Adhesive tape-based thermoplastic sheets are overmolded using repelletized pellets produced from scraps of the same thermoplastic sheet. This consistent single-material approach can keep the composite raw materials completely in circulation.

Thermoplastic composites paved the way for lightweight automobile structures in a circular economy. The processing of organic sheets is currently the most important thermoplastic composite technology. One of the focuses of further development is the recycling of production waste, such as pruning residues. ENGEL has been able to demonstrate that it is possible to process regranulation made from organic sheet waste through injection molding while maintaining the material properties. This makes it possible to keep the composite raw material completely in circulation.

The organic sheet is composed of continuous fibers embedded in a thermoplastic material. Thermoplastic methods can achieve the effective integration of semi-finished product molding and functionalization, thereby reducing unit costs and making the technology particularly attractive to the automotive industry. For processing, the organic sheet is heated, molded in an injection mold, and then immediately completed by injection molding to complete the required detailed geometry. Generally, materials from the base material group of organic sheets are used for injection molding. In the future, scraps recovered from organic boards will also be used. In this way, parts made entirely of single fiber-reinforced thermoplastics can be produced and can be easily recycled at the end of their useful life in the sense of a circular economy.

Even with the best nesting, waste will be generated during the manufacturing and processing of the board. A small amount of waste material accounts for 5% to 10% of the surface of the organic board, but due to poor positioning or complex component structure, the amount of waste material can reach 35% or more. Recycling these production wastes not only helps improve sustainability, but also saves costs.

To prove its feasibility, injection molding machine manufacturer ENGEL and PURE LOOP of EREMA Group conducted a series of tests to study the treatment of organic sheet waste and the subsequent treatment of recycled materials to adapt to the material.

Keep the length of the glass fiber

In the test, organic sheet waste based on unidirectional (UD) tape was used to produce pellets. To this end, by adding an unreinforced substrate, the chips are shredded during the melting process and the fiber content is reduced. The fiber content of organic sheets is usually as high as 72% by weight, which is too high for injection molding processing. When reducing the fiber content, the fibers must be evenly dispersed in the base. In the case of long glass fiber particles, the goal is to re-particles to a performance level comparable to that of the original material.

In the test, the picture on the right is a regranulation made from the scraps of a thermoplastic sheet based on unidirectional (UD) tape, and the picture on the left.

Only by maintaining the length of the fiber and evenly distributing the fiber at the base can the high mechanical strength of the material be ensured. The challenge in recycling semi-finished products of continuous fiber-reinforced organic sheets is to establish a preparation and injection molding process to maintain the longest possible fiber length.

In the test, ENGEL used scraps from Profol's Progano brand organic sheets, which were made of UD tape and contained 72% by weight of glass fibers in a polypropylene substrate.

The composite material used for the test is made of glass fiber and polypropylene by German Profol.

The ISEC evo 302 system from PURE LOOP is used for regranulation. The characteristic of this production line is that the shredder and extruder screws are mounted on a common shaft. The feeding time is approximately. 1.5 meters long organic slats. The aim is to dilute the fiber content to 40% by weight.

PURE LOOP's ISEC evo 302 production unit is used to re-granulate scrap. The shredder and extruder screws of the system are mounted on a common shaft.

Unreinforced PP is fed in pellet form, and the material mixture is plasticized. No filter is used in the extrusion area as this will remove long fibers. Finally, the material is demolded and dried.

The obtained particles were ashed to optically evaluate the length of the glass fiber. It can be seen that enough long fibers are retained. There are glass fibers longer than 4 mm. The adjustment of fiber content can be optimized when cutting decorative parts. The smaller the fragments, the more uniform they will be ground and mixed with the base material. Calculating from the density, the fiber weight content of the recycled material is about 35 wt.%.

The successful post-processing of production waste shows the potential of auto parts at the end of the vehicle's useful life. After cleaning the components and removing any used metal inserts, they can be recycled and pelletized in the same way.

There is no difference between injection molding and original material

In ENGEL's technical center, sample parts are made from re-granulation obtained on an e-victory injection molding machine. The focus of the test is the process behavior of regranulation compared with the series of materials. As a reference, a long glass fiber reinforced original material was processed, namely the GB477HP type PP GF40 from Borealis. Both materials were plasticized at three different screw speeds to evaluate process consistency. From the linear metering curve with almost no decline, perfect metering behavior can be derived.

Regranulation is premixed in a masterbatch mixer to simulate the effect of an optional homogenization step—for example, in a silo with agitator. Compared with the original material, weighing the components showed a slightly higher standard deviation for regranulation. However, in absolute terms, the standard deviations of the two materials are very small.

Since the material is well homogenized during the injection molding process, when evaluating the fiber length and ratio, it is expected that there will be no significant difference between the re-granulated material and the original material, which is confirmed by testing. The additional homogenization in the masterbatch mixer does not further increase the already good value. In short, it can be said that regranulation can be processed in the injection molding process just like the original material.

Aiming at car mass production

After the successful test, the problem of transferability to actual components appeared. ENGEL and PURE LOOP cooperated with the LIT factory in Linz to produce nearly a series of sample parts with an integrated process.

Sample parts close to the maturity of the series were produced in an integrated process at the LIT plant in Linz, Austria.

The continuous fiber-reinforced organic sheet is heated in an infrared oven, inserted into the mold of the ENGEL duo injection molding machine by an articulated arm robot, formed and overmolded with the re-granulation of the trimming waste. The achieved very good component quality and the high efficiency of the integrated manufacturing process prove the great potential of the series of applications for processing organic sheet waste in lightweight automobile structures. ENGEL and PURE LOOP will continue to work together to develop this potential.

The quality of the sample parts produced is very good, and the high efficiency of the integrated manufacturing process proves the great potential of processing thermoplastic sheet scraps.

DI Gerhard Bäck, development engineer at the ENGEL AUSTRIA Lightweight Composites Technology Center in Sankt Valentin, Austria, [email protected]

DI Dr. Klaus Fellner, ENGEL AUSTRIA Plastic Engineer, St. Valentin, Austria, [email protected]

DI Florian Gruber BSc, PURE LOOP Process Engineering Supervisor, Ansfelden, Austria, [email protected]

Dr.-Ing. Norbert Müller, Head of ENGEL Lightweight Composites Technology Center, St. Valentine/Austria; [email protected]

Müller, N., Zwicklhuber, P.: "Blick fürs Essenzielle" (An Eye for the Essential), Kunststoffe 8/2020, p. 26-29

The LIT factory of the Linz Institute of Technology (LIT) of Johannes Kepler University is a networked teaching, learning and research factory. Focusing on the digitization and connectivity of the production process, lightweight engineering technology has been developed and optimized throughout the value chain-from product development and processing to recycling to achieve a circular economy. EREMA and ENGEL are founding members and partners of LIT Factory.

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