Extrusion International 1-2021

28 Extrusion International 1/2021 FILM EXTRUSION Silage fi lms are primarily used in ag- riculture to preserve bales of silage, hay and maize. Wrapped around the bales, silage fi lms prevent over- fermentation processes and main- tain the nutritional value of the con- tents. To ensure easy bale wrapping and proper protection throughout the entire storage period, the qual- ity requirements of these fi lms are relatively high. So far, silage fi lm was mainly made from blown fi lm as the right dart impact, cling level and oxy- gen permeability could be more eas- ily achieved than with cast fi lm. This is now changing. In cooperation with premiummateri- al suppliers, SML recently developed a silage cast fi lm for bale wrapping thatmeets expectations in terms of its properties and even beats the blown product. “Our major concern was the higher oxygen permeability of our cast fi lm,” Thomas Rauscher, Product Manager at SML, remarks. The mar- ket-conform value for the oxygen permeability of silage fi lm should not be higher than 9,000 cm³/m²*24h. “In the past, a typical cast stretch fi lm of 25 micron displayed a value above 10,000 cm³/m²*24h, whereas due to the latest development we achieved cess. As usual, the UV masterbatch is also added to this layer. The UV addi- tive protects the goods against dam- age caused by sunlight. The properties of the inner layers of the silage cast fi lm guarantee the right dart impact and stretchabil- ity. “The dart impact of silage fi lms should be at least over 10 g/ μ m, while the tested cast fi lms reached values of more than 15 g/ μ m,” Thomas Rauscher comments. Silage fi lm usually requires a stretchability of between 50 and 100 %. “For a cast fi lm, this is not an issue and so all the tests produced results exceeding the required stretchability by far, based on 750mm wide rolls,” Rauscher ex- plains. SML’s cast fi lm lines do not only pro- duce silage fi lm, which is a seasonal business: the same lines also manu- facture conventional stretch wrap fi lm. With the cast process, manufac- turers are able to raise the utilisation rate of their extrusion lines further which also has a favourable impact on the pro fi tability. SML Maschinengesellschaft mbH Gewerbepark Ost 32, A-4846 Redlham, Austria www.sml.at Silage Film for Bale Wrap Newly developed formulations from SML make it possible tomanufacture high-quality silage fi lmon its cast stretch fi lm lines. Compared with the manufacturing of blown silage fi lm, this new approach offers some key advantages: higher output capacities, signi fi cantly lower production costs and the possibility to optimise the relevant fi lmproperties values of around 8,500 cm³/m²*24h,” Thomas Rauscher explains. Typically, silage fi lm made on SML’s cast fi lm line has at least 5 layers. The claim to fame of the silage fi lm for bale wrap is that the cling layer has to be at the outside of the roll and the “stickiness”has to be very high, whereas a good cling level is between 5 and 10 N/m. “During the cast process, it is an easy task to put the cling material on the outer side of the roll. And we have several pos- sibilities to guarantee a high sticki- ness,” Rauscher says. The cast fi lm line can either produce the fi lm with PIB in liquid as well as in granule form, whereas the preferred process nowadays is without liquid. Addition- ally, the high cling level can also be reached without PIB. This solution re- duces the handling cost, since silage fi lm on a PIB base normally needs to “rest” at least 48 hours in a heated storage room. The cling value of the PIB-free cast silage fi lm was tested at 8 to 9 N/m. The layer on the other side of the si- lage fi lm has to have a high slip. The ‘slip layer’ helps to ensure that the wrapped bale does not get damaged during the stapling and storage pro- Silage fi lmproduced on SML’s MiniCast stretch fi lm line