MVI supply PFA lined cast steel fittings where possible, we recognize the advantages this lining material has over isostatically lined PTFE components.
Fittings lined in PFA or isostatic PTFE will perform satisfactorily provided that their manufacture and materials of construction are of the highest quality. Poor quality materials and fabrications will produce an unsatisfactory performance and such lined fittings may be subject to premature catastrophic failure.
MVI has chosen to use steel castings lined with high molecular weight PFA’s for its lined fittings. The reasons for this are technical and not cost driven. The use of PFA in castings provides the best combination of lining performance, housing strength and consistency.
These reasons are:
Our key argument against Isostatic molding would be in terms of the repeatability of the manufacturing process – particularly when the number of production variables are considered. The factors that have the most important effect on the configuration and surface finish of the mold are:
• Pre-form shape
• Surface finish of the part
• Pressurization direction
The pre-form shape is determined by the molding bag, the rigid part of the mold and the compaction pressure. The rigid part (in the case of a Tee for example -the steelwork) impacts on the shape by blocking the progression of compaction. The flexible bag assumes a shape -affected by a number of factors: the configuration of the mold, bag properties such as membrane thickness, residual stress and modulus of elasticity of the elastomer, surface finish of the metallic parts, PTFE characteristics, the rate of pressurization and maximum pressure. Mold filling and variables such as the interaction of the rigid and flexible parts and the stability of the pressurization also have some influence. For all of these reasons it is not possible to produce lined components with a high degree of dimensional accuracy. These issues tend to magnify for smaller components as one does not have the ability to increase lining thicknesses to compensate for variation, moreover filling molds is more difficult as filling areas diminish. Thus thicker linings that often appear in isostatically-molded products are because of a process requirement rather than a technical need. A key variable highlighted by PTFE Isostatic molding is the component into which the product is molded. Castings with internal characteristics (such as corner radii) appropriate for isostatic molding should be used. The use of fabricated assemblies has the potential to provide final product variation that is dimensionally out of specification or create unacceptable material stresses in the PTFE. Contrasting this with PFA molding, there are many fewer variables in the PFA molding process. Once molding parameters are established on manual or automatic transfer molding presses they are highly repeatable. In terms of component geometry, castings are still preferable to fabrications, principally for the accurate mounting of the PFA molding tools, but as long as key geometry is met, small variations in products that are made using fabricated steelwork can be managed.
The next key difference between PFA molded components and PTFE is the fact that PFA is translucent rather than opaque PTFE. This means that independent of any physical testing, products are generally examinable by eye for defects, whereas voids, gaps, crevices and stress faults are not visible in PTFE products unless they are present on the internal surface.
Surface finish will always be superior on a PFA molded product vs. isostatic molding. Isostatic molded products take the form of the compressive bag on one side and the material against which they are pushed on the other. In the case of fittings, the bag form tends to be on the inside and here the surface is normally not smooth, as the bag tends to conform to the individual particles of resin, or exhibit join lines showing the construction of the rubber bag.
Surface finish of Isostatically molded PTFE fitting compared to PFA
PFA is the least wettable material available, bettering PTFE and is 137% less wettable than polished stainless steel. This leads to a much slower initiation of biofilms on the surface of materials. It is also hydrophobic and as a consequence it is not possible for a substance to chemically adhere to a surface if the substance is unable to wet that surface. The higher the critical angle of wetting the lower the wettability of the surface.
Wetting Contact Angle of Materials
PFA has lower permeation rates to granular PTFE. Data published by DuPont, other polymer manufacturers and by major processors of Fluoropolymer materials clearly confirms the superior permeation resistance of PFA.
PFA exhibits consistent linear thermal expansion characteristics throughout
its useful temperature range, a feature of great importance when
controlling the physical attributes of a finished lining. This property
compares favourably with the vastly varying expansion coefficient of PTFE’s.
Vacuum performance is a function of the hoop strength of the liner and the
level of support derived from the metal housing. Hoop strength is derived from the mechanical strength of the lining material and the cross sectional
area resisting inwards collapse. Support derived from the metal housing
relies on the proximity of the liner to the housing. Therefore in a properly
designed system these features are used to determine the thickness of liner
required to resist vacuum induced collapse. The System provided by MVI相乐百合子相乐百合子,谷田部和沙谷田部和沙,みかみ ゆあみかみ ゆあ
has been properly designed and tested to perform at conditions of elevated
temperature and high internal vacuums. Isostatically molded components
tend to have large gaps behind the liner – it occurs as part of the
manufacturing process. Also if a pre-formed Isostatic molding is inserted
into a component flattening of the elbow form often occurs on the apex of
the bend causing an even larger gap and a form that is susceptible to
MVI prefers, wherever it is practically possible, to use cast steel housings
with integral flanges for the manufacture of their lined fittings. In addition
to having no fabrication welds machined castings are inherently more
robust and offer greater dimensional accuracy. Dimensional accuracy gives
much improved lining control with lined faces which are square to the line
axis of the fitting bore with bores that are concentric to the flange periphery
and body outside diameter.