Thickness variation

Perhaps the single most important objective for any blown film extrusion operator is to produce film that meets thickness specification. Many resources are devoted to ensuring that thickness measurements are in-specification across the film web and remain so throughout a production run. Variations in thickness can lead to product failure, downtime of conversion machinery, and large amounts of film scrap. Also, large thickness variations can significantly reduce profits through excessive material consumption. 

Thickness variations can be categorized as either time-dependent or position-dependent. For the case of time-dependency, thickness varies in the machine direction (MD) of the film, which may be caused by extruder surging (as described above). Also, temperature drifts on the extruder can lead to longer-term changes in the flow rate of polymer, hence thickness variations. It may be beneficial to document melt temperature on a chart recorder over several hours if this cause is suspected. 

A comprehensive, practical overview of bubble instabilities is given by Waller in an article entitled What to Do When the Bubble Won t Behave [50]. In it he identifies and describes seven different types of instabilities, shown in Fig. 7.1, and recommends remedies for each type. These will be briefly summarized here. 

Draw resonance Helical instability Frost line oscillation Bubble sag 

Bubble tears Bubble flutter Bubble breathing 

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For defect sizing by TOED, use of L waves involves a penalty in resolution of almost a factor of two at a given frequency because of difference in velocity as compared to shear waves and use of SV waves runs into difficulties because of the mode conversion problems. Further, problems due to couplant thickness variations, surface roughness affects, beam skewing and distortion problems in anisotropic welds can also be expected. On the contrary, SH waves are not affected... 

The wall thickness for a typical generator tube in the swaged is 4,2 - 4,7 mm with a typical circumferential wall thickness variation 0,25 mm. The swage on a tube is typically formed in one of four ways ... 

Pre-Production Handling. Salt-cured catde hides, when received at the tannery, are individually bundled to prevent excessive moisture loss. The bundles are tied with ropes that are later cut and removed the hides may be sorted for different weight or quaUty classification at this point. It is best to have hides of similar size and thickness in a given production batch to assure an even reactivity of the processing chemicals and to avoid frequent adjustments in the machinery to compensate for size and thickness variations. In the modem large tannery, the size/quaUty classification is not necessary because the hides arrive in carload quantities under specifications as to size, type, and month of slaughter. 

Standard of deposition such as thickness uniformity, or defined thickness variation and coating defects. 

Film manufacture also requires special considerations in the case of polypropylene in order to ensure small crystal structures and hence high clarity. Chill-roll casting processes may be used and these give films of high clarity and minimal thickness variations at high rates of production. Blown film processes can, however, give superior mechanical properties and in addition equipment costs are lower, and in consequence the process is more economic for lower tonnage production. 

D. Beaglehole. Finite thickness variation of the spreading coefl cient in wetting phenomena. J Phys Chem 95 5900-5902, 1989. 

By extrusion parison control it is possible to minimize the wall thickness variation and the extent of stretching and stretch orientation. These are the province of the processor when the designer is not familiar with BM. Knowledge is required to provide information on what is possible and to select the specific BM process that has the capability to mold the product. The designer should be aware of the possible failure modes and compensate for them in the design. There is little else the designer can do but select the best material and process to make the product. 

In the experiments based on optical interferometer, Kaneta et al. [50] measured the film thickness variation and asperity deformation as an artificial circular bump or latticed asperities pass through the contact zone. They found... 

The product quality, however, is related to the calendering process. Thickness gradients across the calendered product, thickness variations along the product, as well as cord density distribution determine the quahty of the product and are tremendously related to the process parameters. 

Boundary layer similarity solution treatments have been used extensively to develop analytical models for CVD processes (2fl.). These have been useful In correlating experimental observations (e.g. fi.). However, because of the oversimplified fiow description they cannot be used to extrapolate to new process conditions or for reactor design. Moreover, they cannot predict transverse variations In film thickness which may occur even In the absence of secondary fiows because of the presence of side walls. Two-dimensional fully parabolized transport equations have been used to predict velocity, concentration and temperature profiles along the length of horizontal reactors for SI CVD (17,30- 32). Although these models are detailed, they can neither capture the effect of buoyancy driven secondary fiows or transverse thickness variations caused by the side walls. Thus, large scale simulation of 3D models are needed to obtain a realistic picture of horizontal reactor performance. 

FIG. 54. Optimum values for pressure and frequency for uniform (thickness variation <57i) film deposition. The shaded area signifies the process window. Also shown are similar data by Chatham and Bhat [146]. 

Another beneficial effect of modulation of the discharge is the observed improvement of uniformity of deposition. At cw discharge conditions that normally result in thickness variations of 70%, modulation lowers the variation to about 10% [519]. This was attributed to the fact that, as a result of modulation, reactions are not confined to the sheaths, and radicals are produced in the entire volume between the electrodes. 

Dip 20nm-50pm Complex and irregular shapes can be coated, can be a continuous process, very simple Thickness variations, voids, and pin holes Insulating polymer layers, electronic parts, and assemblies... 

Common defects that are observed in this coating technique are voids, pin holes, thickness variations, and wavy surfaces. Voids and pinholes are caused by air entrapment in the film, surface contamination, or dust particles. 

Local thickness variations in a thin specimen complicate the quantitative analysis of a single element in the absence of precise knowledge of specimen thickness and without the ability to compare the measured x-ray intensities with those of thin standards. To avoid this difficulty, the x-ray intensity for the element of interest can be divided either by the intensity of a region of background between peaks as in the Hall method[8], or by the intensity from another element as in the Cliff-Lorimer method[9]. The former is largely used for biological analysis while the latter has become the standard thin specimen microanalysis method for materials science applications. The Cliff-Lorimer method is expressed in the following equation ... 

Thickness variations from the intrados to the extrados and along the length of the bend shall be gradual. The thickness requirements apply at the midspan of the bend, y/2, at the intrados, extrados, and bend centerline radius. The minimum thickness at the end tangents shall not be less than the requirements of para. IP-3.2 for straight pipe (see Fig. IP-3.3.1). 

But even in a homogeneously doped material an etch stop layer can be generated by an inhomogeneous charge carrier distribution. If a positive bias is applied to the metal electrode of an MOS structure, an inversion layer is formed in the p-type semiconductor. The inversion layer passivates in alkaline solutions if it is kept at the PP using a second bias [Sm5], as shown in Fig. 4.16b. This method is used to reduce the thickness variations of SOI wafers [Og2]. Illuminated regions... 

The PS layer thickness may show fluctuations over the electrode area on different length scales. The thickness variations may originate from a waviness of the bulk-PS interface, or the electrolyte-PS interface. The latter case, which is usually due to a dissolution of PS during anodization or a collapse of the PS microstructure due to capillary forces during drying, is discussed in Section 6.5. 

A lateral variation of the anodization current will produce different growth rates and consequently an interface roughness for porous layers. Note that this is not the case for stable macro PS formation on n-type, because here the growth rate is independent of current density. An inhomogeneous current distribution at the O-ring seal of an anodization cell or at masked substrates produces PS layer thickness variations, as shown in Fig. 6.6. Inhomogeneities of the current distribution become more pronounced for low doped substrates, as shown in Figs. 6.3 b and 6.5 d [Kr3]. 

The shape of the parts must be adapted to the material and the process, which dictates certain conditions for example, the maximum thickness, the thickness variations on the same... 

Parts are less sensitive to sink marks, thickness variations and residual stresses. Thicknesses, 5-15 mm, can be higher than with dense compounds bosses and ribs can be oversized. 

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