Viscosity, moulding compound

Increase viscosity, e.g. magnesium oxide in unsaturated polyester resins increases the viscosity so much that a liquid resin becomes a tackfree solid, suitable for making sheet moulding compound (SMC). A relatively high viscosity is also desirable for latex compounds used to coat textiles. 

Figure 4.7 shows the steady- and dynamic-viscosity profiles as functions of shear rate for a filled reactive epoxy-resin moulding compound. Here, interestingly, the Cox-Merz rule provides a better correlation than does the modified Cox-Merz rule. 

Han et al (1997) examined the chemorheology of a highly filled epoxy-resin moulding compound that is characterized by a modifed slit rheometer. Results show that a modified Cox-Merz rule relating dynamic and steady viscosities is established, >7(7 ) = (Tm )-Also the material was shown to exhibit a yield stress at low shear rates and power-law behaviour at higher shear rates. The temperature dependence of the viscosity is well predicted by a WLF model, and the cure effects are described by the Macosko relation. 

Saeki and Kaneda (1988) examined the apparent mean viscosity, minimum viscosity and apparent gel time of an epoxy-resin moulding compound during the transfer-moulding operation by using an instrumented transfer mould. 

These equations have not stood the test of predicting viscosities in filled systems and polymer blends. For calculating the viscosities of PVC-PVAc polymer blends (filled and unfilled) used as record moulding compounds, we have analysed from systematic investigations [40] the influence of... 

Therefore, in order to produce plastic moulding compounds with reasonable melt flow characteristics, as a rule, 15 to 35% plasticizer is added for CA and 3 to 25% for CAP and CAB [22]. In Figure 3.11 the melt flow index (MFI), as a measure of melt viscosity, is shown for typical cellulose ester moulding compounds as a function of plasticizer content. Obviously, as expected, the esters with longer side chains are more flexible and need considerably less plasticizer than the shorter chain esters to reach the same flowability at a given temperature. 

The type and amount of filler have an effect on shrinkage and sink marks more filler reduces both, but at the same time increases the mixture viscosity, which is critical when working with moulding compounds. A combination of coarse and fine particles produces the optimum results. Fillers, such as clay, calcium carbonate and wollastonite, judiciously selected and in relatively high concentrations, can also impart flame retardancy and serve as a stress transfer medium, as well as reduce the total material cost. It should be pointed out that fillers strongly influence the flow characteristics of moulding compounds. 

Curatives have been prepared via condensation of (meth)acrylonitrile with various hydrazides such as carbohydrazide, oxalyl dihydrazide, and succinyl dihydrazide (Eq. 16). When these cyanoethylated and cyanopropylated hydrazides are mixed with DGEBA-type resins, viscosity increases are noted in 4-5 days at room temperature. However, these partially reacted mixtures are still uncrosslinked and flowable at temperatures near their final cure temperature of around 120°C. At this temperature reaction occurs very rapidly for a variety of epoxide-to-amine ratios. Adhesive compositions of DGEBA-type epoxy resins and the cyanoalkylated hydrazides give extremely strong bonds on substrates such as sheet moulding compound (SMC) and steel. 

Non-accelerated, medium viscosity, highly reactive, fast curing, fully pigmentable isophthalic resin particularly suitable for the manufacture of polyester moulding compounds offering excellent mechanical strength at elevated temperatures and resistance to mild chemical reagents. 

For calcium carbonate, polyester (GRP, SMC, DMC) is by far the most important of the thermosetting resins and accounts for about 20% of total CaCOs usage in plastics. Low viscosity is often important, especially for sheet moulding compounds, and surface treated grades are beneficial in this respect. Fine particle size grades are used where good surface finish is required, and colour is often critical. 

What viscosity is required for both liquid systems and moulding compounds ... 

Unsuitable moulding compound. The use of a compound with an abnormally low melt viscosity may well lead to bleed problems and possibly flashing. Filler particle size is also important because flash and bleed increase with decreasing filler size. These two parameters are controlled by the moulding supplier and with persistent problems the manufacturer should be consulted for help with flash and bleed control. 

Melt viscosity (voids). The melt viscosity during a moulding operation is one of the most important parameters that influence a moulding compound s mouldability. An abnormally low melt viscosity can give uncontrolled cavity filling leading to the en-... 

For the methods of application above, resin viscosity must be maintained low, and this precludes the use of particulate fillers as inert diluents to cheapen the system. Sheet-moulding compound (SMC) and dough-moulding compound (DMC) prepolymer forms of UPR continue to be a major growth area of GRP technology. 

The viscosity should quickly reach a suitable value on heating in the barrel. It should not be too high since it may be difficult to fill the mould. At the same time it should not be so low that little heat is generated by friction. At the injection melt temperature of 100-130°C the compound should have a good stability but should cure rapidly at the high curing temperatures as exist within the mould. 

Simple ram machines are cheaper than screw machines having equivalent shot volumes. Although the simple ram injection moulding machine was once quite widely used it was generally obsolescent by 1977. Plasticisation by screw is quicker and more controllable than by heat conduction in simple ram machines. A higher and more uniform compound temperature and thus a more uniform viscosity is achieved. 

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