Process system shrinkage

Let us write this problem in more detail. The process of shrinkage is described by the last equation, which is written for two zones. The motion of the border between the two zones is determined from the condition of equality of the filtration expenses. Then for two zones we have instead of (4) the next system of equations ... 

So the physical process of shrinkage is coupled to some extent with the chemical process of polymerization but shrinkage is not a measure of conversion, as is normally observed with liquid systems. 

Shrinkage is a complex parameter and can be influenced by many interrelated factors, such as machinery, mold design, part geometry and direction of flow, processing conditions, and molding system. Shrinkage is critical to control the dimensions of the finished part. Flow chart 5.2.19 shows the way to approach and solve the problem of shrinkage. 

A variety of thermosetting resins are used in SMC. Polyesters represent the most volume and are available in systems that provide low shrinkage and low surface profile by means of special additives. Class A automotive surface requirements have resulted in the development of sophisticated systems that commercially produce auto body panels that can be taken direcdy from the mold and processed through standard automotive painting systems, without additional surface finishing. Vinyl ester and epoxy resins (qv) are also used in SMC for more stmcturaHy demanding appHcations. 

A Multi-faceted, Hierarchic Analysis cf Chemical Micro Process Technology 1.5.1.4 Shrinkage of Total System... 

One implication of micro reactors on chemical-process engineering concerns the shrinkage of the total system. This is exemplarily discussed for catalyst testing. 

During the process, the solute diffuses into the intercellular space and, depending on the characteristics of the solute, it may pass through the membrane and enter the intracellular space. Differences in chemical potentials of water and solutes in the system result in fluxes of several components of the material and solution water drain and solute uptake are the two main simultaneous flows. Together with the changes in chemical composition of the food material, structural changes such as shrinkage, porosity reduction, and cell collapse take place and influence mass transfer behavior in the tissue. 

Transparent polyolefin systems containing P.R.88 are stable up to 260 to 300°C, depending on the type and on the pigment content. 1/3 SD systems are heat stable up to approximately 240 to 260°C. Some types are recommended only for use in LDPE at low processing temperatures. The lightfastness of such specimens is between step 6 and step 7 on the Blue Scale. P.R.88 considerably affects the shrinkage of injection-molded articles, a feature which somewhat restricts its application in such systems. 

SD HDPE systems (1% TiOz) are thermally stable up to 250°C, while 1/25 SD specimens withstand 280°C. P.Y.182 does not affect the shrinkage of the plastic. 0.37% pigment is needed to formulate a 1/3 SD HDPE sample. Combining P.Y.182 with nickel stabilizers in polypropylene is to be avoided. The pigment dissolves in polystyrene as the temperature reaches 200°C, a process which is accompanied by considerable color change. P.Y.182 is also an unsuitable candidate for ABS. 

In Figure 4 the rate of polymerization, as measured with DSC, is compared with the rate of shrinkage, as measured with TMS. Contrary to our previous observations with HDDA (7) it seems that the shrinkage process is faster than the polymerization. The relative rate of shrinkage not only exceeds the relative rate of polymerization, it also reaches its maximum value at an earlier time. Since TEGDA polymerizes much faster than HDDA, this effect may well have been caused by the inertia of the DSC. As a check we measured the response times of the two systems separately. 

The characteristics of the three most common thermoset resin systems used in pultrusion are compiled in Table 11.2 [3]. It is noteworthy that unreinforced polyesters and vinylesters shrink 7-9% upon crosslinking, whereas epoxies shrink much less and tend to adhere to the die. These epoxy characteristics translate into processing difficulties, reduced processing speed, and inferior component surface finish. It is normal practice to use resin additives to improve processability, mechanical properties, electrical properties, shrinkage, environmental resistance, temperature tolerance, fire tolerance, color, cost, and volatile evaporation. It is normally the resin, or rather its reactivity, that determines the pulling speed. Typical pulling speeds for polyesters tend to be on the order of 10-20 mm/s, whereas speeds may exceed lOOmm/s under certain circumstances. Apart from the resins characterized in Table 11.2, several other thermosets, such as phenolics, acrylics, and polyurethanes, have been tried, as have several thermoplastics (as will be discussed in Sec. 11.2.6). 

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