A detailed description of the bag making machine allows you to better understand and manipulate it.
It consists of two rubber lined rollers which are pressed against each other by pneumatic cylinders and the material is pinched between the rollers. The rollers which are coupled by gears at their ends are driven in opposite directions by a single motor. Typically they are driven by VFD control but in certain very high speed machines they are servo control driven.
Material from the in-feed / unwind is fed to sealers and cutters by the feeders. Feeders ensure the bags of correct length are fed at the right time. They also respond to feedback from print mark sensors to guarantee printed web is fed to the right position. Feeders should move the web only when the cutter/sealer is open position. They also work in tandem to maintain the tension between them so that all accessories mounted between them can work well.
The feeder consists of two rubber lined rollers which are pressed against each other by pneumatic cylinders. Material passes between the rollers and held by the pressure exerted between the rollers. These rollers which are coupled by gears at their ends are driven in opposite directions by a single motor. They are normally driven by servo motors as they need high dynamics [high speed and high acceleration / deceleration] and high accuracy.
Web tension at different locations of the machine is maintained by diverse mechanisms. Web tension between the feeders is maintained by adjusting the feed ratio between Feeders. Dancers facilitate to maintain tension in certain places. Some materials tend to stretch when they are hot. Web tension is sometimes relieved / reduced after the sealers to prevent stretching.
Print Mark Control
Print mark control works on the feeders by helping to position the bags correctly under the cutter and sealers. This section includes the film registration sensor and placement adjustment mechanisms. The film registration is used on film with graphics or pre-printed information. Printing process variations, film stretch, film slippage during acceleration and other factors can allow the graphics to drift away from ideal cosmetic / marketing placement on the finished bag. The registration mark provides a method to make minor adjustments to the actual end placement of the seal and cut on a bag. When there is no printing or graphics on the bag, the process is defined solely on length.
Cutter/sealers are moved up and down during production of the bag. This section is the heart of a bag making process as the bag gets sealed and/or cut here. The design of this section will determine the machine type.
The oscillatory motion is sometimes achieved by using a mechanical cam driven by induction motor. In these machines this is the primary axis to which all other axis is synchronized. It is critical in such places to get the position feedback of the mechanical cam by using an auxiliary encoder coupled to the Cam. In certain configurations servo motor and drive combination is used to achieve this motion. Servo solution provides higher flexibility, accuracy and speed compared to the VFD solution.
The sealers are maintained at right temperature so that the seal on the bags are of right quality. Temperature is maintained by using either standalone hardware or by utilizing special add-on instructions controlling the PID loops. The sealing temperature is primarily determined by the material of the bags and to a certain extent by the bag design.
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