Freeze-off time

The reader may wish to check the predictions of freeze-off time for the data in the above Example. 

From (5.84) using the boundary condition that t = tf (freeze-off time) at... 

For the non-isothermal cases the freeze-off time, t/, may be estimated by the method described in Example 5.11. 

However, in the non-isothermal case the pressure is also high at low injection rates. This is because slow injection gives time for significant solidification of the melt and this leads to high pressures. It is clear therefore that in the non-isothermal case there is an optimum injection rate to give minimum pressure. In Fig. 5.28 this is seen to be about 3.0 x 10 m /s for the situation considered here. This will of course change with melt temperature and mould temperature since these affect the freeze-off time, //, in the above equations. 

By varying the "plunger forward time" Holmes et al. were able to determine the time necessary to just obtain the maximum value of L this time was called freeze-off time (tf). The following approximate relation was found ... 

Fractionation of chain molecules according to their chain length, 727 Fracture mechanics, 472 Free enthalpy of formation, 753, 754 Freely jointed chain model, 247 Free-rotation model, 246 Free surface energy, 229 Free volume fraction, 537 Freeze-off time, 806 Freezing-in process, 151 Frequency doubling, 349 factor, 751... 

The sharp melting poiat 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. Melt decompression, or suck-back, iavolves the screw retractiag slightly after the screw retractioa time, pulling the material back and... 

This equation cannot be integrated as simply as before because h is now a function of time. Fig. 5.26 shows how the depth of the cavity changes with time as the melt flows. Barrie has investigated this situation and concluded that the freezing-off could be described by a relation of the form... 

Once the mold is filled and packed and the gate freezes off, the injection molding machine switches to the cooling stage. The amount of cooling is determined by the cooling time. While the melt in the mold cools to solid. 

Cycle time. When one is adjusting cycle times, it is typically best to use a fast injection speed and a minimum holding time to achieve gate freeze-off and a short cooling time. 

Dimethylhexane-2,5-diol [110-03-2] M 146.2, m 88-90. Purified by fractional crystn. Then the diol was dissolved in hot acetone, treated with activated charcoal, and filtered while hot. The soln was cooled and the diol was filtered off and washed well with cold acetone. The crystn process was repeated several times and the crystals were dried under a vac in a freeze-drying apparatus [Goates et al. J Chem Soc, Faraday Trans 1 78 3045 1982]. 

Five parts of the terpene, 7 of amyl nitrite, and 12 of glacial acetic acid are mixed and cooled with ice and salt, and a mixture of 6 parts of hydrochloric acid and 6 parts of glacial acetic acid added in small quantities at a time. Five parts of alcohol are then added and the mixture allowed to stand in a freezing mixture for a itime. A mass of crystals separates, which consists of the crude nitrosochlorides. This is filtered off and washed with alcohol. When perfectly dry 100 grams of the crystals are digested with 200 c.c. of chloroform for a few moments and at once filtered. The chloroform dissolves a-nitrosochloride, which is precipitated by the addition of excess of methyl alcohol. The crude compound is filtered off, dried and digested with anhydrous ether for... 

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