Manufacturing of Lightweight Parts by Sandwich Foam Injection Moulding using Recycled Thermoplastics

Abstract

Reinforced, high-performance thermoplastics may have moduli of elasticity of up to 40 GPa, depending on the filler content. The filling materials can thereby consist of secondary raw materials such as recycled carbon fibers. By combining these highly stiff materials with unreinforced but material-compatible and foamable thermoplastics, excellent and complex lightweight sandwich structures can be realized in a one-shot process using the sandwich foam injection moulding technology. The core and top layers of these structures are usually made of two different components (2-C). The process is both reproducible and cost-efficient for high production volumes. The foaming process is carried out either by adding a chemical blowing agent or an inert gas at supercritical conditions. The resulting composite consists of a short-fiber reinforced high-modulus material in the outer layer and an unreinforced foamed material in the core. Due to the low requirements on performance, odour as well as surface properties, recyclates are particularly well suited for the use as core material. Three sources of recyclates are presented in the paper, which have been the scope of the specific studies: A) Polyolefin mixed fraction from household waste, B) PET (polyethylene terephthalate) waste flakes, and C) polyamide waste. Depending on the source mentioned, dedicated lightweight structures with specific properties may be produced—with applications in particular market segments. A suitable example in this context is the sandwich foam injection moulded functional support structure for the electro-mobility automotive industry with a design suitable for foam injection moulding. Moreover, a significantly higher specific stiffness was achieved with this semi-structural component compared to an existing reference structure.

1 Introduction

The German Federal Ministry for Economic Affairs and Energy designates lightweight design as a key technology that enables industry to manufacture lighter and more cost-effective products with improved resource efficiency [1]. In the future, automotive mobility will be characterized by the development of intelligent and at the same time sustainable lightweight solutions [2, 3]. Thermoplastic foam injection moulding (FIM) [4] is a promising and future-proof process for implementing lightweight solutions. With its application, particularly lightweight and yet robust, strong and durable components may be manufactured [5, p. 4]. Political and legal frameworks aim to encourage manufacturers to intensify the use of recyclates in order to significantly increase their market share [6, 7].

The Kunststoff-Zentrum in Leipzig gGmbH (KUZ) uses the FIM process to manufacture sandwich components from recyclate. This is intended to contribute to the “European Strategy for Plastics in the Circular Economy” as part of the European Green Deal [8]. In accordance to this strategy, the use of recyclates is to be increased from 4 to 10 million tons by 2025 [6]. However, higher recycling rates can only be achieved if high-quality recyclates are made widely available in sufficient quantities. The availability of sorted post-industrial waste on the European market tends to become increasingly limited, as the market demand for higher-quality recyclates, which may be used in technical applications, is steadily increasing [9, 10]. The KUZ therefore uses waste from post-consumer recycling (PCR) as a source of raw materials. Table 1 exemplarily presents foamed sandwich components from three different waste streams (see Table 1).

Table 1. Types of foamed recycled material, sources and possible applications.

2 Material Structure According to Sandwich Principle

The sandwich principle leads to a reduction in weight with comparable bending stiffness, so that the lightweight design effect clearly has an impact. At the KUZ, component geometry, materials and manufacturing processes are combined in such a way that the sandwich principle can be applied effectively while reducing component costs. Figure 1 schematically shows a bending loaded structural part under load F. Compressive stresses occur on the side where the force is applied (marked in blue), whereas tensile stresses occur on the opposite side (marked in green). In the neutral axis, the normal stresses decrease to zero, whereas the normal stresses in the face layers become maximum. According to Parallel axis theorem, the load-bearing capacity increases as the distance between the face sheets and the neutral axis is increased. The sandwich core essentially acts as a spacer and must absorb the shear stresses that become maximum in the core. Application of the sandwich principle means that high-strength materials are ideally placed far away from the neutral axis and lightweight materials are placed close to it.

Fig. 1.

Schematic representation of the sandwich principle

The high strength and stiffness of the short-fiber-reinforced material of the outer layer contributes significantly to the bending stiffness, while the core made of lightweight but strong foam enhances the weight reduction. The 2-component FIM manufacturing process allows the use of different thermoplastics in the outer and core layers. The outer layer is made of a reinforced plastic that provides both the high-quality visual appearance and the structural function. The core contains a lightweight foam made of unreinforced shear-resistant thermoplastic. In the most basic case, the foam can be achieved using a chemical blowing agent. An option for large-scale production involves physical direct gassing of the melt using nitrogen.

Fig. 2.

Exemplary principle of sandwich-like material structure | a), b), c): Improvement of the foam quality of the recycled material in the core through upstream processing step.

Figure 2 shows a cross-section of a test specimen produced in the 2-C FIM process using the imaging computed tomography technology available at the KUZ. The test specimen serves as a demonstrator for two-dimensional, shell-shaped industrial applications with a structural function. Hereby, the higher weight-specific bending stiffness compared to a part made of compact material may be well illustrated.

3 Manufacturing Process and Machine Requirements

For the processing of two different thermoplastic materials, a 2-C injection moulding machine is required. The two separately adjustable plasticizing units allow the injection moulding parameters to be set in order to fulfill the requirements of the respective part (e.g. skin/core layer distribution). For the specific control of the skin and core melt flow, a commercially available 2-C sandwich spacer plate is used between the mould and the plasticizing cylinder. Its working principle is shown schematically in Fig. 3. During 2-C sandwich injection moulding, the cavity is partially filled with the skin material (1st component) in the first step, and a thin, solidified skin layer is formed in contact with the cold mould wall. The immediately following foamed core material (2nd component) further displaces the remaining plastic core of the skin material until the cavity is completely filled. A needle valve nozzle prevents the gas-loaded melt from leaking. Foam is formed only after the pressure in the mould cavity has dropped.

When using a chemical blowing agent, which is usually added to the core layer material as a masterbatch, a 2-C injection moulding machine with standard plasticizing units may be used. Physical direct gassing with nitrogen is more suitable for higher volumes due to the higher machine-related investment costs. The process also leads to lower foam densities due to higher foam pressures and thus provides increased lightweight design potential [11, 12].

Fig. 3.

Left: Cross-sectional sketch showing the working principle of the sandwich spacer plate, the melt flow of component 1 (skin material) in the vertical plasticizing unit and component 2 (core material) in the horizontal plasticizing unit. Right: Top view of sandwich spacer plate from A&E Produktionstechnik GmbH, Dresden.

Figure 4 provides a schematic representation of the process combination 2-C sandwich injection moulding with physical direct gassing. The experimental tests are carried out at the technical lab of the KUZ on a 2-C injection moulding machine (type: Combimould HM-MK 180/525H/350V with Cellmould® unit for physical foam generation by WITTMANN Technology GmbH).

Fig. 4.

Schematic representation of the process combination

4 Material Preparation as a Prerequisite for Foaming Post Consumer Recycled (Pcr) Material

The challenge in foaming PCR materials is the thermal-oxidative molecular degradation caused by environmental degradation during the life cycle as well as renewed thermal stresses as part of the recycling process. The resulting melt instability often leads to rupture of the bubble webs during the foaming process. This in turn leads to bubble coagulation, which may eventually impair the mechanical

Fig. 5.

Notched test specimen

properties of the structural parts. Since gas is prevented from leaking through the parting line within a 2-C sandwich mould due to the closed shell of skin material, a non-uniform foam structure often occurs. A fine-cell foam structure develops near the sprue, while large bubbles appear at the flow path end. In view of the negative effects on the mechanical properties and dimensional stability as well as shrinkage and warpage, the bubble size distribution of the core layer should be as homogeneous as possible. The necessary processing step of regranulation for reasons of flowability and feedability into the injection unit may be used for material preparation by means of additives or reactive compounding.

5 Test Series for Recipe Optimization

A special notched test specimen, shown in Fig. 5, is used for an initial formulation adjustment of the material composition with respect to impact strength, subjective odour characterisation, foam structure and moulded part surface. Using the test specimen shown above, test series for improving properties are investigated (see Tables 2 and 3) and material modifications are deduced. By adding additives in a preceding compounding step, an improvement of the properties to those of the virgin material is intended. The dimensions of the specimen used for the three test series differ significantly from those of the usual specimen for the notched bar impact test according to DIN EN 179-1. Based on the specific specimen, a rapid evaluation of bubble size distributions across the test specimen cross-section may be obtained using an automated algorithm, developed at KUZ.

6 Preparation Steps to Match the Properties of Fresh Material

In the following chapter, three different PCR materials rPP/rPE, rPET as well as rPA are examined with regard to their respective improvements in foam quality. While in the case of rPP/rPE mixed waste it is mainly the odour that is a decisive exclusion criterion for use in the automotive industry, for example, the focus is mainly on the foam structure in the case of rPET and rPA. In the case of rPET, the degree of embrittlement, demonstrated here by the impact strength, is also relevant.

6.1 Foaming of PE/PP Mixed Waste

The PCR regranulate Dipolen S® from mtm Plastics GmbH in Niedergebra, which originates from large-scale household waste processing and is obtained from the PE-PP mixed fraction by large-scale recycling plants, is used.

Fig. 6.

Cross-sections of the notched test specimen made from the Dipolen S® regrind | without the addition of additives (left), with the addition of additives (right)

A sound and homogeneous foam structure can be achieved even without the addition of additives (see Fig. 6, left). Figure 6 (right) shows an integral foam structure altered by the compound modification as shown in Table 2. The incorporated additives (Additive 1–4) lead to improved foam morphology due to the increase in melt stability. In both cases, a weight reduction of about 30% is achieved. A critical concern with PCR from the household waste fraction is the strong odour, which has so far widely prevented its use in industrial applications. A reduction in odour and emissions can be achieved by adapting the formulation in a preceding compounding step. By using an entraining agent, oligomers are removed from the melt, so that the odour is significantly reduced compared to the reference material (see Table 2). However, this is still not sufficient for an application in the automotive interior sector for reasons of emissions and the part surface, so that encapsulation by processing as a core layer in 2-C sandwich moulding could be an appropriate approach for this purpose.

Table 2. Odour reduction

The results presented in Table 2 for the assessment of the odour level are based on a purely subjective perception. No odour testers specialised in this field were involved. The values should therefore only be regarded as guidelines and would have to be measured again in a qualified manner as the project progresses. Irrespective of the above, it can be stated that the recycling odour typical of PP/PE mixed waste may be reduced by about half when entraining agents and additives are incorporated. The specific type designation of the additives for the odour reduction of PE/PP mixed waste and for molecular chain attachment for rPET (see Sect. 6.2) are subject to confidentiality due to a new development and can therefore not be published.

6.2 Foaming of rPET from Bottle Waste

Figure 7 on the left shows the grinded flakes of MultiPet GmbH Bernburg as delivered, after re-granulation without additives (Fig. 7, center) and as compound processed with additives (Fig. 7, right).

Fig. 7.

Left: rPET flakes as supplied, Center: After re-granulation without additives, Right: Processed with additives

Without the addition of additives, the material with a light brownish discoloration shows a residual contamination by very fine particles (see Fig. 7, center). In addition, the flakes are not of sufficient quality for foaming due to insufficient melt strength. Figure 7 right shows the prepared foamable granulate.

In order to be able to feed the light flakes into the injection moulding machine without difficulties, the necessary regranulation is used at the same time as a reconditioning step for the addition of additives to improve the part surface and impact strength. The additives are incorporated using a co-rotating ZE25Ax47D-UTXi-UG twin-screw extruder (screw: 25 mm, L/D: 47, L: 1175 mm) from KraussMaffei Berstorff. The differences in weight reduction, impact strength and foam structure between the regrind from rPET bottle waste made from pure flakes and the prepared compound are shown in Table 3.

Table 3. Properties of foamed rPET after preparation

Figure 8 Left shows the cross-section of a foamed sample made of rPET in its initial state without the addition of additives. Individual voids, an uneven component surface and a clear yellow discoloration as a result of degradation mechanisms can be seen. The cross-section of a foamed test specimen produced using an additivated compound is shown in Fig. 8, right. A very fine-cell foam structure is formed as shown in the detail magnification of an SEM image (see Fig. 8 below). The bubble size is in the range of about 100 µm. The surface is smooth, the visual impression greatly improved and no yellow discoloration occurs.

Fig. 8.

Left: PET recycled foam before reprocessing with yellowish discoloration, uneven specimen surface and insufficient foam structure due to molecular degradation reaction, Right: After preparation with smooth specimen surface, without discoloration and with improved foam structure, Bottom: Detail picture (SEM) of the compound with additives

6.3 Foaming of PA6 from Fiber Waste, Ropes and Fishing Nets

Due to thermal-oxidative molecular degradation reactions, this recycled material also exhibits insufficient melt strength for foaming in its as-received state. The rheological characteristics such as the melt flow rate MFR (275 ℃/0.325 g) of 10 g/10min and the viscosity number of 135 ml/g indicate a reduced molecular weight and thus poor foamability. During both chemical and physical foaming, large voids with a diameter of approx. 1 to 3 mm repeatedly occur, which predominantly concentrate at the end of the flow path. By adding 1% of the Chain Extender MB AR/PBT-30, Trigon Chemie GmbH [13], the foam structure could be decisively improved (see Fig. 9). The chain extender is available in the form of a masterbatch and consists of a preparation of benzene-1,2:4,5-tetracarboxylic acid dianhydride, pyromellitic acid dianhydride, benzene-1,2,4-tricarboxylic acid 1,2-anhydride and trimellitic acid anhydride. The low molecular weight polymer chains are linked by the addition of the additive to form longer chain polymers while maintaining the original linear structure [14]. The compounding step with reactive compounding is carried out on a twin-screw extruder analogous to the compounding of rPET.

Fig. 9.

Microscopic image of the cross-section of physically foamed test specimens made of PA6 (Left: Without chain extender, Right: With chain extender)

7 Production of Sandwich Component Function Carrier

The findings described in Chap. 6 on improving the foam quality of PCR materials of one-dimensional test specimens can be applied to structurally more complex components, such as the sandwich component functional carrier shown in Fig. 10. Additives are used to improve the foam quality depending on the core material used (rPP, rPET) in the same way as in the previous test series. In cooperation with the Koller Leichtbau-Zentrum Lupburg, a R&D project has resulted in the development of a topology-optimized semi-structural lightweight part (see Fig. 10). It is based on a bionic design and is manufactured using a 2-C sandwich injection moulding process. In order to be able to use the one-shot sandwich process economically for largescale structural and visible components in high-volume series production, a multiple connection with hot runners and cascading filling is a necessary requirement. To this end, Koller has contributed a patent-pending mould development to the project.

Fig. 10.

Topology-optimized semi-structural lightweight part using a 2-C sandwich injection moulding process

Two types of PCR material are processed in the core layer. The material compositions are:

• Material cluster A

  Skin layer: virgin material PP-LGF40 Fibremod GB 402 HP, Borealis AG.

  Core layer: Processed regranulate of PE/PP—mixed waste of the domestic waste fraction

• Material cluster B

  Skin layer: virgin material PC/PET-T15 macroblend VT 235 M, Covestro AG

  Core layer: rPET from processed regranulate from bottle waste of the household waste fraction

8 Possibilities of Industrial Applications and Conclusion

The production of lightweight structures in 2-C sandwich injection moulding is well suited for large-scale production due to the one-shot technique and thus provides the potential for cost-effective lightweight engineering. Ribbed, flat, plate-like or shell-shaped components with a load-bearing function are predestined. For the bending load case, the sandwich effect results in weight reduction while retaining the mechanical properties. For rotating and/or fast-moving components, energy savings can be achieved by reducing the moving masses.

Fig. 11.

Possibilities of industrial applications, such as a) cover elements, b) gripper elements, c) housing components, d) fast rotating components, e) machine elements and f) rotor components

Some examples within the field of automotive applications include: Front end carrier, pillar trim, sill trim, battery carrier, hood for cylinder head, hood compartment cover, step trim, seat pan, trunk floor, rear spoiler. Other technical applications are: High strength planar housings, structures for robot heads, moving closing mechanisms, fast rotating machine elements (see Fig. 11). Applications for foamed rPET are mainly in the field of electrical and electronics. Recycled PA foam may be used in components with increased thermal or mechanical stress.

Based on the presented test studies with the three PCR compounds rPP, rPET and rPA, it is demonstrated that the foam quality, the foam morphology and the impact strength can be substantially improved by a systematic preparation of the materials through the addition of suitable additives. Depending on the material, various mechanism of actions are used, such as increasing the melt stability by adding chain extenders or reducing odour due to the incorporation of entraining agents. For the material PP/PE mixed waste it can be stated that the odour level may be decisively reduced by the incorporation of various additives and entraining agents. A significant improvement of the foam morphology as well as a reduction of degradation effects may be achieved by adding various additives for rPET. For rPA, a significantly improved foam morphology may be obtained by means of the application of chain extenders. In the course of the experiments, the additives are added by means of a preceding compounding process step in order to ensure an effective incorporation of the additives. In the further process of the project, the material properties are to be investigated in comparison with the respective virgin material.