good mold and an average mold is whether the engineer places that line intelligently, keeps it subtle, and flags unavoidable witness marks early.
For cylindrical, housing and outdoor UV parts like our PP+10GF barrels, we prioritize rim-side circular parting lines to preserve clean outer surfaces. If your design has strict cosmetic requirements, talk to our engineering team in the DFM stage — we will map the parting line for your approval before mold build starts.
