SURFACE TREATMENT FOR THE EXTRUSION MARKET
Crown surface treatment is a well known and recognized part of the printing, converting and laminating industries, but it has an even more critical role to play in the extrusion process. Giuseppe Rossi is Vetaphone’s specialist in this sector and explained the need for the process and how the technology is responding to changes in market demand.
WHY IS SURFACE TREATMENT SO IMPORTANT IN THE EXTRUSION PROCESS?
It is not as important in the process itself as afterwards. To ensure good adhesion of inks and lacquers during subsequent conversion processes, the molecular structure of the film surface must be modified, and this must be done immediately after the melt cooling phase before the polymer is post-crystallized. completely. By applying the corona charge at this point to the top layer (1 micron deep), we can break the molecular chains and add more oxygen. This alters the surface tension and improves adhesion. The longer you leave it before treatment, the more difficult it will be to break the molecular chains; in fact, it is often impossible, so time is of the essence.
ARE THERE DIFFERENT REQUIREMENTS FOR THE EXTRUSION OF BLOWN AND CAST FILM?
Blown film is the most common use for corona treatment. Due to the high incidence of LDPE in this process and the relatively slow production speed, compared to cast extrusion or any of the conversion processes, the corona system requires only little energy to achieve a good result. The technology in this sector is well established and mature, and with good control it allows for the constant production and processing of high-quality films.
Cast film is a much more demanding process because the PP material and higher line speeds require a more complex corona system design. Even a single-sided treater (as opposed to two-sided in blown extrusion) will generally need higher horsepower, a cooled backing roll, direct drive, and a pressure roll, effectively a proper pull setting.
There is a third type of extrusion that is applied to bioriented cast films such as BOPP, BOPET, BOPA, where line width and high performance demand that the crown unit be contained within the extruder.
ARE THE REQUIREMENTS DIFFERENT ACCORDING TO THE MATERIAL THAT IS EXTRUDED?
Yes they are. It depends a lot on the material and its intended use. How it will become after manufacturing brings a number of variables into the equation. For starters, each polymer has its own initial dyne level – that’s its ability to adhere inks and lacquers. Some materials, such as PVC or PA, require very little power to treat the surface to the correct level of dyne; PE requires a bit more power and PP is by far the most difficult to treat. You should also allow the additives to mix with the polymers, as these can significantly affect the level of corona treatment required and the energy consumed.
WITH ALL THESE CONSIDERATIONS, WHAT BENEFIT DOES VETAPHONE TECHNOLOGY OFFER TO EXTRUDERS?
It’s all down to good design, which has been central to Vetaphone equipment dating back to the early 1950s, when the company invented and pioneered what’s known as corona treatment. Two principles stand out: simplicity and high efficiency. By designing and building a unit that is easy to use, Vetaphone makes cleaning and adjustment easy with its quick-change cartridge system. If you back this up with high-efficiency generators that use the proprietary resonant circuit, you have the ability to deliver 96% of the input power directly to the electrodes. This reduces the heat level, which is an obvious advantage when handling lightweight extruded films. There are other factors like the refined level of control available through our iCC7 interface, but essentially, simplicity and high efficiency are the main benefits here.