Flow Length

The flow ratio is the flow length in the mould divided by the wall thickness of the moulding. A number of factors can affect the flow ratio such as melt flow rate, the processing conditions and the features of the mould. Thick sections enable higher flow ratios to be achieved while very thin sections have the opposite effect. The flow length of the material provides tire mould designer with a guide to the minimum number or position of feed points which will be advisable. 

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G = mass flux V = specific volume of stream P = stream pressure f= friction factor D = duct diameter L = flow length g = acceleration due to gravity b = angle of inclination from vertical... 

In practice the clamping pressure will also depend on the geometry of the cavity. In particular the flow ratio (flow length/channel lateral dimension) is important. Fig. 4.42 illustrates typical variations in the Mean Effective Pressure in the cavity for different thicknesses and flow ratios. The data used here is typical for easy flow materials such as polyethylene, polypropylene and polystyrene. To calculate the clamp force, simply multiply the appropriate Mean Effective Pressure by the projected area of the moulding. In practice it is... 

Example 5.13 Derive an expression for the flow length of a power law fluid when it is injected at constant pressure into a rectangular section channel assuming... 

A power law plastic is injected into a circular section channel using a constant pressure, P. Derive an expression for the flow length assuming that... 

A polymer melt is injected into a circular section channel under constant pressure. What is the ratio of the maximum non-isothermal flow length to the isothermal flow length in the same time for (a) a Newtonian melt and (b) a power law melt with index, n = 0.3. 

Using the data in Tables 5.1 and 5.2, calculate the flow lengths which would be expected if the following materials were injected at 100 MN/m into a wide rectangular cross-section channel, 1 mm deep. 

The parameters of pipe bore, flow, length of pipe, and specific gravity can be used to determine pressure loss. The calculators are versatile in that they can be used for pressure losses of under 1 mbar and for up to 50 bar. Scales for both cast iron and steel pipe are available for greater accuracy. 

Project Engineer uses numerical input values of maximum flow length, nominal wall thickness, and projected area to describe the part geometry. 

Bond (A-Pg)l g Bo = a gravitational force 3 10-2 1 10-2 Relevant for bubble (droplet) flows. Length scale bubble diameter... 

If the length scales associated with changes in velocity are normalized by Vv (characteristic length scale for Stokes flow), length scales associated with changes in curvature are normalized by Ss (typical striation thickness) and velocities normalized by V (a characteristic velocity), then the normal stress condition becomes,... 

It is also informative to express these results in dimensionless form, i.e., in terms of appropriate dimensionless groups. Because this is a noncircular conduit, the appropriate flow length parameter is the hydraulic diameter defined by Eq. (5.48) ... 

The permeability of a porous medium (K) is defined as the proportionality constant that relates the flow rate through the medium to the pressure drop, the cross-sectional area, the fluid viscosity, and net flow length through the medium ... 

In two-phase flow, most investigations are carried out in one dimension in the steady state with constant flow rates. The system may or may not be isothermal, and heat and mass may be transferred either from liquid to gas, or vice versa. The assumption is commonly made that the pressure is constant at a given cross section of the pipe. Momentum and energy balances can then be written separately for each phase, and with the constraint that the static pressure drop, dP, is identical for both phases over the same increment of flow length dz, these balances can be added to give over-all expressions. However, it will be seen that the resulting over-all balances do not have the simple relationships to each other that exist for single-phase flow. 

There are a number of ways to reduce this nonuniformity. The mandrel can be placed eccentrically in the die, allowing for a wider gap at the remote end from the lead port. Such a design can, in principle, provide uniform flow rate, but shear rate and temperature histories will remain nonuniform. In another solution [Fig. 12.42(b)], an insert directs the flow at the far end upward, reducing the flow length around the mandrel and eliminating slow flow (stagnant) regions. In addition, the mandrel is also eccentric. 

The first correction is for entrance and end effects and was suggested by Bagley (1957). Due to these effects, in the calculation of the shear stress at the wall one has to use an effective flow length Le(p. 

Richardson (1985) obtained, on the basis of an analytical model of the flow in a cooled spiral mould of rectangular cross section at fixed wall temperature, for the flow length... 

Fritch (1986) gives the following advice regarding flow length testing ... 

Ease of care properties, 884 Effective flow length, 591 Effect of... 

Wash-action erosion of cavities is minimal and mold maintenance is low since melt flow length is short. 

Here Ap is the pressure difference along the capillary, rj the viscosity of the liquid, r and w are as before, and L is the flow length in the capillary. The volume transferred in time t is ... 

Part of the capillary is inside the needle holder, so the flow length is longer that that of the protmstion of the needle ... 

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