High-quality Surface from Injection Moulding and Reaction Process Machinery

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1 The buckle cover is flooded with PUR in the SkinForm process (all photos except (1): Krauss-Maffei) High-quality Surface from Injection Moulding and Reaction Process Machinery Process Combination. In a close development partnership and through synergistically merging injection moulding and reaction process machinery, a group of companies has developed a new process for producing parts with high-quality surfaces. This SkinForm technology opens up scope for completely new designs in vehicle interiors. T he current tendency to incorporate a constant stream of designs and ideas into vehicle interiors is forcing manufacturers and partners to adapt their process technology to the demands of the designers. Vehicle interiors are becoming more and more the factor that ultimately governs whether or not customers buy a vehicle. Technologies for implementing new design effects and geometries have to be economical since higher prices for parts will generally not be accepted. Sarnamotive Schenk GmbH is a medium-sized company that mainly develops and produces parts for interiors, especially seat trim. In most vehicles, this is Translated from Kunststoffe 10/2004, pp usually injected in PA or ABS. To an extent depending on the accessories (switches, storage space etc), each seat may require as many as 10 trim parts. Luxury class vehicles have an even higher number of variants (Fig. 1). The surface of seat trim is usually upgraded by softcoating. The pleasant haptic properties and the matting effect give the customer the impression that he has not bought a cheap plastic part. The greatest disadvantage of soft-coating is the low scratch resistance of the coating, which is often only micrometers thick. Yet parts in the vehicle Fig. 1. The various types of seat trim for a luxury vehicle (photo: Sarnamotive Schenk) floor and seat areas are continually stressed by constantly moving legs and feet. Clearly there is a need for parts that offer increased scratch resistance. V 1

2 INJECTION MOULDING Fig. 2. Mould ejector side (core side) with docked mixing head; the part is in the removal position Fig. 3. Sliding table mould in the demoulding position The Synergy Effect Sarnamotive Schenk GmbH, Rühl Puromer GmbH and Krauss-Maffei Kunststofftechnik GmbH launched a joint project one-and-a-half years ago that is aimed at developing an economical process for the production of parts having highquality surfaces. The requirements specified at the outset were: i Manufacturer Machine and Equipment Technology Krauss-Maffei Kunststofftechnik GmbH Krauss-Maffei-Straße 2 D München Phone +49 (0) 89/ Fax +49 (0) 89/ info@krauss-maffei.de Parts and Mould Technology Sarnamotive Schenk GmbH Teckstraße 60 D Esslingen Phone +49 (0) 7 11/ Fax +49 (0) 7 11/ info@sarnamotive-schenk.de PUR Cast Skin Rühl Puromer GmbH Hugenottenstraße 105 D Friedrichsdorf/Ts. Phone +49 (0) 61 72/ Fax +49 (0) 61 72/ puromer@ruehl-ag.com Part surface must have pleasant leather-like haptic properties; High scratch resistance; Suitability for complex, highly curved surfaces with narrow radii and apertures; No warpage of decoration or grain; The enhanced surface should be as thin as possible; Process must lend itself to mass production; No post-finishing of parts; Costs no higher than those of a softcoated part. Standard surface-finishing techniques employed in injection moulding are insert moulding, in-mould decoration and in-mould lamination see [1] for information on the Decoform method of soft-touch film and artificial leather, as well as multicomponent injection moulding and downstream soft-coating of the injection moulded parts. None of these processes meets all the specifications mentioned above. The advantages and disadvantages of the special processes are compared in Table 1. Flooding with a PU skin system is used nowadays, for example, to cast in-mouldlabelled plastic parts in a clear PU coating or to upgrade decorative trim and covers made from burl wood in terms of structure, scratch resistance and depth effect. Already, plastic supports are being flooded with a PU casting system that has a leather-like surface. The processes used so far always require two different stages, i. e. are sequential, and are thus very cost intensive. To combine the advantages of injection moulding and reaction technology, Krauss-Maffei along with Sarnamotive Schenk and Rühl Puromer, has developed the SkinForm process, a genuine onestep process that obviates the need for post-finishing. At K 2004, the three partners will be presenting this world novelty on the Krauss-Maffei stand with the aid of a production cell: a seat-belt buckle lock for a luxury car will be moulded on a KM CX injection moulding machine and flooded with a PU skin (trade name: Puroskin). The Production Principle The new technology is based on the principles of classical multicomponent methods. The user selects the most favourable method, e.g. sliding table, rotary table, index plate, core-back or rotary platen, to suit the part geometry, batch size and production costs [2].At the K show, the partners will be using the simple sliding table for the demonstration. The multicomponent technique makes it possible to flood support parts with a soft component, without the melt flowing behind it to the core side of the support. Furthermore, it fulfils all preconditions as regards high process stability and positioning accuracy of the rigid component. In the SkinForm process, a PU mixing head is docked instead of a second injection unit onto the second cavity in the mould. The sliding table of the mould, which is mounted on the fixed side of the clamping unit, has two stations: one for the thermoplastic polymer and one for the PU component. The core side of the mould is attached to the movable side. The mounting for the mixing head for the PU component is located there (Fig. 2). The mixing head could equally be located on the cavity side. In the mould position Injection mould, the support is pre-injected in PA. The part is centrally injected by means of a hot runner and a needle shut-off nozzle. 2 Carl Hanser Verlag, München

3 Fig. 4. The machine is based on the new CX injection moulding machine series; shown here: the KM CX model When the cooling period is over, the mould is opened and the sliding table traverses into the position Cast PU. After the mould has closed, the PU skin is injected through a mixing head connected to the mixing and metering unit into the enlarged cavity. The gate for the PU casting process is a type of film gate. When the reaction period is complete, the mould is opened again. An LR 100 linear robot (manufacturer: Krauss-Maffei) moves into the mould and removes the part from the ejector side. The sliding table then traverses into the Injection mould position and the cycle begins anew (Fig. 3). Communication Between the Injection Moulding Machine and RIM Unit Krauss-Maffei has overhauled the clamping unit and trim as well as the drive of the standard injection moulding machine in the new CX model series (Fig. 4). The machine communicates with the mixing and metering unit via a special interface. This ensures that one support is always flooded during the casting process. The RimStar Minidos small-volume metering unit has been specially developed for low delivery rates. The chosen high-pressure pumps enable metering to be carried out at a rate of just 3 g/s per metering orifice. Because both the dimensions and the container are small, the unit takes up very little space beside the injection moulding machine. The control system measures and logs all necessary process parameters. To ensure constant temperature control over the components as far as the mixing head, the unit has been fitted with heatable high-pressure tubes. Closed-loop control of the tube heater is integrated into the machine control system. A transfer mixing head of type MK 5/ 8 ULKP 2 KVV (Fig. 5) mixes the two PU components. This process has a throughput of g/s; the diameter of the mixing chamber (5 mm) ensures homogeneous mixing. In order that the nozzles, which can be as small as 0.18 mm, do not become contaminated, small filter elements are mounted directly in front of them in addition to the high-pressure filter. The cartridge heaters in the connecting block provide constant heat control over the components. Puroskin is an aromatic, water-borne two-component elastomer casting system. It complies with the requirements V Fig. 5. Transfer mixing head of type MK 5/8 ULKP 2 KVV Fig. 6. The method copes with different wall thicknesses of the PUR skin without increasing the cycle time; this enables the haptic properties and soft-touch effect to be adjusted Analysis of the process cycle Fig. 7. SkinForm CAD machine concept for the KM CX with integrated LR 100 linear robot Fig. 8. Cycle analysis for the SkinForm process using sliding table mould technology 3

4 INJECTION MOULDING Injection moulding Soft-coating of parts Insert moulding In-mould decoration and in-mould lamination of artificial leather and soft-touch film Multicomponent processes, especially overmoulding with TPE/TPU PU cast skin- flooding in two process steps Leather-like haptic properties k k + k /+ + + Scratch resistance Complex, highly curved surfaces with narrow radii + + k k Apertures /k No warpage of decoration or grain k k/+ /k SkinForm process Enhanced surface as thin as possible k + k/+ k/+ + Part with soft touch /k /k + k + + Partial soft touch /k + + No post-finishing No preforms + + k/+ + /+ + Process must lend itself to mass production Hiding of injection moulding flaws k/+ + + Little part warpage + /k k /k /k + + One-shot process + (pre-shape film) (part off-cut) + + Table 1. Every decoration method has its own specific advantages and disadvantages. Process integration and the development of new technology are aimed at producing methods with the maximum possible breadth of advantages + Readily feasible Feasible to a certain extent Unfeasible of PV 1303 concerning the UV resistance of the seating area and with the specified emission values of VDA 278. Foaming of the material generates a soft-touch effect that has been hitherto unachievable and that can be adjusted via different wall thicknesses (Fig. 6). In addition, it is notable for high abrasion and scratch resistance and very good adhesion. The polyol component can be pigmented in virtually all conceivable variations. The crucial advantage of this skin is that no subsequent coating process is needed and, in conjunction with the SkinForm process, flash-free parts are produced. Automation Improves Economics The economics of a production cell depend critically on the degree of automation. The linear robot from the LR 100 series (Fig. 7) used in the demonstration machine is driven via three servo axes that make for parallel axial movement. Once it has been removed, the part is inserted into a stamping unit that separates the film gate of the PU component. The robot then lays the part in packaging units. Each layer contains four positions offset at 180 degrees to prevent the parts from falling over. When each layer is complete, the gripper takes an intermediate layer from a cassette and places it on the buckle cover. The handling-control system is integrated into that for the injection moulding machine. The machine setter can use the robot menu on the console of the injection moulding machine to program and monitor the sequence. The set-down and palletting menu allows the set-down points to be simply programmed without the axial movements. The password hierarchy on the injection moulding machine also applies to the robot menu. The critical areas of the production cell are controlled and safeguarded via space-monitoring points that can be chosen at will. The robot can only move within the areas that are not supervised. The password Fig. 9. Extensive part ribbing on the rear side with ribs roughly 2 mm thick hierarchy and the monitoring points ensure a high level of safety for the production cell. Design Effects Never Seen Before The complexity of the part demonstrates the advantages of the PU skin. The enlarged second cavity for flooding is designed such that the PU skin exhibits certain increases of wall thickness from 0.8 to 4 mm. This causes the material to be partly foamed [3]. Consequently the surface has areas featuring different softtouch effects. Because the reaction time is constant, the cycle time is not extended; this is a crucial advantage over thermoplastic polymer. Different types of grain on the surface ensure that different surface structures in one part can be reproduced exactly. For example, different leather grains can be separated from technical grains by decorative seams. This creates a new design effect that has never been obtained with soft-coating or inmould decoration. Conclusion The one-shot SkinForm technology makes it possible to produce high-quality surfaces by combining injection 4 Carl Hanser Verlag, München

5 moulding technology with reaction process machinery (Fig. 8). The seat trim produced by the SkinForm method at K 2004 meets all the requirements described above. Further advantages for processors are that the reaction time of the Puroskin material has been adjusted to the injection moulding cycle. Foaming of the PU cast skin produces a genuine soft-touch effect which matches that of a free foam. As a result, it is possible to deviate from the usual rib/wall thickness ratio. This opens scope for increasing part safety (Fig. 9). The fully automated process yields flash-free parts with scratch-resistant, leather-like haptic properties and reproduction grain that meets the design specification, without the need for post-finishing, and stays inside the bounds of economic targets. The buckle cover is already the second mould that has been realised by the development team of Sarnamotive Schenk, Rühl Puromer and Krauss-Maffei. The next goal for Sarnamotive Schenk is to produce further products for the interior, such as a shell for a seat-back. Krauss- Maffei is currently debating whether to transfer the technology to side door and pillar trim, underbody modules and dashboard covers. REFERENCES 1 Mitzler, J.; Jansson, H.; Kaufmann, G.: Preforming and In-mould Lamination in one Operation. Kunststoffe plast europe 94 (2004) 3, pp Hunold, D.; Klotz, B.; Heyer, N.: Better Performance for Two-component Technology. Kunststoffe plast europe 93 (2003) pp Rössler, C.: Unpublished studies within a diploma thesis (tutor: Prof. Dr. Dirk Muskat, Fachhochschule Rosenheim 2004) THE AUTHORS DIPL.-ING. (FH) JOCHEN MITZLER, born in 1973, is the product manager for new processes and automation of injection moulding machines at Krauss-Maffei Kunststofftechnik GmbH, Munich. DIPL.-ING. (BA) GEORG BAUER, born in 1978, works in the field of reaction process machinery within the Applied Technology Department at Krauss- Maffei. JÜRGEN EMIG, born in 1957, is head of the PU pilot plant at Rühl Puromer GmbH. DIPL.-ING. (FH) STEFFEN AMMON, born in 1968, is head of the competence center of Sarnamotive Schenk GmbH. DIPL.-ING. MATTHIAS RADY, born in 1969, is responsible for process engineering in the competence center of Sarnamotive Schenk GmbH. 5