Your storage box moulding runs clean all morning. Then packing reports lids that rock on some boxes and refuse to snap onto others. Box and lid each pass their own drawing, but they shrank by different amounts. The fix is to measure both after 48 hours, hold packing, mould temperature and cooling steady, and correct the steel once from measured data.
On a 180 t machine, this matters more because a box and its lid usually come off separate moulds, often on different days, sometimes in different grades or colours. Each of those differences can move shrinkage.
Key Takeaways
- PP mould shrinkage ranges from about 1.0% to 2.5%, so a 0.3% gap between box and lid moves fit by about 0.9 mm on a 300 mm box.
- PP does most of its shrinkage in the first 48 hours, so judge fit after that, not at the press.
- Packing pressure works only while the gate is open; mould temperature, cooling time and wall thickness also move shrinkage.
- At 180 t, a box and lid share one mould comfortably only when both are small.
Why Do a Box and Its Lid Shrink by Different Amounts?
Because PP is semi-crystalline, and its shrinkage moves with wall thickness, cooling rate, packing and grade. A thin, flat lid and a deeper box with thicker corners cool differently, so even in one resin they can shrink by different amounts. Different grades or colours widen the gap.
A LyondellBasell technical tip on mould shrinkage gives PP a range of 0.010 to 0.025 in/in, or 1.0% to 2.5%, against 1.5% to 4.0% for polyethylene and under 1% for amorphous resins such as ABS. As PP cools, its crystals shrink more than its amorphous regions, which makes the result sensitive to how the part is moulded.
The same note includes a plaque study on a 12 MFR impact copolymer in which shrinkage moved between about 1.0% and 1.6% with part thickness and gate area alone. It warns that laboratory figures like these are for comparison and should not be applied to tool design.
Material and colour choices can open a wider gap. Polyethylene's range sits above PP's, so a PE lid on a PP box needs its own allowance in the steel. EnviroTech Molded Products, in its part design guide, notes that some pigments, such as green, blue and orange, are strong nucleators, so two colours of one grade can give different dimensions.
When Should You Measure Parts in Storage Box Moulding?
After about 48 hours, not at the press. PP keeps shrinking as it continues to crystallise after moulding, and most of that happens in the first two days. Measure box and lid from the same shift together, and record resin lot, colour and machine settings.
EnviroTech's guide is the source for the 48-hour figure. It also says cooling rate and second-stage packing pressure directly affect PP shrinkage, and that thick sections cool slower and shrink more, so uneven walls bring internal stress and warp.
Choose the fit dimensions that matter, such as the outer length and width of the box rim and the inner length and width of the lid skirt, and measure them at the same points each time. LyondellBasell notes that a complex part can shrink at different rates in different sections, so take more than one reading along a long side.
Add a plain fit test to the gauge readings: after 48 hours, snap ten lids onto ten boxes from the same day and record any that rock or will not close.
Which Settings and Grades Pull Box and Lid Back Into Line?
Packing pressure and time, mould temperature, cooling time and wall uniformity. Packing works only until the gate freezes, so gate size matters too. Change one setting at a time on the part that is out, and remeasure after 48 hours before you touch the steel.
LyondellBasell notes that holding pressure reduces shrinkage only while the gates stay open, and names melt temperature, cooling rate, part thickness and gate size as the variables that control it. EnviroTech adds that where walls must vary, they should thin progressively in the flow direction, and stay under about 6.4 mm to avoid voids.
Nucleated grades change the balance. EnviroTech describes nucleated PP as doing most of its shrinkage in the mould, which then acts as a cooling jig against warp, though parts can be harder to strip and may need more draft. Reliance's Repol H110MAS, a nucleated homopolymer for rigid packaging, furniture and houseware with an MFR of 11 g/10 min and BIS designation IS 10951-1-MBL-C, is one Indian example.
Our view: mould the box and lid in the same grade and the same masterbatch wherever the design allows. Two grades or two colour packages give you two shrinkage problems to chase on every colour change.
Can the Box and Lid Share One Mould at 180 t?
Only for small boxes. A family mould adds the box and lid areas together, so at 180 t a pair fits at either PP factor only around 120 × 90 mm. Larger pairs need two moulds, and boxes beyond about 240 × 170 mm are trial jobs even alone.
Clamp needed for box and lid at 180 t (box sizes assumed; lid 8 mm longer and wider; hot runner; PP factor 2 to 3 US t/in²; 10% reserve, a starting margin, not a rule)
| Box footprint | Family mould (box + lid) | Box mould alone | Lid mould alone |
|---|---|---|---|
| 120 × 90 mm | 72 to 108 t: fits | 33 to 50 t | 39 to 58 t |
| 160 × 120 mm | 126 to 189 t: up to factor 2.9 | 59 to 89 t | 67 to 100 t |
| 200 × 150 mm | 194 to 292 t: does not fit | 93 to 139 t | 102 to 152 t |
| 240 × 170 mm | 263 to 394 t: does not fit | 126 to 189 t: up to factor 2.9 | 137 to 205 t: up to factor 2.6 |
| 280 × 200 mm | 359 to 538 t: does not fit | 173 to 260 t: up to factor 2.1 | 185 to 278 t: does not fit |
The clamp figures follow the method in the applications article in this series. A family mould has other costs as well. Box and lid have different walls and volumes, so they fill unevenly from one shot, and shrinkage tuning on one part moves the other. Two moulds cost more steel but let you set packing and cooling for each part separately.
What Should You Send for a Box and Lid Review?
Send box and lid drawings with the fit dimensions marked, both grades and masterbatches, and the cavity plan. Shrinkage measured on trial parts helps more than handbook values. See the 180 ton horizontal injection moulding machine page.
Frequently Asked Questions About Storage Box Moulding
How much clearance should a snap-fit lid have?
It depends on the lid design, how flexible its skirt is and how tight a seal the customer wants, so set it with your designer and confirm it on trial parts. Whatever figure you choose, specify it after 48-hour shrinkage, because a fit that feels right at the press can change over two days.
Why does a lid warp when the box does not?
A lid is flat, wide and thin, so small cooling differences between its two faces show up as bow, while a box's walls stiffen it. Even mould temperature on both halves, a uniform wall and ribs about 75% as wide as the adjacent wall help. Nucleated grades can also resist warp.
Should a storage box lid be gated at the centre?
A centre gate gives the lid balanced flow paths and more even packing in every direction, which helps it shrink evenly. A side gate leaves one end packed harder than the other, which can show as a lid that fits one end of the box better. For large lids, confirm with a fill study.
Can a loose lid be fixed by changing only the process?
Small gaps can sometimes be closed with packing pressure, packing time or mould temperature. Larger gaps come from a shrinkage allowance that does not suit the material, and forcing the process to close them can cost cycle time or bring warp and flash. Correct the steel once trials give stable data.
Do different colours of the same box need different settings?
They can. Some pigments act as strong nucleators and change shrinkage, so a blue box can measure differently from a white one in the same grade. Measure each colour after 48 hours during trials, and keep a settings record per colour rather than assuming one setting suits all.
Related Articles
- What Can You Make on a 180 Ton Household Moulding Machine?
- Bucket Moulding on a 180 Ton Machine: Sizes, Shot Weight and Output
- 180 Ton Household vs Container Moulding Machine for Tubs and Containers
Sources & References
- LyondellBasell: Technical Tip: Mold Shrinkage
- EnviroTech Molded Products: Molded Parts Design
- Reliance Industries: Repol H110MAS Technical Data Sheet
Technical Disclaimer
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