By Ken Forziati
Director of Business Development & Product Management
When we’re discussing Machine Direction Orientation (MDO), we are describing machinery that heat and uniaxially stretch materials such as plastic film and sheet in the longitudinal (i.e. “machine”) direction over a series of rollers. The basic concept is relatively straight forward yet designing the right machine for the application is anything but simple. Over the next few articles I’m going to explain the key aspects of an MDO and how they relate to different applications. There are a lot of considerations that go into designing and building an MDO. I hope that you find these articles helpful in understanding the inner workings of these machines.
Let’s start at the molecular level. Orientation is defined as the straightening and alignment of polymer chains in plastic products like fiber, film, and sheet.The process of orienting plastics involves heating and stretching materials in a solid phase using specialized machinery to produce a less-random, more uniform arrangement of the polymer chains. The goal may be to increase material yield by down-gauging a film’s thickness while retaining or improving its physical properties. But, the process may also be used to attain a wide range of performance characteristics (mechanical, optical, shrink, barrier, porosity, etc.)that would not otherwise be possible.
So how exactly is this achieved? How do you down-gauge the film’s thickness while retaining or improving its properties?
An MDO can be used in line with a melt extrusion-casting process or offline as an independent roll-to-roll operation. In an MDO process, film is heated to the optimal stretching temperature via thermal conduction by direct contact with the surfaces of a series of heated rollers commonly referred to as pre-heat rollers. The optimal stretching temperature is typically just above the glass transition temperature or Vicat softening point of the material. Once the sheet has reached the proper temperature, it is subjected to a longitudinal stretching force imparted by a series of draw rollers running at increasing surface speeds. After the film is stretched, it may be annealed (stress relieved, heat set, etc.) in some fashion by maintaining or increasing the process temperature for some period of time, before its temperature is reduced via surface contact with a series of cooling rollers. An MDO is typically comprised of these three or four primary sections: preheating, stretching, annealing (optional), and cooling.
The first key aspect that you need to consider when thinking about an MDO is the heating medium you’re going to use in your application. There are four common types and I’ll quickly explain each one to give you a better idea of when it’s used.
- Water: Using water or a water-glycol mixture as the circulating fluid in the heating system is relatively inexpensive since water is abundant and water heating systems are common and relatively simple. The primary downside, is that the maximum process temperature of a water system will be limited to just 275°F. This limits the range of polymers that can be processed and is not suitable in applications where higher temperature annealing is required. To achieve temperatures above 212°F the water system needs to be pressurized, which can create a safety concern if components fail. Water quality issues also must be considered and addressed to ensure that the service life of components is not compromised due to corrosion or scale.
- Oil (or Synthetic Heat Transfer Fluids): Using petroleum-based or synthetic heat transfer fluids in the heating system enables much higher process temperatures – typically to 500+°F. However, these systems are more complex and the cost of the fluids themselves is not insignificant.Safety precautions also need to be considered when working with these systems as the fluids are flammable.
- Electric: To achieve even higher process temperatures, rollers that are directly heated electrically – either through induction or resistance heating elements – can be used.These rollers are often jacketed with a self-contained fluid to ensure uniform heating across the roller surface.Due to the complexity of construction, the high operating temperatures and resulting electrical loads, this method of heating is relatively expensive.Installation is often simpler, however, as the MDO will not require any plumbing for the heated rollers.
- IR: The three heating methods listed above are used to heat the rollers from the interior. An infrared radiation element is external and in an MDO application is used in two ways; to heat the web over an extended gap between rollers or combined with one of the above methods to increase heating capacity. Since heating by conduction with the roller surface is inherently unbalanced, in some applications adding heat on the exposed side can be beneficial.
Of course, the choice of heating method will be limited primarily by the process temperature requirements of the polymer(s) being oriented. Some may require high heat while others may require less to stretch the material to the right specifications, and in many applications, you may need a combination of heating elements to achieve the proper results.
In the next article I’ll discuss the different process conditions to consider in MDOs such as stretch ratios, stretch rates, line speed, and single-stage vs multistage stretching.