Can specialized plastic case design services sell drawings without the tool?

Answer: Yes, but the drawing needs datums, tolerances and a resin grade, or the next factory builds a case that does not match it.

Is a thicker wall always stronger?

Answer: No. From 3 mm to 4 mm adds little stiffness but stretches the cycle from 45 to 70 seconds; ribs do more.

The rating says 100 kg, so why does it sag three high?

Answer: Temperature and time were left out. A 100 kg rating at 23 C becomes seven tenths of that at 40 C.

How long does a design cycle take?

Answer: Specialized plastic case design services usually run 40 to 50 days from input to first article, with 25 of those days on the tool.

How are design fee and tooling split?

Answer: A separate design fee plus tooling, credited against goods once volume reaches an agreed level.

Specialized Plastic Case Design Services: Size, Wall and Load

Quick answer: Specialized plastic case design services work backwards from input: cavity from payload plus cushioning, wall and ribs from process and load path, rating from stack height with a temperature derating. Each step ends in a number with a unit.

Specialized Plastic Case Design Services Start With Design Input
Wall Thickness Follows Process and Load
Turning Stacking Load Into Numbers
Tolerance Stack and Drawing Callouts
Deliverables and Prototype Validation
FAQ

1. Specialized Plastic Case Design Services Start With Design Input

The first move in specialized plastic case design services is not drawing but interrogating the input; one wrong entry shifts every dimension downstream.

Input splits in two: a payload of 480 x 320 x 210 mm at 12 kg, two units per case; then freight mode, a four-high stack, minus 20 to 55 C, IP65 and 8,000 units a year.

Input	Example	Specification it fixes
Payload envelope	480 x 320 x 210 mm	cavity length, width, depth
Stack height	4 high	base loading, foot pattern
Storage range	minus 20 to 55 C	resin grade, tolerance band

Cavity follows payload plus allowance: 15 mm cushioning per side and 8 mm clearance give 510 x 350 x 240 mm; a 3 mm wall and 4 mm lid rim make the envelope 522 x 362 x 262 mm.

2. Wall Thickness Follows Process and Load

Wall thickness cannot come from load alone or process alone. The same 522 mm case needs 3 mm in injection moulding, 4 mm in blow moulding and 6 mm in rotational moulding.

Specialized Plastic Case Design Services: black foam liner in a black case, a rounded rectangular pocket with a stepped end and a round hole nearby

The load side is a simply supported plate: 117 kg over 510 x 350 mm is 640 kg per square metre, deflecting a 3 mm polypropylene base by some 2 mm; two 60 mm ribs cut that to 0.8 mm.

Ribs run at 0.6 times the wall and no taller than three times it, with a 0.5 mm root fillet: too thick sinks the outer face, too thin freezes early and the base domes. Draft is 1.5 degrees at 200 mm depth and 0.8 at 60 mm, or the mould drifts after 50,000 shots.

3. Turning Stacking Load Into Numbers

A four-high stack is a promise, not a specification: gross weight times one less than the layer count, times 1.5.

At 26 kg and four layers the bottom case carries 78 kg, so the test is 117 kg for 24 hours at 23 C, sagging no more than 2 mm with residual set under 0.6 mm. Polypropylene loses 30 percent of its modulus at 40 C, dropping to three layers, and half at 55 C, allowing two.

Ambient	Modulus factor	Stack layers	Test load
23 C	1.0	4	117 kg
40 C	0.7	3	82 kg
55 C	0.5	2	59 kg

4. Tolerance Stack and Drawing Callouts

Tolerances are not a matter of writing each one small but of the sum. Three dimensions stack: cavity depth, lid groove depth and gasket compression.

Specialized Plastic Case Design Services: open black hard case, foam cut into square pockets, a coiled cable at left, two latches on the lid

Cavity at plus or minus 0.8 mm plus lid groove at 0.5 mm, against a gasket 6 mm free and 1.8 mm compressed, reaches 1.3 mm worst case and leaves half a millimetre of squeeze. So cavity depth tightens to 0.4 mm.

Datums must be stated: A on the outer base, B and C on the side walls, holes at 0.2 mm from A and B, or the gauge and the mould disagree.

5. Deliverables and Prototype Validation

The deliverable is not a 3D model but five items: a STEP solid, an A2 drawing, a manufacturability report, a mould flow study and a first article report.

The schedule belongs in the contract: first tool 25 days after design freeze, five samples, three assembly rounds seven days apart.

Deliverable	Format	Acceptance
3D model	STEP AP214	matches the 2D drawing
Mould flow study	fill and warp results	weld line off the sealing face
First article report	dimensions plus material cert	zero out-of-tolerance items

Conclusion

Specialized plastic case design services earn their fee by turning fits, holds and seals into numbers: cavity from payload, wall from process and load path, rating carrying temperature and time, tolerances allocated by stack. Numbers agreed early mean fewer trial rounds, which is what specialized plastic case design services deliver beyond a drawing.

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.