Flashing melt viscosity

However, some semiaromatic nylons can give problems as a result of the high melt viscosity. A process for produciag polymers of hexamethylenediamine, adipic acid, terephthaUc acid, and isophthaUc acid has been developed, which iavolves vaporising the salt mixture ia a high temperature flash reactor followed by molecular weight iacrease ia a twia-screw extmder with efficient moisture removal (17). 

There are no generally accepted specifications manufacturers set their own specifications for hydroxyl number range, melt viscosity, water content, etc. Typical properties of Du Pont Terathane PTMEG are Hsted in Table 2, and selected thermal properties are given in Table 3 (260). For all grades, water content is <0.015 wt % ash, <0.001 wt % iron, <1 ppm peroxide, <5 ppm as H2O2 and flash point TOC is >163°C. 

Processing PC resins by extmsion or injection-molding methods requires melt temperatures of 290—320°C. High melt viscosity at low shear rates prevents mold flash and drool. At injection shear rates, apparent viscosities decrease, and easy melt flow allows manufacture of large, complex parts. 

The composition and structure of EPDM and the composition of the ionomer affect the mechanical and flow properties of the ionomer. Another parameter that affects ionomer properties is the concentration of metal acetate, specifically zinc acetate, present in the neutralized sul-fonated EPDM. This is illustrated in Figure 5, where the melt viscosity of zinc sulfonate CR-2504 is plotted as a function of zinc acetate concentration. Zinc acetate was added to a cement of the polymeric acid, and the resultant neutralized polymer was isolated through the solvent flashing of the cement. Essentially all of the zinc acetate that was added to the polymeric free acid was retained by the polymer (cf. Table II). 

Because of the low melt viscosity of LCPs, thin sections can he molded with highly filled (70%) LCPs. The addition of hulk lubricants reduces the melt viscosity. Injection molded parts are produced with a minimum of molded-in stress, little or no flash, and high dimensional stability. Fast cycle times, reduced mold wear, elimination of secondary operations, and other processing and performance gains of LCPs often lead to lower overall finished part costs even below those of less-expensive resins. 

Thermal and physical properties of the material to be heated—specific heat, latent heat, melting and boiling points, thermal conductivity, flash point, viscosity, density, and evolution of dissolved gases/vapours generated during heating ... 

The sharp melting point and the low melt viscosity also mean that nylon can give problems with nozzle drool and/or premature freeze-off. For this reason, it is normally necessary to use either a reverse-taper nozzle (fitted with a heater to avoid freeze-off), a mechanical shut-off nozzle, or melt decompression. The design of molds should also take into account this low viscosity and sharp melting point. Mold surfaces should be well-mated to avoid flash. 

Unsuitable moulding compound. The use of a compound with an abnormally low melt viscosity may well lead to bleed problems and possibly flashing. Filler particle size is also important because flash and bleed increase with decreasing filler size. These two parameters are controlled by the moulding supplier and with persistent problems the manufacturer should be consulted for help with flash and bleed control. 

When processing polyphenylene sulfide, it is necessary to vent injection molds appropriately, because the sulfurous degradation products cause mold corrosion. Because of the low melt viscosity the vent holes have to be narrow (flash formation). That is why poorly stabilized compounds require frequent and quite involved mold purging [573]. 

Second, the low melt viscosity of SPS formulations helps to reduce molded-in stress from the injection molding process. The data in Figure 15.7 show typical melt viscosities of SPS in comparison with other semicrystalline thermoplastics. The low melt viscosity of SPS results in low hydraulic pressures during injection molding, which helps to minimize molded-in stress. Low viscosity also contributes to SPS s ability to fill complex parts with thin walls and its relative lack of flash during injection molding. 

Melting poiat. Density, g/cm Refractive iadex. AH, Viscosity, Flash-poiat, LD q (oral, rat). 

CAS Registry MPT Melting Boiling Density, Refractive Viscosity (t]) Flash... 

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