Bottom construction is the single most common cause of a case erecting machine failing to do what the buyer expected. The machine works perfectly, the board is within specification, and the cases still will not form — because the bottom is not a four-flap bottom.

The three constructions covered here are mechanically distinct, require different forming motions, and cannot generally be run on the same machine without dedicated tooling.

The Short Answer

A standard four-flap bottom — the RSC 0201 construction — folds four flaps around a footprint and is sealed with glue or tape. Any case erector forms it.

A lock-bottom case is die-cut so the four bottom flaps interlock mechanically without adhesive. Forming requires the flaps to be pushed through and locked, which is a different motion and a different mechanism.

A crash-lock bottom is a die-cut construction that locks automatically when the case is erected from flat by opening it, which means it requires the machine to open the case rather than fold flaps. Machines for crash-lock are a distinct category.

A self-locking or auto-bottom case combines both behaviours: it opens into a locked bottom and requires no sealing at all. It also requires the opening motion.

The rule at enquiry stage is simple: state the FEFCO code or send a blank drawing. A four-flap machine and a crash-lock machine look superficially similar and produce incompatible results.

What Each Construction Actually Is

The four-flap bottom (FEFCO 0201). The blank is a simple rectangle with four bottom flaps defined by score lines. The machine squares the case over a mandrel, folds the two inner flaps, folds the two outer flaps over them, applies adhesive between the outer flaps and the inner ones, and compresses. Bond strength depends entirely on the adhesive, which is why sealing specification dominates this construction.

The lock-bottom (FEFCO 0713 and related). The bottom flaps are die-cut with tabs and slots that interlock. Forming requires the flaps to be folded in a specific sequence and then pressed so the tabs enter the slots. No adhesive is required, and the finished bottom is considerably stronger than a glued four-flap bottom of the same board grade, because the load path is mechanical rather than adhesive.

The crash-lock (FEFCO 0427-style constructions and variants). The bottom is pre-glued at the blank manufacturer into a partly formed state. When the case is opened from flat, the bottom geometry collapses into a locked position without any additional folding. Forming is an opening motion, not a folding motion — the machine must pull the blank open and square it while the bottom self-forms.

Self-locking and auto-bottom cases. Functionally identical to crash-lock in machine terms. The difference is in the blank design and in when the interlock engages.

Where the Machine Requirement Diverges

The critical engineering difference is force and travel. A four-flap machine applies adhesive and a modest compression force. A lock-bottom machine must press tabs through slots, which requires a higher forming force over a short travel, and the tooling must support the bottom while the tabs engage or the flaps deform instead of locking. A crash-lock machine must apply a controlled opening force at the blank’s edges, and the squaring motion must complete before the bottom locks or the locked bottom will be out of square permanently.

Speed Consequences

The sealing method determines the achievable speed as much as the mechanism does.

A four-flap machine with hot-melt adhesive is limited by compression dwell — typically 1.5 to 3 seconds depending on board grade — but that dwell is a fixed time, not a fixed distance, so the machine designer can trade speed against compression section length.

A lock-bottom machine has no adhesive dwell at all, which sounds like an advantage. It is partly offset by a longer forming stroke and by the need for the tabs to engage reliably, which requires the board to be within a tighter calliper band. Practical speeds are typically 10 to 20 CPM — slower than a four-flap machine of the same size, because the forming motion is more complex.

A crash-lock machine has no dwell and a short forming motion, and it can be fast — 20 to 35 CPM — but only if the blank opens reliably. Blank quality dominates. A blank that has been stored flat under load, or that has absorbed moisture and taken a set, will not open cleanly, and the failure mode is a partially opened case that jams at the locking station.

Board and Blank Specification

For lock-bottom and crash-lock constructions, the blank is the machine’s real specification.

Calliper tolerance matters more than grade. Because the tabs must pass through slots with a designed interference, board thicker or thinner than the blank was designed for will either not enter or will not lock. A calliper band of plus or minus 0.2 mm is realistic for reliable operation; wider bands produce intermittent failures that look like machine faults.

Score quality determines whether the bottom locks square. A blank with weak or inconsistent scores will fold at the wrong place, and on a locking bottom the error is permanent. Die-cut quality should be specified and verified on incoming inspection, not assumed.

Moisture content should be 6 to 8 per cent. Below 6 per cent board cracks at the score when the tabs are pressed through. Above 9 per cent the board is soft, the tabs deform instead of entering the slots, and the bottom locks loosely.

Compression at the blank supplier matters. A crash-lock blank holds its pre-glued state only if it has not been over-compressed in storage. Pallets stacked too high take the set out of the blank and the machine cannot recover it. The same blank definitions are covered from the machine-selection angle in our article on the difference between a case erector and a carton former.

Sealing Cost: The Real Comparison

The reason lock-bottom and crash-lock constructions exist is that they eliminate adhesive or tape. The saving is worth quantifying, because it changes the machine payback calculation.

A four-flap case sealed with hot-melt at 0.9 grams per case and adhesive at 2.80 USD per kilogram costs about 0.25 cents per case in adhesive. Add the cost of the glue module, its maintenance, its energy consumption and the downtime associated with glue faults.

A lock-bottom case costs nothing in consumables but carries a board premium — die-cutting adds material and tooling cost, typically 10 to 25 per cent over a plain RSC blank. The crossover depends on the relative costs of board and adhesive, and it moves in favour of locked constructions as adhesive prices rise.

The operational argument is often stronger than the consumable argument. A machine with no glue system has no glue faults, no nozzle maintenance, no temperature control and no adhesive storage requirement. For a plant with high downtime sensitivity, that simplification is worth more than the material arithmetic.

Alarm Patterns

E-45 Bottom Lock Failed is a construction-specific alarm and its first three checks are all blank-side checks, which is the point. On a locking bottom, machine adjustments rarely fix a failure that originates in blank quality; the answer is usually an incoming inspection criterion rather than a setting change.

Maintenance Rhythm for Locking Constructions

Every shift — inspect the forming tooling for adhesive residue or board dust build-up at the tab slots, verify that the last ten cases in a run are still locking square, and check blank stack condition.

Every week — inspect the locking tooling for wear at the tab engagement surfaces, verify forming force against specification, and check blank calliper with a gauge on the current pallet.

Every month — measure locked-bottom squareness on five cases, inspect the forming cams or servo profile for timing drift, and review whether any recent failures trace to a specific board delivery.

Every quarter — inspect the locking tooling for deformation, re-baseline forming force, and review incoming inspection data against field failures.

Annually — replace or recondition locking tooling, re-verify forming timing across the case size range, and re-establish the blank specification and inspection criteria with the board supplier.

FAQ

Can a standard erector run a lock-bottom case?
No. A four-flap machine folds flaps; a lock-bottom machine pushes tabs through slots. The motions differ and the tooling differs.

Which is stronger, glued or locked?
A locked bottom is generally stronger, because the load path is mechanical rather than adhesive. For heavy contents, that is a genuine advantage, and the same reasoning drives the heavy-duty case forming specification described on our Double-Head High-Speed Case Former page.

Do locking bottoms need tape on top?
The top of a lock-bottom case is usually still a four-flap top and is sealed normally after filling. Only the bottom is adhesive-free.

Why does my crash-lock machine fail intermittently?
Intermittent failure on a crash-lock machine is almost always blank-side: storage compression, moisture, or calliper outside band. Measure the blanks before adjusting the machine. Where the construction is a multi-piece one with a separate body and lid rather than a locking bottom, the requirement changes again — see the Three-Piece Carton Erector for that forming sequence, and the FAQ for the standard blank codes used across both.

Running a lock-bottom or crash-lock construction? Send a blank drawing or FEFCO code, the board specification and your target CPM to [email protected] or WhatsApp +86 13681839278. Our engineers will confirm whether the construction needs dedicated tooling, specify the forming mechanism and return a quotation basis within 24 hours.