A slit coating enquiry that starts with "we need a coater" usually ends with a machine nobody can commission. The head is bought, the pump is sized, the web runs — and then the film thickness will not hold from the first metre to the hundredth, no one can say which parameter caused it, and every conversation with the supplier becomes an argument about whose assumption was wrong. Requirements documentation is what prevents that. Not a specification sheet, but a record of the coating task in parameters a machine can be built against and a buyer can accept against. The sections below are the ones that decide whether the project lands.
1. Write the coating task as a physical description, not a product name
"Coat perovskite" tells an equipment engineer nothing they can design with. The functional description has to state what happens at the meniscus: which face of the web receives the film, whether the substrate is rigid or flexible, whether it travels continuously or in batches, and whether the coating is one layer or several in sequence with a drying or annealing step between them. For work in perovskite solar, OLED, quantum-dot and OPV development, the layer stack is part of the task definition — a single-layer test coater and a stack-development coater are not the same purchase.
State the substrate family with its dimensions and its handling behaviour: thickness, whether it is brittle, how it is carried (vacuum table, pinch rollers, edge guides, carrier web), and whether it is coated on one side or two. A web-thinning assumption that never gets written down is the most common cause of a delivered machine that cannot run the real material.
2. Specify the ink, with a behaviour, not just a name
Slit coating is only stable when the ink behaves predictably through the die. The documentation should carry what you know and what you have measured: solvent system, solids content, approximate viscosity and how it changes with temperature and shear, surface energy against the substrate, and whether the ink is abrasive, corrosive, degassing or prone to skinning at the die lip. Slurries with solids behave differently from clear solutions, and a machine sized for a solution will not handle a slurry of the same name.
Two properties decide the fluidics more than the rest. First, how the ink wets the substrate and how fast it sets — an ink that levels slowly and an ink that gels in seconds need opposite coating speeds. Second, what the ink will tolerate in a closed flow path: residence time in the manifold, exposure to metal or polymer wetted parts, and whether it can sit idle without settling or clogging. If you cannot measure these, say so in the document and ask for the ink to be characterised jointly before the die is designed.
This is also where the process envelope gets recorded for later troubleshooting: the ink lot, its measured viscosity, the bath temperature, and the make-up date. A thickness drift traced to an ink lot change is a normal, cheap fix; the same drift traced to a rebuilt pump is an expensive week.
Because the ink defines so much, the requirements document should treat it as a controlled input with an owner — not as a consumable purchased separately from the machine.
3. Give the coating window and the tolerance, not a single target
Write the wet film thickness you intend to deposit and the dry thickness you need, with the conversion you are assuming. Then state the tolerance you actually have to meet, and be honest about which one matters: cross-direction uniformity across the coated width, machine-direction uniformity along the length, or step-to-step repeatability between runs. Those three fail for different reasons and are corrected with different adjustments — die and shim work for cross-direction, pump and speed control for machine-direction, bath and ink state for step-to-step.
Record the operating window rather than one number. A coater that is only correct at 12 m/min with one ink at one viscosity is a demonstration machine. Buyers should document the range they expect to work in — coating speed range, wet film range, the substrate widths to be run — and state which parts of that range must be reached on day one and which are future capability. That distinction protects the budget without closing off the roadmap.
4. State the environmental envelope and who provides it
For oxygen- and moisture-sensitive materials the enclosure is part of the coating specification, not an accessory. The document should say what atmosphere the coating must happen in — inert gas or air, the water and oxygen level that must be held, and the dew point the room can actually deliver — and whether the coating happens inside a glove box, inside a closed enclosure on an open line, or in a controlled room. It should also state how long the door can be open, because a chamber that loses its atmosphere every time a roll is changed cannot be held to a written spec.
Temperature and humidity of the room itself belong in the document too. Solvent evaporation, ink viscosity and levelling all move with them, and a result that cannot be reproduced across seasons is a process nobody can trust.
5. Define drying and curing, because coating without it is only half a process
Document the drying method required after the die: heated roll, convection, IR, or a separate annealing step, the temperature profile and the dwell that the ink needs, and any constraint on the substrate — a plastic carrier that cannot exceed a temperature, a wet film that must not skin too fast at the exit. The drying capacity sets the line speed as surely as the head does. Too often a line is bought against the coating head and then throttled to a fraction of its speed because the dryer cannot keep up; that mistake is free to prevent by writing the dryer into the requirement at the start.
6. Specify the web handling in the same detail as the head
Tension, and how it is controlled and measured, decides whether a thin or flexible substrate survives the run. Write down the substrate width range, the tension range the material can tolerate, whether the line is roll-to-roll or sheet-fed, the supply and take-up arrangement, splicing and threading behaviour, and whether the coated side can touch a roller at any point. Add the defect rules: wrinkling, curl, edge bead, scratching and contamination from contact rollers, and how the machine is expected to avoid each. This section is also the honest place to record what the team has already seen fail on the shop floor.
7. Write the acceptance test before the order is placed
The acceptance test is the most valuable page in the documentation, and it is the one most often left out. It should say: with the buyer's own ink and the buyer's own substrate, at the stated speed and wet film thickness, the coated film is measured by a named method at named positions across the width and along the length, and the results fall inside the stated uniformity and repeatability. It should also define the run length of the trial, the number of consecutive runs, and who is present when it is witnessed. A machine accepted on a demonstration with the supplier's ink is a machine accepted on nothing.
Measure the film by a method both parties can reproduce — offline thickness mapping, a coated witness strip, or an inline gauge — and record the raw data, not a verdict. A coater that passes on a single spot reading can still be useless; the profile is the evidence.
8. Record the constraints that are not technical
Footprint, floor loading, door width and the route from the delivery vehicle to the position all limit what can physically be built and installed. Power, compressed air, exhaust and solvent handling, and any flammable-solvent classification that applies to the room are part of the specification, and a solvent recovery or disposal route has to be named before the first run. Add the practical operational constraints as well: how many people will run the line, whether it is a development asset shared between projects or a production asset on a shift pattern, the planned duty cycle, and who does maintenance. A coater specified for a one-person R&D lab and put on three shifts is specified for the wrong machine.
9. Keep one document under change control
Requirements that live in email threads and separate spreadsheets become claims, not records. Keep one document with a version number, a date, a named owner, and a change log that states what changed, when, and what it cost in scope or lead time. Every parameter in it should have a source: a measurement, a target from product development, or an assumption someone signed. Assumptions left unlabeled are the ones that fail.
Before an RFQ goes out, run a review with three people who will live with the result: the process engineer who owns the ink, the engineer who will commission the machine, and whoever owns the budget. Read the document against the acceptance test. If the test cannot be executed from what is written, the document is incomplete — and it is incomplete now, when a correction costs an hour, rather than after delivery, when it costs a rebuild.
10. Share the document, then use the supplier's questions as a test
Send the full document, not a summary. A supplier's questions are the cheapest diagnostic available: an engineer who asks about ink rheology, substrate thickness tolerance, atmosphere recovery after door openings, and the drying profile is working from the physics. A supplier who replies with a catalogue configuration and a price has not read it.
For teams specifying this class of asset, slit coating equipment from Shenzhen PowerDe Automation Precision Equipment is built around the requirements described above, and where the process envelope is genuinely new — an ink that has never run on a slit die, a substrate the line has not carried — the integrated engineering team services cover the process development alongside the machine build, with hardware testing as a defined stage before acceptance. The practical first step is to send the requirements document and the acceptance test to the contact page and ask for the parameters the supplier needs filled in — the ones they would have to know to guarantee the uniformity you wrote down.
