Surface finish extrusion

The low Mg -t- Si content of H9 facilitates the production of complex extrusions with a good surface finish making H9 a natural choice for glazing sections and other architectural features. Higher mechanical properties are obtainable with the H20 and H30 compositions, which are therefore more suitable for load bearing structures. 

In polymer blends, or mixtures, the primary question is whether one of the components segregates preferentially to the surface. One of the reasons this is of interest is because most commercial polymers contain more than one component and a surface segregation of one of the components from a miscible mixture during, for example, extrusion of the material, could affect the surface finish of the product. Because polymer blends are generally dense liquids, from the previous discussion it is clear that packing effects are expected to dominate the surface properties. 

The advantage of calendering over direct sheet extrusion is that a complicated and expensive die is not required. Some additional mixing can be done as part of the calendering process, as well, and a surface finish can be applied by the final roller, if desired. Sheet up to a few millimeters thick and a meter wide can be made using calendering. 

In this process the plastic is directed against the surface of the substrate immediately before its entry into the nip between a pressure roll and a chill roll. The extrusion rate and the line speed of the substrate determine the coating thickness. The chill roll controls the surface finish of the coating. Extrusion coatings are typically applied in thin layers, ranging down to 0.005 mm. 

In the film blowing process where a continuous stable parison is blown from an annular die, it is crucial that the molten polymer exhibits certain elastic extensional properties and it is here that the viscoelastic nature of the polymer is beneficial.If, however, the manufacturer is concerned with profile and surface finish of an extrudate, viscoelastic effects of the polymer may well present difficulties. Both die swell and most polymer extrusion instabilities are linked to viscoelastic effects and as such different levels of viscoelasticity give rise to different extrusion characteristics. 

For hundreds of years sticky surfaces have been dusted with powder (e.g., talc) to keep them separated. Talc is broadly used in cable and profile extrusion to obtain a smooth surface. Similarly, in injection molding, the application of aluminum trihydroxide gives a better surface finish. Talc, CaCOs, and diatomite provide anti-blocking properties. Graphite and other fillers decrease the coefficient of friction of materials. PTFE, graphite and M0S2 allow the production of self-lubricating parts. Here, PTFE, a polymer in powder form, acts as a filler in other polymers. Matte surfaced paint is obtained by the addition of silica fillers. 

The rheological properties of molten polymers influence many aspects of processing. In injection molding and extrusion they affect screw pumping efficiency, generation of mechanical heat, and die pressure (and hence output rate). They also influence die and mold weld lines, surface finish, product dimensions, etc. 

Thin section panel results. The results of the thin section panel evaluation are presented Table 3. Trial mixes PINR and PINT offered very similar workability and mesh bond characteristics. The difference in mesh concrete bond can be seen in Fig. 5 which compares the bond between the mesh and mix PINT with that of the control mix CL. The trial mixes PINR and PlNl gave the best all round balance between workability and finish. As can be seen from Table 3, the trial mix P2NL may suit an extrusion process very well. This possibility was identified during the construction of the thin section panel specimens and will require further research. During the application of the mix mortar it was noticed that additional trowel pressure gave the mortar a putty like appearance which also served to improve the surface finishes of the manufactured specimen. 

Film is usually extruded down onto a chill-roll assembly (Fig. 5 44103) gjjjj ig cooled and drawn by two or more water-cooled rolls while the surface of the chill roll imparts a finish to the film. Alternately, the film can be extruded down into a trough of water (Fig. 5.45 ). In the water bath, the film passes under a guide shoe before it is raised out of the water. Since chill rolls cool more efficiently than the water bath, they increase transparency and gloss, decrease haze, and provide a better surface finish to the extruded film. Consequently, chill rolls are preferred even though they are more expensive than a water bath. Water cooling is limited to films, such as tarps, where good surface finishes are not required. Chill rolls (Fig. 5.46 ) are also used in sheet extrusion. However, the sheet is typically extruded horizontally into nip between two chill rolls and then wrapped around another roll. 

Sovitec of Belgium markets Microperl sohd glass beads for engineering thermoplastics such as polyamides. They are said to act like lubricants and to increase productivity in injection moulding and extrusion. They also provide good surface finish and resistance to scratching. 

The extrusion stage must always be free from any screens and pressure plates to allow ume-stricted, uniform extrusion. The extrusion head temperature ensures a smooth surface finish to the extruded slug (billet). The optimal temperature, anywhere between 60 and 90°C, depends on the line output and product formula (Figure 9.6). 

There was also an indication of severe adhesive fracture occurring during the extrusion process. When the extrusion was performed under lubricated condition or under dry conditions at high temperature, the surface was smooth. High density polyethylene could not be extruded satisfactorily under dry conditions at room temperature, but its extrusion at elevated temperature resulted in a smooth surface extrudate. Thus, from the point of view of surface finish, the extrusion of polymeric materials is recommended only under lubricated conditions or under dry conditions at elevated temperatures. 

Various compounds (often a second polymer) are used to modify the rheological properties of a polymer during processing to provide higher outputs, better surface finish, and easier handling in general. For example, low molecular weight polyethylene is added to PVC to improve extrusion behavior. 

This findings strengthen the need for mirror-polished specimens with an average surface roughness Rj < 1 pm to derive reference S-N curves. Intrusions and extrusions are surface defects of the order of some micron (see Sect. 1.4). If surface finish is rather rough leaving material plicas higher than these few microns... 

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