Wall draw ratio

Another important parameter is the wall draw ratio (WDR). This is the gap between the tip and die divided by the wall thickness of the tubing. 

The draw ratio balance should be equal to or larger than one DRB 1. Yet another parameter that is used is the sizing ratio or SR. This is the wall draw ratio divided by the diameter draw ratio. 

Example 4.8 A small flower pot as shown in Fig. 4.56 is to be thermoformed using negative forming from a flat plastic sheet 2.5 mm thick. If the diameter of the top of the pot is 70 mm, the diameter of the base is 45 mm and the depth is 67 mm estimate the wall thickness of the pot at a point 40 mm from the top. Calculate also the draw ratio for this moulding. 

Plug assist is used principally when the process is likely to lead to undue variation in the product wall thickness. Plug assist supplies essentially a selective or localized stretch that is related to the specific demands of an individual mold cavity. It is likely to be beneficial when the draw ratio of a product feature is high, and when the product includes edges, corners and other features where excessive stretch and thinning is likely to occur. Plug assist is preferred for plastics with high enthalpy and low thermal conductivity such as polypropylene sheet formed in the solid phase. 

An attempt was first made at poling after the neck. The 6 1 machine draw ratio tube (OD 1.4 mm, wall thickness 290 ym) was poled at a corona voltage of +30KV, 15 seconds after it had emerged from the furnace where necking had occurred, but only a low g coefficient of 4.9 x 10 2 Vin/N was obtained. 

Tube No OD (ym) Wall thickness (ym) Machine draw ratio True draw ratio Draw speed (m/min) Corona Voltage (kV)... 

Billow forming n. A variant of thermoforming, in which the hot plastic sheet is clamped in a frame and expanded upward with mild air pressure against a male plug or female die as the plug or die descends into the frame. The process is suitable for thin-walled containers with high draw ratios. 

Thus, the wall and diameter draw ratios determine the area draw ratio in the extrusion process. A high ADR increases orientation and the chance of pinholes and breakaways. A low ADR reduces orientation and increases the chance of melt fracture. 

F. Forming with billow plug is often used to produce thin-wall items with depth-to-diameter-draw ratios up to 1.5 1. 

Fig. 4 Relationship between extrudate (a) tensile modulus and (b) orientation parameter with die L/D. Samples prepared at extrusion temperatures 290°C (filled symbols) and 300 C (unfilled symbols) at draw ratio 4. Samples extruded from a 0.76-mm die at a wall shear rate of 720sec (Redrawn from Ref. 32.)...

FIfl. 5 Relationship between (a) tensile modulus, (b) tensile strength, and (c) orientation with spinline draw ratio of extrudate prepared using dies of L/D = 0.5 ( ), 10 (O). and 133 ( ) at extrusion temperature 290°C, Samples extruded from 0.76 mm-diameter dies at a wall shear rate of 720sec". (From Ref. 46.)... 

Limiting drawing ratio (LDR) for wall breakage of both quenched and tempered sheet (0.7 mm thick). Specimen numbers had the following average properties (see table on next page). 

A small tube is usually one that has an outside diameter of 5 mmf l or less and a wall thickness of less than about 1 mm. A free extrusion technique, similar to the wire coating process, is the method by which small tubing is manufactured. In this case, the desirable drawdown ratio is much smaller than that of a wire insulation process. The draw-ratio balance should be equal to 1. Outside diameter can be controlled by placing a sizing die at the entrance to the quench bath. Air pressure can be employed to expand the tube against the die. The air pressure should be slight due to the low melt strength of the melt cone. 

Figure 5.5 Volumetric density of compressed hydrogen gas as a function of gas pressure including the ideal gas and liquid hydrogen. The ratio of the wall thickness to the outer diameter of the pressure cylinder is shown on the right-hand side for steel with a tensile strength of 460 MPa. A schematic drawing of the pressure cylinder is shown as an inset.

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