Streets ahead of Injection Molding

Transcription

1 One-meter long strips in ABS with a Y cross-section and end caps (photos: IB Steiner) Streets ahead of Injection Molding Exjection. A newly-developed process combines the advantages of extrusion and injection molding. Long profiles with integrated mounting, closure, reinforcing and decorative elements can be economically produced in a single step via a single injection point without the flowability of the plastic being a limiting factor. It is precisely these additional elements that cannot be achieved when profile-like plastic parts are produced by extrusion. PE GOTTFRIED STEINER HERBERT EICHLER I B Steiner engineering consultants and Hybrid Composite Products GmbH, both of Spielberg/Austria, have jointly developed a new technology for producing strips and profiles, taking three years to the point of the first successful test run. The process, designated exjection, is now being launched on the market, following its world premiere at K2007 [1, 2]. The process a combination of extrusion and injection molding is ideally suited to the production of long, thin-walled and structured parts with a low clamping force, including parts in viscous thermoplastics. End caps and functional elements, such as snapfit hooks and screw bosses, are shaped on the profile as it is produced. Potential Translated from Kunststoffe 4/2008, pp users are to be found primarily in the construction and furniture industries, in the automotive and aviation sectors and also in mechanical engineering. Specific examples of products that could be series-produced in this way include building and furniture profiles, LED lighting strips, lamp covers, cable ducts, cable binders and decorative strips with a metallic effect. Products Offering more Functions at a Lower Price The exjection process offers the user technical and economic benefits. The investment in systems and equipment is roughly half that required for classical injection molding, and the production costs are up to 30 % lower. The cost benefits of the new process have been set out on the basis of the piece costs for an ABS strip (Title photo) with the following key data: component geometry with a Y crosssection and end caps, component length 1,000 mm, cross-section 10 mm 16 mm (W H), wall thickness 1.5 to 1.8 mm, annual requirements 100,000 pieces or running meters. A number of different processes were compared with each other (Table 1). The shorter cycle times for conventional injection molding are more than offset by the higher hourly rates for the production plant. Despite the higher mold costs, the exjection process offers the lowest production costs of EUR 1.24/piece when the injection molding processes are compared. When strips are made from extruded profiles, the finishing stages (gluing on the end caps, grinding, painting) entail considerable extra costs, which can be saved with the exjection process. For profiles without functional elements and without surface structures, extrusion is still the most cost-efficient 24 Carl Hanser Verlag, Munich Kunststoffe international 4/2008

2 process. If use is made of high-temperature thermoplastics and there is only a low annual demand, however, the balance will tip in favor of the exjection technique here, too. The cost-efficiency of the process goes hand in hand with advantageous component properties. A large number of materials were sampled on the test mold in close cooperation with the raw materials manufacturers. It proved possible to produce parts measuring 935 mm 10 mm 16 mm (L W H) with a mean wall thickness of 1.5 mm both in commodity plastics (ABS, SAN, PP) and in engineering plastics (including ionomer, POM, PBT, PC, PMMA, PA6 and PA66), as well as in a thermoplastic polyurethane elastomer (TPU). Tests with the high-performance plastics PEI (grade: Ultem; Process long-fiber reinforced and also mineralfilled grades, special-effect pigmentation) all show a uniform distribution of the fillers and pigments over the length of the part. Hot-runner nozzle Mold insert, nozzle side Functional nozzle insert with sword- shaped bar Cavity Shaping jaws Mold insert, ejector side Ejector Carriage Conventional injection molding Cascade injection molding Fix Movable Cycle time Fig. 1. Cross-section of the test mold in the region of the gate with the key mold components required for the process Mold costs [EUR] Manufacturing costs [EUR/piece] 35 s 67, s 87, Exjection 44 s 100, Extrusion (only profile) 180 rm/hour 15, Extrusion with finishing work 180 rm/hour 25, Table 1. Calculation of the piece costs for a 1 m long strip in ABS with the cycle times and investment requirements for different production processes The Gate Follows the Flow Front Once the mold has been closed, the exjection process starts, in the same way as for conventional injection molding, with the injection of the plastic into the cavity. The mold carriage is in its starting position. The entire component geometry is carved into the carriage (Fig. 1). A swordshaped bar on the fixed nozzle-side of the mold partially seals the otherwise open cavity and acts as an extrusion die with a calibration unit. The melt is injected into the cavity via a hot-runner nozzle, filling the end region and flowing away from the gate into the open cavity. A filling pressure of several hundred bar develops at the nozzle in the customary manner. The carriage starts to move counter to the injection direction even before the melt has passed through the heated supply section that is about 100 mm long and emerges into the open air. The injection process is primarily pres- V manufacturer: Sabic Innovative Plastics), PPSU (grade: Radel R; manufacturer: Solvay GmbH) and PEEK (grade: Victrex PEEK; manufacturer: Victrex plc) produced strips in a quality that cannot be attained through the conventional form of production by injection molding or extrusion. A large number of special injection molding techniques can be implemented with exjection, including in-mold labeling, the hybrid technique and multi-component injection molding. Assembly costs can be saved through decoration with films, materials and wood veneer, as well as through the full or partial encapsulation of insert parts made of glass, metal, wood and fiber-reinforced plastics. Inmold lamination and the encapsulation of a wooden insert have already been tried out in the course of process development. The tests with filled materials (short and Variothermic element Internal pressure of the mold p i p max 0 Hot-runner nozzle Supply zone Flow front Frontal flow Filling pressure 0 Solidified surface layer Fix nozzle side Calibration zone Holding pressure Path s Carriage movement Movable ejector side Fig. 2. Filling behavior in the exjection process with a free flow front in the supply zone, the effectiveness of the holding pressure and the cooling in the calibration zone (schematic diagram) Kunststoffe international 4/

3 sure-controlled, with the flow front velocity and the carriage movement being harmonized. This means that the gate follows the flow front. During the filling operation with frontal flow, a uniform pressure profile develops on the basis of the melt viscosity (Fig. 2). This state is a virtually stationary one, and hence, with a constant cross-section, it is independent of the length of the part to be produced. The carriage advance and the injection profile are controlled over the length of the part on the basis of setpoint values and velocity profiles. The controlled variables are the profile cross-section and the part thickness, which determine the cooling rate and hence the carriage speed as a function of the material and the process temperatures. With a cross-section that is modified by functional elements or a variable wall thickness, the carriage speed and/or the injection speed are suitably adapted, thus permitting homogeneous filling of the cavity. Supply zone, approx. 65 mm Material ABS (Terluran) Injection pressure 250 bar Calibration zone, approx. 200 mm 200 mm Pressure Time = s [MPa] Y X Fig. 3. Pressure profile in the cavity for the control strip part in ABS, calculated with Moldflow Plastics Insight (MPI) When the Filling Pressure Acts as Holding Pressure The melt cools down on the cavity surface, proceeding from the flow front, and is moved back to the gate together with the cavity in the carriage. In the course of this process, the filling pressure that acts as the holding pressure is built up in the filled cross-section (Fig. 2), peaking at the gate. Filling pressures (holding pressures) of between 50 and 200 bar develop as a function of the viscosity of the melt, the filling speed and the wall thickness of the Project Partners for the Exjection Process part. These ensure good reproduction of the structure and also mean that sink marks are avoided. Each volume element in the part thus runs through a time-dependent holding pressure profile until the melt solidifies. After this, the extruded surface leaves the calibration zone of the nozzle insert, which can be up to 300 mm long, and emerges into the open air. The minimum width for this surface is the same as the gate cross-section, it can, however, be made wider. Shortly before the gate reaches the end of the cavity, the carriage is slowed down Arburg GmbH & Co. KG Machine manufacturer Böhler Edelstahl GmbH & Co KG Mold steel CAD-Plast GmbH Mold construction, production Dema Engineering GmbH Design, mold construction Engel Austria GmbH Hasco Hasenclever GmbH + Co KG Process analyses, machine manufacturer Standard mold components/ hot runner HTBL Kapfenberg Process analyses Hybrid Composite Products GmbH IB Steiner Process development and part production Process development and process creation Kistler Instrumente AG Measuring technology Moldflow Corporation Process simulation Oerlikon Balzers Coating Austria GmbH Mold coating For further information and contacts: and brought to a standstill. The end zone of the part is filled through a conventional injection molding cycle, shaped and brought to solidify. After the metering operation has been completed, the mold can be opened, the carriage returns to its starting position, and the part can be demolded, ejected and removed. The overall cycle time is equivalent to an injection molding process with cascade control, with the molding being filled from one end of the part (see Table 1). Close cooperation with Moldflow Corporation, Framingham, Massachusetts, USA, made it possible to simulate the process in rheological terms and hence to analyze the individual process steps. The pressure profile in the cavity for the control strip part shows a falling pressure in the direction of flow, starting at the gate (filling to the flow front) and in the direction of the carriage movement (holding pressure to solidification) (Fig. 3). Outside these two boundaries, the mold can be open. The area shown is employed for dimensioning the supply and calibration zone. An expert model based on Moldflow Plastics Insight (MPI) was used for the investigations. The suitability of the simulation tool is verified by systematically comparing the computed results with the parts produced. Self-contained Competence for Series Implementation The companies involved in building the test mold were mold makers CAD-Plast 26 Carl Hanser Verlag, Munich Kunststoffe international 4/2008

4 Fig. 5. Test mold (vertical) with an integral drive unit on an all-electric injection molding machine, model e-motion 200/55 (photo: Engel) GmbH, Aichach, Germany, mold steel specialists Böhler Edelstahl GmbH & Co KG, Kapfenberg, Austria, standard mold component and hot runner manufacturers Hasco Hasenclever GmbH + Co KG, Lüdenscheid, Germany, coating specialists Oerlikon Balzers Coating Austria GmbH, Kapfenberg, Austria and pressure-measurement experts Kistler Instrumente AG, Winterthur, Switzerland (see box on page 26). This meant it was possible to acquire the know-how necessary for the construction of series molds at the process analysis stage already. The molds currently being designed and constructed at CAD-Plast and Dema Engineering GmbH, Wolfsberg, Austria, are being made with process-relevant mold components for the nozzle side that have been developed and built by Hasco. The first set of standard mold parts for series production, which has already been optimized in functional terms, was installed in the test mold at the end of February 2008 and put through a comprehensive functional test. This unit is made up of a hot runner tailored to the process and two extrusion elements with a swordshaped geometry that can be fitted into the mold (Fig. 4). This sword-shaped bar is designed according to the same principles as for an extruder, with a variotherm filling zone (supply zone). The solidification zone is cooled and constitutes the calibration zone. V Kunststoffe international 4/

5 Fig D-view of the mold (horizontal) for a control strip, with a drive unit on the fixed lower clamping platen of an Arburg Allrounder 375 V (picture: Arburg) The process analyses and the process optimization were conducted at the Kapfenberg Higher Technical College, Kapfenberg, Austria, and at Engel Austria GmbH, Schwertberg, Austria. Engel was the first injection molding machine manufacturer to recognize the high potential of the process and presented this world first, together with IB Steiner and Hybrid Composite Products, at the plastics fair K What was shown was the production of the strips in the title photo in the test mold similar to the production mold on an Engel e-motion 200/55 with a clamping force of 550 kn (Fig. 5). The all-electric machine design fits in well with the control requirements of the process. Compared with conventional injection molding machines, the clamping forces required for the new process are comparatively low. The plasticizing unit is relatively big as a result of the high shot volume, whereby intrusion and alternatelyoperating injection units with two cylinders are two different solution approaches that can be adopted here. The mold does not have to be installed in a vertical position as shown at K 2007 (Fig. 5) but can also be of a horizontal design for long parts. Further selection criteria are the space available at the production location, the degree of automation and the machine concept. The Allrounder V series with a vertical clamping unit from Arburg GmbH + Co KG, Lossburg, Germany, offers the advantage of a horizontally moved carriage module that is mounted on the fixed lower clamping platen. With this design, the carriage length, the carriage stroke and hence the part length can be freely selected. Reaching into the mold to remove the part is very easy. For the Arburg Technology Conference at the start of April 2008, IB Steiner, working in close cooperation with the injection mold manufacturer, designed and built a mold for the production of a control strip for use on a vertical machine the Arburg Allrounder 375 V. The drive unit for the carriage movement was integrated in the machine. An adjustable coupling unit means that the servoelectric linear drive can be used for more than one mold (Fig. 6). Prospects A number of companies already secured a technological lead for themselves prior to K 2007 by signing production licenses for the exjection technique. Work is currently underway with these companies on implementing the initial series parts. As far as the requisite molds are concerned, finished designs are available on the basis of different mold concepts. The process know-how necessary for the production of series parts is being constantly expanded in test series. The methods and guidelines for the layout of exjection parts are based on empirical values acquired over a period of several years already, using state-of-the-art CAX tools. The continuous further development of the process and the property rights applied for and granted will ensure a competitive lead for all the project partners in future, too. Fig. 4. Set of standard mold components with functional inserts for the nozzle side of the mold (picture: Hasco) ACKNOWLEDGMENTS The project entitled Exjection The Production of Profiled Plastic Parts by Injection Molding is being conducted in the context of the branch initiative BRA.IN Kunststoffwirtschaft der Österreichischen Forschungsförderungsgesellschaft mbh (FFG), Vienna. Without the financial support granted by the FFG and the Steirischen Wirtschaftsförderungsgesellschaft mbh (SFG), Graz, Austria, for feasibility studies and for promoting the start-up, it would not have been possible to carry out this research project with its overall budget of EUR 1.4 million. REFERENCES 1 Sambale, H.: Injection Molding of Profile-like Plastic Parts. Kunststoffe international Technology Report at 09/ Exjection: Kunststoffe international 97 (2007) 8, p. 35; Kunststoffe international 97 (2007) 9, p. 20; Kunststoffe international 97 (2007) 12, pp. 33 and 38 THE AUTHORS DIPL.-ING. GOTTFRIED STEINER, born in 1958, founded IB Steiner engineering consultants in Spielberg/Austria in The main research and development focus is on the fields of product creation, aircraft interiors, wood and plastic, and also innovative technologies; g.steiner@ibsteiner.com DIPL.-ING. HERBERT EICHLER, born in 1966, has been quality manager and project engineer for assemblies and technology developments at IB Steiner engineering consultants since 1996; h.eichler@ibsteiner.com 28 Carl Hanser Verlag, Munich Kunststoffe international 4/2008