A hot calender is the simplest and most effective solution to transform a standard needled punched felt into a high technical product, giving immediate benefit and extra added value. This is achieved without any modification within the existing line.
Typically a calender is installed in-line, directly after the last needle-loom, but it can be installed off-line in order to serve more than one producing line (Calender working speeds are faster than a standard felt production line)
In one single step, calenders are able to make an excellent finishing job to the felt, with the following advantages:
- Calibrating accurately the product to the required thickness
- Increasing the mechanical properties, by thermofixing the fibers
- Reducing the peeling effect.
- Reducing transport costs (up to 60 – 70%), by reducing the roll diameter of the felt
Felt thickness regulation
The main purpose of a hot calender is to adjust the final thickness of the felt. To do this, rather than working on the pressure, it is by far easier and more accurate to adjust and set the gap between the rolls (say 1,2 mm, or whatever is required). To do this the calender is equipped on both sides of the rolls with motor driven screw jacks with encoders. So, during operation it is possible from the control board to set the rolls gaps in order to achieve the required felt thickness. Adjusting the max working pressure, independently left to right can also be made “on the fly”. With the 3 rolls version, adjustment is independent between rolls 1-2 and 2-3.
2 or 3 rolls
The calender are made normally with 2 or 3 hot rolls. In both models the felt could be passed at S around the rolls or direct between two rolls.
The calender with 3 rolls is able to bring the process to a deeper step, with a final felt thinner and more consolidated. This because the 3 rolls have more heating surface but, more than this, has the capability of pressing the felt twice (between rolls 1-2 and 2-3) with independent setting. And this bring two benefit : less final thickness and gradual compression of the web. The gradual compression (with progressive gap setting in the rolls) reduce the possibility of creating wrinkles for all type of felt.
Accordingly with the product weight and characteristics, the felt could be passed on the rolls or direct or at S.
On S passage the contact time is considerably higher.
The 3 rolls version is able to press the felt twice.
The selection of the passing system is made from control panel.
Rolls bending compensation
Under pressure, rolls bend, this is a fact and in order to avoid variations in the felt thickness (across its width) we build into the calendar, a “Roll Bending Compensation” system. This system is based on the cross-rolls effect, offsetting the shafts of the rolls so that the profile of the rolls will then “kiss” each other and the effective deflection is compensated.
Because the bending is proportional to the effective applied pressure on the felt, the offsetting value is not fixed and can be regulated by the end-user. Regulation can be manual or electrically driven, with encoder.
The rolls are parallel, as the use of crowned rolls for compensation, only works for a specific, fixed pressure (like 10 t) and not in the full range of pressure. Using the adjustable shaft off-setting, the machine is ready to operate from 0 to the maximum pressure with minimal effective roller deflection.
Cooling rolls, after calandering
For specific applications, additional cooling rolls are necessary to stabilize the felt just after the hot rolls. They are placed immediately after the hot rolls and the felt passes through them in an “S” way. Cooling is made by water circulating inside the rolls.
Rolls are heated by the forced circulation of hot oil. Internally the rolls are equipped with a specific oil circuit, developed from our long experience, that accurately distributes the hot oil and provides a uniform real surface temperature across the full working width, from the 2 m to the 7 m rolls widths. Oil entry and exit from the rolls is made from the same side, while their drive is made from the opposite side. The special roller shaft oil seals are engineered by us, and allow safe working with low and easy maintenance (no need to disassemble the joint to change the worn element). Oil heating is made externally to the calendar, with a self standing unit. A centrifugal pump ensures good oil circulation inside the heating unit and the rolls. The heating unit can be fired with gas, or alternatively by oil burner or electric system, each of them of Bombiconstruction. If available and suitable, an existing heating unit can be used and connected with the calender. Electric heaters are controlled with S.S.R. (Solid State Relays), to adjust the temperature uniformly and proportionally. The temperature can be the same in each rolls, or optionally, it is be possible to adjust the temperature independently in each roll. The max working temperature is 240 ° C for the standard version. Higher temperatures can be reached with our special version, available under request.
Accordingly with the fibers blend and with the working temperature, the process may generate smokes. They are easily captured with the use of superior hood, installed on top of the calender. This is an optionally equipment.
Two type of safeguard are available. One this is directly installed on the machine. Other type are installed laterally, between the machines before and after the calender.
Both are extremely sure as stop the rolls when opened.
Best solution will be discussed with customers.
A broken needle coming from the loom, passing through the rolls, can damage/mark them. In order to avoid this, a metal detector is installed before or on top of the calender. This device detects the needles and gives a signal to the control board; when a signal is received the rolls are automatically opened in order to allow the needle to pass between the rolls without damaging them. To reduce the amount of un-calendered felt at every detection, a system of “delayed roller opening & closing” is used. Depending on the actual position of the metal detector and speed of the line, the system waits until the needle or ferrous metal impurity arrives near the rolls to open them and then closes them automatically.
|WORKING WIDTH||600 – 7.100mm|
|PRESSURING SYSTEM||Pneumatic for Model 1, Hydraulic for Model 2 and 3|
|OIL HEATING SYSTEM||Thermal oil circulation inside the rolls. Oil could be heated via gas boiler or electrical boiler with separate unit|
|ROLLER SURFACE||Hard Chromed plated, 62 Hrc hardened, 52 Hrc Hardened, PTFE Coated, Nylon|
|ROLLS GAP ADJUSTING SYSTEM||Electric driven screw jacks|
|COMPENSATION OF ROLLS DEFLECTION||Cross-axis system, adjustable manually or electrically|