The case cracking fracture runs at 45°; what do I check first?

Answer: The impact point. A 45° face is shear tearing, so look for dents at corners, latch seats and hinge seats, then check freight records for a drop above 1.2 m. More than 3 dents puts external load ahead of structure.

Case cracking only appears at −20 ℃. Material or stress?

Answer: Measure cold impact first. Breaking at 4 J at −20 ℃ against 18 J at 25 ℃ marks a high brittle transition; if the same batch also splits at 25 ℃, go back to the corner radius.

Why does 8 g/10min resin make case cracking worse?

Answer: Shorter chains and easier flow cost about 40% of notched impact against 3.5 g/10min. At the same R3 corner and 2.4 factor, the softer grade splits first.

Only one case in the batch split. Is a material test worth it?

Answer: Read the origin map first. One split with an isolated origin out of 12 is usually handling damage; when 8 of 12 sit in one corner, skip the panel and open the radius.

Can a repaired case keep its original stacking rating?

Answer: No. With 70% tensile and 45% impact left, five layers becomes three, and the 5 mm band around the patch stays likeliest to crack again.

Case Cracking: Stress or Material

Quick answer: Case cracking splits on fracture angle first: 45° shear means impact overload, 90° flat means cold brittleness, and the two need opposite actions.

Fracture angle first
Three stress origins
Three material numbers
Plotting the batch
Repair leaves 70%
FAQ

1. Fracture angle first

A 10× loupe and a caliper split case cracking into two families in two minutes. A break at 45° to the wall, with stretched edges and a bright lip, is shear tearing, so the load came from outside. A break at 90°, flat and grainy with no stretch, is brittle: the wall never bent. Get the direction wrong and every test that follows is wasted money.

Three details matter more than the angle. A white, glossy face means the fracture is under 24 h old; a yellow face with grey smears means the crack sat there for months. Silver streaks within 5 mm of the origin show the resin yielded before it failed, which points to stress. A dent at the origin means impact; a clean origin means built-in stress.

Feature	45° shear	90° brittle
Path	Oblique, 1.4× wall	Square to the wall
Edge	Flipped, bright	Flush, no stretch
Surface	Grey-white, matte	Even white, coarse
Origin	Dent or scuff	Clean
Likely cause	Impact, overload	Cold embrittlement
Measure next	Dent count	Impact at −20 ℃

Photograph three frames first: the whole case, the fracture under 30° side light, and the origin at 10×, each with a steel rule in shot.

2. Three stress origins

Stress-driven case cracking starts at three places: the inside corner, the boss root and the rotomould seam. Each is a geometry change where nominal stress is modest and peak stress more than doubles.

A corner at R3 against R1 is a factor, not a detail. At R3 the finite element run gives a stress concentration factor of 2.4; at R1 it rises to 3.6. With 12 MPa nominal on the wall the peak reaches 28.8 MPa at R3 and 43.2 MPa at R1, past the 24 MPa yield point of polypropylene.

Case cracking inspection row of black and olive cases on a pale counter, one blue case upright, wooden shelves with orange and yellow cases behind

The boss root fails on section thickness. A 3.0 mm wall that grows a 2.2 mm root cools fastest there, leaving a 0.3 mm oriented skin that cracks first; a 1 mm fillet pulls the peak back under 30 MPa.

The rotomould seam adds a third path. A 0.2 mm mismatch at the parting line leaves a step, the halves shrink against each other, and the crack runs along that line for up to 200 mm, looking cut.

Sketch the three positions once: corners within 30 mm of each radius, the boss base ring, and the line where the mould halves meet.

3. Three material numbers

Ignore the grade and measure three numbers: melt flow rate, impact energy at −20 ℃ and regrind share. When two are out of spec, case cracking is not a structure problem, and changing resin is faster and cheaper than changing the tool.

MFI 3.5 and 8 g/10min behave like different materials. At 230 ℃ and 2.16 kg the 3.5 g/10min resin has longer chains and better toughness, while the 8 g/10min grade moulds easily but loses 40% of its notched impact. Swap one for the other on the same tool and the failing lots multiply while every machine parameter looks normal.

Cold impact explains most winter failures. A bar conditioned 4 h at −20 ℃ that breaks at 4 J needs 18 J at 25 ℃, a gap of more than four times. Prepare bars at 80 mm × 10 mm × 4 mm with a 2 mm notch, test five per lot and report the median; a square, unstretched bar break puts the cause on the material.

Above 20% regrind the chains have been cut once, impact falls about 25% and colour dulls; the ratio never appears on the delivery note, so back it out with FTIR or a density check at roughly 300 yuan per lot.

Number	Sampling	Limit	Meaning
Melt flow	230 ℃, 2.16 kg	3.5–5 g/10min	Above 8, low toughness
Cold impact	4 h at −20 ℃	≥8 J	4 J is brittle
Regrind	FTIR or density	≤20%	Above 20%, weaker
Density	Immersion, two	0.90–0.91 g/cm³	High, filler or regrind

4. Plotting the batch

One fracture gives a cause; twelve from the same batch give a pattern. Line the cases up by production date and record three coordinates per origin: height above the base, distance to the nearest corner, and which wall it sits on.

Case cracking samples on white shelving, four blue orange yellow and red cases on top, black cases open showing black foam and red inserts below

A cluster is a structure verdict. When 8 of 12 origins fall inside the same 30 mm corner zone, stress concentration is the cause; when origins scatter across four walls at different heights, material and process come back into play.

Inspect the cases that did not crack. Take 4 intact units, look at 10× and find a 2 mm white band at the corner, and the stress is already accumulating: the next lot will split, and a larger radius fixes it faster than a new resin.

Budget 15 min per case, 3 h for twelve: cheaper than a material panel and far cheaper than a second field failure. Three columns carry the record: serial number, origin coordinates, fracture angle.

5. Repair leaves 70%

A repair stops case cracking but never undoes it. Tensile strength lands at 70% of the original and impact at about 45%, and that line decides which cracks are worth mending.

Three limits settle it: length over 40 mm, penetration through the 3 mm wall, or an origin at a boss root. Any one means replace rather than repair; everything else may be repaired and used at 70% strength, with stacking layers recalculated instead of copied from the nameplate.

The repair is a new stress raiser. Most second cracks appear within 5 mm of the patch edge, so re-inspect at 30 days with a 10× loupe, and write the pre-repair strength and the downgraded duty on the case itself.

Conclusion

The order of attribution cannot be reversed. Read the fracture angle first: 45° sends you to external load, 90° to the material, and a case showing both goes back along the 45° face to the impact point. Then measure, multiplying the 2.4 factor at an R3 corner by 12 MPa to get a 28.8 MPa peak, and check whether 3.5 g/10min resin with 4 J cold impact passes at the same time. The batch map decides the last step: 8 of 12 origins in one corner means change the geometry, scattered origins mean change the resin. Repair is a downgrade, and 70% strength will not carry the original stack. Before compensation is discussed, have the photographs and the origin table ready, so every party works from one coordinate system.

Fracture pattern	Main cause	Measure first	Action
45° oblique, flipped edge	Impact overload	Dent count	Claim against transport
90° square, flat face	Cold brittleness	Impact at −20 ℃	Change resin or limit cold use
Straight break on seam	Seam mismatch	0.2 mm step	Rework tool or downgrade
Origins at one corner	Stress concentration	Radius and factor	Open the radius

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.