Stiffness Dimensioning

The tolerances of the molded part are to a large extent determined by the properties of the mold. The most fundamental property for the tolerances is probably the stiffness of the mold. This is particularly important in LCM due to the large forces that can be generated by the injection pressure. As an example, an injection pressure of 4 bar will exert a force of 800 kN or 80 tons on a mold with a surface area of 2 m2. This force has to be reacted by the clamping device and the mold. 

The necessary stiffness of the tool is easy to estimate with standard engineering methods and should therefore always be calculated. The loads on the mold are primarily the internal overpressure from resin and reinforcement. It should be kept in mind that the compaction pressure necessary for closing the mold with dry reinforcement can be as high as or higher than the injection pressure [8]. The mold must be dimensioned for the maximum injection pressure in the whole cavity because this will be the internal over pressure if the outlets are closed while pressure is maintained at the inlet. 

The stiffness of the mold must be both high enough to keep a sufficiently uniform cavity thickness and so high that the sealing surfaces are kept sufficiently close to each other, particularly if small diameter O-ring seals are used. 

---

Next is a list detailing the intended requirements of the final part. Physical considerations regarding stiffness dimensions and tolerances impact resistance surface hardness compressive, tensile, and torque loads light transmittance and so on all need to be quantified. If the part is intended for electrical applications, determine voltage to be encountered, current-induced temperature extremes, arc resistance and surface tracking resistance, dielectric strength, electrical frequency, and the like. List possible environmental... 

DIMENSION IBC (MAXBC), JBC (MAXBC), VBC (MAXEC) DIMENSION RMATl(MAXEL, 13),RMAT2(MAXEL, 13) DIMENSION NOPD (MAXEL,MAXST),RRSS (MAXDF) COMMON/ONE/ STIFF(2000,300)... 

Transverse Dimensions or Fineness. Historically, the quantity used to describe the fineness or coarseness of a fiber was the diameter. Eor fibers that have irregular cross-sections or that taper along their lengths, the term diameter has no useful meaning. Eor cylindrical fibers, however, diameter is an accurate measurement of the transverse dimension. Though textile fibers can be purchased in a variety of cross-sectional shapes, diameter is stiU a useful descriptor of the transverse dimension. Eiber diameter is important in determining not only the ease with which fibers can be twisted in converting them to yams, but also fiber stiffness, ie, fabric stiffness, and, alternatively, fabric softness and drapeabiHty. 

Ultem PEI resins are amber and amorphous, with heat-distortion temperatures similar to polyethersulfone resins. Ultem resins exhibit high modulus and ate stiff yet ductile. Light transmission is low. In spite of the high use temperature, they are processible by injection mol ding, stmctural foam mol ding, or extmsion techniques at moderate pressures between 340 and 425°C. They are inherently flame retardant and generate Httie smoke dimensional stabiUties are excellent. Large flat parts such as circuit boards or hard disks for computers can be injection-molded to maintain critical dimensions. 

Thus, for a given stiffness (F/8), panel dimensions I, b) and edge-constraints (C) the lightest panel is the one with the smallest value of p/E ... 

Example 2.9 A solid polyethylene beam is 10 mm thick and IS mm wide. If it is to be replaced with a sandwich section with solid polyethylene in the two outer skins and polyethylene foam (density = 200 kg/m ) in the centre, calculate the dimensions of the sandwich beam if it is to have optimum stiffness at the same weight as the solid beam. If the foam material costs 20% more than the solid material, calculate the increase or decrease in cost of the sandwich beam. 

The stiffness ratios (i.e. stiffness of the foam sandwich beam relative to the original solid beam) are also given in Fig. 2.21. In both cases the values given are independent of the original solid material or its dimensions, so this provides a good design chart. The design of solid/foam sandwich structures is also considered in Chapter 3 in the laminate analysis. 

Clearly there are many permutations of D,b,h,a, etc and Fig. 2.31 shows how the stiffness enhancement factor, q, changes with various values of these parameters. In each case the angle a has been fixed at 85° and the corrugation dimensions have been expressed as a function of the wall thickness, h. 

To illustrate the correct approach, consider applications in which a material is used in sheet form, as in automotive body panels, and suppose that the service requirements are for stiffness and strength in flexure. First imagine four panels with identical dimensions that were manufactured from the four materials given in Table 3-1. Their flexural stiffnesses and strengths depend directly on the respective material s modulus and strength. All the other factors are shared in common with the other materials, there being no significantly different Poisson ratios. Thus, the relative panel properties are identical with the relative material properties illustrated in Fig. 3-3. Obviously, the metal panels will be stiffer and... 

Material Characteristics Strength and Stiffness Toughness Short-Term Heal Resistance Long-Term Heal Resistance Environ- mental Resistance Dimensional Accuracy in Molding Dimensions Stability Wear and Frictionat Properties Point Subtotal Cost Point Total... 

In the current work a Digital Instmments Dimension 3000 SPM was operated in force-volume mode using a probe with stiffness selected to match the stiffness of the sample. Standard silicon nitride probes with a nominal spring constant of 0.12 or 0.58 N/m were used for recombinant and native resilin samples. These samples were characterized in a PBS bath at a strain rate of 1 Hz. For synthetic rubbers, silicon probes with a nominal spring constant of 50 N/m were used and the material was characterized in air. Typically, at least three force-volume plots (16 X 16 arrays of force-displacement curves taken over a 10 X 10 p.m area) were recorded for each of the samples. 

The latter can be estimated from the side force coefficients obtained during the abrasion experiments for a small shp angle and a high speed. They reflect directly the compound stiffness and since the dimensions are the same also the shear modulus. 

One point, which is often disregarded when nsing AFM, is that accurate cantilever stiffness calibration is essential, in order to calculate accurate pull-off forces from measured displacements. Althongh many researchers take values quoted by cantilever manufacturers, which are usually calculated from approximate dimensions, more accurate methods include direct measurement with known springs [31], thermal resonant frequency curve fitting [32], temporary addition of known masses [33], and finite element analysis [34]. 

---