Valve Gate and Sequential Injection: Weld Line Control

On a part with multiple gates, the melt fronts must meet somewhere. Where they meet, they form a weld line — and in a conventional hot runner, that weld line is a cold weld: two already-frozen fronts pressing against each other with minimal molecular entanglement. It is the weakest line on the part, it shows up as a visible witness line on cosmetics, and it is exactly where a burst or impact test will fail.

Figure 1. Cold weld from simultaneous gating versus hot weld from sequential valve gates.

Sequential valve gate molding changes that. Instead of opening all gates at once, the gates open in a timed cascade, so each new flow front arrives while the previous one is still molten and moving. A cold weld becomes a hot weld, with real entanglement and real strength. This guide covers how it works, how to set it, and what the tool has to do to make it reliable.

The mechanism

Three production benefits come from sequencing:

• Hot welds instead of cold welds. The second front advances into a still-molten first front, so the interface has time and temperature to diffuse.
• Weld line position control. Because the fronts advance in a known order, the final meeting point can be moved into a non-critical area — into a rib, under a label, behind a boss, or off the cosmetic surface.
• Lower clamp force. Filling sequentially means only part of the part is filling at high pressure at any moment, reducing the peak projected pressure and the tonnage needed.

There is a fourth, quieter benefit: balance. In multi-cavity or family tools, sequential control compensates for natural imbalance by letting you give more time and pressure to the cavities that need it.

Valve gate hardware

• Pneumatic or hydraulic actuation. Pneumatic is fast, clean and common; hydraulic gives more force for large pins or high packing pressures.
• Pin and gate geometry. The pin seals on a seat in the gate. Pin diameter, seat angle and gate land length all influence the gate vestige, gate mark, and whether material strings when the pin retracts.
• Sequence controller. A standalone unit or an integrated function of the hot runner controller or the press, capable of opening and closing each gate at a defined time, screw position, or cavity pressure.
• Position feedback. Not essential on every tool, but where part quality depends on the pin actually being open or shut, feedback turns an assumption into a fact.

Thermal gates (hot tips) are cheaper but cannot be sequenced: they stay open, and their flow is only influenced by temperature. Sequencing requires a physically closing valve.

Setting the sequence

The sequence is defined by when each gate opens and closes, and it is set against the fill progression rather than by feel:

1. Start with the fill study. Run a short-shot series with all gates open to understand natural fill order and where the fronts meet.
2. Pick a lead gate. Normally the one that fills the largest, thickest or most pressure-hungry area — often the center gate on a large cover, or the gate furthest from the last-fill point.
3. Open the next gate while the lead front is still molten. Trigger by screw position (which corresponds to fill volume) or, better, by a cavity pressure sensor at the advancing front. Too early wastes the benefit; too late produces a cold weld anyway.
4. Cascade through all gates, then pack. All gates stay open for packing, or close in reverse order, depending on whether you want localized packing or a controlled gate seal.
5. Verify with the weld line, not with the clock. Confirm the weld line has moved where you intended, then confirm it by strength test — a flexural, tensile or burst test on parts with the weld line in the critical location.

Triggering on cavity pressure is the most robust approach because it makes the sequence respond to the actual melt state rather than to a fixed time that drifts with material lot and barrel condition.

Process rules that make sequencing work

• Keep the flow front moving. If a gate opens too late, the front freezes; if the next gate opens before the previous one can establish flow, you get hesitation and race-tracking. Hesitation marks look exactly like a weld line but are much harder to fix.
• Balance velocity, not just timing. Each stage of the cascade should maintain a controlled front velocity. A stage that is too fast will jet; too slow will freeze off.
• Watch the gate vestige. Valve gate marks are more visible than tunnel gate marks. Pin protrusion, pin temperature and closing pressure all affect the vestige — and a cosmetic part may need the gate placed on a non-visible face.
• Control pin temperature. A cold pin chills the gate area and creates a cold slug that shows up downstream. A hot pin can string. Set the tip temperature deliberately.
• Account for manifold thermal expansion. A hot manifold grows as it heats; the pin-to-gate relationship is set cold and must still be correct hot. This is the most common mechanical source of gate leakage and pin wear.
• Mind pack pressure limits. With sequential filling, all gates eventually open, and the full projected area bears on the clamp at that moment. Verify clamp force for the packing phase, not just the filling phase.

Common problems and their causes

• Weld line still visible or weak — the following gate opened too late, or the front had already frozen; also check gate size and melt temperature.
• Blush, blemish or halo around the gate — pin seat geometry, excessive injection speed at the gate, or a cold pin.
• Stringing or drool at the gate — gate land too long, tip temperature too high, or decompression not set.
• Short shot in one cavity — manifold imbalance, undersized drop, or a sequence that starves that cavity.
• Pin wear or leakage — thermal expansion mismatch, insufficient pin guidance, or contamination in the actuation circuit.

Design rules for sequence-capable tools

• Gate size must be generous enough to fill in the allotted time at acceptable shear; do not size a sequenced gate like a single-gate tool.
• Provide space for actuation cylinders, air or hydraulic lines, and pin guidance in the mold base — sequencing adds hardware above the manifold and it has to fit.
• Keep the manifold naturally balanced even though you sequence it. Sequence control should tune a balanced system, not rescue an unbalanced one.
• Place gates so the intended weld positions land in acceptable areas, and design the family/cavity layout so each cavity can be fed properly.
• Add cavity pressure instrumentation at the sensing points you intend to use for triggering, and route sensors and wires so they survive mold handling and maintenance.
• Plan maintenance access to pins, seats and seals. A sequenced tool with inaccessible valve pins will drift out of specification quietly.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist

• Weld line critical locations identified on the drawing and in the quality plan
• Fill study completed before any sequence is programmed
• Gate opening order and triggers defined (screw position and/or cavity pressure)
• Each new gate opens while the previous front is still molten, verified by weld line position
• Weld line strength verified by mechanical test, not visual inspection alone
• Gate vestige assessed cosmetically; gates placed on non-visible faces where needed
• Valve pin tip temperature set to avoid cold slugs and stringing
• Manifold thermal expansion accounted for; pin seats correct at operating temperature
• Clamp force verified for the phase when all gates are open
• Actuation hardware, sensors and wiring integrated into the mold design, with access for service
• Setup sheet records gate sequence, triggers, pin temperatures and pack profile

Sequential valve gate molding is one of the few techniques that improves cosmetics, strength and clamp force at the same time. It rewards tools that are designed for it and processes that are triggered by melt state rather than by timers — and it punishes tools where sequencing was added as an afterthought to a manifold and gate layout that were never balanced.