Konstruktionsbüro Hein GmbH Marschstraße 25, Neustadt (Hanover), Germany

Transcription

1 Controlled reduction of distortions Introduction: Plastic parts often have a high degree of distortion if the geometries have not been optimised. An optimised geometry reduces distortions and tensions within the part. By simulating the distortions re-design is made possible and can be verified. Should a relevant distortion remain we will perform a negative correction while maintaining all tangentialities in order that your part complies with the nominal geometry after the injection moulding process. Distortions are a particularly important issue when semi-crystalline plastic materials are involved. That was also the case with this project for Hirschmann Electronics, a German company in Neckartenzlingen. Picture by Hirschmann Electronics Page 1 of 10

2 Compensation of shrinkage: In addition to the procedure described in the introduction we would like to mention another important procedure in order to give you a complete overview: Gas injection for the compensation of shrinkage. The parts are produced with little shrinkage and distortion. Is the optimisation of geometries costly? The left column shows the old course of action, the right column depicts the new, more predictable, way. When you compare the costs you see the potential for optimising them. Adaptations to the tool after the first sampling are reduced to a minimum. The tool itself will not be damaged by the adaptations. Time saving? An even greater advantage is demonstrated when comparing the project time until the start of production. The left column depicts the excess time needed in red. In most cases, this time has not been calculated beforehand but has to be spent nevertheless. On the right you see a minimum need for extra time in the part development phase but the entire project is ready a lot earlier and can be calculated more accurately. Page 2 of 10

3 Parameters relevant for distortion: Overview: Plastics raw material Semi-crystalline plastic material in particular have a tendency to show high degrees of distortion and shrinkage. Often, manufacturers are not aware of the enormous consequences of the part development phase on the actual production process. The correct geometry of the part allows for a comfortable production time frame for injection moulding. Apart from a separation of tools and the correct point of injection, the temperature control of the tool is a decisive factor for the shrinkage and distortion behaviour of the part. If the parameters mentioned above were considered and the injection moulding machine is optimally adjusted it is possible to produce a part with good characteristics while having a short cycle. Project Cap Page 3 of 10

4 for the German company Hirschmann Electronics in Neckartenzlingen. The innovative course of action regarding optimisation of the article and the ensuing negative correction led to a good product quality of 4 different parts produced in 4 different tools. Hirschmann charged BaHsys of Engelskirchen/Germany to fabricate the mould inserts (for the existing basic tool by BaHsys) in order to test samples?similar to the production items but made from different plastic materials?. Picture by Hirschmann Electronics The material that was actually selected was KEBATER PBT (not reinforced) by Barlog plastics. But, as PBT is a semi-crystalline material it is highly prone to shrinkage and distortion! Without an optimised geometry (illustrated in red) the cap displayed an intense recess in the direction of the x- coordinate. The modifications to the geometry can be observed particularly well at the intersection of the circumferential sealing ridge with the top surface. Controlled modifications of the intersection and the contours following in direction of the end of the flow have a considerable effect on the distortion behaviour. Distribution of wall thickness without Page 4 of 10

5 modification: Generally, the wall thickness should be as homogenous as possible. But, from the injection point to the end of flow they should become reduced.?the relation between wall thickness and ridges have to be exactly correct!? Despite exact specifications regarding the outer contour, the positions of the circumferential sealing ridge and of the screw crowns important corrections were implemented in order to optimise the part. The more homogenous distribution of the wall thickness can easily be distinguished. Many correlations regarding the optimisation of the part geometry are related to the classic corner distortion. The remaining melt is oriented towards the core. The ensuing forces of shrinkage distort the component. Basically, the distortion characteristics can be controlled by means of diverse manipulations of the corner and the ensuing wall thickness. In the simulation without optimisation of the part geometry the two red areas mark Principle layout Page 5 of 10

6 the high degree of lateral distortion of 0,6 mm / side. After the optimisation the lateral distortion is reduced to 0,2 mm / side. As the colour scale is always the same the distortion at the top of the part now comes into view and indicates the new task for the negative correction. Please see also the illustrations on page 1! The superimposed illustration of distortion for the height of the part shows clearly how the part will deform in deviation of the nominal geometry (grey). As the distortion is almost identical with or without an optimised geometry the only solution for obtaining the desired results is a negative correction of the contour?entering the tool.? Page 6 of 10

7 Negative correction Negative correction Cap Before performing the negative correction a comparable part was examined for frozen tensions in the warming cupboard. While maintaining all tangentialities the article is now?simulated onto /displayed on? a correctional spline (see above) and thus corrected using the negative. The result of the ensuing injection moulding process is a part low in distortions which complies with the nominal geometry within close tolerances. The remaining distortion is prestressed within the tool using the negative correction of the part. Picture by Hirschmann Electronics Page 7 of 10

8 Orientations Orientation is another factor that can enable us to influence the distortion behaviour. In the aforementioned example the material involved is unfilled PBT. But, if the orientations in this case were those of glass fibres we d have an important field of operation: It is a well-known fact that parts filled with glass fibres have a greater shrinkage transversal to the fibre than longitudinally. Tool Design / Temperature Control Our latest experiments regarding the optimisation of geometries have shown that even minuscule changes can modify the orientation of the glass fibres and thus result in less distortion. This illustration shows the shrinkage during processing as well as the post-shrinkage as functions of the female mould temperature. That means: If the tool temperature during production was too low there will be a higher degree of post-shrinkage. This may result in tensions that will be released when the injection part is later exposed to high temperature again. Pictures by Walter Formenbau of Sulzbach- Laufen/Germany Page 8 of 10

9 In addition to correct definitions regarding separation and the injection point, a comprehensive concept is needed for an optimum temperature control of the tool. This is a decisive factor for the part quality. F Mould mass H Hotrunner L Contact M. K Convection Str. Radiation TM Medium for temperature control Injection Moulding Tool Simulation The simulation of the temperature distribution within the entire injection moulding tool is an innovative procedure for interpreting and illustrating the temperature control! Here, the influence of adding materials having a good thermal conductivity at the same flow temperatur has been studied. The top picture shows the mould kernels without temperature control (yellow) as hot spots. In the bottom picture the same areas are already too cool. It becomes evident that an efficient temperature control helps to keep temperature differences inside the tool small, thus allowing for the manufacture of parts low in tensions and distortion while having good surfaces. Project: Phoenix Contact Blomberg SIGMA Page 9 of 10

10 Your Partner Design Engineering Design / Prototyping / FEM Calculation Product development using ProE; Catia; thinkdesign Mould filling analyses Calculation of distortions Negative correction 2D+3D Design + NC Programs for: e.g.: GIT / 2C injection moulding tools pressure moulding tools / elastomer moulds Training Project training Courses for product development / design engineering Sales thinkdesign ; deskartes hyperform ; hypermill ; Cadmould; Our team Page 10 of 10