By Ken Forziati
Director of Business Development & Product Management
Machine Direction Orienters (MDOs) are relatively straight forward devices. Their primary function is to induce orientation in the polymer structure of plastic film or sheet in the machine direction by stretching it longitudinally at an optimal process temperature. This results in down-gauging a film’s thickness while retaining or improving its physical properties. The process may also be used to modify performance characteristics such as tensile strength, modulus, elongation, clarity, haze, shrinkage, oxygen barrier, water vapor barrier, porosity and so on. However, just how these characteristics will be affected by orientation will depend on the polymer and overall formulation of the precursor sheet in addition to the conditions at which the material is processed.
As mentioned previously in Part 1, to orient material we need to heat and apply a longitudinal stretching force using a series of rollers running at increasing surface speeds. Depending on the application, the MDO will be designed using a single or multi-stage stretching section. These configurations simply refer to the number of draw rollers used in the stretching process. A single-stage machine employing one pair of draw rollers generally would be less-expensive than a multi-stage machine employing additional draw rollers, but a multi-stage machine may enable the product to run faster and provide more output. However, it’s never that straight forward.
So, which configuration would work best? Does your application require an MDO that utilizes a single-stage or multi-stage configuration?
Before we dive into these questions, we need to first understand the critical process conditions: mainly how the stretch rate, stretch ratio, and the line speed influence one another and ultimately affect the results of the material being oriented.
- Stretch Ratio – The stretch ratio (also referred to as the draw ratio) is one of the primary process conditions that will be adjusted in an MDO. It’s a measure of the overall elongation of the film or sheet. In the simplest terms, it is the ratio of the finished length to the initial length of the material being stretched, and nominally is the ratio of the output speed to the input speed of the draw section of the MDO. If the input line speed is set at 100 fpm and the stretch ratio is set at 2:1, then the output speed would be 200 fpm and we would be doubling the length of the film in the process.
- Stretch Rate – the stretch rate or related strain rate is a measure of how quickly the length of the material is increasing in the stretching process. In the example of a universal tensile testing machine, it would simply be a factor of the speed at which the moving gripper is pulling on the fixed sample or the change in length per unit of time relative to the original length of the sample before stretching. In an MDO it is a little more complicated as the “sample” is not fixed, but continually moving through the machine and the stretching is occurring over a set distance. So, in this case the stretch rate is a factor of the stretch ratio, the line speed, and the distance over which the stretching is occurring; not necessarily easy to define as it is will depend on configuration of the draw rollers, gap between the draw rollers, location of any nip rollers, and frictional forces acting on the web.For a given stretch ratio, the stretch rate will increase roughly proportionally to line speed.
- Line Speed – the speed at which the web is fed through the process is a critical design parameter and will greatly affect the configuration of the MDO. Increasing the line speed at a given stretch ratio for any machine will increase the stretch rate and decrease the residence time on all the heat transfer rollers.
The stretch ratio needed for any given application is typically fixed over a narrow range and dependent primarily on the general family of polymer being used. The strain rate is often a limiting factor as many materials will have an upper limit after which it will tend to break rather than stretch. So, the overall process line speed will be limited by the combination of stretch ratio and strain rate due to the interdependence of these process conditions.
It’s impossible to explain how all materials will perform in a single or multi-stage process in just one article. However, a good rule of thumb is that for applications were the stretch ratio and/or line speed are relatively low, single-stage machines are often adequate. Also, for unique applications where a high strain rate is desirable (for example, PTFE), single-stage machines are preferred. However, for applications running at high speeds and/or where the strain rate needs to be limited (such as the MDO for a BOPP line), multi-stage stretching will be needed.
Another benefit of a multi-stage stretching configuration is that it makes any given MDO more versatile giving you flexibility to run a wider range of materials and process conditions. This can be useful in situations where there is uncertainty in either the process conditions or the range of products you will run on the MDO during its useful life.
We haven’t even touched upon how all material can be affected by the temperature, not to mention the unforeseen primary and secondary effects that can occur. This will be discussed in the next article.