Knowledge Base
Thickness Uniformity in Flexible Film Glossary and Fact Sheet
Key Terms
Caliper – Another term for thickness, commonly used in the paper, film, and packaging industries.
Gauge band / thin spot – A localized area of a film web that runs thinner than the surrounding material, often caused by die or extrusion inconsistency.
Cross direction (CD) – The direction across the width of a film web, perpendicular to the direction of travel through the production line.
Machine direction (MD) – The direction of film travel through the production or converting line.
Mechanical scanning – A thickness measurement method using a micrometer with a flat measuring foot applied under defined pressure, as specified in ISO 4593 and ASTM D6988.
Standard deviation (in thickness testing) – A statistical measure of how much individual thickness readings vary from the mean; used alongside the average thickness to assess uniformity.
Standards Reference Table
| Standard | Title | Scope |
| ISO 4593:1993 | Plastics – Film and sheeting – Determination of thickness by mechanical scanning | Specifies a mechanical scanning method for film and sheeting thickness. Not suitable for embossed film or sheeting. |
| ASTM D6988 | Standard Guide for Determination of Thickness of Plastic Film Test Specimens | Describes methods for determining plastic film thickness where that thickness is used directly in calculating other test properties. Addresses similar subject matter to ISO 4593 but differs in technical content and scope. |
| ASTM F2251 | Standard Specification for Thickness of Plastic Film Test Specimens | Covers thickness specifications for plastic film test specimens. Referenced by both the TMI Büchel 49-86 configurations for plastic and film testing. Confirm applicability to the 49-56 with your technical expert before including in instrument-specific copy. |
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FAQs
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All three address thickness measurement of plastic film, but differ in scope and technical content. ISO 4593:1993 is the international standard commonly referenced outside North America. ASTM D6988 is the equivalent guide widely used in the United States. ASTM F2251 covers the specification for thickness of plastic film test specimens and is referenced alongside D6988 for certain instrument configurations. Which standard or standards apply typically depends on your customer’s specification or region.
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No. ISO 4593 explicitly excludes embossed film or sheeting from its scope, because the textured surface interferes with a flat measuring foot. Embossed or textured films typically require an alternative thickness measurement method.
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A film can meet its target average thickness while still containing localized thin spots. Since barrier and seal performance depend on thickness at every point, not just the average, standard deviation across multiple readings is used alongside the mean to assess whether a film is genuinely uniform.
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Common causes include die or extrusion setup issues (often producing systematic variation, such as one edge running thinner) and raw material or process instability (often producing more random variation). Reviewing the pattern of variation, not just the numbers, helps identify the likely cause.
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The TMI Büchel Digital Micrometer 49-86 is designed for thin sheet materials including film, and the TMI Büchel Digital Micrometer 49-56 is configurable to a range of international thickness standards, including at low contact pressures for compressible or delicate films.
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