Shopping Basket
PDF's from PIM International
3D visualisation of flow patterns in injection moulded ceramic green parts
Technical Paper: PIM International, Vol.5 No.2 June 2011, pages 60-65, 3721 words
Fraunhofer Institute for Ceramic Technologies and Systems, Winterbergstraße 28, 01277 Dresden, Germany
Abstract
Mould filling behaviour affects the green structure of an injection moulded part, since defects can be caused by flow patterns. These defects might be pores, or weak spots such as weld lines. These risks can be minimised by optimising the tool design aiming for a desired flow pattern. Hence, understanding the formation process of specific flow patterns is very important and necessary to characterise. In this paper, a method for visualising flow lines within injection moulded green parts is presented. By introducing an X-ray detectable tracer material, a three dimensional image is produced revealing the flow patterns. Four case studies demonstrate the potential of this method.
Introduction
Defects in finished sintered ceramic injection moulded (CIM)components often have their origins in the green part. Cracks can be initiated at weak points such as pores, but also at weld lines or knit lines which result from jetting. Residual stresses in the green part may lead to distortion or warpage during thermal treatment. Since the green microstructure is created during the mould filling phase it depends on feedstock properties and, most importantly, on the tool design, including sprue and gate number, shape and position.
In order to investigate the flow behaviour experimentally several approaches have been developed to visualise the flow pattern. They follow approaches like direct melt observation, evaluating surface marks, introducing tracer particles into the melt and interrupting the injection process.
For direct observation the melt flow is recorded inside the mould by high speed cameras. Flow around obstacles [1], in-mould shrinkage [2], duration of partial cavity filling [3], changes in melt front shape [4] or jetting [5] can be analysed and time resolved. Special tools need to be applied in which one cavity wall is substituted by a transparent window to allow connecting of the camera. Therefore it is limited to flat specially designed testing geometries.
A method, which can be used for any existing tool, evaluates patterns visible on the parts surface. These are, for example, weld lines examined under the microscope [6] or flow lines and ripples on the surface. By decreasing melt and tool temperature those lines can be intentionally evoked [7, 8]. It has to be taken into account that the test parameters diverge from the common production parameters and might affect the flow lines.
By adding a marker material into the melt the flow paths can be derived. This is especially suited for transparent matrices like many plastics. The markers can be particles like pigments, the orientation of fibres or stream lines when two coloured polymers are mixed [9].
The most common way and widely applied in industry is to analyse the melt progression by producing a short shot study [10, 11, 12]. By interrupting the injection phase at different volumes a sequence of filling stages is created. The obvious advantage is its universal applicability to all tools using normal production conditions. Furthermore it requires only modest time and cost. However, the produced samples show only the state at one moment and do not reflect the flow pattern in the final part..........
Further sections of this paper include:
- Experimental
- CT-method
- Development of visualisation method
- Case studies
- Results and Discussion
- Conclusion
- Acknowledgments
- References
Figures and Tables:
Fig. 1 Principle of computed tomography
Fig. 2 Viscosity of alumina base feedstock and its associated zirconia tracer feedstock
Fig. 3 Test parts used for the case studies: A: bar; B, C: modified ladder; D: two-component gear wheel
Fig. 4 Radiographs of a short shot study visualising the filling process of a bar shaped sample
Fig. 5 Stream lines at different filling stages marking the boundary to the dead water zone
Fig. 6 Positions of weld lines in the testing part modified ladder determined by a) short shot study, b) simulation
Fig. 7 Flow pattern of the modified ladder; a) radiographic image; b) detail in XZ-plane; c) detail in YZ-plane showing the four joined parallel streams in cross-section
Fig. 8 Simulated velocity vectors at the end of filling
Fig. 9 Volume ratios of feedstock streams measured at cross-sections for different injection moulding parameters
Fig. 10 Volume ratios of feedstock streams after each rung for the Catamold® and the wax-based feedstock
Fig. 11 Visualized flow lines in the gear wheel
Fig. 12 SEM image of gear tooth showing binder exclusions which have separated along flow lines
Fig. 13 CT image of a gear tooth after debinding with internal cracks
Table 1 Composition of an alumina base feedstock and its associated zirconia tracer feedstock
Table 2 Variation of injection moulding parameters for the feedstock Catamold®
Search all documents in the PDF Store
Please note that this search only covers document title, introduction / abstract, table / figure captions and author details, as available to view on each document preview page.Or view by topic
