Flow marks

Figure 8-67B. For sieve tray layout arrangements typical one-, two-, and four-pass tray flow patterns with clarifying flow markings by this author. Used by permission, Glitsch, Inc., Bui. 4900-5th Ed.

To meet the products functional needs the wall thickness of the part can be varied. However, to obtain optimum processing cycles and reduce the chance of sink- and flow-mark formation, wall thickness should be kept as uniform as possible. Where variation in wall thickness is unavoidable, gradual transitions in the wall thickness should be provided to ease the polymer melt flow transitions. Part design, mold design, and gate location should work together to allow the material from heavier section into thinner walls for best appearance and process latitude. 

Flow marks -imperfections, cracks in the surface, causes flowing in uncured rubber. 

H. Hamada and H. Tsunasawa, Correlation between Flow Mark and Internal Structure of Thin PC/ABS Blend Injection Moldings, J. Appl. Polym. Sci., 60, 353-362 (1996). 

Equations (14) and (17) can be simplified by introducing a corrected molar flow marked with a tilde, f. The molar flow of the gas may change due to two reasons first, the adding of another flow to this gas stream or the removal of a part of the gas and second, the change in total mole numbers due to chemical reactions. The correction eliminates the second effect ... 

If excess material were added to the bank without lateral transport the bank would grow in size but in fact material is transported towards the edges of a film, beyond the edges of the feed and so increasing the width of the product. Usually, after the ultimate nip, an excess width of film is trimmed away as waste if this is not done care must be taken to make sure there is no variation in the surface finish in the areas of extended width. Occasionally a thin line of roughness will indicate the end of the width of material on Roll 3, and in the extended area flow marks are more likely to be found. 

Circulation within the bank depends upon the position of the currents and these are not necessarily constant across the width of a calender. Examination of a bank may reveal sluggish areas, some rotating in opposite directions, or what appears to be spiralling (the last, because of local variation in the temperature of the stock, often will result in flow marks in the film). Defects of this nature in the flow are influenced by the composition, and it may be necessary to adjust the amount of lubricant however, if a mix is long-established the indication probably is that the conditions of calendering have drifted away from the optimum—and appropriate corrections must be made. The size of a bank can be altered by adjusting the previous nip or by changing the speed ratio of the rolls, and often this will rectify a fault. 

Two-component injection machines, using separate injection units for each of the two components perform the standard sandwich molding process. This process is characterized by a sequential injection of the two components using the same gating system. After filling the mold partially with the skin material with one injection unit the core component will be injected by the second injection unit. In order to avoid flow marks on the part surface a simultaneous phase is inserted between the injection of skin and core components. The final packing phase may be performed either with the core or the skin component. 

Many defects in molded sheet can be related to fillers. Blistering may be caused by moisture but also by insufficient wetting of fibers and entrapped air pockets. Inconsistent cure and thickening is caused by the presence of moisture. Improper fiber orientation may initiate Ifacture cracks. Flow marks, streaks, warpage, and protruding fibers are also caused by improper fiber orientation. 

The open-system calorimeter allows a constant flow to pass through a reaction tube surrounded by a water bath (Figure A5.2). The reaction is initiated and the apparatus is run until a steady state is reached, with the temperature in the centre of the reaction tube held equal to the inlet temperature by a control system (not. shown). The temperature drop across the reaction tube will thus be rendered negligible. The heat flow marked on the diagram is from the water bath to the reaction tube, in accordance once more with the sign convention of equation (13.34), although an exothermic reaction will cause the heat to flow out of the reaction tube and into the water. 

One of the major advantages of compression molding is that it is relatively inexpensive because of its simplicity. Also, since there are no spmes, runners, and gates, material waste is reduced considerably. The consistency of part size is good and the absence of gate and flow marks reduces finishing costs. 

Transparent film without facial flow marks and pits. [Data from Lin, t-h, us Patent 6,649,105, Nov. 18,2003.]... 

The elimination of facial flow marks and pits is accomplished by manufacturing technology rather than formulation. In the invented method, film from the calender directly enters a water bath tank which partially cools material and crystallizes it. Material then passes a set of pressing rollers which preferably include 3 sets of rubber/metal rollers. 

In this example the sample temperature is controlled by circulation of liquid through the outer cylinder housing (flow marked in and out in Figure 2.4) and h denotes the sample height within the shearing gap. The shear rate may be calculated from... 

Flow mark n. A defect in a molded article characterized by a way surface appearance, caused by improper flow of the resin into the mold, which itself may have a number of causes. 

Eliminate stress and flow marks Improve cosmetic surfaces Yield clean parts, ready for finishing In pultrusion, they help ... 

Prototype moulded sight tube in acrylic showing severe weld lines and flow marks (in shadow). 

Plate 23. Witness difficult to see, and weld just visible on pointed core, and dull flow marks at core adjacent. 

Plate 24. Good gloss overall. Hatch marks OK. Welds still visible in reflection, but flow marks very close to them. 

Lubricants Normal reduce resin viscosity without adverse effect on properties better flow at lower temperatures, reduced flow marks and knit lines. Nylon types increased production, higher throughput and improved quality of film... 

As well as improving mould release. Axel s latest Mold-Wiz pellets can reduce the resin viscosity without adverse effect on physical properties. This makes for better flow of the resin at lower temperatures, and reduced flow marks and knit lines on moulded products - all of which make a significant contribution to increased productivity. Compared with commodity lubricants, the MoldWiz... 

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