Amazingly Thin. Manufacturer

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1 INJECTION MOULDING Photos: Engel Expansion Injection Moulding. According to the state of the art, thin-wall injection moulding requires the injection unit to be supported by a hydraulic accumulator. But without a hydraulic accumulator greater precision is achieved in the process technology, energy consumption is lower and amazingly thin walls can be produced. The new state of the art is called X-Melt, the expansion injection moulding process developed by Engel. It is all made possible by the ability to exert full control over the thermoplastic melt. Amazingly Thin GEORG STEINBICHLER ALFRED LAMPL ANDREAS PÖTTLER F or the injection moulding of very thin parts with wall thicknesses significantly below 1 mm and flow length/wall thickness ratios of up to 450:1, use is (still) primarily being made of injection moulding machines equipped with a hydraulic accumulator system. This is apparently the only way to achieve the injection speeds of 1000 mm/s and above that are needed to fill such mould cavities. At least that is the opinion of the Translated from Kunststoffe 12/2004, pp majority of specialists in this field. But is the assumption correct? What really happens when we accelerate a plastic melt to such a speed during injection? Engel has now been able to throw new light on the i Manufacturer Engel Austria GmbH Ludwig-Engel-Straße 1 A-4311 Schwertberg Austria Phone +43 (0) 50/ Fax +43 (0) 50/ injection moulding of thin wall parts with the findings it obtained from a detailed analysis of the overall process and the cavity filling procedure, working with hydraulic accumulators at injection speeds of up to 1000 mm/s and volumetric flows of up to 2800 cm 3 /s. Contradiction Between Theory and Practice Hydraulic accumulators allow fluids to be stored under pressure. Use is made of the high compressibility of gases, mainly nitrogen, to pressurise the fluid (hydraulic oil) at high pressure in a bladder or ram accumulator. With the aid of highly dy- V Kunststoffe plast europe 12/2004 1

2 INJECTION MOULDING plastic melt, and for elastic deformation of the components of the injection unit. As a result of these investigations, Engel has developed an alternative process for the injection moulding of thin-walled parts and precision micro-components, namely the Engel X-Melt process, which can basically be described as an expansion injection moulding method [4]. Lower Energy Consumption, Greater Precision To fill the mould cavity, X-Melt makes use of the expansion volume that arises on removing the pressure (approx. 10 % at hot runner system, as Engel impressively showed at this year s K (see title photo): In one operation, using a standard allelectric machine, four battery housings of PC/ABS blend just 0.17 mm thick were produced for a mobile phone. The flow length was 30 mm, the process was stable from one shot to the next, and the cycle time was 4.9 s. Such thin parts with a flow length/wall thickness ratio of 200: 1 had never succeeded before with any injection moulding machine using a high-speed process. Thin-wall Injection Moulding: What Actually Happens? Testing with a thin-wall specimen Fig. 1. Test set-up for examining high-speed injection with a hydraulic accumulator: The strip test mould (300 mm x 10 mm x 0.4 mm) is equipped with three pressure sensors (DS 2 to DS 4), and an injection unit with a melt pressure sensor (DS 1) in the screw antechamber namic servo-valves, this energy can then be used, for example, to attain high injection speeds during injection moulding. With so-called high-flow bladder accumulators, hydraulic fluid speeds of up to 140 l/s can nowadays be attained [1]. Tests carried out by Engel on the production of thin-wall parts on injection moulding machines with hydraulic accumulators of this kind have, however, revealed a major discrepancy between theory and practice. There is a significant difference between the volumetric flow calculated from the injection speed and the volumetric flow that is actually introduced into the cavity. The theoretical figures are up to six times higher than the actual data [2]. This allows only one conclusion: When injection moulding with the support of hydraulic accumulators, it is impermissible to deduce the position of the melt front in the cavity from the injection speed (screw advance). There are several reasons for this deviation between theory and practice: the compressibility of the hydraulic fluid, the elastic deformation of the machine parts and the compressibility of the plastic melt [3]. Only part of the generated energy is converted into work in order as desired to overcome the filling resistance in the cavity.a large proportion of the energy is consumed unproductively for accelerating the melt, for compressing the oil and the 2000 bar) from a thermoplastic melt that has been compressed under high pressure, in other words without the screw advance that is needed with conventional injection moulding, and thus also without a hydraulic accumulator. In expansion injection moulding, the pressure on the compressed melt (which is blocked off from the mould cavity with a needle valve nozzle) is suddenly removed so that it expands explosively and fills the cavity in fractions of a second. This new Engel technology consumes less energy and is more precise from a process engineering point of view than the traditional highspeed injection moulding with a hydraulic accumulator. With X-Melt, it is now also possible to perform multi-cavity applications with a As an example, we will look at the results of tests carried out on a thin-wall specimen having a wall thickness of 0.4 mm, a width of 10 mm and a flow distance of 150 mm. The tests were carried out on an Engel Victory 200/120 injection moulding machine (clamping force: 1200 kn) equipped with a hydraulic accumulator and a barrier screw (D = 25 mm); the material was a conventional polystyrene injection moulding grade (PS 143 E; manufacturer: BASF). Of the 3.94 g shot weight, the part accounted for 1.24 g and the sprue for 2.7 g. To record the flow front development during the process, the mould was equipped with three pressure sensors (DS 2 to DS 4), while a further pressure sensor (DS 1) measured the melt pressure in the screw antechamber (Fig. 1). The hydraulic accumulator was set to a pressure of 160 bar.with a transmission ratio between the effective injection plunger surface area and the screw crosssection of 15.9:1, this is equivalent to a specific injection pressure of 2545 bar. Fig. 2 shows details of the measured process parameters. After opening the machine shut-off nozzle, the sprue is first filled at a mean injection pressure of approx. 530 mm/s. When the melt front reaches the mould cavity, the flow resistance and melt pressure at DS 1 increase rapidly to a maximum of 2300 bar. The screw advance is then completed, but the cavity is at this point far from being completely filled, as the local pressure development in the cavity shows: position DS 2 is reached with a melt pressure in the screw antechamber (DS 1) of just under 850 bar, DS 3 is reached a hundredth of a second later at 1900 bar in the screw antechamber, and cavity position DS 4 only after the screw has remained at its final position for s (Dt in Fig. 2). After 68 ms,the screw has displaced the entire melt volume. More than 75 % of 2 Carl Hanser Verlag, München Kunststoffe plast europe 12/2004

3 INJECTION MOULDING Process parameters Fig. 2. Process profile for thin-wall injection moulding with hydraulic accumulator (test strip with thickness of 0.4 mm), shown by the measured melt pressures in the screw antechamber (DS 1) and along the flow path in the cavity (DS 2 to DS 4) this time (47 ms) is needed solely to fill the sprue. However, the remaining 21 ms until the end position of the screw is reached account for only just under 50 % of the time needed to volumetrically fill the cavity (without the sprue), namely 43 ms! This temporary volume shrinkage can only have been absorbed by compressibility, acting more or less as an intermediate energy and melt accumulator. Through the rapid pressure build-up with the hydraulic accumulator, the plastic melt is compressed above all in the screw antechamber and the nozzle area. At the same time, however, the hydraulic fluid in the injection cylinder also becomes compressed and the parts of the injection unit under pressure become deformed. The latter has a not insignificant influence on the course of the process (for details of this, see the section entitled: Expansion Injection Moulding with Process Model Simulation, p. 6). A calculation shows that the quantity of melt displaced by the screw is responsible for partial filling of the cavity and for the compression volume. After the screw comes to a standstill and the maximum injection pressure of 2300 bar has been reached, the accumulated compression energy ensures the subsequent volumetric flow into the mould cavity until the cavity has been theoretically filled at a melt pressure of 1000 bar. The subsequent pressure decrease results from the balancing out of the pressure differences existing between the cavity and the screw antechamber at the end of the filling process. The mean flow front velocity when filling the cavity does not remain constant either, but decreases rapidly as the filling level rises. In the filling process up to DS 3, it is still around 5000 mm/s, but drops to nearly half (approx mm/s) by DS 4. These test results show clearly that the filling of the cavity when injection moulding very thin parts with a hydraulic accumulator occurs for the most part without any screw advance, Expansion injection moulding solely through expansion of the pressurised melt. Tests with delayed opening of the machine shut-off nozzle (both time and pressure-controlled) produced similar results. Similar tests on an all-electric Engel E-Motion 200/100 injection moulding machine (clamping force 1000 kn) showed that the strip-shaped cavity (wall thickness 0.4 mm, flow length 150 mm) can also be filled solely by removing the pressure from the previously compressed melt in the screw antechamber. A hydraulic accumulator is not needed for the expansion injection moulding process. A sufficiently large melt volume was compressed to 2500 bar in the screw antechamber with the machine shut-off nozzle closed. At the same time, the sprue weight was reduced to 1g using a longer machine shut-off nozzle and a shorter sprue bush in the injection mould. The pressure decrease and volumetric flow development after opening the shut-off nozzle are shown in Fig. 3. A control measurement of the flow front velocity in the area of the pressure sensors produced a high level of conformity with the calculated data. Expansion Injection Moulding: Many Advantages, New Possibilities Expansion injection moulding can be divided into two process steps, namely com- Fig. 3. Pressure profile in the screw antechamber and volumetric flow in the gate and in the cavity (test strip with thickness of 0.4 mm) during the expansion phase V Kunststoffe plast europe 12/2004 3

4 INJECTION MOULDING Fig. 4. Production cell for manufacturing thinwalled battery housing parts for mobile phones using the X-Melt technology, consisting of an all-electric E-Motion 200/55 injection moulding machine, high-speed parts removal device ERS-1 and an Engel ERC 23/1-FH linear robot for the systematic stacking of the parts in a tray server pression and expansion. For this, the injection unit must be equipped with a controllable shut-off nozzle or the mould must have hot runner shut-off nozzles. From the process engineering point of view, the variant with the shut-off nozzle in the hot runner is preferable because the pressurised melt accumulator can move as close as possible to the mould cavity with the result that only low pressure losses occur in the dynamic filling phase. This makes it possible to achieve the short filling times necessary for the extremely thin walled injection moulded parts. Since the compression phase brings about an increase in the melt temperature, the processing temperature in the plasticising unit can be reduced (on average by around 15 % compared with injection moulding with a hydraulic accumulator). For compression of the melt with the injection unit, all the time it takes for all the mould movements, for removal of the part to the next build-up of the clamping force, is freely available. For this reason, the performance of an injection unit equipped with servo-electric drives does not have to be particularly high. The energy needed to fill the cavities is stored in the melt and the pressurised machine parts. For the injection phase, the shut-off nozzles are opened and the cavities start to fill up without any movement of the screw whatsoever. The servo-electric drive unit of the injection device guarantees the consistency of the preloaded melt volume, because with this, the screw can (due to the system) be axially positioned as desired and kept exactly in this position even under high pressure and sudden pressure changes. The volumetric flow required to fill the cavity it changes during the filling time is governed by the preloaded pressure, the accumulator volume and the flow resistances. From this it is possible to derive an effective venting profile for the cavities, which is an important prerequirement for a fast cavity-filling process. Equally important for a thin-walled project is what filling time will be necessary from a process engineering point of view to fill the planned thin-wall part in the first place. This optimum filling time can be approximated with a simple model for thin-walled injection moulded parts at a constant flow front velocity, also taking into account the frozen skin layer. To determine the thickness of this skin layer, the equations of Dietz and White are used [5, 6]. The optimum filling time t F at minimum pressure requirement is, by approximation: (1) where s = wall thickness of the part and n = flow index of the melt. B takes account of the solidified skin layer thickness δ: and (2) (3) where s = change in the wall thickness, t c = contact time, T NO = no-flow temperature, T W = cavity wall temperature, T M = melt temperature and a eff = effective temperature conductivity. For the two examples described in this article, we get the following filling times: For the 0.4 mm thick strip ( mm 3 ) of PS 143 E (parameters: T M = 220 C; a eff = mm 2 /s; T NO = 110 C; n = 0.24; T W = 30 C), the ideal filling time t F = s. For the 0.17 mm thick battery housing ( mm 3 ) of PC/ABS blend (parameters: T M = 280 C; a eff = mm 2 /s; T NO = 140 C; n = 0.51; T W = 80 C), the ideal filling time t F = s. The results conform very well with the empirical data. A specialist in this field will quickly recognise from these data that an injection moulding machine with a hydraulic accumulator would fail with this 0.17 mm battery housing because such a machine configuration will allow, at best, filling times of minimal 0.05 s, in other words similar to those needed for the 0.4 mm test strip described in detail at the beginning. With the X-Melt process, however, 0.17 mm and below can be reliably achieved in a four-cavity mould. Thin Walls and High Reproducibility The ultra-thin 0.17 mm battery housing made of a PC/ABS blend (Bayblend FR 2000; manufacturer: Bayer MaterialScience), which Engel produced at K 2004 in a fully automated production cell (Fig. 4), is a world first in thin-wall injection moulding. The parts are gated via four submarine gates with sub-runners and a hot runner system with a needle valve (Fig. 5). Pressure sensors in the plasticising cylinder (screw diameter 25 mm), in the hot runner nozzle and in the mould record the pressure profiles at the measuring points. After compressing the plastic melt to 3000 bar, a pressure equalisation of the compressed melt takes place before the needle valve nozzles in the hot runner open (Fig. 6). Abruptly, within the space of just 6 ms, the preloaded pressure in the hot runner drops from 2860 bar to 2180 bar. The resultant volumetric flow fills the cavities up to the pressure sensor position in the mould (approx. 90 % filled). In this first, extremely fast filling phase, the pressure in the screw antechamber remains virtually constant. The pressure equalisation that then takes place between the screw antechamber and the hot runner nozzle leads to a build-up of pressure in the hot runner and, as a result, complete filling of the four cavities. On switching over to holding pressure, the expansion effect of the melt is clearly recognisable by the change in the screw position. As described above, the optimum filling time for the 0.17 mm thick battery housing part is just 33 ms. The basic requirement for high and uniform part consistency in all cavities is, however, absolute synchronous filling of the cavities. Together with the hot runner manufacturer, Mold-Masters Europa GmbH, Baden-Baden/Germany, the hot runner system was optimised in such a 4 Carl Hanser Verlag, München Kunststoffe plast europe 12/2004

5 INJECTION MOULDING way that the needle valve nozzles open at exactly the same time (controlled via a common frame synchronising all four nozzle movements). Individually controlled needle valves are unable to satisfy the requirements of expansion injection moulding. The process stability and reproducibility attainable with X-Melt can be documented graphically and quantitatively with the process monitoring system integrated in the CC 200 machine control unit. It continuously monitors, for example, the pressures in the individual mould cavities (Fig. 7). With the thin-wall battery housing component, fluctuations of only ±15 bar were measured between the four cavities and of only ±10 bar in one cavity from one shot to the next. At a mean cavity pressure of 480 bar, this is equivalent to fluctuations of ±3 % and ±2% respectively. This high reproducibility is also reflected in the weight of the moulded parts. Consistent Further Development of Thin-wall Injection Moulding The conventional thin-wall injection moulding process using an injection unit with a hydraulic accumulator is only in competition to a limited extent with the X-Melt expansion injection moulding process. As stated at the beginning, the mould cavity, even with the conventional process, is always only filled to a certain extent by expansion of the melt. However, because melt compression and injection take place in parallel in one operation with the machine nozzle already opened, Fig. 5. Battery housing of a mobile phone made of ABS+PC with a wall thickness of 0.17 mm, produced using the X-Melt technology with a melt accumulator in the needle valve hot runner nozzle; 1: hot runner needle valve nozzles, 2: tunnel gate with barrier plate filling of the cavities takes longer. As a result, there are certain restrictions with this process with regard to the minimum possible part thickness, even with such high drive-energy application. X-Melt decouples the compression from the injection more or less as a consistent further development of the thinwall injection moulding process with a hydraulic accumulator. The result is a better command of the overall process and thus higher process reliability. During injection, higher flow front velocities can be achieved from the very beginning and, consequently, thinner parts. At the same time, the energy balance with expansion injection moulding is significantly more favourable than with thin-wall injection moulding using a hydraulic accumulator. The inefficient acceleration of melts and the displacement of the fluid from the hydraulic cylinder as in conventional thinwall injection moulding are eliminated completely. X-Melt significantly improves the reproducibility of the process parameters during injection moulding, as a result of which the precision is increased, especially with small parts. Typical shot weights are between 0.1 and 20 g, and wall thicknesses of 0.8 mm down to approx. 0.1 mm (with flow length/wall thickness ratios of up to 400:1). X-Melt can be used with engineering plastics V Pressure profile at three measuring points Fig. 6. Compression and expansion of the melt for filling the cavities (battery housing for mobile phone) shown with the aid of typical pressure curves in the screw antechamber, in the hot runner melt accumulator and in the cavity Kunststoffe plast europe 12/2004 5

6 INJECTION MOULDING! Expansion injection moulding with process model simulation This study is based on a simple theoretical system for a servo motor-driven injection unit and a mould, consisting of a drive spindle, plasticising cylinder with injection plunger, shut-off injection nozzle and mould cavity (Fig. 8). The screw is Geometric model moved by the drive spindle and compresses the melt in the screw antechamber to the required pre-loaded pressure p 1 with the hot runner nozzle closed. On opening the injection nozzle, melt flows into the mould cavity and the pressure in the screw antechamber falls. This change in the melt pressure in the screw antechamber can be calculated in stages by taking into account the expansion of the plastic melt and the elastic deformation of the relevant parts of the injection unit: where p 1x = pressure in the cylinder; p 1 = final compression; V F = volume of the moulded part; V 1 = volume of the compressed melt; κ k = compressibility of the melt; V m = fictitious volume of the deformed parts of the injection unit; E = elasticity modulus of steel. For every volumetric change V F, the corresponding reduction in pressure is determined and the pressure dependence of the melt compressibility taken into account. With the known dimensions of the plasticising cylinder and the screw, it is possible to determine the elastic deformations and the elastic volumetric changes as a function of the prevailing pressures. The total volumetric changes enable a fictitious volume to be determined, which, multiplied by the reciprocal of the elasticity modulus of steel and the prevailing pressure, gives the corresponding elastic volumetric change. For example, to establish the influence of the elastic deformation of the components in expansion injection moulding, a commonly used screw Fig. 8. Theoretical system for an injection unit with electrical spindle drive and mould cavity with hot runner needle valve nozzle; 1: Servo drive, 2: Drive spindle, 3: Screw (length L K, diameter D 1, cross sectional area A 1 ), 4: Screw antechamber (starting volume V 1, pressure p 1 ), 5: Hot runner needle valve nozzle (length L D, pressure loss coefficient K D ), 6: Mould cavity (volume V F, pressure loss coefficient K F, max. flow length L F, instantaneous flow length x) with a diameter of 25 mm and an overall length of 945 mm (shank length 325 mm + screw section length 620 mm) is used as an example. With a mean core diameter of 18.8 mm and a shank p-v diagram diameter of 23 mm we get a mean stressed cross sectional area along the length of the screw of 3.25 cm 2. For a preloaded pressure of 2500 bar, an injection force of 123 kn is required. This produces a compressive stress of σ = 378 N/mm 2 and thus a screw compression of 1.7 mm. In the expansion phase with a pressure drop of 1000 bar, we get an elastic recovery of 0.7 mm, equivalent to a volumetric change of cm 3. With the 0.17 mm thick battery housing described in this article, this is around 16 % of the volume of the moulded part (sprue plus part). Together with the deformation of the drive spindles and the thickwalled pre-plasticising cylinder, it means that elastic deformation of the machine parts accounts for 21% of the full expansion volume! After opening the injection nozzle, a volumetric flow develops that can be calculated with the constantly changing pressure p 1x in the screw antechamber and with the usual equations for pressure losses in the flow of thermoplastic melts. For this, the pressure loss coefficients of the nozzle, the runner system and the cavity, which are dependent on the constructional design, must be known. It must be borne in mind that the flow resistance of the cavity increases with the filling level. The shear rate and thus the viscosity is different in the individual flow sections. The volumetric flow for the simple theoretical system is determined step by step as a function of the pressure p 1x in the screw V (4) Fig. 9. Change of isothermal and adiabatic state for polystyrene. T A : starting temperature; T E : end temperature Isothermal and adiabatic compression (Spencer-Gillmore equation): (v - b*) (p + p*) = RT/W; Temperature change with adiabatic change of state: T = b* p/cp; b*: specific inherent volume of the macromolecule [cm 3 /g]; p*: cohesion pressure [bar]; W: molecular weight [g/cm 3 ]; R: universal gas constant [J/(Mol K)]; c p : specific heat [J/(g K)] 6 Carl Hanser Verlag, München Kunststoffe plast europe 12/2004

7 INJECTION MOULDING antechamber and the pressure losses in the nozzle and mould cavity: where V X = volumetric flow at filling level x; p 1x = pressure in the plunger antechamber at filling level x; x= filling level of the cavity; Κ D = pressure loss coefficient of the nozzle ( for pipe-shaped cross-section with a length L and radius R); Κ F = pressure loss coefficient of the cavity ( for rectangular slit cross-section with a width b and slit thickness s); η D = viscosity in the nozzle area; η F = viscosity in the area of the cavity. (5) variables is described by the Spencer-Gilmore equation of state [7], the structure of which corresponds to the van der Waals equation of state for real gases. Equations of state for liquids and solids, unlike those for gases, have more the character of interpolation formulae that are valid only in a limited range. For polystyrene, starting with the initial state, the isothermal and adiabatic changes of state were calculated step by step and plotted in the p-v diagram (Fig. 9). With the isothermal change of state, the primary state variable T is kept constant. In this way, we get to the isothermal compressibility, which is defined as the volumetric change per pressure unit (bar, Pa) at constant temperature, related to the existing volume (Fig. 10). With adiabatic processes, heat is neither added nor removed. In practice, this always happens when processes take place so quickly that, even without insulation, no temperature equalisation takes place through heat conduction. With adiabatic compression, the temperature rises (with PS, approx. 30 C per 1000 bar). With expansion injection moulding, the compression of the melt nevertheless takes place slowly so that the temperature of the melt rises only slowly due to a polytropic change of state. The expansion, on the other hand, occurs extremely fast and thus without any significant thermal conduction. The isothermal and adiabatic compressibilities can be determined from the relationship between the state variables. They change with pressure. Calculation of the pressure change in the expansion phase must therefore be performed in stages for small volumetric changes. With hydraulic fluid, the change in compressibility in the pressure range up to 300 bar proceeds linearly, the change in the temperature range from 20 to 70 C is negligible. Gas-free oil has a compressibility between and bar -1. With calculations, a compressibility of κ = bar -1 is normally assumed for hydraulic fluid (not absolutely gas-free). The compression module K =1/κ is also often indicated (K = bar). In other words, oil is 130 to 150 times more elastic (more compressible) than steel. Basically, all non-porous bodies behave like elastic bodies under equally isotropic pressure on all sides [8]. The volumetric change can be calculated from the equation for pressure difference: Compressibility (6) If, in a limited range, we assume the compression module K to be constant, then integration and subsequent series expansion if V/V is small produces the familiar equation, taking into account only the first link: (7) Fig. 10. Isothermal and adiabatic compressibility for polystyrene. The diagram shows that, with PS, raising the preloaded pressure above approx bar generates hardly any additional compression volume of the melt that is usable for expansion injection moulding The volumetric flow with the nozzle closed is zero and grows constantly during the time until the valve opens completely (5 to 10 ms) to reach the value of the open valve. While melt flows into the cavity, a pressure difference exists between the screw antechamber and the tip of the nozzle, which is dependent on the pressure loss coefficient of the nozzle. The pressure equalisation takes place when the cavity is full or the nozzle is shut off. Relevant Thermodynamic Principles for Expansion Injection Moulding The state of a single-phase system is, in the thermodynamic sense, determined by the statevariable temperature, pressure and volume, because the melt and chemical composition remain unchanged. The state variables are related to one another as described by a so-called equation of state. For thermoplastics in the molten range, the connection between the state (8) Since, with expansion injection moulding, only small restricted areas can basically be allowed for the change of V or p, the relationship between volumetric change and pressure change can be calculated with sufficient accuracy. The pressure-related compressibility of the plastic melt is considered and the compressibility of the hydraulic fluid is assumed to be constant. The high injection pressures necessitate accurate calculation of the machine parts of the injection unit so that long-term strength data are not exceeded. The elastic deformation of the parts subject to the working pressures must be taken into account in an analysis of the pressure changes and volumetric flow development. With injection moulding machines having an electric drive system, not only the screw and plasticising cylinder but also the drive spindles for moving the screw are subject to high stresses. Kunststoffe plast europe 12/2004 7

8 INJECTION MOULDING Fig. 7. High process reproducibility with X-Melt, documented by monitoring with the new Engel CC 200 machine control system top: pressure curves in the four cavities filled in parallel; below: pressure curves for 50 shots in one cavity Leobener Kunststoff-Kolloquium, Nov Michaeli, W.; Bourdon, K.; Haupt, M.; Hunold, D.; Roberts, T.: Das Volumenstromübertragungsverhalten in der Einspritzphase. Plastverarbeiter 41 (1990) 9, pp Pokorny, P.: Brachliegende Potenziale genutzt: Expansionsspritzgießen eine neue Idee in der Dünnwandtechnik. Kunststoffe 91 (2001) 7, pp Dietz, W.; White, J.: Ein einfaches Modell zur Berechnung des Druckverlustes während des Werkzeugfüllvorganges und der eingefrorenen Orientierung beim Spritzgießen amorpher Kunststoffe. Rheologica Acta 17 (1978) 6 Dietz, W.; White, J.; Clark, E. S.: Orientation development and relaxation in injection moulding of amorphous polymers. Polymer Eng. & Sci. 19 (1979) 7 Spencer, R.; Gillmore, G. D., in: J. Appl. Phys. 21 (1950), p Reiner, M.: Rheologie in elementarer Darstellung. Hanser, München 1968 such as PC, PBT, LCP, PA, POM and ABS+PC. Suitable machines for expansion injection moulding are all-electric machines of the Engel E-Motion and hybrid machines of the Engel Victory electric series. REFERENCES 1 N. N.: Company brochure: Firmenschrift über Hydrospeicher. Hydac International GmbH, Sulzbach, Steinbichler, G.: Präzisionsspritzgießen Maschinen- und prozesstechnische Besonderheiten. 17. THE AUTHORS DIPL.-ING. GEORG STEINBICHLER, born in 1955, is head of Research & Development at Engel Austria GmbH, Schwertberg/Austria; georg.steinbichler@engel.at PROF. DIPL.-ING. DR. MONT. ALFRED LAMPL, born in 1931, is the retired Technical Director of Engel Austria GmbH, Schwertberg/Austria. ING. ANDREAS PÖTTLER, born in 1979, is Technology Manager for the X-Melt process in the applications technology department of Engel Austria GmbH; andreas.poettler@engel.at 8 Carl Hanser Verlag, München Kunststoffe plast europe 12/2004