Big bags go high-tech

Woven technical textiles are now frequently used for innovative applications in the fields of architecture, construction and packaging. Woven textiles made of polyester are mainly used in the construction industry, while woven textiles made of polypropylene (PP) are mainly used in the packaging industry. Due to the special manufacturing process, these PP materials have a very high strength with a low raw material input.

The use of big bags

FIBCs (Flexible Intermediate Bulk Containers), also called big bags, are used in the chemical and pharmaceutical industries. They are also increasingly used in the food industry. Especially for the sensitive areas of the food and pharmaceutical industries, hermetically sealed packaging is essential. Especially in the pharmaceutical industry, so-called containment systems are indispensable. Containment is the isolation of hazardous and/or irritating substances. This is to prevent hazardous substances from escaping from the production process and the packaging or to prevent a product from being contaminated by foreign substances in the environment.

Nowadays, big bags are produced using sewing technology, which means that the production of such packaging is essentially done by hand. The very time-consuming and non-automated production by means of sewing technology leads on the one hand to high production costs and on the other hand to disadvantages in terms of production technology. Another serious disadvantage of the sewing process is that the sewing needle inevitably penetrates the dust- and waterproof fabric, so that at least the dust-tightness can only be achieved by additional costly processes. The use of thermal welding processes also leads to a number of production problems, such as the so-called memory effect. The memory effect is the shrinkage of stretched materials at higher temperatures.

a New process

Within the framework of a cooperation project, a novel laser welding process was developed to produce big bags from single-variety PP fabrics without additional pigmentation and without the use of additional material. In order to achieve the highest possible degree of automation, the new process was planned and executed as a roll-to-roll process within a production plant. After a series of trials, the so-called “V-seam process” prevailed for further progress and a functional production plant was built and set up at a laser centre.

The V-seam process 

In V-seam welding, the two joining partners are fed to each other from two sides via deflection rollers and pressed together pneumatically. With the help of a scanner system, a laser beam is focussed from above into the joining zone through a variable focussing lens and melts both joining partners evenly so that a continuous and tight weld seam is produced. The strength and tightness to be achieved with the new joining process is a decisive requirement. The rhodamine test is used to check the tightness of the weld seam. 

Several process parameters are important for a successful, tight and stable welded joint. These can be summarised in the effectively introduced line energy E. The most important parameters are the laser power P and the scan speed v_scan. The line energy E can be calculated with the following formula:

In order to determine a suitable combination of the various process parameters, many samples were produced and examined. The figure on the right shows the influence of the applied line energy E for a polypropylene fabric with a thickness of d=500 µm. The material feed and the contact pressure remained constant. 

If the focus of the laser beam is positioned exactly in the joining zone (Δz = 0mm), stronger welds are produced with increasing line energy E, but significantly more defects (holes and punctures). 

Furthermore, there is an additional stiffening of the weld seam. By varying the focal position, the line energy E is introduced more homogeneously into the fabric material, so that it melts more gently and remains soft.

The laser welding process

The developed laser welding process can be used to produce high-strength welded joints with an aesthetic appearance while retaining the same material flexibility as the starting material. The laser-welded specimens have significantly higher tensile strength compared to the conventionally sewn specimens. This makes the V-seam laser welding process competitive with the sewing process. The process also makes it possible for the first time to produce dense (vapour- and dust-tight) FIBCs and to substitute the previous sewing process. 

The strength of the weld seams is in the range of or in some cases even above the strength of seams produced by sewing technology, so that there are no restrictions with regard to the load capacity and thus the container size. In contrast to classical thermal processes, the laser process allows the targeted and dosed use of line energy to avoid so-called memory effects. Furthermore, in contrast to manual production, the new manufacturing process can be fully automated and additional online quality control would be possible.

The advantages

Laser welding of unmixed PP fabrics without any further additives creates a single-material packaging that saves many additional materials and accordingly also has a high recyclability. Another advantage of the laser process comes into play in the event of possible damage during transport. If one piece is torn, the rest of the packaging remains almost untouched, whereas with conventional seams, a seam tear means unusable packaging. 

From this point of view, a laser-welded bag, albeit on a smaller scale (see cover photo), is potentially interesting for online retailers. The reusability as well as the recycling possibility through the use of a single-material solution combined with the robustness of the PP fabric make this packaging interesting. The laser process can also be used to seal the big bags.