Can one PP grade for plastic boxes serve both food and industrial use?

Answer: One grade can carry two reports, but food contact needs virgin resin and a migration test, and regrind voids it.

Does higher melt flow always mould more easily?

Answer: For thin walls yes, for thick ones no. At 25 g/10 min in a 3 mm body, weld lines show and a drop splits them first.

Homopolymer or copolymer for a cold store?

Answer: Copolymer, at 6 kJ/m2 at minus 20 C against 3, so corners crack about a third as often.

Is a single PP material certificate enough?

Answer: No. It confirms polypropylene only; without MFR, grade code and migration data the spec never lands.

Is regrind ever acceptable?

Answer: In industrial bodies at 30 percent, yes. For food contact and medical transfer, virgin only.

PP Grade for Plastic Boxes: Five Selection Tests

Quick answer: Choose a PP grade for plastic boxes with five tests: melt flow, food contact papers, heat resistance, additive package and batch consistency.

Start a PP grade for plastic boxes with melt flow
Where food grade and industrial grade split
Heat resistance and cold brittleness
Additives, odour and migration
Five checks in order
FAQ

1. Start a PP grade for plastic boxes with melt flow

Melt flow rate is the first filter for a PP grade for plastic boxes. Two pellets sold as PP run from 2 to 25 g/10 min: the low number means longer chains, better impact and worse flow.

Tier	MFR (g/10 min)	Process	Notched impact (kJ/m2)
High viscosity	2.0 to 4.0	Extrusion, thick wall	8 to 12
Medium	6 to 8	Standard moulding	5 to 7
Low viscosity	20 to 25	Thin wall, fast cycle	3 to 4

High flow fills a thin grid but leaves weld lines in a thick wall; low flow starves a 2 mm rib. At 2 mm wall and 300 mm of travel the ratio is 150, filled at 230 C; a low viscosity tier runs 200 C but takes base impact from 6 to 3.5 kJ/m2. Test MFR to GB/T 3682.1 at 230 C and 2.16 kg, since a 190 C PE condition reports higher.

2. Where food grade and industrial grade split

Food grade is a set of papers, not a resin name. A body that touches food is tested to GB 4806.7 with hexane extraction at 60 C for 6 hours, total migration under 10 mg/dm2, and a three-person sensory panel; industrial grade carries mechanical data and RoHS only.

Item	Food grade	Industrial grade
Resin	Virgin, lot traceable	Virgin or up to 30 percent regrind
Migration	Hexane, 60 C, 6 h	Not tested
Documents	Full GB 4806.7	Mechanical data, RoHS

Resin source splits: food grade wants virgin pellets with a traceable lot, industrial grade can carry 20 to 30 percent regrind, cutting cost 8 to 12 percent but taking 15 percent off impact. An industrial report filed as food is the usual mistake; only migration and sensory pages count.

3. Heat resistance and cold brittleness

A cold-store PP grade for plastic boxes should be copolymer. Homopolymer melts at 160 to 165 C and deflects at 100 C under 0.45 MPa, so service stays at 90 to 100 C; copolymer holds 6 kJ/m2 at minus 20 C where homopolymer is down to 3.

PP grade for plastic boxes: light grey case open, two white latches raised in the lid, a perforated tray of glass vials, moulded front handle

Both ends need a number: thirty minutes of 100 C steam shrinks a homopolymer body 0.3 percent against 0.15 percent for copolymer, and at minus 25 C a 1.2 m drop cracks 30 percent of homopolymer corners against under 8 percent for copolymer. A grade listed at 100 C still sags on a 120 C drying line.

4. Additives, odour and migration

Two boxes can share a melt flow and differ in additives. Antioxidant 1010 and 168 at a 1:1 ratio, 0.1 to 0.2 percent total, plus 0.3 percent light stabiliser outdoors; overshooting yellows the part, and a yellowness index past 3 sends the batch back.

PP grade for plastic boxes: black case corner close-up, rim flange over a seal groove, black foam cut-out with one rounded part, ribs outside

Odour tracks MFR and drying: dry pellets at 80 C for 2 hours, hold melt at 200 to 230 C, keep residence under 5 minutes. Cheap resin with high residual monomer still smells after 4 hours.

5. Five checks in order

Run the five tests for a PP grade for plastic boxes in sequence and stop at the first failure.

Check	How	Pass line	Common trap
Melt flow	Certificate or test	Matches wall and flow	Only PP stated
Food contact	Migration, sensory pages	Full GB 4806.7	Industrial report reused
Heat	Deflection plus real cycle	Under 90 C continuous	Peak temperature only
Additives	Formula, yellowness index	0.1 to 0.2 percent	Overdose yellows parts
Consistency	MFR across three lots	Spread within 15 percent	Silent resin change

Price is the last hint: food grade virgin PP runs 900 to 1,200 RMB per tonne above industrial, and a quote 15 percent under the field means another resin.

Conclusion

Melt flow sets the moulding window, food papers compliance, heat the duty, additives the smell, and lot consistency the batch. Wired together, a PP grade for plastic boxes becomes a line in a purchase order rather than a supplier's word.

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.