Process Variants for Manufacturing Optical Parts

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Ashley Butler
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1 Transparent Pressure A Quality Criterion for Optical Parts. In injection moulding of optical parts, there are limitations to the direct measurementofcavitypressure, since marks on the surfaces are taboo. But to The sensors are introduced into the compression plunger, where they measure the compression of the steel caused by the cavity pressure (photo and graphics: Kistler) allow this important process parameter to be used for quality monitoring, the cavity pressure is measured by a contactless method with special measuring dowels. The flow path is incorporated into the pvt diagram to develop the new process monitoring technique. PE ERWIN BÜRKLE BERND KLOTZ OLIVER SCHNERR T here can be no compromises on the quality of high-end optical components on the contrary: Optical applications can only be produced at the very highest quality level. In lenses, not only do geometry and surface structure play an important role, but the reproduction quality is also strongly influenced by internal properties such as inherent stresses or molecular orientation. Consequently, for an individual case, in order to assess the optical properties at the machine,it is not sufficient just to check the part geometry or visually inspect the surface. It is more important to carry out optical testing, for example to use a Shack-Hartmann sensor (SHS) to test the distortion of the wavefront as a result of the lens. Mathematical methods can be used to derive further important quality functions [1]. Translated from Kunststoffe 5/2007, pp The above-described optical tests are difficult to perform for a continuous production process such as injection moulding. Quite apart from the fact that the necessary sensitive test equipment must be externally mounted, this is a very timeconsuming process. A disproportionately long waiting time is necessary until the results of the measurements are available, which delays the entire sequence, particularly during start up. In general, the machine must be stopped during this time, since rejects are unacceptable in view of the very long cycle times and high machine costs. Process Variants for Manufacturing Optical Parts In the injection moulding of optical parts, the moulding process not only influences the geometry but also has a crucially important effect on the interior properties of the parts. Comparative studies to assess the part quality showed, for example, that changing from injection moulding to injection compression moulding only slightly increases the geometrical accuracy. However, the optical characteristics are significantly improved by a factor of about 7, and, therefore, testing of the part geometry is not of itself sufficient for assessing the optical properties [1]. Injection-compression moulding has therefore proved a more suitable process for manufacturing optical parts [2]. During the pressing, the holding pressure phase directly follows the compression phase. The pressure is applied over the surface, which is equivalent to a homogeneous pressure distribution. As examples of different variants on injection-compression moulding, conventional injection-compression and expansion-compression moulding are discussed below. Conventional Injection-compression Moulding: In contrast to standard injection moulding, injection-compression moulding permits flow path/wall thickness ratios of up to 500:1. It is important that there are no changeover marks on the part surface. The individual processes and movements during a cycle must therefore flow into one another. 10

2 INJECTION MOULDING During the filling phase, the mould is only slightly open. The gap corresponds to the compression stroke. The plastic melt is injected into the mould and the compression process is started as a function of the screw position. Since the mould is open, internal stresses are reduced or eliminated completely during injection. The compression phase is initiated via an adjustable screw position. Before the cavity has been completely filled, the plastic melt is displaced towards the end of the flow path and compressed by further compression stages; this compensates for shrinkage. If the cavity is already filled at the end of the filling phase, the compression phases are initiated directly. With some machine settings, material can also be forced back into the plastication unit, against the holding pressure of screw, in this phase. Idealised Process Control With this method, weld seam critical parts (lenses with a large wall thickness ratio S A / S I 3, e. g. biconcave lenses or diffusion lenses with a negative meniscus) can be produced without a weld seam or the weld marks selectively shifted to the edge regions. Expansion-compression Moulding: Expansion-compression moulding by means of the clamping unit is preferably suitable for parts with a constant wall thickness across the cross-section. The advantage of the process is that it does not introduce stresses into the part after mould filling, since the holding pressure is not applied via the screw as in injection moulding but via the mould. The cavity pressure is thus uniform and the plastic can cool with virtually no stresses, and develop a homogeneous microstructure. Fig. 1. Ideal process injection moulding The filling phase takes place in the closed mould, as in standard injection moulding. In the expansion phase, the mould, possibly with a falling clampingforce profile, is forced open by means of the screw to a defined, precisely reproducible position. The precision with which the position is approached is the crucial factor in achieving a reproducible wall thickness. The expansion phase serves to bring the compression core of the mould into the compression position. When the point has been reached (measured directly on the mould), the compression phase is initiated and the filling process is stopped. The compression and holding pressure phase consists of a multistage clamping force/compression force profile. Both expansion compression moulding and injection-compression moulding can be varied by compressing not the machine clamping unit but a mould core. However, that only makes sense if a part only needs to be partially compressed. Depending on the concept of the injection moulding machine clamping unit, however, compression is also possible just with the mould core. Greatest flexibility is offered by fully hydraulic tie-bar clamping units. Process Control with a Threedimensional Problem Area Practical Process Control Fig. 2. Actual process injection moulding The individual phases are explained in the text In the production of optical parts, a three-dimensional problem-area must be managed: The geometry, surface, and optical properties must be brought into line with the required product quality. This requires precise process control, which presupposes that it is known what states are present in the mould and what processes are taking place there. Understanding this requires a knowledge of the cavity pressure. Because the cavity pressure precisely describes the filling phase, compression phase and holding pressure phase during injection moulding, this information is helpful both during process optimisation and for process and quality monitoring. With idealised, holding pressure-free injection moulding, the mould is theoretically filled isothermally, the melt is compressed and then cooled isochorically, that means that the specific volume remains constant in this phase (Fig. 1). However, such a filling operation would require an extremely high pressure, which is not always possible because of the stresses it puts on the mould and machine. V Kunststoffe international 5/

3 Fig. 3. Actual process injection moulding, supplemented by the flow path Fig. 4. Actual process for expansion compression moulding, supplemented by the flow path The classical pvt diagram illustrates the actual process profile (with holding pressure) (Fig. 2): (1) During the injection phase (1-2), the pressure increases at almost constant temperature (isothermally). (2) At the end of the filling phase, with volumetric filling of the cavity, the compression phase begins. The melt is compressed to ensure proper reproduction of the part contours. (3) The maximum cavity pressure is reached the holding pressure phase begins. It compensates for the high thermal contraction of the polymer, i. e. the reduction of its volume as a result of cooling, by feeding further material. (4) Solidification of the melt in the gate region (sealing point) the progressive thermal contraction allows the pressure in the mould cavity to be reduced to the ambient pressure (5). The phase (4 5) is isochoric. (5) The ambient pressure is reached processing shrinkage starts. (6) The part reaches the ambient temperature the change of specific volume in phase (5 6) is the processing shrinkage. The important point is that during injection moulding, the pressures are transmitted pointwise by the screw, through the gate, to the part. Cooling effects cause the outer layers to freeze along the flow path. This results in a pressure gradient that induces stresses in the part during the compression and holding pressure phases. The important thing about this consideration is that the pvt diagram, as a two-dimensional representation, actually only gives spot information. That means that it does not show the pressure states in the part along the flow path. For optical parts, however, the constancy of the internal properties along the flow path is important for quality no internal stresses must be induced. To determine the actual conditions, the pvt diagram must be extended around the flow path into the third dimension (Fig. 3). Only this makes clear that, in standard injection moulding, there is an inhomogeneous pressure state in the part, which leads to internal stresses. Compensation of shrinkage by the screw holding pressure acting pointwise via the gate has the effect that every point along the flow path is subject to a different pvt state. In this three-dimensional diagram, the process profile is represented as a surface, for the standard process with decreasing pressure and falling temperature along the flow path. For the example of expansion compression moulding (Fig. 4), the ideal process runs with an isothermal filling i Manufacturers Kistler Instrumente AG Eulachstrasse 22 CH-8408 Winterthur / Switzerland Tel. +41 (0) 52/ Fax +41 (0) 52/ Krauss-Maffei Kunststofftechnik GmbH Krauss-Maffei-Straße 2 D Munich / Germany Tel. +49 (0) 89/ Fax +49 (0) 89/ Three-Dimensional pvt Diagram Three-Dimensional pvt Diagram phase (1 2) and then merges into the expansion phase (2). During this phase, the maximum pressure is reached (3) and the process is controlled isobarically. In (4), the compression phase starts, which is also isobaric. From (5), the process is isochoric again until the ambient pressure is reached or until it has cooled down to ambient temperature (7). During isobaric process control, the process flow is a flat surface with constant pressure along the entire flow path. Starting from complete filling of the cavity, the process runs analogously to standard injection moulding during the filling phase (1 2). Constant pressure along the entire flow path also cannot be achieved with compression during the injection phase, since the flow resistance increases along the flow path. While the standard process then transforms into the compression phase with higher pressure, the pressure is kept constant during com- 12

4 INJECTION MOULDING Cavity Pressure Curve in Expansion Compression Moulding pression (3 5 in the pvt diagram). The pressure drop along the flow path is compensated during injection-compression moulding by the large-area pressure application. The aim of the compression stroke, to achieve isobaric process control during the embossing phase, is thus fulfilled. Fig. 5. The test results recorded during the optimisation phase show an optimised cavity pressure curve (top), and its collapse during the expansion phase (centre) and during the compression phase (bottom) Like the ideal process flow in standard injection moulding, this idealised flow can also not be completely realised. That is partly because of the freezing of the outer layers of the part. The actual flow described in the diagram is therefore not usually achieved. Guidelines for Process Optimisation The pvt diagram for the compression of optical parts shows the basic procedure for process control, explained here for the example of expansion-compression moulding. After volumetric filling of the cavity, the injection pressure and force of the compression plunger must be optimised for the expansion phase so as to achieve a constant cavity pressure. After changeover to compression, the force and injection pressure must be set to constant cavity pressure again. If a constant value is not achieved, loss of optical quality can be expected. Measurement of the cavity pressure in the production of optical parts is thus a prerequisite for optimum process control. In addition, measurement of this process parameter makes it much simpler to optimise this complex process. During the optimisation phase, different cavity pressure curves were recorded (Fig. 5): The top figure shows the optimised curve. In the centre graph, the cavity pressure collapses during the expansion phase. In this phase, the force acting on the plunger is too small material is displaced. The force must be increased to achieve a constant rise in the cavity pressure. Fig. 5 bottom shows how the cavity pressure collapses during the compression phase because the shrinkage is not compensated. A higher compression force makes the process more favourable. Besides the pressures and forces being too low, the cavity pressure curve also shows forces that are too high or incorrect transitions between the injection, expansion and compression phases, and provides concrete information about expedient optimisation measures. V Kunststoffe international 5/

5 Contactless Measurement of the Cavity Pressure Obviously, parts for optical applications must not have any marks or other flaws in visible or functional areas. This eliminates conventional sensors because they measure the pressure in the cavity at the part surface and may generate impressions. A possible remedy is to dimension the parts larger to displace the measurement point to the edge region (outside the optically useful zone) and then subsequently mechanically remove the projection. But this approach does not come into consideration because it increases costs and reduces productivity (damage). For contactless measurement of the cavity pressure, Kistler Instrumente AG, Winterthur/Switzerland, has developed special measuring dowel sensors and, together with Krauss-Maffei Kunststofftechnik GmbH, Munich/Germany, has realised this instrumentation in an injection compression mould for lenses. The new sensors are introduced into the mould structure behind the cavity wall or in the compression plunger, where they measure the compression of the steel caused by the cavity pressure. With this measurement set-up, it is possibly to quickly and easily optimise the process control in a two-cavity test mould by means of two measurement dowels. During production, the quality of the lenses was monitored by means of the cavity pressure. Fig. 6 shows the cavity-pressure curve (beige), the pressure in front of the screw (black) and the compression-plunger position (red) for expansion compression moulding. In the injection phase, the cavity pressure increases until volumetric filling. Then the cavity pressure is built up as a result of the holding pressure and expansion force. According to the pvt curve, this remains constant. The compression phase begins, which is ultimately followed by demoulding. Quality Defects are Recognised at an Early Stage Injection-compression moulding has proved a suitable process for manufacturing optical parts. Numerous parameters have a significant effect on the quality, and their variation can only be tracked and controlled if the cavity pressure is known. The cavity pressure curve during injection moulding is a function of the variation of compression force or clamping force with time. Machine Parameters during Expansion Compression Moulding Continuous process monitoring by measuring the cavity pressure provides early information about the part quality without complicated determination of the optical properties. A production shop therefore does not need to produce rejects for an unnecessarily long time before the quality defects are recognised. The above-described results were confirmed in various series of experiments. This demonstrates that a pvt diagram along the flow path, a three-dimensional pvt diagram, is suitable as a reference curve for the quality monitoring in the production of optical plastic parts. The reference curve also offers the advantage that the optimum operating state can be reached much faster on renewed start-up of a mould. The decisive factors are thus the mould trials and the subsequent mould optimisation phase. Classical cavity pressure measurement is required to determine the ideal basic process. Only when a reasonable process flow has been achieved are the parts subjected to the optical quality test discussed above. When the necessary quality is achieved, the reference curve has also be found that ultimately makes possible production monitoring and therefore quality monitoring. Outlook Fig. 6. The graph shows the curves for cavity pressure, for pressure in front of the screw and for the mould position during expansion compression moulding Because this reference curve correlates with the optical properties, it is conceivable to use the cavity pressure in future as a control parameter for the compression process. For this purpose, the current cavity pressure (actual state) must be track the reference curve (setpoint cavity-pressure curve). With a suitable control strategy, it will also be possible in future for a machine to adjust itself fully automatically for production start-up. This could eliminate manual optimisation, since only the maximum permissible cavity pressure would have to be specified. REFERENCES 1. Bürkle, E.: Forschung fördert Durchblick. Kunststoffe 96 (2006) 6, pp Bürkle, E.; Klotz, B.; Lichtinger, P.: Transparency in Injection Moulding. Kunststoffe plast europe 91 (2001) 11, pp Informations from Kistler Instrumente AG THE AUTHORS DR.-ING. ERWIN BÜRKLE, born in 1942, is Head of Preliminary Development, New Technologies and Process Engineering at Krauss-Maffei Kunststofftechnik GmbH, Munich/Germany. DIPL.-ING. (FH) BERND KLOTZ, born in 1956, works in applications development at Krauss-Maffei. DR.-ING. OLIVER SCHNERR, born in 1967, is Head of Product Management in the Plastics Business Unit at Kistler Instrumente AG, Winterthur/ Switzerland. 14

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