What is the minimum sampling for case raw material preparation on arrival?

Answer: Three: MFI, density, one cold impact run. If a dispute continues, add five moulded samples to a drop comparison.

How much drift between declared and measured MFI is normal?

Answer: Parallel runs within 0.8 g/10min and within 15% of nominal. Outside that, change the batch rather than compensate at the machine.

Does a new supplier mean new limits?

Answer: Yes. Catalyst and comonomer distribution can shift cold impact by 5 kJ/m², so hold the first three batches to 0.003 g/cm³ density spread.

How is the regrind ceiling set for in-house granulating?

Answer: By drop height, not by a percentage agreed in a meeting. Five drops at 20% and five at 35%: a gap within 0.15 m releases the higher share.

Six Checks for Case Raw Material Preparation: MFI to Regrind

Quick answer: Case raw material preparation can be judged at the dock: melt flow index, density, impact strength and regrind ratio inside their windows, plus one bag checked for powder and colour.

Six checks on the receiving dock
When the MFI window closes a batch
Density and filler load before price
Impact strength read against temperature
Regrind ceiling set by drop height
Turning the checks into a receiving card
FAQ

1. Six checks on the receiving dock

A 25 kg sack of PP copolymer arrives with little beyond a delivery note: grade code, batch number, 9.8 yuan per kilogram. None of it says whether the resin survives a -30℃ drop in a 420 mm case.

Six measured numbers do. Melt flow index fixes flow, density fixes filler load and part weight, impact strength fixes cold toughness, regrind ratio fixes cost and life, then powder and colour cover the visible lines. All six take two hours, no first-off sample needed.

Check	Method	Pass line	Cost of a miss
MFI	230℃, 2.16 kg weight	within 15% of nominal	wall swings 0.3 mm
Density	density cup, three samples	spread under 0.005 g/cm³	0.4 kg between parts
Impact	notched Charpy, 23℃ and -20℃	cold value at or above 15 kJ/m²	corner crack in winter
Regrind	declaration plus drop test	drop loss under 0.15 m	later cases go brittle
Powder	200 g through a 0.5 mm screen	at or under 0.5%	bridging at the feed throat
Colour	colorimeter, three points	ΔE at or under 1.0	two shades in one order

The six are not equal. MFI and impact are gates: a miss returns the load. Density and regrind are fit questions. A 3.5 mm thin-wall case runs well on 0.905 g/cm³ copolymer; an 8 mm ammunition case packs short and stays soft.

2. When the MFI window closes a batch

Melt index is the first line on a grade certificate and the usual origin of a mixed-batch failure. Drift beyond 15% of nominal is not normal, and two part-used sacks should never share a hopper. Blend them and half the cases come out brittle, half soft.

Case Raw Material Preparation: an open white case on a bench, black gasket in the lid, grey foam with four cut pockets and kraft packets

Resin 3 g/10min above nominal fills the cavity early, so weld-line molecules never entangle and the latch ring breaks first. Resin 3 g/10min below nominal needs 8 extra seconds of holding pressure, stretching a 62 second cycle to 70 and costing 40 cases a shift.

Sampling matters more than measuring. Resin at the hopper top dries overnight and reads 1 g/10min higher than at the sack bottom, so sample the middle. Two runs differing by more than 0.8 g/10min mean an uneven batch: change the lot.

3. Density and filler load before price

Density gets read as proof of strength, when it first sets weight. PP copolymer sits at 0.905 g/cm³; with 20% talc it reaches 1.05 g/cm³, and a case with a 420 by 320 by 180 mm cavity goes from 4.6 kg to 5.9 kg.

More filler buys stiffness and sells impact: each 10% of talc costs 8% of room-temperature impact, more in the cold, so an outdoor case holds filler under 10%. That trade inside case raw material preparation beats 0.3 yuan per kilogram off the invoice.

Density band	Compound	Cases that fit	Cost
0.90–0.91 g/cm³	unfilled copolymer	thin-wall equipment cases	flats bow
0.95–1.00 g/cm³	10% talc	medium-wall cases	8% less cold impact
1.04–1.10 g/cm³	20%–30% filler	load bases	28% more weight

Readings of 0.98 and 1.06 from one delivery are two filler loads, not a broken instrument.

4. Impact strength read against temperature

Impact only means something with a temperature attached. Copolymer at 45 kJ/m² at room temperature falls to 16 kJ/m² at -20℃, a 64% loss, and a winter truck bay sits there before dawn.

Case Raw Material Preparation: two white cases on separate galvanised rack tiers, upper lid half open, lower case showing a moulded handle and four catches

Notched Charpy and drop testing measure different things: crack growth in a cut bar against whole-case deformation. Judge the case by the drop, the resin by Charpy, never one for the other; between them sit wall thickness, ribs and liner.

Read the lower bound. A printed 45 kJ/m² is a mean of ten bars, and the weakest may sit at 38 kJ/m². Writing the minimum into the contract beats arguing the average, and a low cold figure sets the shipping latitudes.

5. Regrind ceiling set by drop height

Regrind is the one knob that trades cost against toughness by hand. Regrind at 20% cuts material cost 8% and drops the case from 1.2 m to 1.05 m, still acceptable.

At 35% and a third cycle, cold impact retention falls to 68% and the corner whitens on the first drop. Each pass shortens the chain; by the fifth cycle the lot goes to base plates.

Cycle	Share	Drop height	Cold impact
1	up to 20%	1.05 m	92%
2	up to 15%	0.95 m	85%
3	up to 10%	0.80 m	76%

A declaration is thin evidence. Drop five virgin cases and five from the suspect lot: a gap over 0.15 m means the higher share.

6. Turning the checks into a receiving card

Six checks fit on one A5 card that a warehouse clerk fills in without knowing moulding: batch and quantity, the six values, the verdict, the disposition. Photograph it, one per batch.

Field	How to fill it	Owner
Batch and quantity	copy the bag code, count sacks at 25 kg	clerk
Six values	write the number, never the word pass	inspector
Verdict	release, concession or return	quality lead
Disposition	re-batch, sieve or downgrade use	buyer

When a lid cracks in service, the card shows whether cold impact was 16 kJ/m² or 20 kJ/m² and regrind 18% or 33%.

Conclusion

Choosing resin ends as a list that can be measured: MFI governs flow, density weight, impact the cold, regrind cost. Each line carries a pass value and a disposition, so a clerk fills the card and a buyer argues from it. That is why case raw material preparation deserves two hours at the dock.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.