Annealing temperatures of polycarbonate

nylons, polycarbonates, etc

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Annealing temperatures of polycarbonate

Postby Louis on Sat Aug 18, 2001 4:11 pm

Try resin manufacturers. This from the GE site

LEXAN® resins Anneal LEXAN at 250° F for as short as time as possible to achieve acceptable part performance (30 minutes at temperature 250° F per 1/8 inch thickness). Determine this time by experimenting with actual end-use testing.

The injection molding process generally results in parts with molded-in stresses. These stresses can arise from a number of sources, but key causes include differential flow patterns in the mold, sharp wall transitions, different wall thicknesses, and machining.

Annealing is a secondary operation that can remedy certain stress imbalances. It is accomplished by exposing the part to elevated temperatures for extended periods of time.

Annealing should not be used for glass-filled part, and may cause increased notched sensitivity and reduced chemical compatibility in some parts. Also the extended time needed for cooling can affect manufacturing efficiency and production rates.

Injection molding plastic parts invariably produces molded in stresses. These stresses arise from a number of sources. Differential flow patterns in the mold, sharp wall transitions, different wall thicknesses, and machining all contribute to nonuniform distribution of inherent stresses.

Although annealing reduces stresses, it should not be considered a cure-all for a number of reasons:

Annealing glass-filled parts may not thoroughly relieve internal stress, because they are composites.
Studies show that post-molded heat histories may increase notch sensitivity and reduce chemical compatibility with certain substances. Therefore, the time at temperature should be the minimum needed to achieve acceptable part performance.
The extended periods of time needed for annealing may prevent it from being used economically in actual production.


Review and examine molding procedures and part designfor possible problems before choosing annealing as a solution

Always anneal parts in an air circulating oven and cool them slowly to prevent thermal "shock," as cooling rates may reintroduce stresses into the part causing warping and cracking. To avoid overly rapid cooling rates, cool the parts by turning the oven off until the parts return to ambient temperatures.

hope it helps . . . <b> <i> Louis </i> </b>
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And heaven in a wild flower,
Hold infinity in the palm of your hand
And eternity in an hour.
-- William Blake



[This message has been edited by Louis (Edited: 08/18/01).]
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Annealing temperatures of polycarbonate

Postby Rich Geoffroy on Fri Sep 07, 2001 12:33 pm

Lynford:

Remember that residual stresses are also caused by differential cooling, therefore, it is a two-way street. Not only do you have to ensure a slow heat-soak cycle to relieve stresses, but the cooling cycle must be equally as gradual, lest you incorporate new stresses during the cooling operation.

Residual stresses do not only result from molding, but from machining and polishing as well. If a truly stress-free part is required, it is essential that stress relieving take place after final processing. If dimensions are critical, it may be necessary to stress relieve once after the majority of the processing is completed, and once again after finishing.

To estimate the amount of residual stress in the part, try looking at the part with polarized sheets, one sheet above a light source but underneath the part, and the other above the part but oriented 90 <sup>o</sup> from the first sheet. The oriented molecules in the stressed regions will create birefringence which shows up as colors under the cross-polarized sheets. The polarized sheets are available from places like Edmund Scientific.

Hope this helps. BTW give my regards to Larry Dawson.


<B><I>Rich Geoffroy</I>
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