Extrusion profile

Inserting Eq. E12.3-12 into Eq. E12.3-11 gets the melt temperature increase of each layer of melt fluid throughout the die. 

We now discuss the residence times and temperature increases of layers close to the die wall using the preceding equations At r — Ra 0.1 mm and r — R 0.01 mm, the residence time and the temperature increase of the two layers are, respectively, 0.008 s, 77°C and 0.082 s, 704°C. On the other hand, on the core surface (r — Rf), the residence time is 0.0005 s, and the temperature increase only 38°C. It is obvious that the closer the melt layer is to the die wall, the residence time is longer and the melt temperature during transit increases in an exponential fashion. Despite the very high temperature increases, the residence time of the melt layers near the wall is short and much shorter than the degradation induction time 0(T) (see Fig. E5.1(a), which is for unplasticized PVC). Thus, degradation is not likely to occur, and the wall melt layer has such a small viscosity that it precludes melt fracture. 

If the melt viscosity is considered as a function of temperature, then the momentum and energy equations will have to be solved simultaneously. Nevertheless, the results concerning the temperature increase of the melt layers near the wall will be only slightly different from that just given. The resulting polymer coat thickness can be calculated by equating the volumetric flow rates inside and outside the die, namely  

Solving for h, the polymer melt-coating thickness, we obtain h — 0.45 mm. Thus, taking into account the density increase upon solidification, the solid polymer coat thickness is hs — 0.37 mm. 

Profiles are all extruded articles having a cross-sectional shape that differs from that of a circle, an annulus, or a very wide and thin rectangle (flat film or sheet). The cross-sectional shapes are usually complex, which, in terms of solving the flow problem in profile dies, means complex boundary conditions. Furthermore, profile dies are of nonuniform thickness, raising the possibility of transverse pressure drops and velocity components, and making the prediction of extrudate swelling for viscoelastic fluids very difficult. For these reasons, profile dies are built today on a trial-and-error basis, and final product shape is achieved with sizing devices that act on the extrudate after it leaves the profile die. 

The final zone of an extruder is the metering section (also known as the pumping zone). The principal function of this zone is to ensure a steady output of molten polymer at a constant pressure. The longer the metering zone, the greater the pressure buUt up within it. Channel depth is constant to ensure a uniform transport rate, vhich helps reduce pressure fluctuations arising in the mixing zone. 

The build up of pressure is aided by the use of a breaker plate, which is a thick disk of perforated metal positioned just downstream of the tip of the screw. A screen pack, comprising layers of woven metal mesh, is placed on the upstream side of the breaker plate. The screen pack increases back pressure within the metering zone and acts as a filter to catch any extraneous material that may have entered the extruder. 

We use profile extrusion to make continuous products that have fixed cross-sectional dimensions, such as pipes, house siding, refirigerator door gaskets, and vindshield wiper blades. During profile extrusion the molten output from an extruder is pumped to a die where it is formed to approximately the desired cross-sectional profile. As the molten polymer leaves the die, we apply the final forming step and simultaneously cool it to yield the product in its solid state. 

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Report 104 Plastics Profile Extrusion, R.J. Kent, Tangram Technology Ltd. 

We use variants of profile extrusion to produce tubing -with diameters of less then 1 mm and pipes with diameters exceeding 1 m, Wall thicknesses can vary from a few tens of micrometers up to several centimeters. Extruded window and door frames are more complex than pipes. Such profiles are largely hollow with internal ribs and fins that reinforce and divide the interior into two or more channels. We use solid rubber profiles in applications such as door seals and windshield wipers. We can produce foamed extrudates by incorporating a blowing agent, such as butane or carbon dioxide, into the polymer in the molten state. As the polymer exits the die, its internal pressure drops and the dissolved gas expands to form bubbles within the product. Examples of foamed extrudates include pipe insulation and automobile door gaskets. 

Sheet and profile extrusion, low density resin for, 20 233t... 

PBT-PC blends show increased melt strength allowing them to be easily processed by blow molding and profile extrusion. The PBT-PC blends have been extruded into sheet and thermoformed into parts. Enhanced melt strength allows PBT-PC blends to be foamed. Structural foam grades for injection molding (10-30% density reduction) are commercially available. 

Matsuoka, T. and Takahashi, H., Finite Element Analysis of Polymer Melt Flow in Profile Extrusion Coating Die, Int. Polym. Proc., 3,183 (1991)... 

A profile extrusion line was required to increase its rate from the current 75 kg/h to about 230 kg/h in order to meet business demand. The extruder was 88.9 mm in diameter and was running a HIPS resin. Operation of the existing extrusion equipment, however, caused the extrudate temperature to be too high at rates higher than about 80 kg/h. The objective of this project was to increase the rate of the profile line from the current 75 kg/h to a maximum rate of 230 kg/h while maintaining the extrudate temperature below 195 °C. 

Table 12.2 Screw Channel Dimensions for a 63.5 mm Diameter Screw Running GPPS Resin for a Profile Extrusion Line...

The industrial use of twin-screw extruders for this purpose revolves extensively, but not exclusively, around intermeshing co-rotating variants. Closely in-termeshing counter-rotating designs are widely used for profile extrusion of UPVC dry-blends since they permit close temperature control and exhibit a high conveying efficiency due to the positive displacement of material where the screws intermesh [150]. 

PVC has a unique ability to be compounded with a wide variety of additives, making it possible to produce materials that range from flexible elastomers to rigid compounds, that are virtually unbreakable. Compounds arc also made that have stiff melts for profile extrusion or low viscosity melts fur thin-walled injection molding. 

Let us examine more closely some of the problems that arise in designing profile extrusion dies whose origin is to be found in the flow patterns. We consider the square-tube flow patterns calculated for a Power Law fluid of n = 0.5 (Fig. 12.47). Although the velocity profiles are symmetric, they are still 0-dependent, 6 being the angle in the... 

Fig. 12.49 Flow channel of a profile extrusion die identification of its main zones and geometrical controllable parameters considered in the definition of the preparallel zone (PPZ).

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