A suction cup that will not pick a blank looks like a vacuum problem. About half the time it is not. The cup is the final link in a chain that runs from the compressor, through the filter, the regulator, the valve, the manifold and the hose, to the cup face — and a fault anywhere in that chain produces the same symptom at the same place.

The efficient way to diagnose it is therefore not to attack the cup but to measure the vacuum at the cup face and then work backwards. This article sets out the eight causes in the order in which they are usually found, with the specific measurement that identifies each.

The Short Answer

If cups are missing blanks, measure vacuum at the cup face with a gauge or a tee’d sensor, not at the pump. Then compare to the machine’s specified pick-up vacuum, which for most case erectors is 0.4 to 0.6 bar below atmospheric or, expressed as an absolute pressure, 400 to 600 mbar.

Four values tell you where the fault is. Vacuum absent entirely points to supply or valve. Vacuum present but low points to leakage, filtration or generator capacity. Vacuum correct at the cup but the blank still not held points to cup condition, cup geometry or blank surface. Vacuum correct and held, but the blank drops in transfer points to timing, acceleration or cup count.

Cause 1: Contaminated or Glazed Suction Cups

The single most common cause, and the easiest to confirm. Cup material — usually nitrile, polyurethane or silicone — hardens and glazes with age, and a glazed cup seals poorly against the board surface.

The test: press the cup against a clean glass or steel surface and pull. A good cup holds; a glazed cup releases or holds weakly. Alternatively measure vacuum at the cup face sealed against a flat plate: a healthy cup reaches the specified value within a fraction of a second, a glazed cup plateaus below it.

The fix: replace cups. Replacement intervals are typically 2 to 3 million picks or 12 months, whichever comes first. In dusty environments, used board, or high ambient temperature, the interval shortens.

Cause 2: Cup Geometry Unsuitable for the Board

Cup diameter, bellows count and lip profile all matter, and a cup that works on C-flute may not work on E-flute.

The reason is stiffness. A thin or low-density board deflects under vacuum, and a large flat cup on a flexible board will pull the board into the cup throat, reducing effective sealing area and, in the worst case, pulling two blanks at once.

The test: compare the pick reliability across board grades. If the machine picks reliably on one grade and not on another, the cup is the variable.

The fix: use a smaller-diameter, single-bellows cup with a firm lip for thin board, and a larger, multi-bellows cup for thick or uneven board. Multi-bellows cups tolerate uneven surfaces better but deflect more, which is the wrong direction for thin board.

Cause 3: Blank Warp Beyond the Cup’s Tolerance

Warp is the curvature of a blank across its width or length, and the usual acceptance criterion is under 3 mm of deviation over the blank width. Beyond that, a rigid cup cannot achieve a seal across the full face; only the high points contact.

The test: place the blank on a flat surface and measure the gap at the highest point with a feeler gauge or steel rule.

The fix: warp is a storage problem, not a machine problem. Correct causes are pallets stored on a damp floor, pallets stacked beyond the supplier’s recommended height, and board that has taken up moisture on one face. The machine fix, where warp cannot be eliminated, is a larger-diameter cup with a softer lip and, in persistent cases, a cushioned or floating cup mount that lets the cup follow the surface. Storage and handling criteria for board are collected in our support section.

Cause 4: Vacuum Filter Blocked

Every erector has a filter between the vacuum generator and the cups. Because the filter’s loading is gradual, degradation is invisible until the machine starts missing picks on the fastest part of the cycle.

The test: measure vacuum immediately upstream and immediately downstream of the filter. A differential above 0.05 bar indicates a restriction worth addressing.

The fix: clean or replace the element. On machines running recycled board, dust loading is high and the interval is short — weekly inspection is realistic, not monthly.

Cause 5: Leakage in Hoses, Fittings and Manifold

Vacuum systems run at low absolute pressure, which means a small physical leak is a large proportional loss. A cracked hose or a loose push-fit fitting that would be minor on a 6 bar air line can destroy pick performance on a vacuum line.

The test: with the cups plugged or sealed, measure the vacuum the generator can hold. If it cannot reach specification with no load, the leak is in the circuit, not at the cup.

The fix: replace hoses showing surface cracking, re-seat fittings, and check the manifold gasket. A practical routine is to replace all vacuum hoses as a set at 24-month intervals rather than chasing individual failures.

Cause 6: Ejector or Vacuum Generator Capacity

Venturi ejectors and vacuum pumps both degrade. An ejector’s throat erodes over time, and a pump’s vanes or seals wear. The symptom is a vacuum that is achievable but slow to build — which matters because a case erector’s pick window may be only a fraction of a second.

The test: measure the time to reach vacuum, not just the final value. A healthy circuit reaches specification in well under a second. Slow development points to a degraded generator even when the final figure looks acceptable.

The fix: service or replace the ejector, or the pump. Where air consumption is a concern, a multi-stage ejector replacement addresses both the vacuum problem and the energy problem.

Cause 7: Pick Timing and Cup Approach Speed

If vacuum is correct and the cups are good, the fault is often mechanical timing. Cups that approach the blank too fast bounce, and a bounce breaks the seal before vacuum has developed. Cups that arrive too late in the cycle have insufficient time to develop vacuum before the lift begins.

The test: observe the cup approach at reduced speed. A visible bounce or a lift that begins before the cup has settled is the signature.

The fix: reduce approach velocity and add a short dwell at contact. On servo-driven machines this is a profile adjustment; on pneumatic machines it may require a flow control or a cushioning adjustment.

Cause 8: Number and Distribution of Cups

Finally, the design itself. A single central cup on a large blank will lift the blank but not keep it flat, and a blank that flexes during transfer can release. Three or four cups distributed across the blank hold it flat and, importantly, provide redundancy — one cup losing seal does not lose the blank.

The test: if the blank is reliably picked but intermittently drops in mid-transfer, count and position are the likely cause.

The fix: add cups and distribute them. This is a specification change rather than a maintenance one, which is why cup count should be discussed at the enquiry stage for large or flexible blanks. The picking head configuration differs across the range, and the cup layout used on the Double-Head High-Speed Case Former is a useful reference for large formats running at high case rates.

Reference Values

Alarms Associated with Pick Failures

E-12 Vacuum Low combined with E-33 Servo Follower Error on the same cycle is the classic signature of a double-blank pick. The second blank adds mass the axis cannot accelerate as designed, and the follower error appears before the operator has noticed the double feed.

Maintenance Rhythm

Every shift — listen for vacuum leaks around the picking head, check that the local filter drain is functioning, and confirm the first picks of the shift are clean.

Every week — clean the vacuum filter element, inspect cups for hardening or cracking, and check hose condition at the flexing points.

Every month — measure vacuum at the cup face and compare against the specification rather than against last month’s reading, and verify the generator’s time to reach vacuum.

Every quarter — measure filter differential, inspect the manifold gasket, and replace any hose showing surface cracking.

Annually — replace the full vacuum hose set, replace or assess cups against a wear standard, service or replace the ejector, and re-baseline pick reliability across the board grades actually run.

FAQ

Why do cups pick on one board grade and not another?
Because cup geometry suits one surface stiffness and not the other. Thin board needs a smaller, firmer cup; thick or uneven board needs a larger, multi-bellows cup. Specifying one cup for all grades is the usual root cause.

Can I increase vacuum to fix a weak pick?
Sometimes, and it is the wrong first move. Raising vacuum masks the underlying fault, increases the double-blank rate and increases energy consumption. Fix the seal first.

How often should cups be replaced?
Plan on 12 months or 2 to 3 million picks. On recycled board or in dusty conditions, inspect monthly and expect a shorter life.

Is a vacuum pump better than an ejector?
A pump gives steadier vacuum and lower air consumption; an ejector is simpler and has fewer wearing parts. The choice depends on duty cycle and on whether the plant’s air system is already at capacity. Typical vacuum configurations for each machine type are listed across our product range, and the common board and cup questions are answered in the FAQ.

Chasing an intermittent pick failure? Send your board grade, blank dimensions, cup configuration and the vacuum reading at the cup face to [email protected] or WhatsApp +86 13681839278. Our engineers will identify the likely cause and recommend a cup and vacuum configuration within 24 hours.