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How to Measure Plastic Film

How Do I Measure Plastic Film Thickness?

SolveTech, Inc. has 36 years of experience in measuring plastic film using our proprietary gauging technology. This page is dedicated to educating our customers on the different methods available to measure the thickness of plastic film.  If you have any questions, feel free to contact us here.

We are typically either looking to measure film across the web or in the machine direction so that we can see the performance of our production process.  Film can vary in either direction depending on process conditions.  Plastic film presents some unique challenges, and there are many tools available to help you.

how to measure plastic film

Update 2017: New ASTM Standard D8136 For Measuring the Thickness and Variability of Plastic Film available here.

This article will discuss off-line gauging and on-line gauging for plastic film, including blown film and cast film.

Off-Line/Laboratory Thickness Gauging


 Should I Use Contact or Non-Contact?

Measuring in the lab is usefully a good first step for quality control.  Many plastic film producers or buyers will want to know the thickness and thickness variability of their film for performance and quality reasons.  The average thickness can be calculated using the density, weight, and cross-sectional area of a piece of film.  To get film variability however, you will need to measure multiple points along the film to see how much variance there is in thickness. Typically, there is a specification that buyers want and a producer will need to meet.

SolveTech is currently working with ASTM to develop a new standard for non-contact thickness gauging which will have much better R&R (repeatability and reproducibility).  It will also allow buyers and seller to define film variability in a standardized way.

There are several different things to consider when measuring film in the lab which are presented below.  With a thin material like plastic, you will need special tools to get an accurate measurement which can be either contact or non-contact.   Below, we will compare the contact and non-contact methods and explain when each is appropriate.

Contact  Gauging

Micrometer measuring plastic
Micrometer Measuring Plastic Film

Contact gauges are an easy and simple solution, but have some drawbacks.  A micrometer can be used to measure thickness fairly easily in the lab, but  the issue is accuracy when it comes to thin films under 2 mils.

Benefits:

  • Easy to use (minus cleaning)
  • Inexpensive

Drawbacks:

  • Resolution is low for thin films (0.05 mils for a 0.5 mil film gives +/- 10% error)
  • Takes a lot of time to measure multiple points
  • Not very repeatable on thin films (See the case study here)
  • Not practical for taking a complete profile

 

ASTM Standard D6988 discusses using a dead weight caliper for measuring the thickness of plastic film.  Section 10 discusses the precision statement, and there it states that no precision statement can be made because of the all the factors involved.  In SolveTech’s experience, and micrometer can read accurately to +/- 0.05 mil.  However, when measuring a 0.5 mil film, this would give you an error of +/-10%.  With non contacting gauging, you can be much more accurate at around +/- 0.2% for this thickness of film.

Additionally, a surface finish on a film will typically give a micrometer an error.  Usually it will skew the reading high because the device will rest of the peaks of the rough surface.

Non-Contact Gauging
non contact thickness gauge

For most plastic films and especially thin films, non contacting gauging is the method of choice to get an accurate and quick reading.  Many capacitance gauging systems are set up as profilers, such as SolveTech’s PR2000 Precision Profiler.  Profilers are helpful because they can provide a picture of the film thickness across the web or in the machine direction.  You can bypass the calipers by calibrating based on the average thickness of a strip.  This is known as the gauge by weight method and it is built into the PR2000 software.

Plastic Film Thickness Profile Measurement
PR2000 Non Contacting Thickness Gauge

Capacitance is the preferred method for the following reasons:

  • Repeatable and Accurate to 0.001 mils
  • Will pick up almost all defects with high resolution
  •  Measures a samples quickly and easily which saves time and money.
  •  Can accurately deliver the average material thickness while showing the variability profile

Drawbacks:

  • Higher cost initial cost than a micrometer
  • Calibration can be required

 

Capacitance technology will tell you the overall thickness of the material you are trying to measure.   However, coating thickness can still be measured if you are able to measure the material before the coating is applied and after the coating is applied.  You can then do  a subtraction of the before and after to get the delta.

There are other technologies on the market that can be helpful when trying to read individual film layers using NIR for example.  Capacitance has a drawback in that it can’t measure conductive materials and it can also be sensitive to variation in moisture levels.

 

On Line  Thickness Gauging

Measuring on line is a great tool for monitoring your quality in real time.  Catching an error can help reduce waste and improve the products you are producing.  Additionally, producers can sometimes down gauge their average thickness if they produce a more uniform film.  A thin spot in the film is like the weakest link in a chain, so if it is eliminated, the film becomes stronger overall.  Down gauging can save some producers a great deal of money in material costs.

In many cases, live feedback control is used to help make automatic adjustments to the extruder.   Contact gauging is typically not practical in this case due to the high speed of the line, so non contacting methods have been widely adopted.

 Cross Web vx Machine Direction 2 How To Measure Plastic Film

It is important to understand some key concepts for on line gauging, and different processes will have some different needs.  You can measure both in the machine direction and in the cross web direction.  Cross web measurements are more typical, but machine direction gauging is gaining in popularity.

Blown Film  Gauging
Blown Film Thickness Gauging

There are several styles of blown film production equipment: no rotation, and oscillating die, and an oscillating haul off.

For on line gauging, there are two basic methods, on the bubble gauging, and gauging after the lay flat.  To learn more, read our article “The Impact of Thickness Gauging on a Blown Film Plant”

On The Bubble

Thickness gauges that measure on the bubble are used to provide the thickness of the film as it is being produced.  They typically use a capacitance based technology, although it is different the technology SolveTech uses for it’s capacitance gauging systems.  These systems scan the bubble using a circular slide and  give a relative measurement, meaning that they measure the variability across the web.  They have difficulty measuring absolute thickness because they have trouble holding calibration.

They are however popular on many blown film lines because they can provide feed back control to the die.  If the density is known, they can use gravimetric control to calculate the average thickness.  However, in practice, the exact density is not always known or entered correctly, and the dimensions of the bubble also have to be considered.  Some blown film companies also find these machines to be complex to operate and maintain.

After the Lay Flat

A simpler and lest costly approach is to measure the thickness after the lay flat, meaning after the bubble is collapsed.  SolveTech’s BF200 measures material after the lay flat using a spot along the edge or both edges.  If either the die or the haul off is rotating, the system can map the entire bubble by measuring one point along the edge.  This is because the bubble profile will rotate through this one point over time, and the rotation speed can be tracked.

BF200 Blown FIlm Gauge
Polar plot of the bubble profile using the BF200

Besides being simpler to operate and more economical, these systems are easily retrofitable to existing blown film lines and can hold calibration for an absolute measurement.

The disadvantages are that there is more process time between the creation of the material and when it is measured, and if there is no rotation on the line, you can not get a bubble profile.  There can also be some challenges when doing feedback control when using a oscillating haul off.  This is because twisting of the bubble can distort the profile.

Overall, these systems have been widely adopted, and SolveTech is a leader in this area with well over 150 installed.

 

Cast Film  Gauging

 

For cast film, producers will often measure thickness across the web and make adjustments to their die accordingly.  For wide webs, many customers will use a scanning gauge of some kind.  Scanning gauges take a measurement head to scan it back and forth across the web to create a thickness profile.   Some typically technologies include x-ray, gamma backscatter and beta ray gauges.  There are also some capacitance based technologies and optical technologies that are used.  For measuring a coating thickness, IR is popular.

On Line Thickness Gauging for Cast Film

For narrow webs less than 60 inches, many producers use a C Channel capacitance gauging system.  SolveTech offers our unique Multichannel which does not scan and instead offers full and contact coverage of the web using multiple channels.  This system has been widely adopted as the standard in narrow web since it has superior performance to the other scanning gauge technologies, and it can give a high percentage of coverage.

Measure Plastic Film On Line
3D Map of Film Thickness

 

Biax Film  Gauging

 

Biaxial Film is stretched in two directions.  Therefore, along with measuring the cross web like in cast film,  it may be important for the producer to measure the thickness variability in the machine direction.  This is a SolveTech specialty due to our unique gauge design, and for more information, you can visit our page on machine direction gauging.

 

In summary, there are several methods available to measure plastic film thickness, and the one you select depends on your need and process.   You can learn more about what thickness gauging can do for a blown plant here.

For more information on thickness gauging for plastic film, please contact us at info@gauging.com or at 1-302-798-5400, and we would be happy to answer your questions.

 

 

 

 

 

How to Measure Plastic Film  | Plastics Film Thickness Gauging  |  Measure Plastic Film Thickness

SolveTech, Inc. -2017

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