Which grade should a PP Plastic Case use?

Block copolymer PP-B below minus 10 C, at 7 to 9 kJ/m2. Homopolymer PP-H indoors, where stiffness and price matter.

Is a higher melt flow always better?

No. It solves short shots and costs impact: under a ratio of 200 use 8 to 12 g/10min, and only pass 20 above 280.

Is 1.6 percent shrinkage compensation accurate?

It is a compromise. Split it into 1.4 along flow and 1.8 across, then add 0.3 mm of allowance.

How many cavities should a small batch run?

One in 718H under three thousand units, two up to ten thousand. Four cavities will not amortise below fifty thousand.

Can a PP Plastic Case use regrind?

Up to 20 percent, same-grade sprue only. Every ten percent costs about 0.8 kJ/m2 of impact and nudges shrinkage up.

PP Plastic Case Selection: Grade, Shrinkage and Batch Size

Quick answer: PP Plastic Case selection runs in three steps. Grade follows service temperature: block copolymer below minus 10 C, homopolymer indoors. Melt flow follows flow ratio, so a shell above 280 needs 20 g/10min. Tooling follows batch size, from pre-hardened steel under three thousand units to a hardened mould past a hundred thousand. Shrinkage is the step most often skipped, and the one that stops lids closing.

Three Decisions Behind PP Plastic Case Selection
Three Grades and Their Melt Flow Trade-offs
Melt Flow and Flow Ratio Decide Fill
Shrinkage and Mould Dimension Compensation
Batch Size, Tool Steel and Cost per Unit
FAQ

1. Three Decisions Behind PP Plastic Case Selection

Three variables lock in order: temperature sets the grade, depth and wall set melt flow, order size sets steel. Below minus 10 C only a block copolymer survives; indoors homopolymer is enough.

2. Three Grades and Their Melt Flow Trade-offs

Homopolymer crystallises hard at 1500 MPa, stiff enough for a PP Plastic Case indoors, but glass transition sits near 0 C and 5 to 15 g/10min suits shallow shells.

Block copolymer carries the market: 7 to 9 kJ/m2, embrittlement below minus 25 C, melt flow 8 to 25 g/10min, and a hazy finish.

Grade	Melt flow	Notched impact	Embrittlement	Fits
Homopolymer PP-H	5 to 15	3 to 4 kJ/m2	minus 5 C	Shallow indoor shells
Random PP-R	8 to 20	4 to 6 kJ/m2	minus 10 C	Clear covers
Block PP-B	8 to 25	7 to 9 kJ/m2	minus 25 C	Cold store, outdoor
Glass-filled PP-GF20	6 to 12	5 to 7 kJ/m2	minus 15 C	Heavy pallet bins

3. Melt Flow and Flow Ratio Decide Fill

A 620 mm cavity, gated off centre, has 420 mm to travel; at a 2.8 mm wall the ratio is 150. Gate the short side and the path reaches 700 mm, a ratio of 250. Under 200, 8 g/10min is enough; 200 to 280 wants 12 to 15; above 280 needs 20 or more.

Clamp force: 0.24 m2 at 30 to 40 MPa needs 720 to 960 tonnes for a PP Plastic Case.

4. Shrinkage and Mould Dimension Compensation

Shrinkage is a set of numbers. Along flow a PP Plastic Case runs 1.2 to 1.4 percent, across flow 1.6 to 1.8, and a 3 mm wall shrinks 0.2 percent more than 2 mm. One 1.6 percent figure is a compromise.

Post-shrinkage adds 0.3 percent over 30 days. Lid and body must come from one tool with one coefficient. A 620 mm cavity cut for 1.2 percent measures 627.5 mm in steel; at the 1.8 percent the short side really shrinks, the part comes out 616.2 mm and the latch never engages. At 1.6 percent the steel measures 630.1 mm and the error closes.

Feature	Measured shrink	Compensation	Cost of ignoring it
Long side, along flow	1.2 to 1.4 percent	1.4 percent	1 to 2 mm short
Short side, across flow	1.6 to 1.8 percent	1.8 percent	Too wide, lid will not sit
3 mm wall section	plus 0.2 percent	plus 0.2 percent	Local sink
After 30 days	plus 0.3 percent	Allowance	Loose after a month

5. Batch Size, Tool Steel and Cost per Unit

Under three thousand units, one cavity in 718H at 30 to 34 HRC costs 80,000, lives 300,000 shots, or 27 per PP Plastic Case.

Ten thousand to fifty thousand takes two cavities in S136 at 48 to 52 HRC, 3.6 per PP Plastic Case. Past a hundred thousand, four cavities in H13 run 1.5 million shots at 1.2 per PP Plastic Case.

Batch	Cavities	Steel	Hardness	Life	Tool cost
under 3,000	1	718H	30 to 34 HRC	300,000 shots	80,000
10,000 to 50,000	2	S136	48 to 52 HRC	1,000,000 shots	180,000
50,000 to 100,000	2 to 4	S136	50 HRC	1,200,000 shots	260,000
above 100,000	4	H13	48 to 52 HRC	1,500,000 shots	350,000

Conclusion

Grade from temperature, melt flow from flow ratio, tooling from batch size. A wrong grade splits corners in a cold store, a low melt flow leaves the base short, and a wrong compensation puts the lid off. Selecting a PP Plastic Case is arithmetic done early.

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.