3D makes all the difference

Mark Fletcher looks at a simulation package that pulls plastic moulding and composite analysis into a true multi-dimensional realm


Plastic moulding has slowly evolved – with the help of steady advances in software – from a black art into a much better understood science. This science should now be further enhanced with the UK introduction of Sigma, a simulation tool based on 3D volume elements.
Developed off the back of Magma – a software suite for metal casting – Sigma should give designers a much better understanding of what is actually taking place in the deep, dark depths of mould tools. This encompasses flow, temperature and curing as well as a great many other things such as fibre alignment.
Mould simulation is certainly nothing new but this package "goes beyond the bounds of 2.5D – taking the industry into 3D," says Martin Woodley who is sales manager, polymer technology, at Rapra Technology. This is the recently appointed sole distributor of Sigma for the UK and Ireland.
Conventional programs use geometry representations that describe the mid-plane surface of the real part – simulation codes that are normally referred to as 2 1/2D. The technique used by Sigma has been developed for parts which have a high surface area to volume ratio – demanding a full 3D simulation. And the basic premise behind the development is to optimise a mould design before any metal is cut.
The advantages on offer are quoted as including the following:
real 3D volume calculations;
seamless import of existing CAD geometry data (STL);
fully-automatic mesh generation for both the part and the mould;
calculation of 3D flow effects;
jetting prediction;
coupled calculations of fluid flow and heating/cooling based on heat transfer in the mould; calculation of thermal distortion of the part and mould due to thermal fields;
and air pressure calculations for optimum venting sites. Sigma can also be used to calculate gate positions, runner balancing, flow fronts, local curing, die temperatures and cycle times.
The software can handle a lot of real-life variations as well, such as mould inserts. Metallic inserts can have a large impact on the way materials flow and cure and are, therefore, given the same treatment by the software. In fact, Rapra demonstrates a number of examples where the software has the potential to save a lot of design time. Realistic 3D flow simulation, for example, indicates where cool spots may emerge in moulds, contributing to early curing and cracking, or where air pressure is too high for the material to flow producing unfilled areas of the mould. Some of the more visually stunning examples show how, with amplified movement for demonstration purposes, the software predicts the flexing and twisting of components under the influence of residual stresses. One particular example highlights encapsulated electronics exhibiting bond failure when over moulded with plastic. The case in point indicated that the stresses built up on the connection paths due to the cooling of the plastic could cause the components to break. However, this stress creation could be designed out using the new software's analysis capabilities to modify the flow path, injection point and localised cooling – before any metal was cut.
Anyone currently working with a 3D CAD system will be at home with the results displayed by Sigma as they have a very familiar 'feel' to them. The injection animations and the way the materials flow provide a sense of realism that is immediately understandable. And being able to simulate both the mould and the part should prove a massive benefit, saving huge amounts of both time and money.
One final point: Shawbury-based Rapra is offering the package either 'out of the box' or with materials consultancy and training as required.
Fibres point the way
Fibre reinforced plastics throw a proverbial curved ball at certain moulding applications as the fibres change the way the base material behaves when processing, flowing and cooling. Fibre orientation can play a significant part in the overall structural performance of moulded components with even a simple repositioning of the injection point having a tremendously beneficial effect.
One Sigma module which is still under development but is commercially available clears up many of the mysteries of fibre orientation and its effects. This particular software demonstrates to the user not only the fibre direction but also, thanks to the use of colour, the degree of orientation.
Users can analyse the overall fibre orientation in either the X, Y or Z planes and then calculate any warpage which may result from one particular moulding sequence. This, like the main body of the software, uses exaggerated animation to great effect, leaving the user in no doubt that a problem may exist.
To demonstrate this Rapra uses an injection moulded model that is injected in two different ways. When injected from a central flange the model exhibits a less concentrated fibre orientation in the X plane which could lead to substantial warpage upon cooling. But when injection moulded from the end of the nozzle, not only are the fibres dispersed far better but the warpage is substantially less. Before the evolution of this new software a similar trial end error approach would have accounted for a lot of expensive metal and development time. It’s worth noting that fibre orientation details can be sent directly to the Abaqus FEA package.
: The software offers fully-automatic meshing in 3D
: Full 3D simulations can be performed and coupled for parts, inserts and moulds
: The software allows the part and the mould to be optimised before any metal is cut

Author
Tom Shelley

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