Suction cups not picking up boxes is the most common call our service team takes — and in 9 out of 10 cases it is not the machine that broke. It is a vacuum system, a board change, or a maintenance habit drifting out of spec. This guide walks through the failure in the exact order our engineers check it, with the diagnostics you can run yourself before you pick up the phone.

Work through the steps in sequence. Each one takes under ten minutes, and together they resolve the majority of pick failures on the first pass.

Step 1 — Check the Vacuum Level at the Cup, Not at the Gauge

The panel gauge can read full vacuum while the cups get nothing, because the restriction sits downstream. Measure at the cup manifold with a vacuum gauge or a hand-held meter.

If the reading at the cups is 20 kPa or more weaker than at the pump, you have a restriction or a leak between them. Look for kinked hoses, a clogged vacuum filter, or a cracked fitting. A vacuum filter that has never been replaced is the single most common cause.

Step 2 — Inspect the Suction Cups for Wear

Vacuum cups are wear parts, not lifetime parts. A cup that has lost its lip seal picks air instead of board.

• Check the lip edge. If you can feel a flat spot or a tear, replace the cup. Lip wear of 1 mm on a 30 mm cup drops effective grip noticeably.
• Check for silicone/oil contamination. Board dust plus release agents make the lip slip.
• Check cup hardness. Cups that have gone hard from heat or ozone lose conformability to warped board.
• Sizing rule of thumb: cup diameter should be roughly 1/5 to 1/4 of the shortest blank dimension you run. Too small a cup and heavy blanks drop.

Step 3 — Verify the Board Is Within Spec for the Pick

A machine that picked perfectly last month can fail today because the board changed, not because the machine changed.

• Porosity. High-porosity recycled board bleeds vacuum through the blank itself. Lightweight E-flute (1.5–1.8 mm) at 200 gsm can leak more vacuum than the cups can pull.
• Moisture. Board stored in a damp warehouse absorbs humidity; heavier board resists the bend at the score, and the blank can drag on the hopper instead of lifting clean.
• Warpage. A cup flat can’t seal on a bowed blank. This is the classic cause of “it picks on the left but not the right.”

A quick field test: hold a cup face against the blank and feel for suction retention. If the blank holds for 5+ seconds, the board is within range; if it drops quickly, you either need bigger cups, more vacuum, or a different blank.

Step 4 — Check the Feed Hopper Setup

The pick issue often starts upstream at the blank magazine:

• Side guides should be set to blank width plus 3–5 mm, not pressed tight. Too tight and the blank binds; too loose and it skews before the cups arrive.
• Back stop should sit at blank length plus 5–10 mm so the leading edge stands proud for the cups to reach.
• Hopper tilt of 10–15° keeps the stack leaning into the pick point.
• Stack weight. A too-heavy stack (thousands of blanks) can wedge at the bottom; keep stack height below the level recommended in the manual.

Step 5 — Check Pick Timing and Cup Stroke

If vacuum and board are fine, look at the sequence:

• Pick delay. If the cup contacts the blank and the vacuum valve opens too late, the blank slides.
• Lift stroke. The cup must lift the blank clear of the hopper lip before the erecting cycle starts. On servos this is parameterized; on pneumatics it is the cylinder stroke.
• Vacuum vent timing. If the vent happens too early, the blank drops mid-transfer. The Siemens PLC parameters for pick delay, lift time, and vent are in the HMI service menu — log in, watch one cycle on slow motion, and note the timing.

Step 6 — Decode the PLC Alarms

Our machines surface most pick problems as an alarm code before you ever notice the dropped box. Common ones:

Reset the alarm, then run a single-cycle test. If the alarm returns immediately, the root cause is still present.

Step 7 — Know When It Is Time for Parts

Do not let a pick problem run until it becomes a jam problem. Replace in pairs:

• Suction cups: every 6–12 months depending on board type and duty. Double-wall BC (6–7 mm) and rough recycled board wear cups faster.
• Vacuum filter: every 1–3 months, or when the gauge shows a drop.
• Cup lines and fittings: inspect quarterly; replace cracked ones immediately.

Our recommended spares kit covers cups, filter, and seals so the fix is a same-day swap, not a week of downtime.

Quick Reference: Pick Failure Checklist

• [ ] Vacuum at the cup manifold within 10–15 kPa of the pump reading
• [ ] Cups clean, round, no flat spots or cracks
• [ ] Board porosity and moisture within range (5+ second hold test)
• [ ] Hopper side guides +3–5 mm, back stop +5–10 mm
• [ ] Stack height within manual limit
• [ ] Pick/vent timing verified in PLC
• [ ] No active alarm codes E-102 / E-103 / E-105

The 10-Minute Field Pick Test

When you are not sure whether the board or the machine is the problem, run this structured test once, at the start of a shift:

1. Set the machine to single-cycle mode through the HMI service menu.
2. Load a fresh, flat blank from the middle of the stack — not the top or bottom, where warpage is worst.
3. Hand-press a spare cup onto the blank. If the blank holds for 5+ seconds with the vacuum generator running, the board is acceptable.
4. Run one single cycle. Observe: does the blank lift clean, drag on the hopper lip, or drop mid-transfer?
5. Repeat with a blank from the top and bottom of the stack. Stack-edge blanks are the ones that warp, so this isolates storage issues from machine issues.

Record the result per blank position. If only stack-edge blanks fail, the fix is stack handling — cover the blanks, limit stack height, or improve hopper tilt. If all blanks fail, the machine side (steps 1–6) is the problem.

Myths That Cost You Downtime

Three beliefs we hear constantly, and the reality behind them:

• “More vacuum always fixes it.” No. Excess vacuum on a thin E-flute blank (1.5–1.8 mm) pulls the liner down into the cup and can cause double feeds or blank damage. Match vacuum to board, don’t max it.
• “Cups last until they look worn.” They fail at the lip long before they look damaged. A hard, shiny lip loses grip silently. Calendar-replace them every 6–12 months.
• “The board never changed.” It did. Converters change liner suppliers, recycled content, and moisture profiles seasonally. Re-check the board whenever pick problems appear, before touching the machine.

Prevention Is Cheaper Than Repair

The lines that never call us about pick failures do three things: they log the vacuum reading weekly, they keep the cup spares kit stocked, and they run the daily 5-minute inspection. That is the whole secret.

If your machine is outside the parameters above and the problem persists, send us your alarm history from the PLC — it is timestamped, so we can often diagnose remotely before a technician travels. Where remote diagnostics are not enough, we offer video-witnessed sessions and, for complex cases, on-site support.

• Running a single-piece RSC line with frequent blank changes? The Single-Piece Case Erector is engineered for exactly this class of pick reliability.
• For heavy three-piece shipping boxes, the Three-Piece Case Erector uses a larger cup pattern and stronger vacuum, so the same failure rarely appears twice.

Need a Second Opinion?

Send us your box size, board grade, and a photo or video of the failed pick — our engineers will point you to the specific step. We respond within 24 working hours.

Email: [email protected]
WhatsApp: +86 13681839278
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