Companies which pack goods, machine builders, and automation suppliers are all working towards one goal: the best way to move vulnerable goods from producer to consumer. To extend the shelf life and to protect products is the main essence of packaging. We all want products to arrive at the consumer in a perfect state.
Form, fill and seal (FFS) machines are often used to pack large quantities of goods at high speed. Often these goods are food items like nuts, chocolate bars, and candy. There are different factors which can limit the speed of this type of primary packaging machine. They are: product feed, packaging material feed, and seal-time. The flexible packaging material (e.g. foil) used in FFS machines is formed and sealed on the fly, often around the product while in motion.
Apart from variation and quality, producing more packs per time is fundamental in an FFS packaging machine. With more consumers and the target to reduce costs, the need for more output is obvious. This can be reached not only by faster machines, but also with better quality sealing right from the first pack produced.
In this article, the solutions provided offer better seal quality, higher throughput and less waste at start-up. Conclusion, the sealing of packaging needs to be fast but, most of all, good in all stages of machine transition.
Constant seal-jaw temperature
The packaging material requires an exact temperature at a given pressure to ensure a good seal. In the sealing process, the temperature, pressure and time are equally important, as seen in Fig. 1. The motion control is responsible for the timing and pressure of the sealingjaw. This is important, but not a subject discussed in this article.
Fig. 1: Packaging quality depends on three parameters.
Packaging seal quality is affected not only by the sealing temperature, but also the room temperature, packaging material temperature and the product temperature. In many cases it comes down to only the sealingjaw to compensate for all of these factors. Pre-heating the foil can be an option, but is not always possible.
During operation, at each seal, the film absorbs heat from the seal-jaw causing it to drop in temperature. Before the next seal can be done, the seal-jaw needs to be at the right temperature again. Omron’s 2-PID algorithm and auto-tuning is fully developed to recover fast when temperature drops after each seal.
Control is normally good when the machine is in production stage, but what about when the production needs to start or stop? When moving from the idle stage to production stage, the balance in the PID control loop is disturbed heavily. The big drop in temperature at startup often results in a few or even many bad packages (Fig. 2).
Fig. 2: Bad seal at startup.
When the machine is paused the seal-jaw temperature rises unnecessarily high. Especially with fast running machines like flow wrappers, you want to run at full speed from the start, packing all products with an equally good seal.
Here is the solution; we know when the machine is going to start – the operator gives that signal! Knowing this start moment, we can “prepare” the seal-jaw temperature for the upcoming drop and pre-compensate for that. The technology used to achieve this is called direct power control (DPC). It provides heating output feed forward control in conjunction with Omron’s 2-PID temperature algorithm and is in-sync with the motion of the machine.
Easy-to-implement technology
To improve machine performance one should consider all parameters of influence and apply an integrated way of controlling them. This can be achieved using a machine controller. As mentioned before, the integration of timing, motion and temperature control is mandatory for good seals at high speed.
To stay focused on the temperature control part, three steps are needed to apply DPC to a FFS packaging machine. First set-up and tune the PID control loop, then from the PID values a few parameters need to be derived. Finally, load all these parameters into the matrix which is used by the DPC function block in the controller.
Set up the PID loop
2-PID temperature control is a big advantage over standard PID control. The built-in auto-tune will optimise the system for fast-disturbance-response. Sealing is a fast-disturbance process. Set value (SV) is fixed, but each sealing makes the temperature of the sealing-jaw (PV) drop. The faster the controller recovers the PV to SV, the faster the next seal can be made.
Calculate the parameters for DPC
From the PID parameters that the auto-tuning found, the dead time can be calculated. The DPC function block also needs to be fed with information which can easily be found from the temperature trend chart or with a simple manual test. These values are mainly power levels of the temperature controller output when the jaw is open and when it is closed with the foil clamped. From these calculations a seal temperature profile and a seal timing profile can be composted.
Fig. 3: Functional diagram of DPC interaction with PID.
The DPC function block
With the parameters found and the DPC operation matrix filled, all can be merged together. The PID algorithm, DPC data matrix and the DPC function block are now ready to produce. The result will be that the seal- jaw temperature will show less deviation from the set point (SP) as before, especially during machine transitions from idle to production.
Business benefits
The benefits for using DPC technology is mainly in FFS machines like flow wrappers, HFFS and VFFS machines. Slower moving intermitted (stroke) machines like tray sealers of bulk-bag-sealers are less likely to benefit from this technology.
With most flexible material, especially when they exist of laminated layers of barrier and carries foils, the applied temperature is critical. Two degrees too high could lead to a burned seal and 2°C too low is badly or not sealed at all. For example, easy-to-open packaging is made of foil with complex layer structure and is very sensitive to heat deviation. The goods which are packed could also influence the sealing – for example if the product is wet, frozen or very cold. This product will cool down the film and sealing can fail. Tight control is the only way to automatically compensate for this. Another application would be pharmaceutical. Packing medication is very temperature sensitive and with DPC just enough heat is applied to the seal.
The heat loss compensation is done by the controller, so there is no need for bigger sealing-jaw which hold more heat inertia. The machine can operate with lighter and cheaper jaws. Smaller sealing-jaws also means less energy use.
Fig. 4: Result when using DPC, no bad sealing so good packs right from the start.
Conclusion
Easy is key for the end user, the packer, the OEM and the supplier. Omron Sysmac is the solution to integrate temperature control and seal-jaw timing and pressure. Sysmac machine controllers integrate logic and motion as well with some clever analogue and application specific function blocks. The DPC function block integrates the temperature control with the flow of the machine motion.
Contact Laetitia de Jager, Omron, Tel 011 579-2600, laetitia.de.jager@eu.omron.com
