A die to die for
Q: How are dies different for blown versus cast film? Where is the technology advancing the most?Carl Johnson: Blown film dies tend to run a wider and, at many times, more generic set of materials. Blown film dies must not show port lines/weld lines, but this is not an issue with cast film dies.
Both blown and cast die manufactures continue to advance the state of the art. For example, profile controls have been enhanced for ease of use. Cast dies have also seen a resurgence in micro layer feedblock technology. And blown film dies for high-barrier applications have been getting much larger. I would call them generation II designs.
Jim Campbell: Blown film dies are circular. They produce film in a tubular form that is slit for sheeting. Cast film dies are flat and produce film in sheets. Technology is advancing in both areas. However, blown film is generally a less expensive process. So if the film can be produced in a blown configuration, that is usually preferred.
Q: What recent advancements have been made in extrusion dies and why were they needed?Johnson: Blown dies continue to become more streamlined, even as they get larger. There are more layers available now than ever before and the dies become more flexible in the number of materials they can process successfully. There are many different combinations of technology being introduced, from traditional pancake and spiral dies to hybrids of both. GEC has introduced its encapsulated feed technology for spiral mandrel dies. Superb material distribution and good gauge control are “musts” for today’s high-end processors.
As raw materials and end product become more expensive no one wants to order more than they need right away. This means smaller lot sizes/more frequent orders, all with a requirement of short lead times and on time delivery. This forced processors to transition from one material to another more quickly. They needed a die that would purge quickly and handle a range of viscosities. Even as the dies become larger they wanted to be able to prolong run times on thermally sensitive materials.
Nick Nigro: The designs are constantly evolving. Manufacturers are always looking for new alloys and materials for functionality and cost.
The many options for potential die coatings are also quite plentiful. At times it’s not the metallurgy or process that drives certain changes or evolution. The advancement is, machine tools that perform the actual cutting, drilling and polishing of the die parts from steel are as much a contributor to the design necessities as the other factors.
With the availability of these new machines and machining practices, designs that were conceived but couldn’t be built previously are now possible. This is true for both applications.
For processors, it is demand from their customers as well as competition that constantly pushes the technology. Contributors are:
- Production mix-more for blown than cast, but both exist.
- Quick cleaning or purge and changeover from product to product.
- Newer or more demanding polymers.
- Newer structures-combinations of materials that were never combined before.
- Consistent repeatable product performance.
Q: What are film converters asking for from their extrusion die suppliers and why?Nigro: For blown film, competition and costs are the driving factors. The cost of resin is the biggest culprit today.
Most commodity film producers usually cannot pass along any increased costs. This makes efficiency imperative to staying competitive and profitable.
The same concerns apply to higher end film producers. Although able to pass along some increases due to the unique structures and markets, they are also being pressured by costs and competition.
As early as 10 years ago, production of 7-layer barrier films were left to a few “large” experienced players. Now 7-layer is almost commonplace and the industry leaders have had to raise the bar to 9- and even 11-layer structures to protect market share. But, at the same time, they haven’t been able to pass along the additional costs associated with the improvements.
In the U.S., the three major markets for cast film are (in order of market share percentage): pallet stretch, bakery goods and hygiene applications. Basically, all these are commodities.
Unlike blown film, the scrap generated in the cast film process is fixed for edge trim and oscillation. Whether you manufacture a 30-inch or a 300-inch wide product, the scrap is the same. So, efficiencies of scale would dictate that wider is better.
While it sounds logical and can in many cases work, it, too, has limitations. Web handling and winding are two. Certainly deflection concerns and design approach are critical. Although flat dies have been manufactured and installed successfully over 300-inches wide, the size and complexity of these large lines necessitates much more automation. Not only in process but with finished roll handling, palletizing and shipping.
Q: What new materials are dies being made of and why, and how are new die designs helping to save energy consumption?Johnson: In blown film, most suppliers use 4140 or 4340 steel due to its thermal stability, availability, machinability and satisfactory cost/performance ratio. Some customers prefer stainless steel for non-barrier dies as they never need to be replated. We are continuously looking for new plating solutions to provide better polymer release, lower pressures and plating longevity.
We use less raw material for a given thickness requirement. We are continuously evaluating the die trying to achieve higher throughputs for a given diameter. We run thermal analysis to better understand how to insulate internal bubble cooling (IBC) and air rings from the die. We want the cold air to stay cold. We control this by the type and the design of the insulation, to minimize conductive heating.
Gloucester Engineering Co.
Hosokawa Alpine American
Windmoeller & Hoelscher Corp.
BE ships world's first 11-layer blown film lineBrampton Engineering has shipped the world’s first 11-layer blown film line from its plant in Brampton, Ontario, Canada. The SCD multilayer, streamlined coextrusion die became the world’s first successfully commercial 7-layer die and later led the way to the first 8-, 9-, 10- and now 11-layer blown film lines.
The 11-layer SCD die is an industry first, as is the solution that BE developed to accommodate the 11 extruders around the die. As the number of layers goes up, the extruder arrangement becomes increasingly constricted. BE engineers discovered that arranging 11 extruders radially around the SCD die would require longer adaptors and result in a crowded hot section. A suitable extruder arrangement needs to accommodate a die access platform and leave enough room between the extruders for servicing access.
Ideally, for barrier film coextrusion, the adaptors should be short and straight. BE designed piggybacked extruders, or two extruders with one mounted on top of the other. The design of the SCD is suitable for piggybacked extruders since the extruder center line heights are different for each layer. The configuration for the 11-layer line then became two piggybacked extruders plus seven normal extruders, creating a radial extruder configuration similar to a 9-layer line but with two of the extruders being doubles.