Duplex Center Differential Rewinding Technology
SP Ultraflex Systems (P) Ltd. manufactures slitting and rewinding machines of the universally proven duplex center differential rewinding type for the conversion of flexible packaging substrates and laminates.
Types of Converting Slitting and Rewinding Machines
When classified with respect to the rewinding technology applied, there are broadly two types of converting slitting and rewinding machines-
- Duplex center rewinding machines
- Duplex center-surface rewinding machines
The term "duplex rewinding" implies the use of two shafts for rewinding the slit substrate. This ensures a positive separation between adjacent slit reels, which are wound alternately on the two shafts.
In "center rewinding", the winding force is derived solely from the rewind shafts and is transmitted to the web through the core and layers of material which have already been rewound. The rewind shaft is usually driven by a motor.
In "surface rewinding", the web is driven by the winding drum which is always in contact with the outer layer of the material being rewound. Here, it is the winding drum which is driven, also by a motor in most cases.
A hybrid of the two rewinding techniques described above is the "center-surface rewinding" machine which derives the winding force from both the rewind shaft and the contact with the winding drum. Here, both the Rewind shaft and the winding drum are driven.
The machines using center-surface rewinding technology are simple in construction with a minimal role of electronics and have offered invaluable service to the flexible packaging and converting industry in its nascent stages.
Center-surface rewinding technology was commonly used because of its ability to produce large diameter slit reels tight enough to handle the rigors of transportation to the customer. Since the winding drum is not required to transmit increasing torque with the build up in reel diameter, tightness was achieved without the need for complicated drives and controls.
Limitations of Center-Surface Rewinding Technology
- Center-Surface Rewinding technology is not suitable for Delicate Substrates which can easily get damaged or blocked due to the contact pressure with the winding drum.
- It is difficult to overcome problems arising from the differences in adjacent reel diameters arising out of Thickness Variations which are very common in most flexible packaging substrates, especially in the case of composite structures / laminates commonly used today.
- In the Rewind section, the two Rewind shafts are mounted on a pair of levers pivoted on either side of the winding drum and pressured against the drum through pneumatic cylinders. In most such machines, the shafts have to be removed during finished reels changeover. A fall out of this construction is the increased time and effort for changeover between sets, bringing down the output levels.
Over a period of time, center winding evolved to overcome the limitations of center-surface winding by the use of better technology and refinement in the following areas:
How duplex center differential rewinding technology overcomes the limitations of center-surface rewinding technology
The Differential winding technique afforded by ball shafts effectively compensates for web thickness variations by allowing each coil to rotate at the rpm suited to its diameter.
A Rewind diameter sensing device provides the feedback to reduce the motor RPM while at the same time increasing the slip torque with increasing reel diameter. The rate at which rewind torque increases can be preset with the taper function to consistently obtain finished reels with uniform tightness.
Pneumatically pressured lay on rolls enable the user to vary the contact pressure and thereby produce rewind reels with the required tightness. A self alignment mounting of these lay on rolls also compensates for a gradation in reel diameters within reasonable limits.
Cantilevered construction of the Rewind differential shafts, laser core positioning, and a pneumatically retractable slide door have drastically reduced the changeover time between sets which is an issue of concern since on a high speed Slitter rewinder, more than 50 percent of the machine time is used up in changeovers. Several more advanced options are available to further mechanize the various changeovers and thereby increase output levels.
The vertical arrangement of the two Rewind shafts makes possible various web configurations by changing the position of the Unwind. A careful comparison between the alternate configurations will enable the customer to identify the configuration best suited for the application.