Haze determination is one of the most important optical quality control tests for plastic cast film, quantifying the degree of light scattering that affects transparency, see-through clarity, and packaged-product presentation. This technical article explains how haze in cast film is measured in accordance with internationally recognized standards such as ASTM D1003, ISO 14782, and GB/T 2410, covering sample requirements, instrumentation, calculation procedures, and practical applications in the flexible packaging industry.
Technical Overview
Plastic cast film is manufactured by a chill-roll extrusion process in which molten polymer is extruded through a flat slot die and immediately quenched on a cooled roller. This rapid solidification produces films with excellent thickness uniformity, high gloss, and low haze compared with blown films. Common materials include cast polypropylene (CPP), cast polyethylene (CPE), and multi-layer co-extruded structures based on PP, PE, PET, and PA.
Haze is defined as the percentage of transmitted light that deviates by more than 2.5 degrees from the direction of the incident beam as a result of forward scattering within or at the surface of a specimen. It is calculated as the ratio of diffuse transmittance to total luminous transmittance:
- Haze (%) = (Diffuse Transmittance / Total Transmittance) × 100
A higher haze value indicates a milkier, less transparent appearance, while a lower value indicates better see-through clarity. For cast film used in food packaging, printed laminations, and display applications, haze is a decisive quality parameter because it directly affects product visibility and print aesthetics. Factors such as resin grade, additive loading (slip and antiblock agents), crystallinity, thickness, chill-roll surface finish, and processing temperature all influence the final haze value.
Testing Samples
Monolayer Cast Polypropylene (CPP) Film
Single-layer CPP film, typically 20–100 μm thick, is the most frequently tested cast film type. It is widely used for textile overwrap, stationery lamination, and as the heat-seal or inner layer of retort pouches.
Multi-layer Co-extruded Cast Film
Three-layer to five-layer structures (for example, PP/EP copolymer sealant layers) are tested to evaluate the combined optical contribution of each layer. Because haze depends on thickness, each tested structure must be reported together with its total thickness and layer configuration.
Cast Polyethylene (CPE) Film
Cast LDPE and LLDPE films used for flexible packaging and protective laminations are tested using the same optical methodology, with typical haze values influenced by density and alpha-olefin content of the resin.
Sample Condition and Quantity
Specimens should be representative of the production roll, free from wrinkles, creases, gels, and surface contamination. Prior to testing, samples are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity (ISO 291 / ASTM D618), normally for a minimum of 40 hours. A minimum of five specimens per sample is recommended, each cut to a size suitable for the instrument aperture, commonly 50 mm × 50 mm or larger.
Testing Items
A typical optical characterization program for cast film includes the following measurable items:
- Total haze (percent) — the primary reported parameter
- Total luminous transmittance (percent) — overall light passage through the film
- Diffuse transmittance (percent) — the scattered component of transmitted light
- Specimen thickness (micrometers) — mandatory supporting data, since haze increases with thickness
- Clarity or see-through quality (optional) — the ability to resolve fine detail through the film, which is distinct from haze and is evaluated under standards such as ASTM D1746
Testing Methods
Applicable Standards
- ASTM D1003 — Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics
- ISO 14782 — Plastics: Determination of Haze for Transparent Materials
- GB/T 2410 — Chinese national standard for light transmittance and haze of transparent plastics, technically aligned with the above methods
- JIS K 7361 — Japanese standard corresponding to ISO 14782
Instrumentation
Measurements are performed with a dedicated hazemeter or a spectrophotometer equipped with an integrating sphere and a photodetector system that complies with the photopic response of the human eye (CIE photopic luminosity function). The geometry uses an unscattered incident light beam and defines the scattered component as light deviating more than 2.5 degrees from the normal transmitted direction. Instruments are calibrated and verified using reference standards traceable to national metrology institutes.
Measurement Procedure
- Condition the specimens to equilibrium in the standard atmosphere before testing.
- Measure and record the thickness of each specimen at multiple points.
- Calibrate the instrument by measuring the incident light flux (T1), total transmittance with no specimen, and the instrumental scatter correction (T3).
- Place the specimen flat against the transmission port without wrinkles or trapped air, with a defined surface facing the light source.
- Record total transmitted flux (T2) and total scattered flux (T4).
- Calculate haze as Haze (%) = (T4/T2 − T3/T1) × 100.
- Repeat for all specimens and report the arithmetic mean, standard deviation, thickness, and test standard used.
Quality Control Points
Surface contamination, fingerprints, static attraction of dust, and poor flatness are common causes of artificially elevated haze readings. Specimens should be handled with gloves, and the same orientation and surface should be used consistently. Because haze is thickness-dependent, comparisons between samples are only valid at equal nominal thickness.
Applications
Haze data for cast film support quality control and product development across many sectors:
- Food packaging — bread bags, produce wrap, confectionery, and frozen-food lamination where product visibility is a selling point
- Retort and sterilizable pouches — CPP sealant layers requiring a defined balance of optical and thermal properties
- Printing and lamination substrates — low-haze films ensure accurate color reproduction and good gloss after printing or metallization
- Stationery and document lamination — high-clarity cast film for book covers and protective sleeves
- Medical and hygiene packaging — transparent films allowing visual inspection of contents
- Process optimization — correlating haze with chill-roll temperature, die gap, line speed, and resin formulation during film development
Manufacturers, converters, and accredited third-party laboratories rely on standardized haze determination to verify incoming material conformity, monitor production consistency, and support certification for exports where documented optical performance is required.
FAQ
What is the difference between haze and light transmittance?
Transmittance measures the total percentage of light passing through the film, while haze measures only the scattered portion of that transmitted light. A film can have high transmittance yet high haze, meaning it appears bright but not clear.
Which standard should be selected for testing?
ASTM D1003 and ISO 14782 are the internationally dominant methods and give comparable results; GB/T 2410 is the accepted Chinese national equivalent. The choice typically follows customer specification or the target market region.
Why must thickness be reported with the haze value?
Haze increases with thickness because scattering occurs throughout the bulk and at both surfaces. A haze value without thickness is not meaningful and cannot be compared against specification limits.
How many specimens are required?
At least five specimens are recommended for statistical validity, with results reported as the mean and standard deviation for the defined nominal thickness.
What are typical haze values for cast film?
General-purpose CPP film of 25–40 μm commonly shows haze in the range of 2% to 6%, while thicker lamination or sealant grades may be higher. Exact acceptance limits are agreed between supplier and customer based on end use.
How can haze in cast film be reduced?
Measures include selecting low-haze resin grades, minimizing antiblock and slip additive loading, lowering crystallinity through faster quenching, maintaining a polished chill-roll surface, and controlling melt temperature and gauge uniformity.