ThermoformingA/C Taimoor, A/C Salman, A/CWaqar Theoretical Background:Firstof all, thermoforming is a process in which a sheet is first heated and thenmolded into a required mold shape. Here we will see what is the backgroundprocess, means that how certain inputs are varying and how are they changing,because in software (Ansys) we are just giving the inputs not the formulas orthe relations by which they are varying.· Beforepolydata there is geometry as well as meshing, but these donot involve anyformulas.· Inpolydata we create some tasks as well as their sub tasks and we have to createmold as well Velocity or Force Driven: In polyflow, when we define acontact we do it with the help of penalty technique. In this way we define thefluid velocity and wall velocity are related by the condition (in normaldirection) which involves the penalty co-efficient k, i-e, fn=-k(v-vwall).n Similarly, we can use this equationfor the tangential direction but we have to take the slip co-efficient intoaccount, i-e fs=-Fslip(vs-vs,wall)We haveseen how the contact force is applied, now here we have some selection ofwhether we want our problem to be velocity imposed or force driven. If ourproblem is velocity imposed we can use the above mentioned equations but if itis force driven then we have to solve the corresponding momentum equation, Fm+Ff =Ma· Fm= force applied on mold· Ff = resistance from the fluid· a = acceleration of the mold· M=massof the moving partNow hereis the question that why we use this equation? It is because we want to definea limit for the maximum displacement, because when the deformation increasesshear force and hence the motion of the mold is decreased. So if displacementtends to increase beyond its limit its motion is stopped.
That’s why maximumdisplacement is calculated. Isothermal or non-isothermal: If our simulation is isothermal thenthe conditions (thermal boundary conditions) are same before and after thecontact but if our simulation is non-isothermal then the flow conditions arenot the same before and after the contact,i-e Q=a(T-Tmold) where ‘a’ is convective co-efficient · similarlythe viscosity changes with change in temperature as shown by the followinggraph. Constant viscosity and straindependent viscosity: Most of the times we take constantviscosity for shell models but sometimes it is more desirable to take viscosityin terms of local strain. For this case the fluid constitutive equation iswritten as follows: T=2?(?)DInsimple traction experiment, we, at constant stretching velocity, stretch thesample of initial length L0 and record the tensile stress as afunction of deformation. After some manipulation we can take viscosity as aratio of stress to strain rate. ?*= V0/(L0+?L) ?=exp(?*0t) ?(?)=?0+a ?2+bexp(-((?-?p)/ ?w)2) Typical Viscosity Curve Exhibiting StrainHardeningTypicalViscosity Curve Described with the Smooth Ramp Function · Nowhow postprocessor things are calculated.
Mass of the blownproduct: mblown=?A?hdA· ?=densityof the parison· h=layerthickness· A=surfaceareaPermeability of theblown product: Permeability is important to be calculated in thepacking of pharmaceuticals where moisture content is important. p= ? /h· ?is permeability co-efficient· his local thickness