Auto lock vs crash lock bottom cartons: which to choose
An auto lock bottom is a snap lock base of four interlocking flaps that you fold by hand in one motion, with no glue. A crash lock bottom is glued at the factory, so the base springs into shape the moment you squeeze the carton open. Both carry far more weight than a tuck flap; choose crash lock when cartons are erected in volume and every second counts, and auto lock when the run is small or you pack by hand.
How an auto lock bottom works
The base is four flaps die-cut to interlock. You push the carton square, fold the two side flaps in and press the front and back flaps down until their tabs catch. It takes a second or two once you have done a dozen, and it holds without glue. You will also see it called a snap lock or 1-2-3 bottom.
Because nothing is glued, the base can be opened again. That is occasionally useful, for example when a carton is filled from the bottom, but most customers will never notice.
How a crash lock bottom works
A crash lock bottom is folded and glued during manufacture, then the carton is collapsed flat to ship. At your bench you squeeze the side panels and the base pops into place. There is nothing to seat, nothing to get wrong and no flap that can be locked half in. The trade also calls it an auto bottom, which is why the two names get mixed up.
The glue pass is the trade-off. Every unit goes through an extra step at the carton maker, and that shows up in the quote.
Side by side
| Auto lock bottom carton | Crash lock bottom carton | |
|---|---|---|
| How the base closes | Four flaps folded and seated by hand | Glued at manufacture, pops open |
| Time to erect | A second or two per carton by hand | One squeeze |
| Error risk | A flap can be seated badly on a rushed line | Almost none |
| Reopens | Yes | No, not without tearing |
| Cost drivers | No glue pass on the base | Extra glue pass on every unit |
| Our board | 20pt SBS | 20pt SBS |
| Common sizes | 3 x 3 x 5 to 6 x 4 x 8 in | 3 x 3 x 5 to 6 x 4 x 8 in |
| Minimum and lead time | 200 units, 10 to 14 business days after proof | 200 units, 10 to 14 business days after proof |
| Best for | Hand packing, short runs, products filled from the bottom | Larger runs, many people packing, speed at the bench |
When you need a lock bottom at all
A tuck bottom is fine for light products. Our rule of thumb is about 12 ounces: below that, a straight tuck end carton holds, and a lock bottom is board and cost you do not need. Above it, the weight of the product pushes a tucked bottom open, especially inside a shipping carton that gets dropped. That is when you move to a lock bottom.
Candles, glass jars, bottles and anything dense belong in a lock bottom. Our candle boxes use an auto lock base for exactly that reason. If the product is heavy enough to bow the panels of a 20pt carton, a lock bottom will not save it; that is a rigid box or a mailer with an insert.
Choosing by how you pack
- You pack by hand, a few hundred at a time. Auto lock. Folding four flaps is quick once the team has the rhythm, and you skip the glue pass on every unit.
- Several people pack, or volume is high. Crash lock. One squeeze per carton, every carton identical, and no badly seated flaps reaching a customer.
- Cartons go through a co-packer. Ask them. Many prefer crash lock because it removes a step and a source of mistakes.
The top end is a separate choice
Both bottoms pair with a tuck top. A straight tuck top keeps the front panel clean; a reverse tuck top tucks toward the back. Either can carry a window. See straight tuck vs reverse tuck for that decision.
When to use neither
If the carton should never be opened before purchase, for example a dry food that must show tampering, a seal end carton glues both ends shut, like a cereal box. It is a one-time closure, so it is wrong for anything the customer opens and closes.
Questions people ask
Is an auto lock bottom the same as a crash lock bottom?
No, although the names get mixed up. An auto lock, or snap lock, bottom is folded by hand from four interlocking flaps and has no glue. A crash lock bottom is glued at the factory and springs into shape when the carton is opened.
Which one is faster to pack?
Crash lock. The base forms in one squeeze. An auto lock takes a second or two per carton to fold and seat the four flaps, which only matters when the volume is high or several people are packing.
Which bottom is stronger?
For the products that fit these carton sizes, both hold well on 20pt SBS. The practical difference is consistency: a crash lock is formed the same way every time, while an auto lock depends on the flaps being seated properly.
Why does a crash lock carton cost more?
The base is folded and glued at manufacture, which is an extra step on every unit. On short runs packed by hand, that step often is not worth paying for. On larger runs the time saved at your bench usually pays it back.
Can either style have a window?
Yes. A window sits on a side panel and is independent of the bottom, so both styles take a clear film window as well as a straight or reverse tuck top.
What is the minimum order?
200 units for both the auto lock and the crash lock bottom carton, in 10 to 14 business days after proof approval.