Internal coefficient of friction

The coefficient of friction of the bulk material is another very important property. One can distinguish both internal and external coefficient of friction. The internal coefficient of friction is a measure of the resistance present when one layer of particles slides over another layer of particles of the same material. The external coefH-cient of friction is a measure of the resistance present at an interface between the polymeric particles and a wall of a different material of construction. The coefficient of friction is simply the ratio of the shear stress at the interface to the normal stress at the interface. Friction itself is the tangential resistance offered to the sliding of one solid over another. 

Figure 7.25 shows cpb as a function of the screw helix angle when 4 = 4 = 0.2, fj = 0.6, and Hj = 15.24 mm (0.6 in). The optimum barrel helix angle increases with the screw helix angle and with reducing barrel groove depth. The optimum barrel angle is relatively insensitive to the internal coefficient of friction as shown in Fig. 7.26. 

Optimum barrel groove helix angle versus the internal coefficient of friction... 

Dekker [3] studied the effect of various flight geometries on solids conveying performance. He proposed that many extrusion instabilities might be due to internal deformation of the solid bed. Internal deformation is more likely to occur when the internal coefficient of friction of the polymer particles is low. Spherical particles tend to have a lower internal coefficient of friction than non-spherical (e.g. cylindrical) particles and are, therefore, more susceptible to internal solid bed deformation. This may explain the often observed difference in extrusion behavior between strand pelletized and die-face pelletized material. 

The performance of an extruder is determined as much by the characteristics of the feedstock as it is by the machine. Feedstock properties that affect the extrusion process inciude buik properties, meit flow properties, and thermal properties. Important buik flow properties are the buik density, compressibility, particle size, particle shape, external and internal coefficient of friction, and agglomeration tendency. Important melt flow properties are the shear and eiongational viscosity as a function of strain rate and temperature. The commonly used melt indexer provides only limited information on the meit viscosity. Important thermal properties include the specific heat, the glass transition temperature, the crystalline melting point, the latent heat of fusion, the thermal conductivity, the density, the degradation temperature, and the induction time as a function of temperature. 

Important bulk properties are bulk density, compressibility, internal coefficient of friction, external coefficient of friction, particle size and particle size distribution, and particle shape and particle shape distribution. 

The internal coefficient of friction is the friction between the plastic particles themselves. The external coefficient of friction is the friction between the plastic particles and another surface, such as the barrel surface. For efficient conveying in the feed hopper, it is required that both the internal and external frictions should be low. For efficient conve5ung along the extruder screw, it is required that the barrel friction to be high and the screw friction should be low this is discussed later. 

To ensure a good flow of a bulk solid, a silo must be properly designed from knowledge of the wall friction coefficient of the powder, its internal coefficient of friction and its cohesion [1]. If the storage is long, the temporal evolution of these quantities must also be measured to avoid future blockage by consolidation. 

Here, [L is the coefficient of internal friction, ( ) is the internal angle of friction, andc is the shear strength of the powder in the absence of any applied normal load. The yield locus of a powder may be determined from a shear cell, which typically consists of a cell composed of an upper and lower ring. The normal load is applied to the powder vertically while shear stresses are measured while the lower half of the cell is either translated or rotated [Carson Marinelli, loc. cit.]. Over-... 

Additive, slip An additive modifier that acts as an internal lubricant which exudes to the surface of the plastic during and immediately after processing providing the necessary lubricity to reduce or eliminate coefficient of friction in molded parts, film, etc. products. 

Anodized layer Internal tension Adhesion (Ring-Scher-Test) (always aluminum breaking) Pore-free coating from ca. Coefficient of friction (with lubricant)... 

Systems biology will be important to medicine because data such as expression arrays may need to be interpreted in order to understand internal processes in the patient. Electronic simulation applies whenever the physician must model what cannot be easily seen. This is a general statement that holds for simulation in general. Models of the body with elasticity, plasticity, and coefficients of friction applied to nerves, muscles and organs might be needed to interpret magnetic resonance image movies for osteopathic and sports medicine. 

Processing aids to assist material flow during extrusion or injection molding. The internal and external lubricants are recognized. Internally lubricated resins use oils. Teflon , M0S2, or other materials to give the molded part a lower coefficient of friction. The external lubricant can be a sohd, such as sodium or zinc stearate, a fluoropolymer, or silicone resin or liquid. 

The frictional and wear properties of the polymers used for gears can be greatly enhanced by the addition of internal lubricants. The most commonly used additives are polytetrafluroethylene, PTFE (Teflon, Fluon) silicone fluids graphite molybdenum sulphide, M0S2. PTFE, with its exceptionally low coefficient of friction is very effective as an internal lubricant. Initial shear causes it to form a high-lubricity film over the surface. It is, of course, expensive, and will generally reduce the mechanical properties of the material. 

When 2% silicone is added to acetal copolymer, as an internal lubricant, the compound has good rigidity and toughness, creep resistance, low coefficient of friction, and resistance to most solvents but it also has disadvantages associated with unmodified acetal copolymer, high mold shrinkage, and poor resistance to UV radiation [14]. Typical thermal properties for 2% silicone lubricated acetal copolymer include of 190 to 210°C... 

---