Melt front

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

Solution (a) Isothermal Situation. If the volume flow rate is Q, then for any increment of time, dt, the volume of material injected into the cavity will be given by Qdt). During this time period the melt front will have moved from a radius, r, to a radius (r -I- dr). Therefore a volume balance gives the relation... 

Consider a straight tubular runner of length L. A melt following the power-law model is injected at constant pressure into the runner. The melt front progresses along the runner until it reaches the gate located at its end. Calculate the melt front position, Z(f), and the instantaneous flow rate, Q t), as a function of time. Assume an incompressible fluid and an isothermal and fully developed flow, and make use of the pseudo-steady-state approximation. For a polymer melt with K = 2.18 x 10 N s"/m and n = 0.39, calculate Z(t) and Q(t)... 

Answer We begin with the relationship for flow rate of a power law fluid, as given by Eq. (7.72), and recognize that the length the fluid has traveled, L, is now replaced by the melt front position, Z(i), such that... 

Figure 4.46. Evolution of the melt front in the mold filling process in isothermal (a) and non-isothermal (b)...

Melt front node (d). These are the nodes that are temporarily on the free flow front during mold filling, and are therefore partially filled (0 < /< < 1). During that specific time step this node is assigned a zero pressure boundary condition, pd = 0 (essential or Dirichlet boundary condition). 

Figure 8.54 Experimental and simulated (FAN) melt front advancement in a shallow rectangular mold.

Melt front nodes - Nodal control volumes containing the free flow front (0 [Pg.493]

The element side surfaces are formed by lines that connect the centroid of the triangular side and the midpoint of the edge. Kim s definition of the control volume fill factors are the same as described in the previous section. Once the velocity field within a partially filled mold has been solved for, the melt front is advanced by updating the nodal fill factors. To test their simulation, Turng and Kim compared it to mold filling experiments done with the optical lenses shown in Fig. 9.34. The outside diameter of each lens was 96.19 mm and the height of the lens at the center was 19.87 mm. The thickest part of the lens was 10.50 mm at the outer rim of the lens. The thickness of the lens at the center was 6 mm. The lens was molded of a PMMA and the weight of each lens was 69.8 g. 

Experimental and predicted melt front advancement at fill times of 1.2, 2.4 and 3.6... 

Melt front prediction at 4.6 seconds filling time, and weld line location in the final... 

Figure 9.38 Melt front prediction at the end of filling after raising the mold temperature by 50 K [10].

Rose examined the flow pattern in a capillary tube where one immiscible liquid displaces another one. In the front end of the displacing liquid the flow pattern is one he termed fountain flow, and in the other reverse fountain flow. In polymer processing the significance of the former was demonstrated in the advancing melt front in mold filling (see Chapter 13). 

We discuss some of these regions in detail below. In addition, we concern ourselves with the overall flow pattern during filling. Recall that the manner in which a mold is filled— that is, the location of the advancing melt front—affects the weld-line location and the orientation distribution and may be responsible for poor mold filling conditions. 

Fig. 13.18 Predicted velocity field showing fountain flow around the melt front region for non-Newtonian fiber suspension flow at about half the outer radius of the disk. The reference frame is moving with the average velocity of melt front, and the length of arrow is proportional to the magnitude of the velocity. The center corresponds to z/b = 0 and wall is z/b = 1, where z is the direction along the thickness and b is half-gap thickness. [Reprinted by permission from D. H. Chung and T. H. Kwon, Numerical Studies of Fiber Suspensions in an Axisymmetric Radial Diverging Flow The Effects of Modeling and Numerical Assumptions, J. Non-Newt. Fluid Mech., 107, 67-96 (2002).]...

Figures E13.2d and E13.2e show the melt front and bulk temperature distribution.

Most practitioners deflne the flow behavior of polymers based on the melt flow index however, this property is not entirely relevant to the rotational molding process because it is essentially a shear-free and pressure-free process. The use of zero-shear viscosity has been proposed as a better way to assess the coalescence behavior of resins. Resins with lower zero-shear viscosity coalesce at a faster rate and can thus be processed using a shorter molding cycle.The coalescence of individual powder particles is initiated as the particles stick and melt onto the mold surface or melt front. As the melt deposition process continues, pockets of air remain trapped between partially fused particles and lead to the formation of bubbles. In the rotational molding process, the coalescence of particles occurs at a temperature range close to the melting point of the material thus, from a processing standpoint, low values of zero-shear viscosity at low temperatures (i.e., close to the temperature at which the particles adhere to the mold surface) are preferable. 

---