Can case raw material preparation run PP and HDPE through one mould?

Answer: The structure can be shared, the dimensions cannot. HDPE shrinks about 0.6 points more, which is 7 mm on a 1.2 m case.

Why does a case crack more often after switching to ABS?

Answer: Above 0.1 percent moisture the melt carries vapour, and a drop opens the latch boss.

Can a PC case carry a clear window only where it is needed?

Answer: Yes, with a PC pane over-moulded into the body. Leave 0.3 mm of overlap and hold the tool at 90℃.

What grade suits a case needing both -30℃ and 0.2 mm flatness?

Answer: ABS with a cold-impact modifier, or PC/ABS alloy. Plain PP is down to 2.5 kJ/m² at -30℃, HDPE will not hold 0.2 mm.

How should a threefold price gap be shown in a quote?

Answer: Per case, not per kilo. A PP case carries 4.8 kg at 44 yuan, a PC case 6.4 kg at 166 yuan; the 122 yuan gap is what a buyer acts on.

-30℃ Drop Test: Case Raw Material Preparation Across Four Resins

Quick answer: Run four resins through one mould and weight, shrinkage and cold impact split into four curves. Case raw material preparation settles the grade first; the tool then gets its shrinkage number.

Where the four grades diverge
PP cost line: 9.2 yuan a kilo with three conditions
HDPE and ABS: when a project must switch
PC and alloys: the boundary of a dearer grade
When PP is the wrong call
FAQ

1. Where the four grades diverge

Last winter 600 cases went to a northern cold store. All passed a 1.2 m ambient drop test; three months in, 11 showed 2 to 3 mm splits at the latch bosses. Rebuilt in HDPE, they came back clean at -30℃. Case raw material preparation fixes temperature before price.

A 1.2 m internal span weighs 4.8 kg in PP and 5.6 kg in ABS.

Shrinkage sets tool compensation: PP runs 1.4 to 1.8 percent, cut at 1.7, ABS at 0.5, so one gate dishes 0.4 mm in PP and bulges 0.15 mm in ABS. PP also falls from 6 kJ/m² at 23℃ to 2.5 at -30℃, where HDPE holds 8.

Grade	Density g/cm³	Shrinkage %	Impact at -30℃ kJ/m²	Price yuan/kg	Weak point
PP copolymer	0.90	1.4 to 1.8	2.5	9.2	Brittle cold, poor fuel resistance
HDPE	0.95	2.0 to 2.5	8	11.5	Low stiffness, hard to paint
ABS	1.05	0.4 to 0.6	6	14.8	Poor weathering, needs drying
PC	1.20	0.5 to 0.7	12	26	High moulded-in stress

2. PP cost line: 9.2 yuan a kilo with three conditions

At 9.2 yuan a kilo PP is the cheapest, but only while three conditions stand, and those are what case raw material preparation skips.

Case Raw Material Preparation: a white case open on a steel bench, grey foam, paper bags, racks behind

The first is a span under 1.2 m: 3 mm wall, ribs at 120 mm, 40℃ tool. At 1.5 m the ribs need 4.5 mm. The second is service above -10℃, below which impact falls under 3 kJ/m². The third is no aromatic contact: 72 hours in diesel and PP gains 8 percent in weight, opening latch clearance from 0.2 mm to 0.5 mm.

Meet all three and the list is short: dry 2 hours at 80℃, hold moisture under 0.05 percent, keep regrind under 20 percent.

3. HDPE and ABS: when a project must switch

The question is not which resin is better but which requirement PP cannot meet.

Field requirement	What PP does	Switch to	Preparation change
Store at -30℃ for 12 months	Latch bosses split, drop fails	HDPE at 4 mm wall	Dry 1.5 hours at 70℃
Lid-to-base offset of 0.3 mm	Sink marks push it to 0.8 mm	ABS	Dry 3 hours at 80℃, regrind 15 percent
Clear viewing window	Cannot be moulded	PC	Dry 4 hours at 120℃, 90℃ tool

HDPE gives three times the cold impact of PP and pays in stiffness: at 3 mm it bulges 6 mm under 46 kg where PP bulges 2 mm, so the wall goes to 4 mm and the unit to 5.9 kg.

ABS wins on dimensions, holding a 1.2 m diagonal within 0.8 mm, but above 0.1 percent moisture the parts show silver streaks. Preparation differences surface before moulding ones, so case raw material preparation signs off the schedule and the first shot.

4. PC and alloys: the boundary of a dearer grade

PC alone gives clarity and impact at once: 65 kJ/m² at room temperature, 12 at -30℃, a 1.2 m drop at -40℃ without a crack.

Case Raw Material Preparation: a half-open white case on a galvanised rack, three latches, two handles

The cost sits upstream: dry 4 hours at 120℃, hold moisture at 0.02 percent, and return granules to the dryer after 30 minutes in the hopper, since hydrolysis costs 15 percent of molecular weight.

A PC case also needs an hour of annealing at 120℃.

PC/ABS alloy pulls the price from 26 yuan to 18, cuts impact to 45 kJ/m² and gives up clarity, suiting cases under 800 mm. That is the boundary of a dearer grade in case raw material preparation: clarity, or staying intact below -40℃.

5. When PP is the wrong call

Any one of these conditions should take PP off the case raw material preparation sheet.

Condition	Threshold	Alternative	What it costs
Cold-store storage	Service below -30℃	HDPE	1 mm more wall, 5.9 kg unit
Tight assembly	Flatness at 0.2 mm	ABS	5.6 yuan a kilo more
Fuel and solvent contact	100 wipes without softening	HDPE or a coating	Extra coat, 8 s added to the cycle

Over-specifying happens too: one project moved a 1.2 m case from PP to PC, taking resin from 9.2 to 26 yuan a kilo and each unit from 44 to 166 yuan, while the coldest reading was -8℃. The extra 122 yuan bought nothing.

Switching grades also rewrites the card: PP to ABS adds an hour of drying and 25 minutes of purging.

Conclusion

None of the four grades is universally better, only better matched. PP is cheap and easy to mould but brittle cold; HDPE takes cold impact but flexes; ABS holds dimensions but demands drying; PC buys clarity with price. Write temperature, flatness and contact media first, and case raw material preparation will not come back for rework.

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.