Cavity flow, characteristics

As reviewed in other chapters, different plastics have different melt and flow characteristics. What is used in a mold design for a specific material may thus require a completely different type of mold for another material. These two materials might, for instance, be of the same plastic but use different proportions of additives and reinforcements. This situation is no different than that of other materials like steel, ceramics, and aluminum. Each material will require its own cavity shapes and possibly have its own runner system. 

The flow characteristics can be determined using the model shown in Figure 7.79. During the time interval nnder consideration, it is assumed that the polymer has been heated to a uniform temperatnre, T , that is equivalent to the mold wall temperature. As long as the preform radius, R, is less than the radius of the outer wall of the mold cavity, Ro, we can treat the problem as an isothermal radial flow of an incompressible power law flnid flowing between two disks that approach each other at a constant rate, h. In this way, the velocity field, Vr z, r, t), the pressnre distribution in the mold, P z, r, t), and the plnnger force, Fn z, r, t) can be obtained as follows ... 

Thermoplastics have non-Newtonian melt flow characteristics it means that their viscosity will change dependent on their velocity or the amount of shear that occurs in the melt (Chapter 1). This non-Newtonian characteristic is a key in thin wall molding. As in any molding setup one cannot just simply ram the melt into the cavity. It s flow characteristic, gate size as well as position, and venting have to be balanced in order to obtain the desired structural part and meet tight tolerance requirements. 

Some tests are affected by core orientation and some test properties are more influenced by surface orientation. Since orientation is not uniform, but has a gradient through and along the flow path, it is difficult to predict directly the effects of process conditions on part properties, without a complex model of the part geometry and estimation of flow characteristics in the cavity. 

Die fill characteristics depend upon material flow properties that are primarily affected by particle size and shape. Additionally, high interparticle friction can have a detrimental effect on die fill characteristics due to powder bridging and non-uniform flow characteristics. A non-uniform particle size distribution may also lead to material segregation resulting in uniformity problems. Tablet presses employ volumetric filling of the material into the die cavity. Most high-speed tablet presses are equipped with force feeders, which use rotating paddles to promote uniform die fill characteristics. For certain materials, attention must be... 

Although the simplicity of the direct compression process is obvious, if pharmaceutical blends are pre-treated in one way or another prior to tabletting, flow characteristics can be improved, compressibility adjusted, and a good particle size and distribution can be selected that yields an optimal feed bulk density. However, the pretreatment of dry dosage formulations becomes more and more complicated to render them suitable for use with the sophisticated high-speed tabletting machines [B.34, B.41, B.48, B.66, B.71, B.97, B.99, B.105]. In simple terms, for those presses it is a requirement that the feed flows quickly and uniformly into the die cavities where it can be converted into well-shaped and firm compacts in a very short time. Tablet quality is a result of the blend s compressibility . This characteristic not only includes an excel-... 

Cavities persist when the gas flow is stopped, so power depends on the history of gassing. The cavities are larger than those for the same speed and gas rate in low viscosity turbulent systems and Figure 15.18 shows the effect of this on power. Viscoelastic liquids give even larger cavities of characteristic anvil sha ... 

Special injection grades of UF molding powder have been developed for injection-molding applications which call for molding materials with good flow characteristics between 70°C and 100°C, unaffected by long residence time in the barrel but capable of almost instant cure in the mold cavity at a higher temperature. 

There are a variety of different types of particle that can be synthesized using microfluidics and the exact fluidic format used defines the flow characteristics of the synthesis environment which needs to be carefully matched to the synthetic requirements of each particle type. Broadly speaking, the main particle types of interest include monodisperse microgels, metal nanoparticles and nanocrystals, core-shell particles, microemulsions, and microcapsules. Typically these particles should be homogeneous in size and are composed either of an organic polymer matrix or a metal or metal mixture at the micron or nanometer scale. In addition to these simple particle architectures, we may also consider more complex particles composed of multiple layers or components such as core-shell particles or using a central cavity to encapsulate small molecules, proteins, or other particles, and these are called microcapsules. 

Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. They occur when melt flow fronts collide in a mold cavity. Material characteristics can affect the knitting of the melt fracture. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of part strength. 

This class of material is utilized by the rubber compounder when he has a problem concerned with improvement of material flow in such products as intricate mouldings manufactured by compression or more often by injection moulding. To be economic, injection moulding needs compounds which will fill the mould rapidly and which will have consistent flow characteristics to ensure that all cavities of multi-cavity moulds are filled to make good products. 

In contrast to regular planar electrodes, the geometry and mechanical properties of rod-shaped electrodes enable unique three-dimensional microfluidic cell architectures. An array architecture fuel cell was developed by Kjeang et al. [55] based on a hexagonal array of graphite rods mounted in a single cavity, as depicted in Fig. 4.11. In this case, the flow area between the rods exhibited microfluidic laminar flow characteristics similar to those of a planar unit cell. The array cell had 12... 

Viscosity-conhol additives can improve the flow characteristics of the fiber-reinforced resin matrix during the pressurization stage. Specifically, thickeners minimize fiber separation from the resin itself. If the viscosity of the charge is too low, the resin tends to flow farther and faster, and it easier fills tight cavities and sections compared to the flow of the fibers. 

Compounds for press curing must be carefully designed to flow properly without scorching before the desired shape is reached, and to cure rapidly so that a maximum turnover is obtained. They must be easily removed from the molds after cure and exhibit attractive surfaces in the finished form. The flow characteristics depend on the plasticity of the uncured stock as well as selection of the proper acceleration to permit complete filing of the cavity before the cme begins. 

Individual treatment variables in PD include membrane permeability and blood flow, behavior of the abdominal cavity, biological response to dialysate, and flow characteristics of the catheter. All other variables should be controlled by the machine. 

PBT, PP, ABS, and PC are the fonr materials being tested. These materials were selected as they rqiresent a common mix of amorphous (ABS and PC) and semi-crystalline (PBT and PP) materials. They also represent materials with different flow characteristics regarding their reaction to both shear and temperature. Ten samples were taken from each material at each of the three injection speeds. A total of 120 samples were taken from the 2-cavity mold insert. 

The basics observed in molded products are always the same only the extent of the features varies depending on the process variables, material properties, and cavity contour. That is the inherent hydrodynamic skin-core structure characteristic of all IM products. However, the ratio of skin thickness to core thickness will vary basically with process conditions and material characteristics, flow rate, and melt-mold temperature difference. These inherent features have given rise to an increase in novel commercial products and applications via coinjection, gas-assisted, low pressure, fusible-core, in-mold decorating, etc. 

Part 1 Boiling characteristics. Int J Multiphase Flow 11 269-281 Hsu YY (1962) On size range of active nucleation cavities on a heating surface. J Heat Transfer 84 207-216... 

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