Blog RSS Feed en Copyright 2017 2017-08-15T15:45:00+00:00 <![CDATA[Web Spreaders are a Converting Operator’s Friend]]> By Joe Connelly

Product Manager – Winding and Slitting

converting machine operator has the unenviable task of producing wound rolls of acceptable quality from imperfect web materials. It takes an incredible amount of technology and equipment to produce some of the high-tech structures used in today’s flexible packaging applications, and things don’t always go as planned. Process variability can lead to subtle variations in the properties of the web being produced. Variations in thickness, especially when wound layer-upon-layer in a master roll, can cause permanent distortions in the web that aren’t apparent until the material is unwound into a subsequent converting process. The converting operator now must deal with this imperfect web and produce finished rolls that must meet quality standards acceptable for either the end-user or perhaps the next step in the manufacturing process.

How do you correct imperfect web to meet quality standards?

There are components that exist to make the converting process more tolerant of these material inconsistencies. Whether printing, laminating, slitting, or simply conveying a web through a nipped pull roller, keeping the web flat and wrinkle-free entering those processes is typically a requirement. This can be accomplished with the help of web spreading devices that impart a cross-machine web tension in the material to help smooth it out. These come in various forms of specialized rollers, such as bowed rollers, concave rollers, and slat rollers, as well as bent bars and pinch rollers. Each type has its own set of pluses and minuses and some work better with certain materials than others.

In our next article we’ll cover the various types of web spreading devices and how they’re used.

<![CDATA[Safety Starts at the Front of the Line]]> By Joe Connelly

Product Manager – Winding and Slitting

When web materials are manufactured in a continuous process, winding them into the form of a roll or reel has been recognized as a very efficient method to transport them either to the consumer or to the next step in the manufacturing process. Secondary manufacturing and converting operations, such as printing, extrusion laminating, or  slitting and rewinding, start by unwinding material from a roll into the downstream process. Process operating speeds and type of unwind selected can have an impact on the level of safeguarding required to meet current safety standards.

A parent roll of product is most commonly supported in an unwind by its core. This is accomplished by a full width shaft or by short shafts (or chucks) that are inserted into each end. Torque is applied to the shaft or chuck by either a brake or motor to induce the desired amount of tension in the web as it is being pulled by the downstream process. A drawing-in or trapping hazard can exist between the parent roll and the floor or machine member. Nip hazards can also exist between the web and either the rollers that support it or nearby machine members.Automatic unwinds that attach the start of a new roll to the tail of an expiring roll can have in-running nip and knife hazards in addition to the aforementioned hazards. Barrier guards that prevent access to these hazards must be of adequate height and positioned at a distance deemed safe by today's standards. Where barrier guards make routine access for roll loading and unloading cumbersome, sensitive protective equipment in the form of light curtains or laser scanners can be used provided their sensing field is at a safe distance from the hazard. This distance is calculated using a formula that takes machine operating speed and stopping time into account to ensure that when the safety device is triggered, the machine comes to a complete stop before an operator can reach the hazard.

Manufacturers of unwinding equipment must employ a risk assessment process to identify the necessary safeguarding for their machines. Buyers in search of unwinds should be aware of today's safety regulations and ensure that what they're buying meets these standards. Although unwinds may pose less of a risk to operate than the equipment they are feeding web into, in almost every case hazards are present and must be addressed to ensure a safe workplace for your operators.

Incorporating the necessary safeguards to your unwind is an essential requirement to a safe workplace. Please be sure to keep a watch for follow-ups to our machinery safety series where we discuss safety guidelines for other equipment in your converting operation. 

<![CDATA[The Key Aspects of a Machine Direction Orienter - Part 4 Custom Machinery]]> By Ken Forziati

Director of Business Development & Product Management

In our previous articles on Machine Direction Orienters (MDOs), we reviewed aspects of machine configuration and considerations related to process conditions that affect the design and efficacy of these machines. The takeaway that should be most obvious is that, while any given MDO may appear to be a simple arrangement of heated, driven rollers, determining the optimal MDO configuration for a specific application and capacity is where the real challenge lies.

Typical products made through an MDO process include barrier films and other flexible packaging applications, adhesive tapes, decorative ribbons, high temperature microporous membranes, optical films, draw straps and tear tapes, MD shrink films, and other specialty films. Independent from the application-specific process conditions (process temperatures, draw ratios, neck-in, etc.) that we discussed previously, an MDO needs to be correctly designed to deliver the desired line speed and throughput as well.

How can I ensure I’m purchasing the correct MDO for my application and production requirements, or at least minimize the risk associated with my MDO project?

A good place to start is working with a machine builder that not only has experience building MDOs for the desired application, but also can demonstrate a range of products that reflects an understanding of the varying requirements of different applications. If a machine builder only has a single MDO product solution, chances are that it is at best going to be a compromise for your application, and at worst, may not provide the results for which you are looking.

Working with an experienced machine builder that also has facilities in which lab or pilot scale trials can be conducted further increases the probability that the machine you order will perform as expected. From a customer’s perspective, trials may be thought of primarily as a means of determining feasibility and refining product/formulation development. For the machine builder, however, results from trials can be used to verify process condition assumptions, confirm material-specific heat transfer calculations, and ensure that other design considerations are correct.

Other considerations when designing an MDO are:

  • Space requirements – Would a vertical or horizontal arrangement work better in a customer’s facility?
  • Energy sources – Are there local advantages/restrictions to using one source of energy for heating over another? E.g. electric, natural gas/oil fired boiler, plant steam, etc.
  • Building some flexibility into the design – a trade-off typically of added capital expense that will minimize project risk and provide versatility for future product needs.

The reality is that for most customers, a custom designed and built MDO will deliver the best value and maximize return on investment. The key is to make sure that you choose a machine builder with the experience and know-how to determine the optimal configuration for your situation.

<![CDATA[The Key Aspects of a Machine Direction Orienter - Part 3 Dimensional Changes and “Neck-In”]]> By Ken Forziati

Director of Business Development & Product Management

In our previous articles on Machine Direction Orienters (MDOs), we reviewed key process parameters and examined how their impact on stretching conditions would affect the selection of machine configuration for various applications. One aspect of the stretching process that we haven’t yet addressed is how the stretch ratio relates to dimensional changes in the oriented web.

When longitudinally stretching materials in an MDO, the general presumption is that the nominal change (reduction) in thickness observed will be inversely proportional to the stretch ratio. For example, if we orient a 12 mil (0.012”, 120 gauge, or 305 μm) thick sheet at a stretch ratio of 3:1 (or 3X), we would anticipate that the finished thickness would be approximately 4 mil, or one-third the starting thickness. If we orient that same sheet at a stretch ratio of 6:1, we would anticipate that the finished thickness would be approximately 2 mil. However, this expectation assumes the web maintains a constant volume and that the material is being stretched uniformly in length with no change in width through the process, neither of which is necessarily the case.

In reality, the dimensional changes that occur in a material being stretched are governed by a complex set of property and process relationships. These include the Poisson effect, which describes how most materials subject to longitudinal stretching (positive tensile strain) will exhibit a lateral contraction (reduction in thickness and width). The ratio of the lateral contraction of a material to the longitudinal extension is defined as Poisson’s ratio.In the case of a sheet, this would imply not only a change in thickness, but also a change in width.

We always observe some reduction in width accompanying the reduction in thickness in materials that have been stretched in an MDO process. Our industry generally refers to this phenomenon as “neck-in,” which can be attributed to the Poisson effect described above. MDO design, process conditions, and material properties play a big role in determining the magnitude of the neck-in that is observed.

How do these factors influence MDO neck-in?

First off, while Poisson’s ratio is often considered a material dependent constant, for the polymeric materials with which we work, it is not. So, not only will Poisson’s ratio vary from material to material but also it will vary for any given material depending on the degree of crystallinity of the material, time, temperature, strain, and strain rate.

This last factor is one that will vary with both process conditions and with the design of the MDO being used to orient the material. If a web is stretched over a wide gap, the strain rate at any given stretch ratio and line speed will be much lower compared to a machine with a narrow stretch gap. Poisson’s ratio decreases with increasing strain rate, so a narrow-gap machine will exhibit lower neck-in for any given combination of material and process conditions when compared to a wide-gap machine.

Another factor affecting the magnitude of neck-in is that when the web is stretched over a narrow gap, not only is the strain rate relatively high, but there is substantial resistance to neck-in from frictional forces that exist between the roller surface and the web. This may be further strengthened by use of nip rollers and the typically elevated process temperatures. So, while internal forces in the material are attempting to cause the material to neck-in, the transverse frictional forces on the roll surface resist this and reduce the magnitude compared to what would naturally occur if it were not restrained. This would be the case in an MDO with a wide stretch gap where the material is being stretched suspended over a wide span between stretch rollers.

One implication of this interaction between the material and roller surface is that for MDOs with narrow stretch gaps neck-in tends to be an edge effect rather than a uniform process across the width of the material. Material close to the center of the web is restrained from necking by the frictional forces of all the material outboard of it. The edges, on the other hand, are unrestrained by any outboard material, and therefore, will be where neck-in is most pronounced.

If neck-in were uniform across the width, we would expect that the neck-in observed for a given set of material and process conditions could be expressed as a percentage of the overall width and scaled up with increasing web width. This is something that we are often asked when customers are looking to scale up pilot trials to commercial-scale production.Unfortunately, this is not the case. The upside is that neck-in will be less for wider webs than would be expected if it were uniform. The down side is that it results in thicker edges that typically need to be trimmed and makes it difficult to predict with certainty how wide MDO rollers need to be to guarantee a specific finished width.

Once again, this drives home the point that while an MDO may at first appear to be a relatively simple machine, it takes experience (and often experimentation through lab trials) to ensure that the MDO is designed correctly for your application.

<![CDATA[Dealing with Breaker Plate Contamination]]> By Justin Marriott

Product Manager – Key Filters

Continuous-belt screen changers are an economical and efficient method of filtering out contaminants in thermoplastic material due to their ability to provide constant melt extrusion pressure to downstream processes over long periods of time. The absence of process interruptions needed to change screen packs in other types of filtration translates into increased production and decreased scrap, bringing maximum efficiency to your extrusion operation. However, if you notice that the pressure drop in your continuous belt screen changer is gradually increasing over time, despite regularly advancing the screen at recommended intervals, you may have a flow restriction from the gradual buildup of contaminants on breaker plate surfaces, also known as “plate out.” For many thermoplastic materials this will not be an issue. For some, however this will cause restriction in flow requiring removal and cleaning of the breaker plate.

Breaker plates are common in the extrusion industry and serve two main purposes: to help mix molten polymer as it exits the extruder (eliminating rotational memory and creating longitudinal memory); and to support the filtration media. Filtration media does not have the strength to support itself across the flow channel and the breaker plate on which the media rests provides the needed strength to withstand the high extrusion pressures seen during operation. A manufacturer must balance the design requirements of a breaker plate so that the cumulative area of the flow passages (typically circular holes) are large enough to minimize pressure drop across the breaker plate while also being small enough so that the screen won’t dimple into the holes.In a continuous style screen changer this dimpling will cause the screen to “pin” itself to the breaker plate creating issues when advancing the screen.

While the breaker plate is a necessity to provide support to the filtration media, it also presents its own complications. The holes in the breaker plate provide a large surface area on which material can gradually build up to the level where “plate out” can occur, thus reducing the cumulative area through which polymer can flow. This occurrence ultimately results in an unacceptable pressure drop across the breaker plate to the point where it needs to be replaced with a clean one.

Another area of concern is when making a material, grade, or color change. Many times, this requires replacing the breaker plate and/or filtration screen to minimize cross-contamination.

To address these issues, Key Filters developed the KCH-QC, an innovative continuous-belt screen changer with a “quick-change” breaker plate. The KCH-QC was engineered to drastically reduce the amount of time it takes to replace the breaker plate and introduce clean screen into the process. The screen changer’s unique design allows rapid change-out of the breaker plate in under five minutes, which normally could take up to four hours of downtime. Without the use of tools, the clean breaker plate is actuated into position while pushing the contaminated breaker plate out of the melt stream. Then the hydraulic puller, which is featured on all KCH models, allows the operator to pull clean screen into the melt stream in a matter of minutes. The used breaker plate can then be cleaned, allowing it to be reused at the next change-out cycle.

Plastic extrusion processors are always searching for ways to increase productivity and improve product quality and yield. The KCH-QC fulfills this need by providing a quick and easy solution that substantially reduces downtime and scrap when plate-out and cross-contamination risks are present. 

<![CDATA[Development of Safety Standards for Film and Sheet Winding Machinery]]> By Joe Connelly

Product Manager – Winding and Slitting

As a manufacturer of machinery for the plastics industry, one of our main concerns when designing, engineering, and building these machines is to always keep safety in mind. Operating machinery can always carry the risk of injury. Workers can be struck by moving parts, trapped between rollers, belts, and pulleys, and cut or punctured by sharp edges. That’s why safety standardization efforts are developed to define, for example, the type of guarding, fencing, or torque limitation. Safety standards will often include requirements for both the manufacturer and user of the machinery.

Currently, the Plastics Industry Association (PLASTICS) who is accredited by ANSI to develop, approve, reaffirm, revise, or withdraw American National Standards and Technical Reports, is seeking additional participants for efforts related to machinery safety. They are particularly in need of voting members that are not primary equipment manufacturers.

Current projects of interest include development of standards for safety of film and sheet winding machinery, granulators, strand pelletizers and dicers as well as review of the robot integration safety standard. The Machinery Safety Technical Committee serves as the consensus body for all B151 standards activities and any other technical standards work related to machinery safety. Voting on PLASTICS standards will be done on a company basis.

Each company that is interested in becoming a voting member on the technical committee must complete a membership application. There is no fee for participation in PLASTICS standards development efforts. Most of the work will be done via teleconference, web conference or email, although an occasional in-person meeting is possible. As an active member of the Machinery Safety Technical Committee, Parkinson Technologies feels that a diverse collection of viewpoints and experiences allows for the development of comprehensive, yet practical safety standards.

If interested, please contact Megan Hayes ( or fill out the application available on the PLASTICS Machinery Safety Standards Website.

<![CDATA[Importance of Tension in an Unwind Stand - Part 2 Control System]]> By Joe Connelly

Product Manager – Winding and Slitting

Maintaining the proper web tension at the unwind is critical to the success of a web process. In most situations, two components work together to achieve and maintain the desired tension. In the previous article we discussed one of these components, the braking system, which plays the vital role of applying torque in proportion to the desired web tension. We will now focus our attention on the control system, which provides the required signals to the braking system to regulate the web tension. Control systems are open loop or closed loop and various configurations of each of these exist.

What are the differences between open and closed-loop tension control systems for unwind applications?

In the most basic form of open-loop systems, the operator manually sets the tension by continuously adjusting the braking torque as the unwinding roll’s diameter changes. This is accomplished by manually adjusting the supply pressure to the pneumatic brake, or in the case of a motor driven unwind, adjusting the torque setpoint to the drive. A more refined open-loop method is computer based and requires some upfront calibration during commissioning to determine how much braking torque is generated at the unwind spindle when a known supply pressure or motor torque is set. The control system uses this information to adjust the braking torque as the unwinding roll’s diameter changes. Without a direct measurement of this instantaneous diameter, an estimated diameter can be calculated by knowing how big the roll diameter was at the start, how thick the material is, and the length of material that has been unwound since the start. 

Although the cost of this method is higher as the machine needs a computer-based control system and a device to measure the length of material that has been unwound, it’s not always perfect.Since the roll’s diameter is calculated in the program and not actually measured, improper entry of material thickness or starting diameter can lead to instability in web tension, which can cause variations in finished roll formation. An open-loop tension control system can be an economical alternative for less demanding applications, but to ensure finished roll quality, requires accurate operator entry of processing parameters plus the skills and experience needed to make real-time adjustments to the process.

A closed-loop system, also known as a feedback control system, uses some form of continuous tension measurement to automatically adjust commands to the braking system to maintain the desired web tension.It uses the concept of an open loop system as a forward path but uses tension feedback to compare to the tension setpoint value and makes automatic adjustments to maintain web tension without operator involvement. Feedback is most accurately accomplished with a force measuring roller installed in the web path to measure the actual force, or tension, that’s been applied to the web. This device, also called a loadcell roller, provides a signal to the control system that is compared to the desired setpoint and the process is adjusted to minimize the difference, or error between these two values. 

Another feedback method is to use a translating dancer roller (more commonly pivoting but linear designs exist) that has been calibrated to exert a force to achieve a desired tension in the web, and uses a device to measure its position along its translating path. Because the force exerted is constant, any change in position is a result of either an increase or decrease in web tension. This position signal is then used in a similar manner as the loadcell signal to adjust the process to keep the dancer roller in a fixed position, oftentimes in the mid-stroke of its total travel. Although not a direct measure of how much tension is being applied to the web, if properly calibrated a dancer system can be used quite effectively in combination with a closed-loop tension control system to deliver web at a desired tension to the downstream process. The translating feature of a dancer can be an advantage in certain applications, such as automatic splicing unwind systems, to absorb minor tension disturbances that occur during the splicing sequence, and not create web handling defects.

The act of unwinding material into a downstream web process may seem simple on the surface, but to achieve the required tension control accuracy there are often more complex requirements that aren’t immediately apparent.If your process is being impacted by unstable unwinding tension, please contact us for assistance.

<![CDATA[Importance of Tension in an Unwind Stand - Part 1 Braking System]]> By Joe Connelly

Product Manager – Winding and Slitting

Many web processing applications need a way to unwind web material into a subsequent operation. Whether used inline in an extrusion laminating process or offline in a slitting and rewinding operation, an unwind stand must deliver the material into the process while maintaining the proper web tension to ensure reliable web transport. It’s the tension control requirements that can complicate the simple act of unwinding the material.

Why does an unwind need to maintain tension in a web processing operation?

Maintaining tension in the web results in process stability. If the tension goes slack, web traction is compromised resulting in web wander, improper lamination, or wrinkling. Too much tension can also cause wrinkling or can permanently distort or even tear the material. Maintaining the proper web tension at the unwind can be a critical component to the success of a web process.

To control the correct amount of tension, there are two components at play: a braking system that does the physical job of regulating the tension and a tension control system which acts as the command center to signal how much braking to apply. These two systems need to work together, one as the brain and the other as the brawn.

Braking systems commonly used on unwinds come in two variations, mechanical braking and motor generated braking. Each one has advantages and disadvantages but they both provide the vital role of applying torque in proportion to the desired web tension to the shaft or chuck that supports the parent roll.

Mechanical Braking – this type of braking can either be achieved through pneumatic pressure or electrical power, but the concept is similar. They use either air operated brake calipers or electro-magnetic force to generate the correct amount of braking force (or drag). Mechanical brakes are typically the most cost effective to purchase, install, and operate. However, there are wear items, like the brake pads, that need to be replaced from time-to-time. One disadvantage is that sometimes the torque required to accelerate the parent roll exceeds the minimum controllable torque of the mechanical brake. In this case a motor is required to provide assistance.

Motor Driven –when a parent roll must be quickly accelerated to operating speed, the material is very elastic, or the roll is severely out of round, then a motorized unwind should be considered. Since the motor is driving the unwind shaft, it’s able to feed the material into the web processes while maintaining the correct amount of tension. This is especially beneficial when bringing material up to speed in an automatic splicing system or if the material is too delicate to be pulled. When the unwind spindle is being overhauled by the downstream process, the motor regulating torque to the unwind spindle needs to hold the web back and in doing so, regenerates power that must be dissipated somewhere. One big advantage when using today’s more sophisticated AC motor control systems with a driven unwind is the ability to harness this power and feed it back onto the supply buss, which can then redirect this power to pull rolls or winding spindle drives, thus saving on overall power consumption.

Every unwind system is material dependent. The type of material, thickness, speed, width, size of the mill rolls, and how the roll will be used in the web process all need to be considered when engineering an unwind.

In Part 2 of this series, we’ll discuss the types of control systems used to regulate the tension in the braking system. 

<![CDATA[The Key Aspects of a Machine Direction Orienter - Part 2 Process Conditions]]> 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.

<![CDATA[The Key Aspects of a Machine Direction Orienter - Part 1 Heating Methods]]> 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.