This concluding part on ‘How to Solve Issues With Calender Gauge Control?’ is a continuation from Part 2 on force induced variations in calendered gauge (or thickness) and focuses on the three tips.
- Limit the temperature rate of change at all times to a maximum of 3 degrees per minute.
- Minimize speed changes.
- Mill management
3. Limit the temperature rate of change at all times to a maximum of 3 degrees per minute.
Because faster rates can damage or warp the rolls.
The control system on the TCU should be designed and operated to limit the rate of temperature change for each calender roll water circuit to 3 degrees per minute. Faster rates of change do not allow for equalization of mechanical expansion throughout the rolls, bearings, and calender frames. Differential expansion can stress various components of your calender. In extreme cases, they can actually crack or warp the rolls, or even damage the bearings.
Heating usually is limited by the available heating capacity but the cooling system frequently has the capacity to rapidly cool the rolls and therefore also has the potential to cause damage if not limited to 3 degrees per minute.
Again, it is important to keep the calender running during warm-up and during cool down. Stopping
the calender and turning off the TCU circulation pumps before the calender reaches ambient is not considered good practice.
If the temperature control system has rate of change limits properly set, cooling down is easy, since all the operator must do is to lower the temperature set points to ambient and then 60 to 90 minutes later return back to the operator console to turn off the TCU.
4. Minimize speed changes.
Because changes in your calender speed result in gum wall gauge thickness variations that are undesirable.
When the calender is running there are forces generated that try to separate the rolls, as explained earlier. And these
separating forces are proportional to calender speed (although the relationship is not linear).
The hardness of your rubber stock and bank temperature determines the magnitude of separating forces. The
thickness of the gum wall being calendered also affects the separating force – thinner gum walls generate higher separating forces. Separating forces of 160,000 to 300,000 lbs at each actuator literally stretches the calender frame. When the forces vary, the amount of stretch varies. This further results in the roll-gap varying that in turn causes the gum wall gauge to vary.
Hence, speed changes should be avoided to the extent practical. A speed reduction from 45 mpm (meters per minute) to 9 mpm can result in a gauge decrease of approx. 0.002 inches. Similarly, the vice-versa holds for a gauge increase with roll speed increase.
Calenders are of different types. And different calendars have different degrees of frame stiffness so the quantum of the gauge change for a given speed change will vary accordingly.
5. Mill management
You can substantially reduce gauge variations by feeding the calender consistently.
A consistent, uniform, dwell times on the cracker and feed mills, as well as consistent bank size, will give consistent rubber stock temperatures and work history. The stock temperatures, bank sizes, and work history directly influence calender roll-separating forces.
When you minimize variations in these 3 important process parameters, you will minimize variations in roll separating forces, and this in turn will minimize gauge variations of your calendered sheet.
Further, good mill and bank control practices also give improved uniformity for other final calendered gum and/or fabric properties.
When you feed to the calender, you must also ensure that the rubber bank does not have areas that could stagnate. Because, if rubber remains in the bank on the calender for an excessive amount of time it will partially cure and become much harder. Edge trim process that returns to the bank and stagnates is a common source of this problem.
Such cured and hardened compound will introduce gauge variations as well as negatively influence other properties of your calendered material. In extreme cases, burnt lumps will appear in the calendered gum.
Presence of hard stock makes it difficult to properly penetrate the cord and may cause cord disturbance and improper cord distribution.
In such cases the cushion force causes lateral movement in the nip and this further results in cord displacement that is visible. Paired cords are often the first indicator. You will observe that the calendered thickness will be heavy and there will be cord displacement or irregularities. However, the weight of test samples will not be significantly affected.
Localized cord disturbance in cord distribution does not usually affect fabric weight, whereas redistribution of the cord does. Overall cord distribution problems can affect fabric weight in the case of fabric calenders.
For example, incorrect cord distribution may reduce the average EPI (Ends Per Inch) in the center of the fabric and increase the EPI (Ends Per Inch) on the outer edges. This will affect fabric weight and thickness – the weight of the center would be light, while the outer edges would be heavy.
In steel cord calendering, visible cord displacement and cord pairing after the cushion nip is to be expected in the areas where the cord EPI count is excessive.
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