Articles Large-Part Injection Molding: Eliminating Weld Lines on an 87-Inch Running Board

A long, textured exterior part can’t be filled from a single gate, and filling it from several gates creates weld lines. On an 86.8-inch automotive running board molded on a 2,000-ton press, UPM eliminated visible weld lines by switching from conventional multi-gate filling to a 3-drop sequential valve-gate hot runner and raising the mold texture from 25 µm to 35 µm. That 35 µm depth was the maximum the part’s 1.5° draft would allow. The program launched on time and now runs on a regular production schedule, with a cycle time about 27% faster than originally estimated.

Overview

An automotive accessory supplier asked UPM to mold the main plastic running-board components, left and right hand, for an OEM accessory program. Each part is a long, narrow polypropylene (PP) exterior piece with a molded-in grain texture. The OEM sets and checks texture depth itself.

The Challenge: Long Flow Lengths Make Weld Lines Unavoidable

At nearly 87 inches long and under 6 inches wide, the part has a very high length-to-width ratio. The first sample parts (T1) had visible weld lines across the textured show surface.

The cause is basic flow physics:

• One gate can’t fill the part. The distance the plastic has to travel from a single gate to the far end of an 87-inch part is longer than the material can flow before it freezes off.
• More gates create weld lines. Each extra gate starts its own flow front. Wherever two fronts meet, they knit together and leave a line that shows through texture, most of all on a dark, grained exterior part.

UPM first tried changing the gates. It ran trial sets with two and then four gates per cavity, added vents, opened the gates and ran the mold hotter. Each change helped, but none brought the part to an OEM-acceptable exterior surface.

UPM’s Approach

UPM and its tooling partner made three changes.

1. Sequential valve gating

UPM fitted the mold with a 3-drop sequential valve-gate hot-runner manifold. A standard multi-gate system opens every gate at once. A sequential system opens the gates in timed order:

1. The first gate opens and fills the part’s first section.
2. Once the flow front has moved past the second gate, that gate opens and adds fresh melt behind the moving front.
3. The third gate opens the same way.

Because each gate opens after the flow front has already passed it, two separate fronts never collide. The part fills as one continuous front, so visible weld lines were eliminated on this part’s textured show surface. The manifold retrofit took about three weeks.

2. Texture depth within the limit of draft specific to this project

Deeper texture hides surface flow marks better, but it also needs more draft to release from the mold without scuffing. UPM and its tooling partner re-textured both cavities from 25 µm to 35 µm. At the part’s 1.5° draft angle, 35 µm was the deepest texture that would still pull cleanly. Re-texturing took about seven days.

3. Process refinement

UPM also added vents and adjusted gate size and mold temperature on the 2,000-ton press. This improved fill and surface finish alongside the hardware changes, and brought the production cycle time down about 27% from the original estimate.

Results

Key Takeaways for Large-Part Programs

• Settle the gating strategy at DFM, before steel is cut. On long cosmetic parts, ask early: “Where will the flow fronts meet?” If the answer is “on the show surface,” plan for sequential valve gating from the start. Retrofitting a manifold costs more and takes longer than designing it in.
• Specify texture depth and draft together. Texture depth is limited by draft. If a part might need a heavier grain later, add draft at the design stage.
• Large parts need large-press process capability. On a 2,000-ton tool, hot-runner sequencing, mold temperature control and venting all affect each other. That makes process engineering, not just press size, the deciding factor in cycle time and cosmetic quality.

Frequently Asked Questions

UPM (Universal Plastic Mold) is an ISO 9001:2015 certified, IATF 16949 compliant custom plastic injection molder based in Baldwin Park, California, operating 30 injection molding presses from 150 to 2,000 tons.