Injection Molding Box Sink Mark Troubleshooting for Thick Sections

2026-06-23 14:47:44

　　For thick ribs, bosses, and hinge areas on a box, a sink mark is not a cosmetic flaw—it is a stress concentration point that can lead to cracking under load. Here is the systematic, tiered troubleshooting guide, starting with the most effective fixes.

　　Phase 1: The "Pack & Hold" Fix (The Primary Weapon)

　　Sink marks occur because the outer skin freezes, but the inner core is still molten and shrinking. You must force more material into the cavity while the gate is still open to compensate.

　　Increase Hold Pressure (Not Injection Pressure): Crank up the 2nd stage (hold/pack) pressure to 80–90% of your maximum machine capability. Critical: Do not confuse this with 1st stage (fill) pressure—that only affects filling speed.

　　Extend Hold Time: The #1 mistake is cutting hold time too early. Rule of thumb: Weigh the box. Increase hold time by 0.5-second increments until the part weight stops increasing. The moment weight plateaus is your optimal hold time. If you cut it shorter, the core shrinks inward, creating a sink.

　　Slow Down the Hold Velocity: Pack the material in slowly during the hold phase. Fast packing compresses the melt but doesn't give molecules time to settle, leading to spring-back and sink marks when pressure releases.

　　Phase 2: The Gate & Flow Path Fix (The Root Cause)

　　If the gate freezes off before the thick section is packed, no amount of pressure will help.

　　Relocate or Enlarge the Gate: Position the gate directly opposite the thickest section (e.g., at the base of a thick rib). The flow path from the gate to the sink-prone area must be as short as possible.

　　Increase Gate Diameter: If your gate is too small, the shear heat freezes the material at the gate surface. Enlarge the gate by 0.2–0.5mm to keep it "open" longer, allowing pressure to reach the thick core.

　　Check the Sprue/Bushing Heater: A cold sprue will freeze prematurely. Ensure the nozzle and sprue bushing heaters are active and set 10°C–20°C hotter than the barrel rear zone.

　　Phase 3: The "Rib Design" Rule (The Permanent Fix)

　　Mold design dictates whether you will ever eliminate sink marks. For a box, you cannot treat thick sections as solid blocks.

　　The 60% Rule: The thickness of a rib or boss (at its base) must never exceed 60% of the nominal wall thickness of the box.

　　Example: If your box wall is 3.0mm, your rib base must be ≤ 1.8mm. If it's thicker, tell the toolmaker to steel-safe the ribs and machine them down.

　　Core Out the Thick Sections: For large thick pads (e.g., feet or handle inserts), add a through-hole or a deep recess on the non-cosmetic side. This acts as a "void" that the core can shrink into without pulling the outer surface inward.

　　Add Radius to Corners: Sharp 90° corners at the base of a thick rib act as heat traps. Add a large radius (R 0.5–1.0mm) to allow smoother heat transfer and more uniform packing.

　　Phase 4: Material & Thermal Strategy (The Process Tweak)

　　Since you previously asked about crystallinity, this ties in directly:

　　Lower the Melt Temperature: High melt temps mean more thermal shrinkage. Drop your barrel temperatures by 5°C–10°C (only if the material flow allows). Cooler melt = less total volumetric shrinkage = smaller sink.

　　Raise the Mold Temperature (Counter-Intuitive): Wait—doesn't high mold temp cause more shrinkage? Yes, but it also delays skin freeze-off. By raising the mold temperature (e.g., from 40°C to 70°C), the gate stays fluid longer. This allows the hold pressure to pack the thick section fully before it crystallizes. Once it crystallizes under high pressure, it won't sink. Warning: This increases cycle time, but it is highly effective for PP and PE boxes.

　　Use "Sequential" Cooling: If your mold has independent cooling circuits, turn off or reduce cooling directly under the thick section. You want that thick section to stay hot longer so it remains "packable" while the thinner walls freeze off and seal the cavity.

　　Phase 5: The "Stripping" Test (Diagnostic Action)

　　Before changing steel, run this 10-minute diagnostic to pinpoint the exact cause:

　　What you doWhat happensThe Verdict

　　Increase Hold Time by 2 seconds.Sink mark disappears.Solution: Increase hold time permanently.

　　Increase Hold Time by 2 seconds.Sink mark remains, but box weight increases.Problem: Gate is freezing too early. Enlarge gate.

　　Increase Hold Time by 2 seconds.Part sticks to the core and ejects with a "pop" sound.Problem: Over-packing. Reduce hold pressure, but add core-out holes to the mold to relieve shrinkage.

　　Reduce melt temperature by 10°C.Sink reduces slightly, but short shots occur.Problem: Rib is too thick (>60% of wall). Must modify the mold steel.

　　Phase 6: The "Gas-Assist" Alternative (For Extreme Cases)

　　If the thick section is unavoidable (e.g., a heavy-duty stacking lip) and you cannot core it out, consider running gas-assisted injection molding or micro-foaming (MuCell) .

　　Injecting a tiny amount of nitrogen gas into the thick core during packing creates an internal cellular void. The material shrinks into this internal void, leaving the external cosmetic surface perfectly smooth. This is a permanent, geometry-based solution.

　　Your Immediate Action Plan for Today

　　Weigh 10 consecutive parts immediately after ejection. Write down the average.

　　Increase Hold Time in 1-second increments until the part weight stabilizes (stops increasing).

　　If sink marks persist at max hold time: Reduce the cooling time by 20% (ironically). This ejects the part hotter, allowing it to post-shrink evenly in a fixture rather than against the cold steel, which often hides the sink until it's too late.

　　Final test: Put the box under a 50kg load for 1 hour. If the sink mark deepens, you have a structural design flaw—the rib is too thick, and you must modify the mold steel by reducing the rib thickness to ≤60% of the wall.

　　Never try to fix a structural sink mark by increasing injection speed—that only creates flash and burns the vents. Stick to hold pressure and time.