Definition
Determining Performance
Optimizing Film Yield
Test principles
Industry Terminology

The term Yield is used as a means of measuring a film's coverage per unit weight. Values are presented   in²/lb in US standard and m²/kg in metric (or SI) units.

Unit weight is figured in units of lb/ream or g/m² as the reciprocal of the yield. The film industry generally favors the use of yield values, while the paper industry tends to use unit weight. While this page will use the term yield, bear in mind that the same principles hold true for unit weight.

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While films are marketed by the pound or kilogram, it is the actual area of the film that will determine how many packages it will ultimately produced. Therefore, when determining the amount of film required for a project, ultimately it is the yield that remains the most crucial factor.

OPP is a high-yield material. It is therefore the most economical in comparison to other alternatives. Table 2 provides a pound-for-pound comparison illustration at equal gauges. Notice how OPP offers 53% more coverage than OPET and 28% more coverage than oriented nylon.

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For solid, uncoated films, yields are determined by resin density and average film gauge. Resin density may sometimes be negligibly altered by process conditions. Yield is therefore only controlled to the degree that thickness is controlled. Advanced on-line gauge measurement and control systems can maintain machine and transverse direction thickness profiles tight to the target. In general, films are rejected when found to be outside the specification differential of plus or minus 5% of the target. Better leading brand film yield measurements are always well within 3% of advertised yield.

With Cavitated white films (OPPalyte) and coated films there are additional variables that can affect yield. The cavitated core of OPPalyte films lowers film density, sometimes to as low as 60% of solid OPP (as low as .53 g/cc). Thus, OPPalyte films offer even higher yields per unit thickness, than solid OPP films. Ultimately, it is the yield that is the controlled property. So if cavitation density varies slightly with process conditions, the thickness – not the yield – will similarly be impacted.

Specialized coatings are applied as thin layers onto OPP base films. These coatings are denser than the base films. Therefore, gauge for gauge, yields are slightly lower for coated films.

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Yield is measured by cutting a precise area of film and then weighing it on an analytical balance. The area is divided by the weight, expressed as in²/lb or m²/kg and reported to three significant digits. The recommended minimum specimen size is 300 in² (1940 cm²) and should be obtained by folding a single layer of film multiple times in the machine and transverse directions.

Example: By folding the film twice in the machine direction and then twice in the transverse direction, sixteen layers of film will be cut at once. The folded film is precision cut with a five-inch or .127 m. diameter round die and the measured weight is 3.496 grams. Based on these specs, the following equations demonstrate the yield and unit weight calculations in US standard and metric units. Yield (in2/lb) = Total specimen area = π(2.5)2 in2 x 16 x 453.59 g/lb = 40,761 in2/lb Weight of specimen 3.496 g Yield equals 40,800 in2/lb. Yield (m2/kg) = Total specimen area = π(.0635)2 m2 x 16 x 1000 g/kg = 57.976 m2/kg Weight of specimen 3.496 g Yield equals 58 m2/kg. Unit weight (lb/ream) = 1 = 1 lb x 432,000 in2 = 10.598 ~ 10.6 lb/ream Yield 40,761 in2 ream Unit weight (g/m2) = 1 = 1 kg x 1000 g = 17.249 ~ 10.3 g/m2 Yield 57.976 m2 kg

CAUTION: While yield is related to thickness, it is imprecise to use a micrometer to assess yield. Micrometers lack the required accuracy, as they only measure thickness at a small point. The method outlined above measures average yield over a large area. In addition, thickness cannot be converted to yield without knowing the exact density of the film.

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Industry terminology

Ream: Ream is a general term used by the paper industry equivalent to a coverage area of 3000 ft² or 432,000 in².

Basis weight: Basis weight is a common paper industry term for unit weight, or weight per unit area. Units are identified in lb/ream. "Ten pounds of poly" means 10 lb/ream, which is equivalent to 43,200 in²/lb. A ream equals 432,000 in²) or .70 mils of LDPE – at the specific gravity of .92.

MSI: Film pricing is commonly quoted in $/lb or $/MSI, and yield can be used to convert from one set of units to the other.MSI stands for thousand square inches. Yield can be converted from in²/lb to MSI/lb by dividing by 1000.

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