Date | 2026-10-16 14:17:53
Six steps, what each one decides, and why two of them cannot be undone.
A drawing arrives by email. Some weeks or months later, production parts ship. In between, six stages decide whether the finished part matches the drawing — and most of that work is finished before a molding press is ever switched on.
Design review (DFM) — is the part moldable at all?
Material selection — which compound fits the service conditions?
Mold design — where tolerances, gates and parting lines are fixed
Mold manufacturing — steel, hardness, surface finish
Molding — the process window
Trial run, then mass production — proving consistency, then holding it
Two of the six are effectively irreversible once they are done: mold design and mold manufacturing. A tolerance decision taken in stage three cannot be corrected by adjusting the press in stage five.
Design review for a thermoset part is not the same as for a thermoplastic one. Compression and injection molding of BMC and SMC impose their own rules, and the review is where they get applied:
Wall thickness, and how thickness transitions are made — abrupt changes are where sink marks and voids start
Draft angles for demolding
Shrinkage allowance, set per compound rather than from a generic table
Inserts, and how they are anchored against pull-out and rotation
Gate and ejector positions
Undercuts, and whether they justify the tooling cost
Tolerance stack — which dimensions actually matter to assembly, and which are generous
This stage is where projects save most of their money. Changing a feature on screen costs a fraction of changing the same feature in a mold that has already been cut.
We work from native CAD (Pro/E, SolidWorks, 3D CAD) and can read DWG, DXF, IGES, STEP and STL files.
The service conditions select the series before the drawing is finalised:
Creepage and arc stress in switchgear, contactors and breakers — material group I, PTI ≥600 V
Permanent humidity, condensation or chemical exposure — hydrolysis-resistant grades
Anti-static or explosion-proof environments — controlled surface resistivity
Continuous high temperature — high-temperature series
Large, flat, structural panels — SMC rather than BMC
Heat and flame resistance first — phenolic (PF)
The molding process follows from geometry and volume: injection molding for high-volume complex parts, compression molding for small to medium batches and the best surface finish, transfer molding for multi-cavity molds and intricate inserts. The three processes are described on our Molding page.
Series and property ranges are listed on the Material page. The final grade should be confirmed against the service conditions, not against the closest familiar grade.
Mold design converts the drawing into steel: cavity layout, runner and gate system, ejection and cooling, and the tolerances that must hold for the life of the tool. The figures we work to:
Cavity tolerance: ±0.02 mm
Parting surface flatness: ≤0.03 mm
Surface finish: mirror polishing to Ra ≤0.1 μm, with optional PVD coating (CrN or TiAlN) for longer mold life
Machining: 5-axis CNC
Thermoset shrinkage varies with formulation and filler loading, so the shrinkage allowance is set for the specific compound. That is one reason material selection comes before mold design in the sequence rather than after it.
Core and cavity inserts are made from 2344 (H13) or 5CrNiMo steel, vacuum-quenched to ≥HRC 58. Heat treatment matters as much as geometry: an under-hardened cavity loses its tolerance long before it wears through, and parts drift with it. Tooling engineering and manufacture are covered on the Tooling page.
Compression mold temperature typically runs between 135 °C and 170 °C, depending on the formulation. Closing is staged — fast approach, slow contact, controlled final compression — and venting is multi-stage, because air trapped in a thermoset cavity becomes a void that no later step removes.
For injection molding, inserts are loaded robotically and post-treatment is automated, which is what makes cycle times repeatable rather than operator-dependent.
Every mold goes through a 100-shot trial run before acceptance. The criteria: fill rate ≥98 %, demolding success rate ≥99.5 %, dimensional stability per ISO 2768-mK. Production starts once dimensions have stabilised — not at the shot count that happens to look good.
When material, mold and molding are bought from three different suppliers, a defect in the part has three possible owners and no single one. The material supplier points at the mold, the mold maker points at the compound, the molder adjusts the press and ships.
Under one roof the chain closes: the compound is chosen against the service conditions, the mold is designed around that compound's shrinkage and flow, and the process window is set on the press that will run it. When something is out of specification, it can be traced to a stage instead of argued about.
If you are still at the earlier step — deciding which suppliers to shortlist — our checklist on evaluating a BMC manufacturer in China covers the selection side.
A. Yes. Send the drawing with its dimensions and tolerances. Design review, material selection, mold design, molding and the trial run are all handled in sequence, from the drawing to production parts.
A. It depends on part complexity and the number of cavities. The earlier the drawing is available for review, the earlier a realistic schedule can be given — we would rather discuss timing before the design is frozen than after.
A. Either. We can recommend a BMC or SMC series against the application, or mold to a grade you specify. If you specify, we still review the grade against the process window before cutting steel.
A. Yes. BMC bonds well to metal inserts, so insulation and mechanical fixing can be achieved in a single shot rather than as a secondary assembly step.
A. A 100-shot trial with defined acceptance criteria (fill rate ≥98 %, demolding ≥99.5 %, dimensional stability per ISO 2768-mK), followed by production on the same press and process window that was qualified.
A. Native CAD — Pro/E, SolidWorks, 3D CAD — and DWG, DXF, IGES, STEP, STL.
Wenzhou Jintong Complete Appliances Co., Ltd. has produced SMC and BMC components in Wenzhou, Zhejiang Province since 2001, combining material formulation, mold making and molding under one roof.
Its qualifications can be checked individually:
National standard: contributed to the revision of GB/T 23641-2018, the national standard for fibre-reinforced unsaturated polyester moulding compounds for electrical purposes (SMC/BMC).
Standards committee: member of SAC/TC51/SC1, the national sub-committee for thermosetting moulding compounds for electrical use.
Management systems: ISO 9001:2015, ISO 14001:2015 and ISO 45001:2018 certified.
Product certification: BMC materials hold a UL Yellow Card (file no. E504820, for grade BMC 1616), searchable in UL Prospector under "Wenzhou Jintong" or "smcbmc".
Manufacturing: a 10,800 m² main facility integrating mold manufacturing, compression molding and injection molding, plus a 24,000 m² plant brought into use in 2020.
Further detail is on our About Us page; the three business lines — tooling, molding and material — are described under their own sections.
Most of the cost of a custom thermoset part is committed before the mold is cut. DFM review and material selection are the two stages where a change is still cheap, and mold design and mold manufacturing are the two where it is not.
Knowing which stage owns which decision is what makes a delivery date and a unit price defensible — for the molder and for the buyer.
📧 wendy.qiu@smcbmc.com 📞 +86-13868305300
