Two weight-per-meter control systems whose configuration sheets look almost identical can come back with quotes that differ by a factor of two. It is not that someone is inflating the figure: split the quote into six blocks and every part of the difference can be pointed to a specific place.
What follows covers only what each of the six blocks contains, under what circumstances it gets more expensive, and how to judge whether it is worth the money. No figures are given. The same weight-per-meter control system, installed on different extruders and delivered in different ways, should rightly be quoted differently.
Why the quote for the same machine can differ by a factor of two
The difference is concentrated in roughly four places.
- Hopper: hopper volume runs in steps from 7 L to 60 L. A big line fitted with a small hopper, or a small line with a large hopper, looks similar from the outside, but the structural strength and the installation footprint are completely different.
- Sensor: the weighing unit is the largest single item in the cost of the machine. Two units both marked ≤0.001% FS may differ in principle, in range and in temperature compensation, and stability diverges even further.
- Interface: a new line has an external setpoint reserved, so wiring is only a matter of finishing off; an old line needs a frequency converter added, the control cabinet modified and the cables re-run, and all the quotation says is one phrase, "interface retrofit included".
- Service: on-site installation or remote guidance, how many commissioning visits are capped, whether the after-sales promise is verbal or written into the contract - each of these is moving the boundary.
When two quotes differ by a factor of two, most of the time it is not the same thing being expensive in one place and cheap in another, but the sum of two different scopes. Align the scope first: what is included, what is not, and what is billed separately according to site conditions. The selection sequence is set out in more detail in the 6 questions to ask before signing.
Block one: model and hopper volume
What it covers: the hopper, the frame, and the feeding and cleaning structure. Hopper volume is 7–60 L, in five steps of 7 L, 22 L, 48 L, 52 L and 60 L, corresponding to NHY-M-7L, NHY-M-22L, NHY-M-48L, NHY-M-52L and NHY-M-60L. In a loss-in-weight feeding system the hopper is also the weighing vessel, and its volume decides the refill interval.
When it gets more expensive: one step up in volume brings the structural parts, the weighing load-bearing capacity and the installation footprint up with it, and the impact of refilling has to be handled afresh.
How to judge whether it is worth it: work backwards from the extrusion output and the refill interval; bigger is not better. Choose too large and the money goes on space you never use; choose too small and refills become frequent, breaking the continuity of the loss-in-weight feeding system. The correspondence between the steps is given under technical specifications.
Block two: the weighing unit
What it covers: the sensor, the scale body and mounting structure, sampling and filtering, and the calibration tooling. The ceiling of ±0.25% weight-per-meter control accuracy is decided here; ≤0.001% FS describes its measuring capability.
When it gets more expensive: the sensor works on a different principle; range and resolution, temperature compensation, vibration and corrosion protection, whole-machine weighing or local measurement; whether calibration needs dedicated tooling, and who carries it out.
How to judge whether it is worth it: ask for three numbers - repeatability, temperature drift, and the calibration interval together with the person who executes it. A percentage that cannot state its test conditions is treated as not supplied. This is the block in the price of a weight-per-meter control system most worth reading word by word.
Block three: interface and signal retrofit
What it covers: the setpoint and feedback to the extruder's screw speed, the haul-off speed, the frequency converter or the PLC; analog or communication; modification of the control cabinet and cable routing. The closed loop of extruder weight-per-meter control closes at this point.
When it gets more expensive: old equipment has no external setpoint, so an actuation-side component has to be added; the protocol is not open, so a converter module is needed; one controller serves several machines; the shutdown window is tight.
How to judge whether it is worth it: do not accept the verbal assurance that "it can connect to anything"; require a visit to the site to look at the interfaces first, and have the scope and the division of responsibility written into the agreement. The controller's power supply requirement is modest (AC 220 V, about 50 W); the difficulty lies on the signal side. If the interface cannot be made to work, the weight-per-meter control system is only a display. The signal flow is described in control principle.
Block four: line adaptation and commissioning
What it covers: single-loop or dual-loop control; which quantities are controlled - screw speed, haul-off speed, feed rate; building parameter sets by product; loop tuning and trial running; the acceptance criteria.
When it gets more expensive: dual-loop control brings another set of actuation and tuning; controlling several quantities at once makes the loop coupling harder to tune; more specs mean more parameter sets; trial running takes up machine time.
How to judge whether it is worth it: ask how many days the commissioning takes, how many people, how many visits are capped, and how many products the parameter sets are built for. One complete set of readings on the interface tells the story: weight per meter 0.1413 kg/m, reference wall thickness 1.650 mm, wire diameter 17.50 mm, material density 1.42 g/cm³, wall thickness compensation 0.020 mm, screw speed 41.24 rpm, haul-off speed 60.013 m/min, extrusion output 508.70 kg/h. These quantities pull on one another, and acceptance checks them one by one; "parameter settings" stores the tuning results, and "spec check" is the closing step of a spec change.
Block five: installation and training
What it covers: mechanical installation and alignment, electrical wiring, power-on self-test; operator training, covering start-up, spec change, reading the display and handling alarms; handover of parameter sets, wiring diagrams and documentation. On-site installation versus remote guidance makes most of its difference here.
When it gets more expensive: the site is cramped, dusty and subject to strong vibration; site temperature and humidity fall outside the range of −10 to +50 ℃ and ≤ 90% R.H. non-condensing; training spans several shift teams or requires a stay away from home.
How to judge whether it is worth it: ask how many people and how many shifts are trained, and whether after the training they can change a spec independently and carry out a "spec check"; then ask whether the commissioning record, the parameter list and the wiring diagram are left with the buyer. Training is the item most easily cut when a weight-per-meter control system is put into service.
Block six: after-sales service
What it covers: response time (telephone support and site attendance written separately), remote support, spare parts supply, the software upgrade policy, and the warranty scope together with its exclusions.
When it gets more expensive: a shorter attendance commitment means keeping a person or a part near you; spare parts held in local stock; remote support that has to be able to reach into the workshop network.
How to judge whether it is worth it: this block is the most easily compressed and the most likely to become trouble once production starts. When only the total price is compared it is usually the first thing cut, and it is at the moment of a stoppage that people discover the contract says "respond as soon as possible". Most of the friction after a weight-per-meter control system goes into production comes from here. The test is crude: write each item as a sentence with a time limit and a scope, and see whether the other party is willing to sign it.
The six blocks side by side
Fill the same quote into these four columns; the blanks are the places that were never discussed.
| The six blocks | What it covers | When it gets more expensive | What happens if you do not ask |
|---|---|---|---|
| Model and hopper volume | Hopper, frame, feeding and cleaning structure | One step up in volume, non-standard installation space | Hopper too small, refills interrupt feeding |
| Weighing unit | Sensor, scale body, sampling and filtering, calibration | High resolution, wide temperature compensation, on-site calibration | Accuracy stated with no test conditions, drift that nobody can prove |
| Interface and signal retrofit | Setpoint and feedback to the extruder, frequency converter and haul-off | No external setpoint, closed protocol | Retrofit costs added afterwards, the unit can only be a display |
| Line adaptation and commissioning | Single-loop or dual-loop control, controlled quantities, parameter sets, acceptance | Dual-loop control, many controlled quantities, many specs | Only one parameter set built, spec changes done by trial running |
| Installation and training | Mechanical installation, wiring, training, handover | Poor site conditions, training across shifts | Operators cannot change a spec and wait for the supplier every time |
| After-sales service | Response times, remote support, spare parts, upgrades | Shorter attendance, parts in stock, free upgrades | Response written as "as soon as possible", nobody reachable when the line stops |
Take this checklist to the supplier
The order matters: ask about scope first, then about configuration, and talk about price last. Comparing prices before the scope is aligned makes the difference meaningless.
- Which of the six blocks does the quote cover? Mark each one "included", "not included" or "billed separately according to site conditions".
- Is the hopper volume calculated from the extrusion output, or simply taken from the standard fit? How long can one refill last?
- What is the principle of the weighing unit, and what is its range? What are the repeatability, the temperature drift and the calibration interval?
- Can the extruder and the haul-off accept an external setpoint? Analog or communication? Who does the retrofit, and who pays for it?
- Single-loop or dual-loop control, which quantities are controlled, and how many parameter sets are built per product?
- How many days and how many people for commissioning, and how many visits are capped? Which items are measured at acceptance?
- How many people and how many shifts are trained? Afterwards, can they change a spec independently and carry out a "spec check" on their own?
- How many kinds of response time are there? Are spare parts held in stock? Are upgrades chargeable? Can all of this be written into the contract?
Once the eight questions are answered, put the answers on a single sheet and attach it behind the technical agreement. That sheet is of more use than a verbal promise.
Which block cannot be cut
Only two blocks of a weight-per-meter control system genuinely cannot be cut: the weighing unit and commissioning. The other four can be trimmed to suit the conditions of the line.
The weighing unit cannot be cut
It sets the ceiling on accuracy. The ±0.25% control accuracy for weight per meter and the ≤0.001% FS weighing system accuracy rest on the repeatability and stability of the sensor, not on an algorithm in the interface. Save here and the price is drift in the readings, calibration at ever shorter intervals, and the finished weight per meter creeping upward; by the time it is noticed, installation and calibration have to be redone as a whole.
Commissioning cannot be cut
With the parameter sets built in detail and the loop tuned stably, the system settles into "auto control active" soon after start-up; tuned roughly, the operator does not trust the readings, switches back to manual after a few days, and the equipment degenerates into a display screen. Re-tuning means stopping the line, and this is the block hardest to make up afterwards.
The enclosure and the interface can be cut
The cabinet material and ingress protection only have to be adequate for the site, and the size of the touch screen does not change the control result; it is enough that the entries for spec check, wall thickness compensation, material usage statistics, parameter settings and auto control active are all present.
But the distinction between "cutting the configuration" and "cutting the scope" has to be kept clear: the configuration can be reduced, the scope must not be left vague. Delete interface retrofit from the quote and the money has not disappeared, it has merely moved further down the line. How to work out whether it is worth it is given as a formula you can enter your own figures into in the arithmetic of input and output.
Frequently asked questions
Why are some weight-per-meter control system quotes especially low?
The saving is in one or two of the six blocks, and usually not in the machine with the screen: the weighing unit is a grade lower, interface retrofit is not included, commissioning is compressed into remote guidance. Ask them to mark each item "included" or "not included" first; the blank items are where the difference went. A low price is not the problem; an unclear scope is the problem.
Can I buy only the controller and not the service?
Yes, but accept the consequences. Nobody will finish installing the weighing unit, wiring the actuation side, building the parameter set library or tuning the loop for you, and on power-up it will do nothing but display readings. Where the plant has electrical and instrumentation staff, remote guidance plus self-installation is workable, provided the wiring diagram, the parameter list and the acceptance method are all in hand. Extruder weight-per-meter control is not buying a device; it is closing a loop.
Can it be retrofitted to old equipment?
Mostly yes; the cost lies in the interface. Confirm three things first: whether the screw speed can accept an external setpoint; whether the haul-off motor can be controlled externally; and whether the control cabinet still has room to add components. If all three are yes, the retrofit is routine work; if one of them is not, the actuation-side component has to be replaced, and that cost and the associated downtime are often harder to absorb than the equipment itself.
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