Poor Mold Filling

We discuss some of these regions in detail below. In addition, we concern ourselves with the overall flow pattern during filling. Recall that the manner in which a mold is filled— that is, the location of the advancing melt front—affects the weld-line location and the orientation distribution and may be responsible for poor mold filling conditions. 

The distribution of sizes is at least as important as the mean size. Very large and very small particles both tend to be detrimental to the properties (Fig. 23.2). Small particles result in high viscosity and therefore loss of processability as evidenced by poor mold filling and loss of extruder throughput. Large particles act as flaws. Stress concentrations are high around large particles and they lead to a dramatic reduction... 

Macroscopic product problems that can result from poor control in injection molding include, but are not limited to voids and sink holes on the surface generally due to poor mold filling or low pressure, incomplete mold filling, weld lines and flow marks, warping or distortion of parts, high shrinkage, and so forth. 

Figure 4.70. Processability diagram (moldability) for the stage of mold filling in reactive injection molding dependence of material temperature (or average temperature on flow rate G). I - premature filling II - poor impregnation mixing III - flow instabilities.

Turbulence proprieties. Used to describe an erratic, tumbling flow of liquid elastomer through a mold or cavity during filling. Usually caused by poor mold design, incorrect location of the polyurethane pouring point, obstructions in the mold or cavity, or high polyurethane viscosity. 

Eq. (12) has commonly been used, e.g. in the analysis of mass and gas transfer in gas-filled systems. From this relationship it may be deduced that varies from 0 to 00 and that = 1 when G = 0.5. Systems for which G 4 0.5 are poorly gas-filled ( low-porous ) and those with G > 0.5 are highly gas-filled . The rule of reciprocals ( reversal rule ) facilitates the analysis of gas-filled structures such as foamed plastics by enabling the use of the so-called complementary gas-filled (porous, cellular) systems. The complementary systems relate to each other as a mold and casting or negative and positive . 

Common problems like insufficient filling-packing and poor dimensional control are often related to the gate size and design. Similarly, gate location is another important factor. They should be located in areas having heaviest cross-section of the part to assure fill-out and elimination of sink marks. Also their position should not facilitate the residual molded stress formation in the part, knit line formation. 

In RIM processes, two or more reactive components are mixed together, starting the reaction between the components before the mixture is dispensed into the mold. This tends to increase the viscosity of the liquid that is dispensed due to an increase in molecular weight of the polymers or pre-polymers formed in the initial reaction. An increased viscosity can prohibit complete filling of the mold and permeation of the preform. This tends to decrease the adhesion between the matrix and the fibers. Poor interfacial adhesion between the reinforcement and matrix phase can cause a material to have less than desirable stiffness and strength. 

The notched Izod impact strength of the kenaf system is much lower than that of the glass fiber-filled PP but about the same as all other fillers and mica systems. Short fiber lengths present in the kenaf system due to the compounding system used and molding are probably responsible for the poor impact strengths [30]. 

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