Shineway (Shuanghui) Group in Luohe, Henan, China has successfully started up the first commercial 6-layer PVDC line in China.

The BE line is equipped with an ISOtherm® SCD® die. ISOtherm® dies provide super temperature isolation of the PVDC module of up to 100°C. This isolation means that the PVDC can be processed at its own ideal temperature without effect from hotter adjacent layers. An oil-circulating temperature control system maintains the module at its pre-selected temperature by either removing or adding heat as required. The ISOtherm module allows the PVDC to be treated as one more resin in the SCD® die, providing a PVDC layer uniformity second to none without overloops or weld lines. Its precise temperature control extends the time between cleanups significantly, increasing uptime and reducing waste. Otherwise the die has all the same features of a standard SCD® with stackable modules, each with melt flow channels and spirals optimized for the desired structures.

Shineway Group is producing the PVDC film for meat packaging. PVDC is the material of choice because it is practically impervious to both moisture and oxygen and therefore is ideal for increasing the shelf life of Shineway’s meat products. This packaging will withstand long distribution times under refrigeration only. It does not have to be frozen. As a result of this new packaging, Shineway will be able to open new markets in China because they can extend their geographic distribution and still maintain the integrity of their product.

The 11-layer SCD® die is an industry first, and so is the solution that BE developed to accommodate the 11 extruders around the die. BE’s innovative solution was to design piggybacked extruders, or two extruders with one mounted on top of another.

The design of the SCD® provides a good marriage for piggybacked extruders since the extruder centre line heights are different for each layer. The configuration for the 11-layer line then became two piggybacked extruders(4) plus seven normal extruders, creating a radial extruder configuration similar to a 9-layer line but with two of the extruders being doubles.

BE’s engineering group knew that arranging 11 extruders radially around the SCD® die would require longer adaptors and result in a very crowded hot section. A suitable extruder arrangement needs to accommodate a die access platform and leave enough room between the extruders for servicing access. Ideally, for barrier film coextrusion the adaptors should be short and straight. The piggy-backed arrangement meets these needs.

Thirteen years ago, during the 1995 Polymers, Laminations and Coatings Conference, BE presented a paper titled “An 11-Layer Blown Film Line: Outlining the Advantages of Multilayer Coextrusion.” In this paper we provided theoretical justification for the possibility that one day the market would request an 11-layer blown film line. That prediction has come to pass in 2008 and proves that BE continues to lead the way in blown film coextrusion. The SCD® multilayer streamlined coextrusion die became the world’s first successfully commercial 7-layer die and later led the way to the first 8, 9, 10 and now, 11-layer blown film lines. BE has shipped more 7 - 11 layer SCD® systems than any other machinery manufacturer in the world. At latest count its 77.

Unstable melt is another troubling problem that results in lower output rates and poor quality film.

In designing film structures to achieve certain physical and/or barrier properties, consider the economics of the raw material costs AND the economics associated with productivity rates on a given film line. Balancing these physical properties, blow-up ratios and the available equipment configurations is critical in obtaining and maintaining product quality consistency and profitability.

When designing film structures for a multilayer blown film line it is particularly important to look at each specific material’s processing temperature requirements, the physical properties, barrier properties, as well as melt strength.

Consider extruder sizing by layer, required layer percentage or output, processing conditions to ensure that the polymers are not overheated from excessive extruder RPM or processing temperatures.

Product layflat and blow-up ratio requirements, as a function of die lip diameter, are critical. Too large a blow-up ratio with weak melt strength resins will result in lower production rates, which can have a negative effect on profitability. Large blow-up ratios can also contribute to poor film flatness and finished roll quality. This can have a negative effect on secondary processes such as printing and high speed converting.

For troubleshooting haze lines look at processing temperatures, temperature gradient, screw and barrel conditions and die design.

Avoid melt instability by designing your structure for specific resin characteristics, layer ratios, BURs and your machinery capabilities.

In the next issues of Between the Lines we will discuss gels, unmelts and interfacial instability.