Cast film extrusion covers wide range of markets

Production of cast film is a continuous process where a thermoplastic material is extruded from a slot die onto a chill roll, quenched, and wound.

The resulting film has machine direction orientation compared to a somewhat limited bidirectional orientation achieved in blown film processing. Cast film extrusion, because of the high speed and output, can be limited to dedicated applications such as lamination web, printed packaging film, and a growing market- stretch wrap.

In recent years, the ongoing move toward automatic wrapping of pallets has shown a steady increase in demand for stretch-wrap film. Due to its processing advantages, cast film today dominates the market in this sector. Equipment producers offer high-speed cast film lines for the manufacture of coextruded linear low-density PE-based films with up to nine layers. Coextrusion also allows downgauging to save raw materials costs. The processor can also manufacture a product where he determines the level and location of cling-on the inside, outside, or on both sides. Agricultural wrapping films as well as stretch film for manual wrapping applications can also be extruded on cast film lines. Another interesting market possibility is food-wrapping film, normally a blend of LDPE and LLDPE.

In October 2004, the German processor Orbita Film (Weissandt Gölzau) started production with a new, fully automated five-layer stretch cast film line supplied by Austrian machine producer SML Maschinengesellschaft (Lenzing). It has an output of 2250 kg/h and production speed of 600 m/min. This line can increase a processor''s output from 100,000 to 112,000 tonnes/yr.

Cast film is traditionally produced in either polyethylene (PE) or polypropylene but can also include EVOH and/or nylon for barrier in multilayer materials. It is not limited to these resins. Materials can also be vinyl, or polyester, although PE offers the best economies of scale. In 2004, the stretch-wrap market in Europe showed 5% to 7% growth with an annual production quantity of about 1 million tonnes. This market is divided into 40% hand wrap, 48% machine wrap, and 12% power stretch. These applications generally cover a thickness range from 12 to 50 µm.

Film layer structure is moving toward more uniformity. Europe has traditionally stuck to three-layer stretch-wrap structures, while in North America five to seven layers dominate. Market observers say five-layer lines may be becoming the global norm. Multiple layers also offer a plywood effect said to provide better strength and bonding power.

Processors also are headed toward a more unified approach to resins used. North American processors have traditionally processed cheaper butene (C4) LLDPE with a blend of hexene (C6) to get the right stretch retention, clarity for barcode reading, as well as gloss and cling-all at low prices. European processors tend to concentrate on octene (C8), very low-, ultralow-density PE, and metallocene (mPE) grades.

But some processors say there aren''t substantial benefits to using expensive mPE today compared to new C8 grades. Also mPE tends to cut poorly. Alternatives are coming onto the market that offer processors property benefits as well as lower costs. One such product is a recently introduced super hexene LLDPE grade for Super Power stretch (270-350% prestretch used to machine-wrap odd-shaped objects). It gives the mechanical performance of C8 and mPE grades, but better processability than metallocene and at lower costs.

Another growth market is cast polypropylene (CPP), which is finding a niche as competition to biaxially oriented polypropylene (BOPP) film. The equipment investment for CPP is substantially less than the machinery needed for tenter frame-processed BOPP.

The cast film process offers better gage control than its direct competitor, blown film. Generally, cast film thickness variation is ±3-5%, whereas blown film can be ±7-10%. Cast film shows its advantage over blown film processing by its high output and increased widths to produce mass product in high volumes. In the case of equipment supplier SML, the company can deliver stretch film lines up to 4500 kg/h output in widths of up to 3m.

Stretch wrap, which is coextruded, as in the case of the line delivered to processor Orbita with its five extruders for an output of 2250 kg/hr in a width of 2500 mm, provides a tough, thin film. Innovations, such as edge encapsulation, limit film neck in, reduce unusable scrap, and allow high-speed, stable production.

One trend that persists with this technology is the effort to downsize stretch-wrap film to reduce polymer use, all the while maintaining or improving stretch and impact properties. From 1975 to 2003, the market saw a thickness reduction of an average stretch wrap film from 30 to 20 µm. Also, prestretch has been increased from 50% to 300%, thanks to new polymer grades.

Other changes include specifying slip and cling properties in the same basic raw materials as the core layer. Cling can be introduced by dosing an additive amounting to as little as 10-30% of the overall layer structure. The same principle is used for the slip layer. By such inline addition, the processor can optimize his material handling storage with large lots of less costly standard polymers and minimal additive stocks.

Contact information

SML Maschinengesellschaft mbH

Lenzing, Austria

www.sml.at; [email protected]

Bettina Dreher, marketing manager, SML Maschinen GmbH,

[email protected]