With shrink wrap, it is not always easy to decide which wrap will suit your product best. With the use of shrink wrap, a choice should be made between the varied range of different shrink wraps. We keep several aspects in mind in our advice, which we would like to explain further:
1) Poly Ethylene (PE), Polyvinylchloride (PVC) and Polyolefin (PO)
2) Mono or biaxial
3) Double bubble blown process
4) Cross linked
5) Specific gravity
7) Micro perforation
9) High Friction
There are three main types of shrink wraps: Polyethylene (PE), Polyvinylchloride (PVC) and Polyolefin (PO).
PE is the most common shrink wrap. It involves a usually thick (25 - 150 mÁ) supple foil that is easily processable.
When thinner wraps (< 20 mÁ) are needed that covers the product virtually invisibly, the choice would be a PVC or PO foil. PVC consists of a combination of NaCl and H2. PO is a co extrusion of which the layers are structured from PP and PE. PO therefore comprises of various oil derivatives.
The term mono or biaxial refers to the shrinking direction of the foil. Sometimes it is necessary for the foil to only shrink in one direction. For example when long objects (laminate parquet) are packed or when a product is packed where the foil remains open on two sides (a tray of cans).
When a reasonably uniform product must be enclosed in its entirety, the foil must shrink in two ways, or biaxial. Here one can think of a cake tin, a boxed game or a pallet of bricks.
PVC and PO foils are biaxial extruded using a so-called "double bubble" blown process. This blown process is a variation of the blown process with which cling wrap is made. The degree of stretch during the blown process is however not only of importance for the strength of the foil but also a deciding factor in the shrink properties. Here there is a difference between the blown processes of PVC and PO.
With PVC, a bubble is formed directly after the extrusion which is pulled horizontally at ground level. This bubble ensures the shrink properties in the lengthwise direction. After this first stretch, a second vertical upwards bubble is formed that ensures shrinking width wise. With PO something similar is done, but the first bubble is also blown down in a vertical direction. This co extrusion of 70 mÁ thick is then once again blown in a vertical direction to a final thickness of 12 to 25 mÁ.
The foil thus formed, is wrapped on a large "mother roll". This mother roll is stored for seven days to ensure an optimal migration between the various layers.
After the rest period, the mother roll is cut in various widths and then the flat foil may still be folded double on a separate machine.
With some applications, it may be extremely important for the foil to offer a very high perforation or "dart drop"-resistance because of the shape of the product to be packed (sharp corners). For such applications, a PO foil which is 100% PE, is automically irradiated thus forming a structural change in molecules. Thus the molecules are joined to each other or "cross linked". The optimal effect is reached when this irradiation takes place in the production line immediately after the extrusion. The special properties of this foil is also appreciated when the foil has to be processed quickly and when shut-downs due to breakage must be avoided. A "cross linked" foil is therefore excellent for applications on high capacity production lines.
PVC has a specific gravity of 1.3 tot 1.35 g/cm? opposed to only 0.91 g/cm? with PO.
Just this difference in specific gravity alone already implies that with a similar foil thickness, 45% more will be required in PVC than with PO. Thus, although PO is much more expensive per kilo, the price per packaged product can often be a lot more attractive.
When the products to be packed consist of the same material as the shrink wrap, it is possible during heating that a bonding may develop between the product and the foil. Thus the foil remains stuck, which could lead to damage with removal. To prevent this, a co extrusion can in that case be applied whereby the composition of the material from which the inner layer is manufactured, can be chosen that no bonding takes place. This is then called a repulsive foil.
To allow the air to be released during shrinking, the shrink wrap is provided with a large number of tiny holes, called micro perforation. There are various forms of micro perforation. Depending on the required density of these holes, a choice has to be made. In products with a regular form, the number of holes may remain limited. It is enough to guide the foil on the shrinking machine along a perforation roll with sharp points.
When the product is irregularly shaped, it may be necessary for a higher density of holes in specific places. The perforations must then be applied in advance, during the production process of the shrink wrap. Because of the high feed-through speed of the foil during this process, these perforations look almost like tears. It may also be that the foil will have to breathe after shrinking. Here, the perforation density must be as high as possible. With a large density in perforations, the foil could tear. This can be prevented by doing the perforation process during the production of the foil using hot needles. This "hot needle" perforation method provides nice round perforations with sturdy molten edges.
With exposure to sunlight, the foil can age much faster. This is mainly due to the UV radiation. By adding a UV-stabilizer, this process can be delayed. When it is known that the foil will be applied for the packaging of products that would be kept outside for longer periods, it is advisable to use a foil with such an addition.
When packages or bags are stacked on a pallet, the pallet load as a whole will have to be stable. This can be ensured by securing the load after stacking using aids such as layer sheets, shrinking and stretch wrap or strapping. To limit the use of these aids, it is also possible to choose a High Friction shrink wrap developed by Contimeta. This is a profiled foil that shows an extremely rough surface after shrinking. This foil is used amongst other in extremely cold environments, therefore even possible ice formation on the products do not pose any problem.