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"Hot Measurements"

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joeprocess (100 posts) 04-01-08, 01:04 PM (GMT) "Hot Measurements" What do you think? I cringe when asked about establishing a hot measurement for parts that take a long time to cool. Have any of you had success with this method?   Alert Moderator Edit | Reply | Reply With Quote | Top

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Subject     Author     Message Date     ID    RE: Hot Measurements Brent 04-01-08 1    RE: Hot Measurements Sam 04-01-08 2        RE: Hot Measurements joeprocess 04-01-08 3            RE: Hot Measurements Brent 04-01-08 4                RE: Hot Measurements Sam 04-01-08 5

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Brent (891 posts) 04-01-08, 02:18 PM (GMT) 1. "RE: Hot Measurements" With shrinkage history of a part molded with a semi-crystalline resin, you should be able to establish a "hot measurement". The process must be stable and under control,that is,setup according to the controlled cycle. brent   Remove | Alert Moderator Edit | Reply | Reply With Quote | Top

Sam (136 posts) 04-01-08, 02:51 PM (GMT) 2. "RE: Hot Measurements" I agree with Brent. Many jobs require a molded measurement and a shrink adjusted measurement when it gets to the customer. Its best to establish a controlled shrinkage study so that the data gathered is meaningful. That means you need to know the cooling rate, melt temp/ejection temp and cycle of the part - and keep them constant. Just as important is the collection method. I always try to duplicate what the part will see in normal production and handling. Make the collection method duplicate the production process, even if it means bagging a shot and retuning it to a production parts bin to duplicate what the part will see until the tote is full. Don't try to measure the part for an hour after it is cooled. but measure on that hour or a consistent time frame. Measure at 2-4 hour increments, at 24 hours, three days and at one week. Use the same person to measure the parts if possible to keep the R&R portion the same. This method works well on mating products that my customers are measuring 8 days after manufacture.   Remove | Alert Moderator Edit | Reply | Reply With Quote | Top

joeprocess (100 posts) 04-01-08, 05:26 PM (GMT) 3. "RE: Hot Measurements" Given that the parts shrink gradiantly from the time of ejection to final dimension I agree that when the parts are measured is critical and a study is needed. Measuring the part right out of the press doesn't seem to be a good way to do this especially with multi-cavity tools. I'm thinking of creating a "warm" measurement instead. Something that can be measured after a set time like 10 or 15 min. The shrinkage should be more stable by then I would think. I'll graph it out to find out for sure. Matching the process is also critical, but how accurate can this be in real life molding situations? What if the part is packed out a little more one run then another, wouldn't this sway the hot measurement making it inaccurate? Of course we all know that we should pack out the part the same run to run, but in reality this is not always possible. Thoughts?   Remove | Alert Moderator Edit | Reply | Reply With Quote | Top

Brent (891 posts) 04-01-08, 06:50 PM (GMT) 4. "RE: Hot Measurements" Joe, Yes, with semi-crystalline resins, even packing can influence shrink to a great degree. The resin companies provide shrink nomographs, those along with a history can help predict shrinkage.I remember running cap molds at 3 second cycles, you didn't want to wait hours to find your dimensions were off, you'd have made thousands of rejects. brent   Remove | Alert Moderator Edit | Reply | Reply With Quote | Top

Sam (136 posts) 04-01-08, 09:13 PM (GMT) 5. "RE: Hot Measurements" I followed what you said Joe, that's one reason I waited the full hour to measure. That initial number will be more stable if an hour is used and the part is cool. I'm molding small containers with lids. The containers are not packed to gate seal, they would be at least a full gram heavier. I'm hitting a moving target but could not have come close without the shrinkage study. What the study did tell me was that the initial shrink and the final shrink are indeed linked to the degree of pack and part weight. That part packed out to a larger dim will still shrink at a fairly predictable rate and so your study can still predict final dimension.   Remove | Alert Moderator Edit | Reply | Reply With Quote | Top

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