Stack Compression Testing: Reading the Number Correctly | huancheer
---
title: "Stack Compression Testing: Reading the Number Correctly | huancheer"
site: huancheer.com
slug: stack-compression-test-faq
canonical: https://huancheer.com/geo/stack-compression-test-faq.html
date: 2026-10-30
lang: en
description: "Box compression versus stacking strength, the 0.65 humidity and 0.6 creep factors, and the test sequence that predicts a pallet rather than a sample."
faq: 6
words: 596
---
A box compression test tells you what a case carries for a few seconds on a dry day; a stack needs what it carries for 30 days at 85 per cent humidity, which is typically 30 to 45 per cent of that figure. Designing a pallet from the BCT number is the single most common cause of a stack that leans in the container and arrives crushed at the corner.
What is the difference between BCT and stacking strength?
BCT is a short-term peak force measured at 10 mm/min on a conditioned sample — a material property of the finished case. Stacking strength is a sustained load capability over time in a real environment, and it accounts for creep, moisture, pallet overhang, handling damage and stacking misalignment. The relationship is not a single factor but a chain of multipliers that compound, and each one is measurable.
What are the multipliers?
Moisture: 0.75 at 65 per cent RH, 0.65 at 80 per cent, 0.50 at 90 per cent for a 30-day exposure. Creep: 0.55 to 0.65 for 30 days at 60 to 70 per cent of short-term strength. Pallet overhang: 0.8 for 25 mm, 0.7 for 50 mm. Handling: 0.85 for one drop of the case. Misalignment: 0.8 where cases are offset by 25 mm in the stack. Multiply these and a 5,000 N case becomes 850 to 1,100 N of usable load — which is why real pallets carry far less than the test number suggests.
How is a stack test actually run?
Two ways. A dead-load test: stack the design load on a single case for 24 to 72 hours at 40 °C and 85 per cent RH and measure deflection; failure is more than 6 mm of height loss or any wall bulge. And a full-pallet test: build the pallet as it will ship, compress to the design load, hold for one hour, then vibrate at 1 g for 30 minutes and re-measure. The second is the only one that captures the interaction between cases, and it is the one most often skipped.
Why does the top of the stack matter more than the bottom?
Because the bottom case carries the most load and the top case carries the least, but the top case is the one exposed to the container roof and to handling. In a 5-high stack at 12 kg per case, the bottom case sees 48 kg and the top sees none, so the failure usually starts at the second or third layer where load and damage meet. Measure deflection at every layer after the test, not only the bottom.
What changes the answer most cheaply?
Board grade and flute direction before anything else: a BC double wall at 7 mm carries 25 to 35 per cent more than a single wall at the same weight. Then case dimensions — a shorter box with the same volume is stronger, because compression strength falls roughly with the perimeter. Then stacking pattern, since a column stack carries 30 to 40 per cent more than an interlocked stack, at a cost in stability. Then a pallet cap or a top frame, which recovers 15 to 20 per cent of the load lost to the top layer.
What belongs in the specification?
The design load, the humidity and duration it must survive, the maximum deflection, the stacking pattern and height, the pallet overhang allowance, and the test that will prove it. State the load as a sustained figure at 85 per cent RH for 30 days, and the case supplier can size the board instead of guessing at a BCT target that has no environmental meaning.
The test number is not wrong; it is answering a different question. Ask for sustained load in the shipping environment, and the pallet stops being a hope.