The stack, back to front
| Layer | What sits there | What it decides |
|---|---|---|
| Tail cap and port | USB or Type-C contact, sealed plug; or nothing on wireless models | Charging style and one of the leak risks |
| Battery bay | Lithium cell, cylindrical or pouch, 3.7 V, 800-1,400 mAh typical | Runtime, weight balance, transport paperwork |
| Control board | PCB with mode logic, timer, charge management, optional pressure sensor and radio | Modes, pacing, smart features, approvals |
| Motor bay | Eccentric-weight micro motor or linear drive | Frequency, amplitude, noise, feel |
| Neck and shaft | Drive shaft, seals, snap collar | Head fit, water path, rattle |
| Shell | Two-shot or overmoulded housing, IPX7 sealed | Grip, colours, drop survival |
Read from tail to tip, the stack is also the assembly order on the line, which is why plant tours follow it the same way. Each layer has its own supplier base, its own test items and its own share of field failures - and a quotation that lumps them together without itemisation is hard to audit. The sections below take the layers one at a time.
The stack also explains why toothbrush quotations vary so much at the same claimed specification. Two handles can print identical strokes-per-minute figures while sitting on different cells, different motor grades and different seal systems, and the price difference lives in those layers rather than in the printed number. When the desk compares partner plants, it prices the stack, not the box: each layer gets its own line, its own supplier options and its own test evidence, so a client can see exactly where a cheaper quote is cheaper and decide whether that is a saving or a deferral of trouble.
The motor bay
The drive is the heart of the product and the largest single cost variable. In sonic handles the usual arrangement is a micro motor with an offset weight: cheap, proven, and responsible for the buzz users associate with the format. Linear magnetic drives cost more and give finer control of the stroke. In rotary handles a geared drive turns the round head instead. What a buyer should extract from the plant is the motor specification - rated frequency and amplitude, noise target at or below the 55-60 dB band typical across the category, and the factory's own life-test hours - because motor choice cascades into battery drain, shell vibration and perceived quality more than any other part. Mounting matters too: the same motor damped into a ribbed, overmoulded shell reads solid in the hand, while the identical part floating in a thin-walled shell reads hollow. Two plants building the same motor can therefore ship handles that feel a class apart, which is another reason samples are brushed with, not just measured. The sonic motor page lists the options and their trade-offs.
PCB and firmware
The board is small and its job list is not: charge management for the cell, mode switching across two to five programs, the two-minute timer with thirty-second quadrant prompts, and on mid and premium tiers a pressure sensor and, on smart models, a Bluetooth radio. Firmware lives here too - mode names, prompt behaviour, low-battery signals - and firmware is where inexpensive handles quietly save money, with coarse mode steps and abrupt prompts. For smart models the radio brings FCC Part 15 or CE RED into the approval file. Buyers specifying custom modes or app behaviour should treat firmware as a deliverable with version numbers and a written change process, not a black box. The practical test is behavioural: switch modes, time the prompts, drain a charge and watch how the handle signals low battery - a board either behaves consistently through that sequence or it does not, and no datasheet line substitutes for the exercise.
Battery pack and charging path
Rechargeable handles carry a 3.7 V lithium cell - cylindrical or pouch - in the 800-1,400 mAh range, delivering the 14-30 day runtime typical at two two-minute sessions per day. The cell brings the heavy paperwork: UN 38.3 for transport and IEC 62133-2 for safety, and the same tests apply to polymer packs. Charging enters either through a sealed USB or Type-C port (4-6 hours to full) or wirelessly through the shell on a dock (12-16 hours), and travel cases with charging or UV-cycle features extend that path for higher tiers. Every entry point is also a water path, which is why the battery bay and the seals are designed together. The battery pack page and the charging dock and case page carry the component-level details.
Seals, shell and the head joint
Waterproofing is a system, not a gasket: sealed charging contacts or coil coupling, O-rings at the tail and neck, ultrasonically welded or glued shell halves, and a membrane over the button. IPX7 - one metre of water for thirty minutes - is the norm the whole category builds to, and it is verified by immersion on assembled units, per model. The head joint is the last seal and the most worked: a snap collar that must hold the head firmly through a full session yet release for replacement, cycled thousands of times in development. Retention force, rattle and head-moisture paths after use are the three field-failure sources here, and they are worth watching during a pre-shipment inspection. The construction details sit in waterproofing seals explained.
Where a buyer can intervene
The stack is also the customisation map. On a catalog mold, a buyer normally touches shell colours and finish, button layout, mode set in firmware, packaging and the head's bristle specification - while motor, cell and seals stay on the plant's platform. A private-mold program can go deeper: shell geometry, screen or light effects, custom dock, and a head collar cut to the buyer's own platform so heads become a brand asset. Each step deeper adds tooling, lead time and minimums - typically 500-1,000 pieces for catalog models against 1,000-3,000 plus tooling for private label - and the trade is laid out in the OEM and private label page. The desk's role is to keep the interventions honest: every changed layer re-enters testing, and every agreed option is confirmed in writing before tooling is cut. A teardown-informed brief can be filed through the quote form.