There are a wide range of different kinds of sealing machine available on the market, all of which under the right conditions will produce satisfactory seals. Aside from personal preference between one manufacturer and another, it is important to ensure that the equipment selected is capable of delivering a sufficiently powerful induction field to heat the foil to sealing temperature within the length of time that the container takes to pass under the induction sealing head. Manufacturers may use power ratings in kW or provide tables to show output in terms of containers per minute or linear speed at different foil diameters. As mentioned above, there are many different types and thicknesses of foil and even more variations in cap design all of which will impact upon sealing performance so there is no substitute for having your chosen equipment supplier undertake sealing trials or providing equipment for line trials in your own facility.
Whilst performance is the key criteria in machine selection, there are many more relevant issues. For example, different sealing head designs are available, some have a ’tunnel’ type construction whereby the cap passes through the induction coil whilst others have a flat coil profile providing a less targeted induction field. Generally tunnel coils are only necessary when sealing flip-top closures such as dispensing caps and sport caps (where the foil is located anywhere from 8mm through to 20mm down from the top of the cap – hence further away from the induction coil) such that the coil can be located closer to the foil. The main disadvantage of tunnel coils is that they are extremely limited in terms of adjustment, since the spacing of the coil windings is mechanically pre-set during the construction in the head. Should you change the cap size by anything more than a few millimetres, you are more than likely going to have to purchase another sealing head if you are running a tunnel coil. A flat coil provides a wide induction field which in some cases can have a coverage area which would accommodate closures from 20mm through to110mm (provided the generator can deliver sufficient power) and thus provide maximum flexibility for future product lines being run on the same machine.
Correct induction sealing head design is critical to obtaining consistent and reliable sealing performance and you should ensure that your selected manufacturer has the experience and capability in this area before making machinery purchases. A typical example of using the correct sealing head with the correct kW rating is when sealing larger diameter two piece wax bonded foils. These applications not only require the correct output to seal the foil at the production speed required but to also completely dissipate the wax bonding between the two parts. This is important as wax that has not fully separated can appear to be okay when just sealed but will harden afterwards. The foil can then be damaged as the cap is opened or the cap simply cannot be removed.
One final point on equipment selection is the cooling system. The high electrical current running within electrical components is such that heat is an inevitable by-product. All induction sealing equipment has to have a means of dissipating this unwanted heat. When looking at 100% duty cycle equipment – for example an in-line unit which is constantly running, the heat produced can be significant. Typically there is a choice between having a fan which simply draws in surrounding air to blow over the electronics and a water-cooling system (a smaller version of that found in a car) which will circulate water through a heat-sink at the electronics and then through a heat exchanger to remove the heat. Whilst a good quality re-circulating water cooling system is significantly more expensive than just a fan unit, it provides two principle advantages. Firstly, water cooling is a vastly more efficient means of removing unwanted heat which results in lower running temperatures and improved reliability. Secondly, since airflow is restricted to the heat exchanger, in a water cooled unit, the electronics are in a sealed enclosure and are not at risk of damage due to contamination by airborne particles. That said, not all water cooled systems are equal and some cheaper units can either waste water or require excessive maintenance and inspection so it also pays to look at design, safety measures and build quality. For intermittent duty machinery which operates on a cycling basis, natural cooling occurs during the quiescent period and it is often sufficient to have airflow directed over external fins to provide cooling for this kind of equipment.