Extrudate Thickness

A variety of methods are available to measure extrudate thickness. The methods can be broadly classified into contacting and non-contacting techniques. The contacting thickness measurement techniques are generally simple and inexpensive however, the contact of the transducer with the extrudate can adversely affect the extrudate surface quality. In cases where the requirements for surface quality are very high, non-contacting thickness measurement is generally preferred. 

In the contacting measurement techniques, the micrometer caliper is a common instrument. The micrometer, however, can only be used for pot measurements and this is done manually. A spring-loaded dial gauge can be moved over the extrudate if the thickness variations are small. Thus, the dial gauge can be used to monitor the variation of thickness with time, i. e., in the extrusion direction. If an accurate traversing mechanism is constructed, the dial gauge can also measure the thickness variation perpendicular to the extrusion direction. At the point of measurement, the opposite side of the extrudate has to be firmly supported to avoid measurement errors. 

The micrometer and dial gauge are simple mechanical devices. In many cases, one would like to have a continuous record of the thickness measurement. The LVDT (linear variable differential transformer) provides an electrical signal that can be used to monitor the thickness on a recorder. The LVDT is a device in which the displacement of an iron core changes the inductive coupling between primary and secondary coils see Fig. 4.20. 

Movement of the core produces an AC output signal that reflects the amount and direction of movement. If an LVDT is used in thickness measurement, one has to check the effect of temperature on the accuracy because the extrudate is generally at an elevated temperature that may not be constant. The LVDT can be quite accurate it can measure to an accuracy of about 1 pm. 

Another more or less contacting thickness measurement technique is pneumatic gauging. The device consists of a nozzle fixed in position relative to a stop. Air at a constant supply pressure passes through a restriction and discharges through the nozzle see Fig. 4.21. 

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Figure 10 Mechanism of extensional flow efTects in dies and die swell from elastic deformation, extrudate thickness or diameter D2...

Calvert et. al. describes a theoretical model to calculate the bum out time as a function of the extrudate thickness without bubble formation [Cal 90]. By increasing early development of porosity within the sample, higher debinding temperatures can be used and consequently faster burnout can be achieved. Furthermore the products of the binder decomposition should have a high vapor pressure and should diffuse rapidly through the extruded body. 

The turbo-tray dryer can handle materials from thick slurries [1 million (N s)/m (100,000 cP) and over] to fine powders. It is not suitable for fibrous materials which mat or for doughy or tacky materials. Thin slurries can often be handled by recycle of dry product. Filter-press cakes are granulated before feeding. Thixotropic materials are red directly from a rotary filter by scoring the cake as it leaves the drum. Pastes can be extruded onto the top shelf and subjected to a hot blast of air to make them firm and free-ffowing after one revolution. 

The body is impact extruded from a cold forming steel. The characteristic dimension to be analysed in the tolerance stack is the base thickness of 3 mm (on a 020 mm bore) and this dimension has been assigned a tolerance of 0.02 mm. 

If the extruder is to be used to process polymer melts with a maximum melt viscosity of 500 Ns/m, calculate a suitable wall thickness for the extruder barrel based on the von Mises yield criterion. The tensile yield stress for the barrel metal is 925 MN/m and a factor of safety of 2.5 should be used. 

There are many applications in which it is necessary to put a plastic coating on to paper or metal sheets and the extruder provides an ideal way of doing this. Normally a thin film of plastic is extruded from a slit die and is immediately brought into contact with the medium to be coated. The composite is then passed between rollers to ensure proper adhesion at the interface and to control the thickness of the coating (see Fig. 4.26). 

Example 5.8 It is desired to blow mould a plastic bottle with a diameter of 60 mm and a wall thickness of 2 mm. If the extruder die has an annular slot of... 

During extrusion blow moulding of 60 nun diameter bottles the extruder output rate is 46 X 10 m /s. If the die diameter is 30 mm and the die gap is 1.5 mm calculate the wall thickness of the bottles which are produced. The flow curves in Fig. 5.3 should be used. 

During the blow moulding of polypropylene bottles, the parison is extruded at a temperature of 230°C and the mould temperature is 50°C. If the wall thickness of the bottle is I mm and the bottles can be ejected at a temperature of I20°C estimate the cooling time in the mould. 

A polythene tube of outside diameter 40 mm and wall thickness 0.75 mm is to be extruded at a linear speed of 15 mm/s. Using the 170°C polythene flow curves supplied, calculate suitable die exit dimensions. 

Thus approximate dimensions of extrudate would be 42.3 mm deep x 21.2 mm wide x 3.98 mm thick. 

Many other polymers compete with cellophane such as polyethylene which is extruded as a tough film or in greater thickness as a nonbreakable bottles. Vinyl products used in films are polystyrene, polyesters, and nylon. A chemical derivative from nature rubber, chlorinated rubber, gives films of extraordinary stretch ability. 

For the composite spoiler design, the bottom is a variable-thickness skin on one side in Figure 1-33, but with composite materials that construction is not difficult. We do not have to chem-mill a composite material to change its thickness. All we do is stop building up the material in layers in the middle, but continue to build it up at the sides. That s a very natural process for composite materials and does not involve a costly machining operation. Instead of machined extruded stiffeners, a honeycomb core is placed on the inside of the laminae. That honeycomb... 

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