Plastics Melts

To illustrate what viscosity means, we ean imagine two eontainers of equal size. In the bottom of each container is a elosed hole, eaeh with the same diameter. If we fill one eontainer with water and the other with a viseous lacquer, then open both holes at the same time, a given quantity of laequer will require much more time to flow out of the eontainer than will the same quantity of water. We ean see that the laequer is more viseous than water. If a liquid possesses an espeeially high viseosity, it is said to be highly viscous. 

Shear stress results as the flowing layers of macromolecules slip past one another. The shear rate is the difference in the rates of flow or the shear gradient-that is, the change in the rates of the flowing layers across the radius of the cross section. The viscosity decreases with increasing shear rate. 

Viscosity decreases as temperature increases, so the plastic melt begins to flow in a less viscous manner. In the case of polyethylene, for example, the viscosity measures approximately 138 Pa s at a shear rate of 1000 1/s and a temperature of 150°C (300°F). However, at the same shear rate and a temperature of 270°C (520°F), the viscosity measures approximately 52 Pa s. This example applies to polyethylene (PE). 

Chemical name composition ISO 1043 DIN 7728 Density [g/cm ] Processing temperature [°] Flowability g = good m = medium p =poor Shrinkage in processing [%] r=glass-fiber reinforced 

It can be seen from the table that the range of processing temperatures is very narrow for some thermoplastics but very broad for others. For example, PVC-U (xmplasticized or rigid PVC) can only be processed within a very narrow temperatiue range (190-210°C, 374-410°F), while POM can be processed between 180 and 230°C (356-446°F). Cif no. 10 and no. 11 in Figiue 1-10). 

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In coextmsion, two or more plastic melts from different extmders are combined into a single die in which the melts are joined. Coextmsion permits precise, small quantities of plastic materials to be intimately bonded to each other. 

The drag flow or volumetric conveying capabiUty, for the plastic melt is dependent only on screw speed. A/, and the geometry, M, of the screw ... 

In practice there are a number of other factors to be taken into account. For example, the above analysis assumes that this plastic is Newtonian, ie that it has a constant viscosity, r). In reality the plastic melt is non-Newtonian so that the viscosity will change with the different shear rates in each of the three runner sections analysed. In addition, the melt flow into the mould will not be isothermal - the plastic melt immediately in contact with the mould will solidify. This will continuously reduce the effective runner cross-section for the melt coming along behind. The effects of non-Newtonian and non-isothermal behaviour are dealt with in Chapter 5. 

Equations (5.21), (5.22) and (5.23) are useful for the high strain rates experienced in injection moulding or extrusion but unfortunately they do not predict the low strain rate situation very well where plastic melts tend towards Newtonian behaviour (ie n -) 1). This is illustrated in Fig. 5.7. 

JO The viscosity, t), of plastic melt is dependent on temperature, T, and pressure, P. The variations for some common plastics are given by equations of the form... 

Other seemingly solid fuel flames such as those from the burning of plastics are actually more like liquid pool flames because the plastic melts and vola-tizes ahead of the advancing flame front. 

The mechanical behavior of plastics is dominated by such viscoelastic phenomena as tensile strength, elongation at breaks, stiffness, and rupture energy, which are often the controlling factors in a design. The viscous attributes of plastic melt flow are also important considerations in the fabrication of plastic products. (Chapter 8, INFLUENCE ON PERFORMANCE, Viscoelasticity). 

The recommended radius not only reduces the brittleness effect but also provides a streamlined flow path for the plastic melt in the mold cavity. The radiused corner of the metal in the mold reduces the possibility of its breakdown and thus eliminates a potential repair need. Too large a radius is also undesirable because it wastes material, may cause sink marks, and may even contribute to stresses from having excessive variations in thickness. 

Flow pattern Ultimately, product quality can be considered a direct outcome of a plastic melt s flow behavior in its mold cavity or cavities. Excessive restrictions and obstructions to the flow of material spell trouble in injection molding. 

When the plastic melt flows across the small hinge gap, frictional heat will be... 

When water (a Newtonian liquid) is in an open-ended pipe, pressure can be applied to move it. Doubling the water pressure doubles the flow rate of the water. Water does not have a shear-thinning action. However, in a similar situation but using a plastic melt (a non-Newtonian liquid), if the pressure is doubled the melt flow may increase from 2 to 15 times, depending on the plastic used. As an example, linear low-density polyethylene (LLDPE), with a low shear-thinning action, experiences a low rate increase, which explains why it can cause more processing problems than other PEs. The higher-flow melts include polyvinyl chloride (PVC) and polystyrene (PS). 

When reviewing the subject of plastic melt flow, the subject of viscosity is involved. Basically viscosity is the property of the resistance of flow exhibited within a body of material. Ordinary viscosity is the internal friction or resistance of a plastic to flow. It is the constant ratio of shearing stress to the rate of shear. Shearing is the motion of a fluid, layer by layer, like a deck of cards. When plastics flow through straight tubes or channels they are sheared and the viscosity expresses their resistance. 

Non-Newtonian flow Plastic melts are non-Newtonian. They have basically abnormal flow response when force is applied. That... 

Production molds are usually made from steel for pressure molding that requires heating or cooling channels, strength to resist the forming forces, and/or wear resistance to withstand the wear due to plastic melts, particularly that which has glass and other abrasive fillers. However most blow molds are cast or machined from aluminum, beryllium copper, zinc, or Kirksite due to their fast heat transfer characteristics. But where they require extra performances steel is used. 

The non-Newtonian behavior of a plastic melt makes its flow through a die somewhat complicated. One characteristic of plastic is... 

Fig. 8-17 Example of pressure loading on the plastic melt during IM.

With PBAs the compressed gases often used are nitrogen or carbon dioxide. These gases are injected into a plastic melt in the screw barrel under pressure (higher than the melt pressure) and form a cellular structure when the melt is released to atmospheric pressure or low pressure. The volatile liquids are usually aliphatic hydrocarbons, which may be halogenated, and include materials such as carbon dioxide, pentane, hexane, methyl chloride, etc. Polychlorofluoro-carbons were formerly used but they have now been phased out due to environment problems. 

The calender was developed over a century ago to produce natural rubber products. With the developments of TPs, these multimillion dollar extremely heavy calender lines started using TPs and more recently process principally much more TP materials. The calender consists essentially of a system of large diameter heated precision rolls whose function is to convert high viscosity plastic melt into film, sheet, or coating substrates. The equipment can be arranged in a number of ways with different combinations available to provide different specific advantages to meet different product requirements. Automatic web-thickness profile process control is used via computer, microprocessor control. 

The next step involves heating the mold while it is rotating. Molds can be heated by a heated oven, a direct flame, a heat-transfer liquid (either in a jacket around the mold or sprayed over the mold), or electric-resistance heaters placed around the mold. With uniform heat transfer through the mold, the plastic melts to build up a layer of molten plastic on the molds inside surface. 

Viscosity, apparent Defined as the ratio between shear stress and shear rate over a narrow range for a plastic melt. It is a constant for Newtonian materials but a variable for plastics that are non-Newtonian materials. 

However this solution is not always convenient and may prevent high productivity and/or the production of intricate forms which require high temperature processing. Fortunately, some fire retardants dramatically increase the flowability of fire retardant plastics melts. For example ABS, flame retarded with F-2016 or F-2016M (brominated epoxy) has much higher flowability, melt flow index (MFI), and spiral flow index, than virgin ABS (Fig. 1). 

This increase in the flowability of fire retardant plastics melts compensates for lack of heat stability by providing the same technological potentialities as high processing heat stability using low temperature processing (Fig. 2). 

Fig. 2. Potentials provided by high heat stability and by flowability of plastics melt.

The 7-form of P.V.19 is also applicable in injection-molded and extrusion-made polyamide. It satisfies not only the high thermal requirements in connection with these purposes but has the added advantage of being, like P.R.122 and 209, chemically inert to the slightly alkaline and reducing plastic melt. 

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