Bottom drop-out is not a packaging cosmetic issue. It is a safety and liability issue. A case carrying 50 lb of freight that fails at the bottom seam dumps the product onto the floor or, worse, onto a worker’s feet. In LTL and parcel shipping, the bottom of the case carries the full weight of the load plus the dynamic forces of sorting conveyors and vehicle vibration, and it does so on just two layers of board and one glue or tape joint.

We build case erectors for heavy freight lines, and the machines behind Mars and Duracell handle loads in this class every day. This guide explains exactly how bottom strength is created, where it fails, and what the case former itself does to make the bottom survive a 50 lb load. The short version: the erector does not add strength that is not in the board, but it can easily destroy the strength that is.

The Physics of a 50 lb Bottom

When a case with a 50 lb load sits on the floor, the bottom panel transfers the weight to the outer flaps. Those outer flaps bend at the crease lines and carry a bending moment proportional to the panel span. Two board properties decide survival:

• Edge Crush Test (ECT): the resistance of the corrugated flutes to crushing under edge load, measured in kN/m or lb/in. ECT drives the vertical stacking strength of the case.
• Burst strength (Mullen): the resistance of the liner to puncture and flexing. Mullen drives the ability of the bottom flaps to stay flat under the load.

For a 50 lb load with a case footprint of roughly 600 x 400 mm, we recommend a BC-flute double-wall board at 6-7 mm caliper with a minimum ECT of 32 (N/mm or equivalent) and a Mullen rating of at least 200 psi. Single-wall board in this application is a false economy: the bottom flaps flex, the creases fatigue, and the bottom rounds out before the case reaches the end customer.

Board specification we recommend for 50 lb freight:

Moisture matters. Board at 12% moisture loses roughly 20% of its ECT. A case former that runs board too wet produces a bottom that passes the static test at the plant and fails in the truck.

What the Case Former Does to the Bottom (and Can Do Wrong)

The erector’s job is to create a bottom that preserves the board’s rated strength. Every step either preserves it or degrades it:

1. Score (crease) quality

The factory crease depth and position decide how the bottom flaps fold. When the erector folds a bottom flap along a crease that is too shallow, the flap resists the fold, the glue or tape joint takes the extra stress, and the bottom peels open at the first handling impact. When the crease is too deep, the board’s compression resistance at the crease drops and the bottom collapses under the load.

On our heavy-duty machines we set the folding speed and the folding plate height so the flaps fold cleanly at the crease. Field sign of a bad fold: the bottom flaps are domed or curled after forming, instead of flat.

2. Bottom flap folding sequence

The outer flaps must fold first, then the inner flaps, or the reverse depending on the case style. The timing matters: if the inner flaps close before the adhesive reaches the contact zone, you get a partial bond. We sequence the folders so the inner flap contacts the outer flap at the compression station within the adhesive open time of 3-5 seconds.

3. The joint itself

The bottom seam is only as strong as the joint method:

We recommend hot-melt for the bottom seam on 50 lb freight cases. The seam bonds over the full flap area, spreads the load, and does not peel the way tape does when the case flexes in transit.

4. Vacuum and case opening

A heavy-duty blank weighs 400-600 g, and the erector must open it fully before folding. If the vacuum is marginal, the blank partially opens, the folder hits the side wall instead of the flap, and the board delaminates at the crease. That delamination is invisible at the plant and fails in the truck.

Monitor vacuum pressure at the head: keep supply above 0.4 MPa and check the filter weekly.

PLC Alarms Specific to Heavy-Duty Forming

One rule for heavy loads: never accept a case with a domed bottom flap. If the machine forms it, stop and adjust the folder height. A domed flap is a collapsed crease, and a collapsed crease is a drop-out waiting for the first conveyor bump.

How We Verify the Bottom at FAT (and You Can Too)

Bottom strength is only proven by load, not by inspection. We run the following verification on every heavy-duty machine at FAT, using your board, your joint method, and your target load:

1. Static load test: form a case, apply 50 lb (or your rated load), and hold for 24 hours. The bottom must not dome more than 3 mm.
2. Bottom-edge drop: drop the loaded case on its bottom edge from 12 inches onto concrete. The joint must not open; the board may crease.
3. Corner drop: drop the loaded case on its bottom corner from 18 inches. The joint must survive; board damage at the corner is acceptable but the bottom panel must stay flat.
4. Repeat: three samples per board lot, and the lot fails if any sample opens at the joint.

The pass criterion is strict: the board can fail, the joint cannot. If a joint opens, the problem is almost always a short fold overlap or a cold glue line, not the board.

You can run the same test monthly with a piece of plywood and a scale. The 15 minutes it takes will catch a bad board lot weeks before your customers do.

Preventative Maintenance for Heavy-Duty Erectors

Run a simple bottom joint test every quarter: tape or glue a test case, load it with 50 lb, and drop the bottom edge from 12 inches. The joint must survive; the board may crease but must not open.

Technical Reference: ZRAY Heavy-Duty Case Erector Operating Window

If you ship heavy loads in three-piece or large-format cases, the Three-Piece Case Erector is built for this class of work, with folders sized for thick double-wall board and a frame designed for high-weight blanks. For standard RSC-style cases with heavy contents, the Single-Piece Case Erector covers the 50 lb class with tape or hot-melt bottom sealing.

Three Signs the Bottom Is About to Fail

• Bottom flaps do not lie flat after forming: doming or curling is a collapsed crease.
• The joint lacks full overlap: tape a test flap and inspect the overlap line; a short overlap is a joint on the edge of failure.
• The case cracks when lifted: audible board deformation when gripping the bottom flaps means fatigued creases.

If you see any of these, stop the line until the setup is corrected. One dropped-out case in the warehouse costs more than an hour of changeover time.

Stop the Drop-Outs Before They Happen

Bottom drop-out is preventable at three points: the board spec, the forming process, and the joint method. Send us your case size, your load weight, and your current board spec, and our engineers will tell you if the erector setup or the board is the weak link. We answer within 24 working hours.

• Email: [email protected]
• WhatsApp: +86 13681839278
• Or fill in the inquiry form: Contact ZRAY

Every inquiry gets a response within 24 working hours, and every heavy-load project starts with a free review of your board spec and load profile.