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Decorative strips are the least glamorous tool in the interior set and the one most likely to be rejected at PPAP. They are long, thin, multi-cavity, and measured against a straight edge — three conditions that expose every imbalance in the tool. By steel weight they are the smallest tools in the Automobile Molds range; by rejection rate they are among the highest.
Because bending and length variation have different causes, and a tool tuned for one usually worsens the other: bow comes from a top-to-bottom cooling split, while length spread comes from melt temperature and pack differences between cavities.
The geometry makes both worse. An 800–1,200 mm strip at 1.5–2.0 mm wall has a length-to-thickness ratio of 500:1 or more, and the ribbed back shrinks against a grained face that is held flat by texture and polish. Any imbalance shows up as a bow rather than a sink, because there is nowhere else for the strain to go.
Multi-cavity tools add the second failure. Four or eight cavities fed from one hot runner will not fill identically unless the runner is balanced and each drop is trimmed. A 5% fill imbalance is enough to leave one cavity 8–15 °C cooler at pack, and that difference moves part length by 0.02–0.05% — 0.2–0.5 mm on a 1 m strip.
| Source of variation | Effect on a 1 m strip | Control |
| Fill imbalance between cavities | 0.2–0.5 mm length spread | Balanced runner, individual tip temperature |
| Cooling circuit imbalance | 0.1–0.3 mm bow | Flow meter per circuit, ΔT ≤2 °C |
| Pack pressure difference | 0.1–0.2 mm width and rib fill | Separate pack profile per drop |
| Ejection timing | 0.05–0.15 mm local set | Simultaneous ejection, stripper plate |
The same balance logic applies at smaller scale to Home Appliance Parts such as decorative frames and handle inserts, where a 4-cavity tool produces four slightly different parts unless the runner is trimmed cavity by cavity.
✅ Bottom line: Length spread is a runner and temperature problem; bow is a cooling problem. Fix them separately.
Balance the runner to within 3% fill difference, hold every cavity within 2 °C of the others, then pre-bend the cavity 0.3–0.8 mm in the direction opposite to the measured bow.
Balance comes first, because pre-bend applied to an unbalanced tool corrects one cavity and ruins the rest.
Pre-bend is the second lever. Cut the cavity 0.3 mm proud over 600 mm, 0.5 mm over 900 mm and 0.8 mm over 1,200 mm, opposite the measured bow, and always toward removable steel. A strip is thin enough that 0.3 mm of pre-bend survives polishing and texturing; a 2 mm correction does not, and it will show as a wall-thickness variation when the part is sectioned.
| Strip length | Typical pre-bend | Check point |
| 600 mm | 0.3–0.4 mm | Granite plate, three points |
| 900 mm | 0.4–0.6 mm | Granite plate, five points |
| 1,200 mm | 0.6–0.8 mm | Granite plus V-block, 24 h after moulding |
Measure on a granite plate with the part supported at two points, not laid flat — laying a bowed strip on a surface plate hides half the error. Read it 24 hours after moulding, because a strip that measures 0.4 mm at ejection often reads 0.7 mm the next morning.
Texture is the last variable. VDI 3400 grade 18–24 on the visible face adds 15–25 µm of etch depth, and etching releases surface stress unevenly on a long thin part; etch before final straightness correction, never after.
Jusheng builds these tools in 718H at 33–36 HRC with 4–8 cavities on a balanced hot runner, cuts cavity blocks on five-axis centres to hold length within 0.05 mm, and verifies straightness on a granite plate before shipment. Zhejiang Jusheng Intelligent Technology Co., Ltd. keeps a dedicated strip cell for long, thin work so a 1.2 m cavity does not wait behind bumper blocks. The pre-bend discipline is shared with Home appliance molds for long handle profiles, where the same 0.3 mm allowance decides whether the part clips in flush.
✅ Bottom line: Balance the cavities before you bend the steel; a pre-bend applied to an unbalanced runner corrects one cavity and spoils the others.
Q: How much does a decorative strip mold cost? A: A 4-cavity 718H tool with a balanced hot runner runs 18,000–40,000 from a China factory. An 8-cavity tool with individual tip control reaches $55,000. Cavity count and runner type drive the price; the strip's size barely matters.
Q: What straightness tolerance is realistic? A: 0.5 mm per metre is achievable on a 1.5 mm wall with balanced cooling and a 0.5 mm pre-bend. Below 0.3 mm/m the part needs a stress-relief anneal after moulding, which adds a process step and 30 minutes per batch.
Q: Can cavities be added to an existing strip mold? A: Rarely. Adding cavities changes the runner balance, the clamp layout and the cooling manifold, so Jusheng quotes a new cavity block on the existing frame instead. A rebuild usually costs 60–70% of a new tool with none of the risk reduction.
Send the STEP file, the straightness callout, the resin grade and the annual volume, and Jusheng returns a cavity layout with runner balance, pre-bend allowance and factory price within three working days. OEM and ODM programmes are quoted per cavity, so the cost of the fifth and sixth cavity is visible before you commit. Ask for the pre-bend table with the quote — it is the difference between a strip that clips in and one that springs back out.
Mobile: +86 13516880625
Email: info@zjjsmould.com
Website:www.zjjsmould.com
January 07, 2027
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January 07, 2027
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Privacy statement: Your privacy is very important to Us. Our company promises not to disclose your personal information to any external company with out your explicit permission.