What is the minimum order for a customized blow molded plastic case?

Answer: It follows the tool: a single-cavity steel mold starts at 500 units, an aluminium trial mold at 100.

Can shrinkage simply be taken as 1.8 percent?

Answer: On regular walls yes, but thick sections run to 2.0. Measure all three axes on the first samples.

Should a 4 mm wall be as even as possible?

Answer: Even overall, deliberately uneven locally: corners and the base carry the hardest loads.

Can a customized blow molded plastic case take metal inserts?

Answer: Yes, heat staked after molding or placed in the parison. M6 inserts are checked at 8 Nm of torque.

Blow molding or rotational molding?

Answer: Volume decides. Blow molding suits runs above 500 at 270 seconds per cycle; rotational molding suits small runs and very large parts.

Customized Blow Molded Plastic Case: Size, Wall and Load

Quick answer: A customized blow molded plastic case is specified in three moves: scale the cavity for 1.8 percent shrinkage, set the wall through parison control points, and write the rating as a stack test with a temperature derating.

Customized Blow Molded Plastic Case Sizes Come From the Cavity
Wall Thickness Comes From the Parison
Load Rating and Stack Test Conditions
Mold Line and Pinch-Off Trim
Sample Validation Before the Tool Is Cut
FAQ

1. Customized Blow Molded Plastic Case Sizes Come From the Cavity

A customized blow molded plastic case is dimensioned from the cavity back: the drawing states cold dimensions, so shrinkage comes off first.

Polypropylene shrinks 1.6 to 2.0 percent in blow molding, 1.8 as a working value, more on thick walls. A 1,000 mm length loses 18 mm, so the cavity is cut at 1,018 mm and 420 mm width becomes 427.6 mm.

Item	Example	Range	Method
Cavity L W H	720 x 420 x 380 mm	plus or minus 1.5 mm	tape and gauge
Outside L W H	742 x 442 x 402 mm	plus or minus 2.0 mm	surface plate
Wall	4.0 mm	3.5 to 4.5 mm	ultrasonic, six points

Three allowances sit outside the cavity: 0.3 mm trim stock, a 5 mm rim flange, 12 mm feet.

2. Wall Thickness Comes From the Parison

Wall thickness is set by the parison, not by the mold. A 9 mm tube drops from the die, blows at a ratio of 2.5 and lands at 3.6 to 4.0 mm.

Customized Blow Molded Plastic Case: black case corner, a red catch under a black cam latch, the parting seam crossing the frame, matte texture

Distribution comes from 25 axial control points and 8 circumferential segments: 7 mm on thin sections, 11 mm on thick ones. Corners stretch hardest, so the parison is pre-thickened 30 percent there, or the corner lands at 2.5 mm.

Air enters at 0.5 MPa with the mold at 25 C; a 4 mm wall cools 90 seconds inside a 270 second cycle. Too little pressure leaves the corner short of the surface; too much thins the pinch-off.

3. Load Rating and Stack Test Conditions

For a customized blow molded plastic case, a rating is a test condition, not a lone number: at 30 kg and five layers the bottom carries 120 kg, so with a 1.25 factor the test load is 150 kg.

Hold it 24 hours at 23 C with base sag under 3 mm and residual set under 1 mm, then drop 0.9 m on six faces at minus 20 C.

Derating is milder than for injection molded parts, but above 50 C take 15 percent off: 128 kg, sag limit 4 mm.

Case	Condition	Pass line
Stack, ambient	150 kg, 23 C, 24 h	sag 3 mm or less
Stack, hot	128 kg, 50 C, 24 h	sag 4 mm or less
Drop, cold	0.9 m, six faces, minus 20 C	no crack, lid shut

4. Mold Line and Pinch-Off Trim

The mold line is unavoidable, so it is specified rather than hidden: no more than 0.4 mm proud and 0.3 mm mismatch between halves.

Customized Blow Molded Plastic Case: black case open, red gasket inside the rim, foam pockets holding red-handled pliers and two black tools, red corner bumpers

The pinch-off is 2 mm wide with 0.6 mm of flash, trimmed by a profiling knife run 0.5 mm from the edge: too far leaves a sharp lip, too close cuts the sealing plane.

Position matters more than finish: on the side centre the line escapes the stack load, and it is fixed before the tool is cut.

5. Sample Validation Before the Tool Is Cut

First samples are not for appearance but for three measurements: wall distribution, the thinnest corner and rim flatness.

Three cases, five checks: six wall readings each, 24 hours of water fill, a 150 kg stack, a 0.9 m drop, 8 Nm on the inserts. All five pass before volume; otherwise the parison program changes first, which costs far less.

Feature	Allowance	Action
Mold line	0.4 mm proud max	scrape flat, no sanding
Corner wall	3.0 mm min	thicken the parison corner
Rim warp	1.2 mm max	cooling fixture

Conclusion

A customized blow molded plastic case is specified by three sets of numbers: a cavity scaled 1.8 percent, a wall held between 3.5 and 4.5 mm by 25 parison points, and a rating written as a 150 kg test with a temperature derating. Get those right and both sides sign the same customized blow molded plastic case.

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.