Shrinkage, 5.25

Shrinkage happens when a plastic-based board, extruded and pulled from the die, cool too fast. Too fast means that stretched long polymer molecules, coming from the die, do not get enough time to settle, to come back to their thermodynamically favorable coiled form. They are trapped in the board solidihed matrix in an unsettled, stretched shape. 

To be exact, these distorted in space polymeric molecules continue to get rearranged into their energetically minimized shape, but at ambient temperature rates of this rearrangement are too slow, about 100 million times slower than those at hot melt temperature. If it would take 5 s for a polymer molecule to coil from its stretched shape at hot melt temperature, at ambient temperature it would take about 16 years. 

However, on a deck on a hot summer day, it might take only a few weeks. In the North, it might take a year or two (see the insert). 

How were those temperature-dependent figures for deck shrinkage obtained  

Let s take hot melt temperature (HDPE-based WPC) as 300°F (about 150°C). The temperature coefficient for polymer molecules conformational rearrangements, which is a change in speed of the process by each 10°C, approximately equals to 4. This value for so-called cooperative processes is significantly higher than common temperature coefficients, typically between 2 and 3. 

The shrinkage behavior of different resins and the part geometry must be considered. Generally shrinkage is the difference between the dimension of the mold at room temperature (72°F) and the dimensions of the cold blown part, usually checked 24 hours after production. The elapsed time is necessary to allow the part to shrink. Trial and error determines what time period is required to ensure complete shrinkage. Coefficients of expansion and the different shrinkage behaviors depend on whether plastic materials are crystalline or amorphous (see Chapter 1). 

The most common special feature of the mold is the quick change volume control insert. Rigid volume control is necessary for certain products, such as dairy containers. Here HDPE is used (for many excellent reasons), and the container slowly shrinks and changes in size for many hours after molding. Because of production volume control requirements, some dairies must fill containers molded half an hour before fill and then switch to filling containers molded several days previously. Volume-control inserts that displace the difference in volume between the two types are added to the mold, usually as a disc in the side wall, to ensure that the volume and fill levels are the same in both containers at the time of filling. The device works because HDPE shrinkage is reduced virtually to zero for the life of the container when filled with milk or juice and stored at cold temperatures. 

Plastics will sometimes thin out at an insert or parting line. With resins such as LDPE, this phenomenon is of little importance, but with HDPE and similar resins, it can be a serious problem. When inserts are used, they should be fitted tightly. The parting line miter should be as close as possible, and sufficient clamping force should be provided to prevent partial opening 

With either EBM or IBM, the requirement for a thermally controlled parison at the lowest heat increases the demand on internal pressure/vol-ume to ensure adequate movement or expansion of the parison in the transverse direction. It is possible that vacuum-aided stretch blowing can provide faster expansion rates of the parison and improve performance at the higher stretch-blow ratios. With PVC, PS, and other low shrink alloyed resins, the vacuum assist may need little or no pressure beyond one atmosphere. The function of vacuum (—14.7 psi) has the basic effect of — ( —14.7 psi) 

WALL THICKNESS IN. (MM) BLOW PRESSURE (PSD NORMAL CYCLE TIME (S) CYCLE TIME WITH VACUUM ASSISTANCE (S) CYCLE TIME VACUUM ONLY (S) 

All types of polyurethanes show some shrinkage after removal from the mould, typically in the region 0 5-0-6mm per 25 mm. 

One factor associated with tolerance is shrinkage. Generally, shrinkage is the difference between the dimensions of a fabricated product at room temperature and after cooling,checked usually twelve to twenty-four hours after fabrication. Having an elapsed 

Shrinkage can also be dependent on such climatic conditions as temperature and 

These tolerances do not include allowance for aging characteristics of material. 

Parting line must be taken into consideration. 

Pert design should maintain a wall thickness as nearly constant es possible. Complete uniformity in this dimension is sometimes impossible to achieve. Walls of non-uniform thickness should be gradually blended from thick to thin. 

Concerning the mechanical properties, high crosslinking of the material after UV exposure may induce improved plasma etch resistance [54] as well as improved mechanical properties. This last point is important for the demolding process. However, a long UV exposure time may cause excessive shrinkage and brittleness 

On the other hand, insufficient UV curing will lead to low cohesive strength of the polymer, increasing the probability of pattern distortion and collapse [55]. This implies that the irradiation dose will have to be optimized for each material and pattern type in order to achieve appropriate mechanical properties. 

In addition, some resists have been developed for special applications and with special properties. This is the case for example in microelectronics, where special hybrid organic-inorganic materials with low dielectric constants and high thermal and mechanical stability are being developed to simplify the dual damascene electrical interconnection process [56]. Also, resists loaded with functional nanoparticles can be patterned using NIL [57]. 

In order to limit shrinkage, resist formulations have been made vdth special monomers. For example, steric hindrance might be used. Another possibility is to include an oligomer or a polymer in the resist formulation as a binder [67], but one has to be careful not to prohibitively increase the resist viscosity. 

The formed polymer layer can be used as a mask in a plasma etching equipment. 

Polyketone glass fiber mica wollastonite CaCOs 0 20 20 20 20 105 210 165 120 95 23 

Polypropylene glass fiber carbon black talc CaCOs aluminosilicate 0 30 30 40 40 50 65 148 130 135 116 120 12 74 20 20 43 

Proper choice of filler allows to modify shrinkage of UV curable transparent adhesives.The shrinkage of UV curable adhesive was reduced by factor 2-3 by incorporation of quartz which has better UV light transparency than conventionally 

Addition of glycerol tends to reduce the glass transition temperatures of extruded starches and to narrow the transition regions. However, with glycerol contents of up to 25%, the Tg values are not lowered below 20 °C for the starches used. Lower 

3 Donovan, J.W. (1979) Phase transition of the starch-water system. Biopolymers, 18, 263-75. 

4 van Soest, J.J.G. (1996) Starch plastics structure-property relationships. PhD Thesis. University of Utrecht, The Netherlands. 

5 Rubin, l.I. (1972) Injection Moulding Theory and Practice, John Wiley Sons, Inc, New York. 

6 Bader, H.G. and Gdritz, D. (1994) Investigations on high amylose com starch films, part 3 stress strain behavior. Starch/StSrke 46, 435-9. 

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Volatile oils are known as high shrinkage oils because they liberate relatively large amounts of gas either in the reservoir or the separators, leaving relatively smaller amounts of stabilised oil compared to black oils (also called low shrinkage oils). 

The oil and gas samples are taken from the appropriate flowlines of the same separator, whose pressure, temperature and flowrate must be carefully recorded to allow the recombination ratios to be calculated. In addition the pressure and temperature of the stock tank must be recorded to be able to later calculate the shrinkage of oil from the point at which it is sampled and the stock tank. The oil and gas samples are sent separately to the laboratory where they are recombined before PVT analysis is performed. A quality check on the sampling technique is that the bubble point of the recombined sample at the temperature of the separator from which the samples were taken should be equal to the separator pressure. 

The purpose of this exercise is to identify what parameters need to be further investigated if the current range of uncertainty in reserves is too great to commit to a development. In this example, the engineer may recommend more appraisal wells or better definition seismic to reduce the uncertainty in the reservoir area and the net-to-gross ratio, plus a more detailed study of the development mechanism to refine the understanding of the recovery factor. Afluid properties study to reduce uncertainty in (linked to the shrinkage... 

A Scan at 7mm depth B Scan at 10mm depth C Scan at 13mm depth Fig. 11 Tomography scans of an internal shrinkage at different depths (A B C)... 

The oscillating jet method is not suitable for the study of liquid-air interfaces whose ages are in the range of tenths of a second, and an alternative method is based on the dependence of the shape of a falling column of liquid on its surface tension. Since the hydrostatic head, and hence the linear velocity, increases with h, the distance away from the nozzle, the cross-sectional area of the column must correspondingly decrease as a material balance requirement. The effect of surface tension is to oppose this shrinkage in cross section. The method is discussed in Refs. 110 and 111. A related method makes use of a falling sheet of liquid [112]. 

Because densification occurs via tire shrinkage of tliennodynamically unstable pores, densification and microstmcture development can be assessed on tire basis of tire dihedral angle, 0, fonned as a result of tire surface energy balance between tire two solid-vapour and one solid-solid interface at tire pore-grain boundary intersection [, 78, 79 and 80],... 

During pressure sintering, interiDarticle compressive stress, approximated by the externally applied stress and nonnalized by the relative density of the compact p, supplements the surface tension driving force for pore shrinkage ... 

We can imagine the coil tightening up as this point is approached from better conditions. This is not a shrinking to the vanishing point as suggested by the u = 0 criterion, but a contraction to the point where intramolecular exclusion effects are offset by shrinkage. 

Fig. 31. An acrylic terpolymer designed for chemically amplified resist applications. The properties each monomer contributes to the final polymeric stmcture are for MMA, PAG solubility, low shrinkage, adhesion and mechanical, strength for TBMA acid-cataly2ed deprotection and for MMA, aqueous...

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical (72—74) and radiation-initiated (75) polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of acrylic monomers is first order with respect to monomer concentration and one-half order with respect to the initiator concentration. Rate data for polymerization of several common acrylic monomers initiated with 2,2 -azobisisobutyronittile (AIBN) [78-67-1] have been determined and are shown in Table 6. The table also includes heats of polymerization and volume percent shrinkage data. 

Acrylate and methacrylate polymerizations are accompanied by the Hberation of a considerable amount of heat and a substantial decrease in volume. Both of these factors strongly influence most manufacturing processes. Excess heat must be dissipated to avoid uncontrolled exothermic polymerizations. In general, the percentage of shrinkage decreases as the size of the alcohol substituent increases on a molar basis, the shrinkage is relatively constant (77). 

Crimp. The tow is usually relaxed at this point. Relaxation is essential because it gready reduces the tendency for fibrillation and increases the dimensional stabiUty of the fiber. Relaxation also increases fiber elongation and improves dye diffusion rates. This relaxation can be done in-line on Superba equipment or in batches in an autoclave. Generally saturated steam is used because the moisture reduces the process temperatures required. Fiber shrinkage during relaxation ranges from 10 to 40% depending on the temperature used, the polymer composition used for the fiber, and the amount of prior orientation and relaxation. The amount of relaxation is also tailored to the intended apphcation of the fiber product. 

Other elastomeric-type fibers iaclude the biconstituents, which usually combine a polyamide or polyester with a segmented polyurethane-based fiber. These two constituents ate melt-extmded simultaneously through the same spinneret hole and may be arranged either side by side or ia an eccentric sheath—cote configuration. As these fibers ate drawn, a differential shrinkage of the two components develops to produce a hehcal fiber configuration with elastic properties. An appHed tensile force pulls out the helix and is resisted by the elastomeric component. Kanebo Ltd. has iatroduced a nylon—spandex sheath—cote biconstituent fiber for hosiery with the trade name Sidetia (6). 

Bicomponent fibers have also provided a route to self-texturing (self-crimping) fibers. The crimp results from the length differential developed during processing caused by differential shrinkage in the two polymers in side-by-side or eccentric core—sheath configurations (50). 

At HOY speeds, the rate of increase in orientation levels off but the rate of crystallization increases dramatically. Air drag and inertial contributions to the threadline stress become large. Under these conditions, crystallization occurs very rapidly over a small filament length and a phenomenon called neck-draw occurs (68,75,76). The molecular stmcture is stable, fiber tensde strength is adequate for many uses, thermal shrinkage is low, and dye rates are higher than traditional slow speed spun, drawn, and heat-set products (77). 

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