Polymer Testing Newsletters

Don Fleming (front left in photo) attended the Compuplast winter meeting in January and was pleased to experience the range of new features version VEL (virtual extrusion laboratory) 6.2 has to offer.  VEL 6.0 marked the most radical change in the software for some years, offering a completely new front end and post-processor.  The user experience has been greatly enhanced since VEL6.0 and, in keeping with the Compuplast ethos, is easier to use and considerably more powerful.  Of special interest is the new Leonov viscoelastic model, which in detailed terms means elasticity can be predicted with greater accuracy.  The benefit for users has been the discovery of a relationship between the first normal stress difference (the measurement of elasticity) and interfacial instabilities.  This is especially useful in coextrusion where stress differences at the interface can cause significant problems.  The Leonov model places a greater emphasis on the accurate measurement of elongational viscosity, something Fleming PTC is well positioned to provide having specialised in this measurement for over ten years.  Consequently, with materials correctly characterised in both shear and elongation, prediction accuracy has been significantly improved.  Compuplast also unveiled their new Injection Moulding Plastification Program for injection moulding.  Other improvements/additions include:

• 2DFEM-deformation of particles can now been seen as an animation in a flow field
• Extruder-an evacuation screw can now be completely solved
• Spiral die-conical dies can now be solved
• Profile die-automatic assigning of the elements of a profile can now be invoked, considerably reducing project time.  Profiles can now be solved by length as well as by gap.
  
  
  

The program has been designed to simulate, and thus optimise, the first part of the injection moulding process where the polymer is prepared for injection.  The module has effectively taken elements of the very successful VEL 6 Extruder program and applied them to injection moulding.  It works in a similar fashion to the rest of the Compuplast programs, whereby a project is defined, containing all of the relevant parameters;

• screw diameter
• stroke time
• full cycle time
• desired temperature
• screw stroke

Where the difference between the full cycle time and the stroke time is the maximum time taken for the material plastification, and the he screw stroke and the screw diameter define the required material volume.

Materials can, as usual, be taken directly from the Compuplast material database enabling users own grades to be simulated.  Also in keeping with the extruder module, different extruder geometries can be linked to any project, again enabling the exact user screw geometry to be simulated.  Once this has been achieved the program solves for required plastification time and injection temperature.  The solution takes account of the fact that the screw moves back and therefore becomes shorter during the plastification process.  It also recognises the residence time between plastifications and its influence on material melting.  One of the key outputs is the resulting temperature variation; it is flagged as red if undesirable, facilitating the changing of input conditions for optimisation.  The solution also shows the relationship between mass flow rate, end bulk temperature and plastification time as a function of RPM; this has been found to be especially useful for the optimisation of RPM to eliminate unduly long residence times.

January also saw the purchase of a sapphire reference disc for optimising specific heat capacity measurements using the DSC.  Differences between the heat flow of the baseline and the polymer sample can be corrected by using the known behaviour of a sapphire reference disc. It is thought using the sapphire disc to correct data in this way enables absolute measurements to be refined to better than 99% accuracy.  The system has already been used to calculate Cp values for a range of polymers for a flow simulation project where a comprehensive characterisation was required. 

Following much interest in the TGA service, several customers have inquired as to the possibility of adding a gas-switching system to the instrument in order to allow additional gases to be used; clearly oxygen or air can be instrumental in investigating the oxidative behaviour of materials under heating.......watch this space!