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Lightweight Design worldwide

, Volume 11, Issue 5, pp 58–63 | Cite as

One-step Series Production of Hybrid Components with Visible Surfaces

  • Mesut Cetin
  • Christian Herrmann
  • Stefan Schierl
Design
  • 119 Downloads

Using a center armrest as an example, Krauss Maffei is presenting the first series application of the Fiberform process synchronized with a swivel plate injection molding machine. Processing organo sheets and over-molding a thermoplastic elastomer in one production step reduces the costs and weight of the component and is ideal for visible components.

Soft Surface without Post-mold Processing

The high cost pressure on OEMs and their suppliers demands a high level of functional integration in the production of components, together with very short cycle times. At the same time, requirements regarding vehicle weight are also increasing. Electrification and increasing demands on comfort and safety in the automotive industry are leading to an increase in weight that has to be offset by reducing the weight of components.

Continuous Fiber-Reinforced Plastics (FRP) offer good weight-specific mechanical properties. Thanks to their high strength values and low weight at the same time, continuous fiber-reinforced plastics have taken a leading role on the market for lightweight components in various applications, such as seats, module carriers, battery mounts, frontend components, brake pedals, frontend carriers and underbody trim. In order to be capable of manufacturing these applications at a production volume of more than 50,000 components per year, an efficient manufacturing process was developed back in 2011 within a publicly funded project with Audi, IVW Kaiserslautern, HBW Gubesch, Krauss Maffei and Lanxess as partners. This combines the thermoforming of organo sheets, i.e. blanks with continuous fibers made of glass, carbon or aramide that are embedded into a thermoplastic matrix made, for example, out of polyamide (PA) or polypropylene (PP) and injection molding in a single process. Thanks to the integration of organo sheets, the component can be locally reinforced and benefit from the mechanical properties of the reinforcing fibers. The combination with the injection molding process makes it possible to implement intricate component geometries. Furthermore, the Fiberform technology has low cycle times of less than 60 s, which gives it an economic advantage over conventional FRP manufacturing processes.

However, the organo sheet component surface of the components produced in this process is not always desired. This can be because a soft surface is required or the surface requirements are only achieved in downstream processing steps such as painting. Consequently, post-processing stations, which prolong the process chain, are necessary for the completion of such components.

Spinform Technology for Visible Applications

Therefore, Krauss Maffei has combined the Fiberform technology described above with the Spinform technology for multi-component injection molding for the National Plastics Exhibition (NPE) 2018 in Orlando, Florida (USA). The Spinform technology not only allows for a specific surface lining of the Fiberform components, but also enables an expansion of the application area of the technology. Consequently, Fiberform components for visible applications or, for example, hard-soft combinations can be manufactured efficiently because the process chain is not prolonged substantially. At the trade show, the close-to-series production of a center armrest, Figure 1 and Table 1 were shown. The center armrest consists of two components:

Figure 1 Design of the center armrest (© Krauss Maffei)

Table 1 Machine and material data (© Krauss Maffei)

Injection molding machine

GXW 450-2000/1400 Spinform with reversible plate technology

Clamping force [kN]

4500

Screw diameter [mm]

70 (inner shell)/60 (outer shell)

Injection molding material [-]

PP ADX5344 ZHEA (inner shell)/TPE-80IQ817(outer shell)

Organo sheet material [-]

Tepex Dynalite 104-G601(x)/47 %

Tooling cavity [-]

4-cavity (1+1+1+1-cavity)

Cycle time [s]

Approximately 60

Automation [-]

Kuka KR60-KS + Kuka KR60

  • an inner shell made of PP and thermoplastic elastomer (TPE)

  • an outer shell made of PP, TPE and a cut organo sheet.

For the outer part, an continuous fiber- reinforced organo sheet from Lanxess is used, which is encapsulated in PP and over-molded with a thermoplastic elastomer, Figure 1.

The fully automated manufacturing cell for the production of a center armrest with organo sheet core and TPE outer layer is displayed in Figure 2 and Figure 3. Besides the two jointed-arm robots, the infrared heating station from Krauss Maffei, the reversible plate injection molding machine and a conveyor belt, the cell features a material feeding unit with optical control and centering of the organo sheets and a QR code printer.

Figure 2 Configuration of the entire Fiberform system (© Krauss Maffei)

Figure 3 Access to process and component data using a QR code (transfer time: end of heating until build up closing-force) (© Krauss Maffei)

A standard drawer system feeds the organo sheet into the production cell. Optical inspection of the organic sheet ensures a accuracy transfer to the mold. During this inspection, the contour, fiber orientation and position of the organo sheet are determined and evaluated. Semifinished products outside the defined tolerance are discharged from the process. Furthermore, the actual position of each organo sheet (position in X- and Y-direction and angle offset) are transmitted to the insertion robot (robot 1) at the transfer so that the organo sheet is always picked up in the same position. Then, the insertion robot conveys the organo sheet into the infrared heating station, above the fixed mold platen. There, the organo sheet, which is vertically suspended, is heated on both sides and transferred to the mold system after heating. Thanks to the location of the infrared heating station on the fixed mold platen, both the transfer distance and the transfer time of the organo sheet into the mold are as short as possible. Consequently, cooling of the organo sheet is minimal, which allows for selecting a heating setpoint temperature only a few degrees above the forming temperature. The target temperature can also be set lower than for production concepts with longer transfer times because the longer transfer time cools the organo sheet more and this can only be compensated for by a higher target heating temperature. To sum up, Krauss Maffei's production solution achieves shorter heat-up times because a lower target temperature can be set. This also reduces the thermal stress on the organo sheet blank.

In the first cycle step, the basic components of the center armrest (outer and inner shell) on the A side are manufactured from PP, Figure 6. Then, the mold is opened, and the centrally located reversible plate is rotated 180° so that the basic components are positioned opposite the B side. For the second cycle step, the mold is closed and the basic components are over-molded with TPE. In this step, the outer shell, which is to include visible and functional surfaces, is completely over-molded with TPE, and TPE is applied to the inner shell only in some places. Then, robot 2 demolds the two finished parts.

Figure 6 Mold configuration (© Proper Tooling)

To document the manufacturing process, all process data, for example heating curves of the organo sheets or curves of the injection pressure, are not only collected and evaluated in the DataXplorer of the Krauss Maffei MC6 control system but also individually assigned to each component via QR code, Figure 3. The QR code is printed specific to each cycle and attached to the component before it leaves the manufacturing cell. This enables uninterrupted tracking of all product and process data, such as heating curves and injection molding pressure, at the component level—worldwide, online by mobile phone, tablet or computer.

Part Design

Component design and component configuration follow a reverse engineering approach, Figure 4. To begin with, reverse engineering includes 3-D digitalization and data conversion of 3-D point information into a CAD model (1). Then the component is redesigned (2) in conjunction with the FEM analysis (3) until the final design meeting the requirements for weight and component unit cost has been defined.

Figure 4 Procedure in design and component configuration (© Proper Tooling)

For reducing the weight and the component unit costs, ABS (Acrylonitrile Butadiene Styrene) material, from which the reference component was made, is replaced by PP with a continuous fiber-reinforced organo sheet as an insert. For this, in the FEM analysis, the HiAnt service analyzed various component geometries and organo sheets by Lanxess and compared them to the reference component with regard to component tension and displacement after force effects, Figure 5. Semifinished organo sheets with a balanced (50/50) fabric and with a unidirectional (80/20) fabric in various material thicknesses were considered in this analysis.

Figure 5 Structural analysis for the reference part and the new design (© Proper Tooling)

Both component tension and displacement after a force impact could be reduced by the organo sheet insert. In relation to the total weight of the component, the weight overall is reduced by more than 20 %, Table 2. Most of the weight reductions were made possible by substituting materials and by the organo sheet insert. Due to the high stiffness and strength of the organo sheet insert, it was possible to reduce the thickness of the material wall significantly. Furthermore, the overall stiffness and strength are influenced mostly by the organo sheet insert, which made substituting PP for ABS possible in the first place.

Table 2 Comparison of component weights between the reference component and the new design (© Proper Tooling)

Component

Material

Reference part [g]

Material

New design [g]

Outer shell

ABS

409

PP

276

Inner shell

PP

224

PP

264

Overmolding outer shell

TPE

446

TPE

263

Overmolding inner shell

-

-

TPE

3

Organo sheet

-

-

PP/Glas

38

Locking

Steel

87

Steel

87

Screws/small parts

-

21

[-]

16

Total part weight

-

1187

-

947

Weight reduction

-

-

-

approx. 20 %

Mold Concept

The mold concept for the production of the outer and inner shells was implemented in such a way that both components could be manufactured at the same time. Consequently, on the A side, Figure 6, the basic structures of the two parts are first manufactured in individual cavities (1+1-cavity). This involves inserting the organo sheet, which is heated above its melting temperature, for the outer shell into the cavity before the injection mold is closed. The organo sheet is transferred using positioning pins. The mold is closed in the intermediate position, and a robot inserts the organo sheet into the mold area. Machine and robot movement take place at the same time in order to save cycle time. As soon as machine and robot have reached their respective transfer positions, the positioning pins clamping the organo sheet in place are extended from the mold. One function of the positioning pins is to enable a repeatable transfer of the organo sheet. Another is to prevent excess cooling of the organo sheet because the sheet is only in contact with the positioning pins.

Next, the mold closes and, simultaneously with the closing movement, the organo sheet is thermoformed. Then the injection process with PP takes place for manufacturing the outer and inner shells, Figure 7 (left). In this step, almost the entire surface of the organo sheet is back-injected in order to mold on elements such as screw bosses. Next, the mold is opened. The two components remain on the core side, and the reversible plate rotates 180°. The components are then opposite cavity B. Now, the mold closes again, and the TPE component is injected, Figure 7 (right). In this step, the surface of the outer shell and the function of the inner shell is finalized by molding on damping elements. The steps described above take place on the A side at the same time. Finally, the finished part is demolded. Last but not least, this mold concept allows for a cycle time of less than 60 s because the procedures on the A and B sides take place at the same time.

Figure 7 Mold layout and concept: cavity A (left), cavity B (right) (© Proper Tooling)

Flexible Concept for the Series

The demonstrated trade show application is an example of a successful lightweight construction project in a cooperation of mechanical engineering company Krauss Maffei, mold maker Proper Tooling and material supplier Lanxess. It has been possible to achieve a weight reduction of 20 % with the demonstrated manufacturing concept, the component design and the component configuration. Besides the weight, the component unit costs have been reduced by approximately 15 % as well. The Fiberform manufacturing cell from Krauss Maffei is not restricted to just one automation task or one organo sheet geometry. It also allows for a fast product change-over without major and complicated rebuilding. The optical centering system makes testing of different 2-D organo sheet geometries possible, which results in a correctly positioned transfer to the insertion gripper. Furthermore, the individual infrared heating fields can be selected, deselected and assigned to different heating zones as defined by the user. This allows for individual adaptation of the infrared heating station to the geometry of the semifinished product. The clamping elements of the insertion gripper may be shifted in the horizontal or vertical direction for handling various organo sheet geometries. This is of particular significance for a smooth series start since the insertion gripper can always be exactly adapted to the current blank geometry during optimization of the organo sheet blank.

When using continuous fiber-reinforced semifinished products (organo sheet inserts) in the injection molding process, the mold concept, the structural design and the draping process in the mold are of major importance. A large selection of different organo sheet inserts and support from the material manufacturer in structural design allow for the implementation of a solution exploiting the available potential for weight reduction.

The presented concept for the series production of Fiberform components is available at the Krauss Maffei TechCenter in Munich (Germany) for manufacturing prototypes, proving molds, and development projects, Figure 8. The infrared heating field with dimensions of 100 cm × 125 cm and the Krauss Maffei control system allow for processing organo sheets of different sizes, thickness and various matrix and fiber materials. The injection molding machine of the GX 650/4300 type has a clamping force of 6500 kN and consequently covers a broad spectrum of components. |

Figure 8 Production solution for series production of Fiberform components at Krauss Maffei's technical center in Munich (© Krauss Maffei)

Thanks

We would like thank all participating project partners for this successful collaborative effort and implementation of the project: Proper Tooling, Lanxess, Audia Elastomers, UCC Uniform Color Company, US Farathane, Tenibac, HRS flow, Advanced Composites and Tool Stats.

Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2018

Authors and Affiliations

  • Mesut Cetin
    • 1
  • Christian Herrmann
  • Stefan Schierl
  1. 1.Krauss Maffei Oberding-SchwaigGermany

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