The specification sheet for a hot-bending machine arrives in two very different states. One buyer sends a page of rounded numbers and assumes the supplier will fill in the gaps. Another sends a sheet on which every figure carries a measurement condition, and where each blank is a question rather than an assumption. The first sheet turns into a long argument after commissioning; the second turns into a quotation that can be compared. The machine used throughout this article is a 3D curved-glass hot-bending machine, shown here as a 3D curved glass hot bending forming machine, and the parameters published on its own page serve as reference values that show how a specification line should read. None of those figures is a promise about a future order. Write your own numbers, with your own measurement conditions, into the same structure.
Workpiece envelope and part-to-part variation
The size line on a machine datasheet is an envelope, not a licence to run anything inside it. The reference machine lists an applicable size of 14 inches within. Two buyers sit inside that envelope and mean different things: one runs a single large curved panel close to the limit, the other runs mid-size parts with a changeover at every shift. The first case puts demand on tooling area, chamber gas volume and uniformity across the whole field. The second puts demand on how fast the fixture is changed and how repeatable the first good part is after that change.
Specify the incoming stock as well as the finished part: thickness and its tolerance, thickness variation across one piece, curvature before the cycle, and how much that curvature drifts from batch to batch. Glass that leaves the previous station outside the tolerance you assumed will leave the bending station outside it too, and the machine will be blamed first.
Write the tolerance chain explicitly. List the tolerance you are handing to the station after bending and the tolerance that station needs from you. A downstream process that states its own conformity accuracy - the neighbouring bonding machine on the same category page, for instance, publishes a conformity accuracy of 0.1 mm - is what that line looks like when it is filled in rather than left blank. Put the number you owe your downstream process into the specification, together with how you will measure it and on what sample.
Cycle time and the number of tooling positions
The reference machine lists a cycle of under 180 seconds per piece and 30 moulds in the normal cycle. Those two lines have to be read as a pair, and the specification should say whose assumption sits behind each one. Multiply the two and you get a theoretical hourly figure; then decide what fraction of it your line can actually use, given loading and unloading, a break for inspection, and the time lost whenever a fixture is changed.
Ask for the cycle time as a range with the conditions attached: which part geometry, which glass thickness, which target forming temperature, and whether the loading and unloading step falls inside the number or outside it. A specification that records one clean cycle with an empty fixture is not comparable with a supplier who measures ten consecutive cycles with parts in place.
State your own demand in annual pieces and in peak-hour pieces, and let the supplier show the arithmetic that connects the two. Where your peak is higher than a single station delivers, say whether the answer may be a second machine or a shorter cycle. That choice changes the quotation, so it belongs in the specification rather than in the negotiation.
Heating method and the temperature window
The reference machine specifies dual high-frequency heating with a forming temperature of 600 to 850 degrees Celsius. Those two lines answer the same question from two sides: how energy reaches the glass, and over what range the glass softens enough to take the shape.
A window on paper can hide a machine that reaches it unevenly, which is what shows up later as a part that matches in curvature and fails in optical quality. Ask for the measurement method behind the window: how many thermocouples, on which points of the tooling, at which moment in the cycle, and whether the reading is taken on the glass or on the mould.
Write down the ramp behaviour you need, not only the final temperature. Some glass families tolerate a fast rise and pay for it in bubble formation; others need a slower climb and pay for it in cycle time. That trade-off is yours to set, and it should be in the specification before the supplier picks a default.
Atmosphere control and how it is verified
The reference machine lists a cavity oxygen content of 50 ppm or less, and describes circulating oxygen detection on four nitrogen-filled chambers together with a controller. Those lines are worth reading as a model for how an atmosphere specification should be written, because they name the limit, the method and the location.
A specification that says only "nitrogen atmosphere" invites a machine that reaches the concentration in an empty chamber and loses it as soon as a part is inside. Write when the reading is taken: before the cycle, at the moment of forming, or on a sample of cycles. Write where the sensor sits, and what the machine does when the reading leaves the limit - pause, alarm, or a diverted part. Write what the oxygen figure means for your glass, and what it costs you per hour of gas consumption.
Record the gas supply you can offer: pressure, flow, dryness, and whether the plant already has the piping to reach that station. Atmosphere problems in production are frequently supply problems in the building, and they will not be solved by the machine.
Yield figures and the definition behind them
The reference machine lists a product pass rate of 80 percent or above and an equipment failure rate of 2 percent or less. Those two numbers are commonly read as one, which is the fastest way to a misunderstanding. One describes parts, the other describes the machine.
For the pass rate, define what counts as a pass: the geometric criteria, the optical criteria, and which inspection method settles a disagreement between your inspector and the supplier's. State the sample the figure rests on: pieces from a trial, pieces from a production shift, or pieces from a week of running. For the failure rate, define the counting period, the clock and the denominator, and say what counts as a failure rather than a stoppage that was planned.
Ask for the two numbers as measured on your glass, not as measured on whatever the supplier had in the building. Where that is not possible before order placement, ask for the trial in which they will be measured, and put the trial itself into the specification.
Tooling: the figures that belong to it
Tooling carries specifications of its own, and they are routinely left out. State the material, the finished surface condition, the dimensional tolerance of the mould cavity, and the expected life in cycles with the wear limit that ends that life. State the replacement lead time and whether the supplier keeps the drawing on file. Where the machine itself performs tooling checks - as in the reference machine's mould-thickness detection before discharge - say in the specification which readings you expect from it and what happens when a reading falls outside them.
Fix the ownership question in the specification too: who owns the tooling drawing, who may copy it, and what happens to it if you later move production. That paragraph is cheap to write before the order and expensive to negotiate after it.
Process recipe: who sets it and who holds it
The reference machine has a touch screen for setting and displaying heating temperature and heating time, which means the recipe lives inside the machine and is editable. A specification that ignores that fact hands the process to whoever stands at the panel. State who may change a recipe, whether change is permission-controlled, and whether the machine keeps the parameters it actually ran for each piece.
State how recipes are stored and moved. Ask what the file format is and whether a recipe set can be exported, because an exit from a single supplier should not mean re-tuning every parameter from scratch in a new building. Also decide whether a second machine, inside your own plant or at a partner, has to run the same file.
Inspection points and the record you need
Decide what happens to a bent part next. On the same category page, the following machines include a 3D surface vacuum bonding machine and a fully automatic 3D surface laminating machine, so a downstream station may be bonding a film to the surface you just formed. Any defect you leave on that surface becomes visible and permanent one station later.
Write the inspection into the specification: what is inspected at the machine exit, with what instrument, at what frequency, and what the machine does with a part that fails - hold, mark, or reject. Then write the record. Say which fields you need per piece or per batch, in what format you want them, and how long they have to survive. A record written inside the machine's own storage disappears with the machine; a record written in a format you can read does not.
Utility and layout constraints from your side
High-frequency heating ties the station to a building. The specification should carry your electrical reality - supply voltage, available capacity, harmonic limits if your plant has them - alongside compressed air, exhaust, cooling, and the footprint and floor loading the machine must fit inside. A machine specified only on production figures is often the part of the line that turns out not to fit the room.
Add the operator reality: how many people will work the station, whether they will load parts by hand, and what interface language and permission levels they need. Then add the maintenance reality: who performs the first-level service, what the expected interval is, and which wearing parts you are expected to keep on a shelf.
Changeover and small series
If the plan is to run many different parts in short series, changeover becomes the dominant specification rather than a footnote. State the changeover time you need, measured from the last good piece of one part to the first good piece of the next, including the trial piece. State how many tooling sets you will hold and how they are identified at the station. State whether a new part geometry may be introduced by your own engineers or only by the supplier, and how long that introduction may take.
Short series also change what an acceptable pass rate means, because the loss on the first pieces of a series is spread over fewer parts. If your production is organised that way, say so in the specification and let the pass-rate figures be discussed on that basis.
Documentation that completes the specification
Attach the parts of the specification that are not numbers: the drawings and dimensional tolerances of your part, the glass grade and its supplier, your inspection criteria with the instruments named, and the trial plan in which the process figures will be measured on your parts. Name the standards your industry expects, and name the documents you want delivered with the machine: wiring diagrams, spare-parts list, parameter list, operating manual, commissioning report.
The documents you attach decide the quality of the answers you receive. A supplier who knows your glass grade and your inspection criteria can put real figures against the parameters; a supplier left to guess will answer with rounded ones. Where the full process has to be designed around this station rather than fitted to it, the same integrated engineering team services that carry a line from concept to commissioning will also read a specification like this one and tell you which lines still need numbers.
How to read the answers side by side
Build a comparison table whose rows are your specification lines, not the suppliers' own headings. Where a supplier declines a row, the blank is information. Where two answers differ, look at the measurement condition written next to each before comparing the numbers - a pass rate from a trial week and a pass rate from one clean run are not the same row.
Close the sheet with what has to be in place before the machine arrives: the building preparation, the trial plan with named parts, the training you expect, and the commissioning acceptance with its measured criteria and its consequence if a criterion is missed. The specification is finished when no line is left to the supplier's interpretation and no line is left without a method. That is the point at which the machine you are told you will receive and the machine you install start to be the same object. For questions about a specific part geometry, Shenzhen PowerDe Automation Precision Equipment, the company behind the reference machine, can be reached through the contact page of the same site.
