Rotation ratio

The ratio of translational partition functions (Qjr/Qtr) is 1 here, and for all unimolecular reactions, because the mass and number of molecules of the reactants is the same as for the transition state. The rotational ratio (Q, /Q, ) is given by the ratio of the moments of inertia (/ //t/2/3)1/2- The moments of inertia are probably slightly higher in the... 

In rotational molding, also known as rotomolding, the mold (or cavity) is filled with material, either as a solid powder or liquid. The mold is closed, placed in a heated oven, and then rotated biaxially. The mold is then cooled, opened, and the article recovered. Powders of about 35 mesh (500 pm) are typical though different sizes are also employed. The distribution of particles and additives is determined by the mixing/rotation ratio. 

Examples of such kinetic treatments were provided by work on chiral 1,1,2,2-tetramethylcyclopropane-d630 and rran -l-ethyl-2-methylcyclopropane146 148. At 350.2 °C, the first substrate approached cis, trans equilibrium with rate constant, and suffered loss of optical activity with a rate constant k The /c, /c, ratio was 1.7 130. The second substituted cyclopropane, at 377.2 °C, exhibited kinetic behavior dictated by kf.ka = 2.0 1. Using steady-state kinetic treatments and the most-substituted-bond hypothesis, these rate constant ratios were calculationally transformed into (cyclization) (rotation) ratios of 11 1 and 0.29 1, ratios different by a factor of 38. 

Original Rotation. Ratio at Original Rotation. Ratio at ... 

A measured amount of powder or liquid thermoplastic (TP) is placed in the cavity that is mounted on a turret arm capable of rotating the mold. The mold in the oven spins biaxially with rotational speeds being infinitely variable, usually ranging up to 50 rpm on the minor axes and 12 rpm on the major axes. A 4 1 rotation ratio generally is used for symmetrically shaped parts. A wide variety of ratios are necessary for molding unusual and complex shapes. 

Based on the product distribution of their computed trajectories. Doubleday and Hase found that = 2.3-3, depending on the initial state sampling. Borden and Carpenter obtained the following 12/ 1 ratio for trajectories initiated at the conrotatory, disrotatory, and monorotation TSs, 5.8-6,2.S-3.3, and 0.35-0.4, respectively, giving an overall value of 4.73. TST predicts a double-to-single rotation ratio of less than 1.5, dramatically lower than the value from two MD studies. 

This section provides examples of temperature and time cycles for rotomolding select fluoroplastics. The most frequently molded fluoroplastics, by this technology, are PFA, FEP, PVDF, ETFE, and ECTFE. Rotation ratios for these resins follow the general guidelines that have been listed in Table 6.31. In summary, the rotation ratio ranges from 1 5 for cylinders to 8 1 for oblongs. A typical rotation ratio for a medium part is 4 1. The exact value of rotation ratio for each resin must be determined experimentally. 

Table 6.31. Typical Rotation Ratio of Various Shapes ...

Shapes Rotation Ratio Rotation Speed of Major Axis... 

Wall thickness uniformity Incorrect mold rotation because of equipment problems or the wrong rotation ratio Variation in the thickness of the mold wall resulting in uneven heat transfer, thus improper melting and fusion rate of the resin Variation in heat transfer to the mold... 

Plate speed (mm) Arm speed (rpm) Rotation ratio Standard deviation of part wall thickness (mm)... 

Table 14.4 shows the standard deviation of part wall thickness for DOP plastisols at different plate speeds, arm speeds, and rotation ratios. The analysis of effect of these vari-... 

Wall distribution is determined by the rotation ratio. The fewer rpms in a certain ratio, the more uniform the wall thickness. Resin, molds, and final application must be taken into consideration to establish the most efficient cycle. 

The axes in rotational molding have different lengths. The speed of rotation of the mold and the ratio of the two axes are very important variables of the rotational molding process. Rotation ratio is the ratio of the speed of the mold around the two axes according to Eq. (9.1). Speed is a function of the flow properties of the polymer and the ratio is related to the shape of the part. Shorter cycles and thinner part walls amplify the sensitivity of these two variables. 

The rotation ratio of the two axes depends on the shape of the molding. Symmetric parts can be molded at a ratio of 4 1 examples include spheres, cubes, and regularly shaped artieles. Table 9.8 shows rotation ratios for a few different shapes. Irregular objects require ratios ranging from 2 1 to 8 1. It is impractical to calculate the rotation ratio of a new part. A starting value can be estimated based on the shape of the part. The optimal ratio is then determined by trial and error and evaluation of the part made at a given value of the ratio. 

Frequently, the flow properties of adhesives and sealants are measined with a Brookfield viscometer at two different spindle speeds at a rotation ratio of 10 1. The ratio of the low spindle speed viscosity to the high spindle speed viscosity is termed the Shear Thinning or Thixotropic Index and is a measine of shear sensitivity (pseudoplasticity) see Figure 3. 

Using the mixing method described in the previous paper in this series, which produces reproducible data, the authors investigated each factor that affects the black incorporation time. Various carbon blacks, oil-extended rubber and oil were used as the materials in the tests. The mixing conditions used were the rotational speed, rotational ratio of the rotor, fill factor and ram pressure. The authors studied the effect of these factors on the black incorporation time by the new theory based on static electricity phenomena and rabber viscoelasticity. 7 refs. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

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