By Suhas Kulkarni, FIMMTECH
Q.Should CPET be dried before extrusion of a color masterbatch for, specifically stretch blow molding of beverage bottles, to avoid cloudy parts and loss of physicals?
A.There are no general rules for drying the different grades of CPET or with the use of a masterbatch. It is best to look at the processing datasheet or contact the manufacturer. The reason one should always look at the datasheet is because every grade of a particular plastic is different. For example, GE makes a polycarbonate that must process only between 580F and 630F and another polycarbonate that must process only between 480F and 520F. There is no overlap between these grades although they are manufactured by the same manufacturer. Use of datasheets is imperative – if you don’t have a Prospector account from IDES, you can get one here.
Q.Can you recommend a moisture tester that is user friendly?
A.Most moisture analyzers available on the market are ‘weight loss’ based moisture analyzers and are very friendly and easy to use. Unfortunately, we cannot name any of the companies here. Accurate moisture analyzers are based on the Carl-Fischer titration method. They are very accurate but expensive and need trained personnel to use them. Search ‘Moisture Analyzer’ on the IDES website as a resource.
Q.With Polycarbonate, will automatic temperature (in/out) monitoring in the hopper and automatic temperature adjustment (lowering) reduce risk of overdrying?
A.Temperature is an important parameter in drying. However the dew point of the air is also equally important. Both must be monitored. When ever we talk about ‘overdrying’, it is not only related to removal of moisture but also to the possible removal of the low molecular weight additives. Monitoring of the outlet temperature will not give you an indication of overdrying.
Q.I know that the PBT needs to dry until 0.02% of moisture level, but with the drying parameters: temp 250F@4hrs and Dew Point Temp -40F the material had a moisture level of 0.07%. Why do you think that those occurred?
A.There are a lot of factors that can contribute to a higher than normal reading. In my (long) experience with PBT (polybutylene terephthalate), faulty equipment, leaky (not air tight) hoses, distance of the feed throat from the dryer, residence time in the hose that conveys the material, unusually high humidity on the particular day causing the initial moisture to be high requiring higher drying times, could all be reasons for the 0.07% moisture level.
Q.Is the consequence of drying cumulative — ie if I dry a material for one hour at a time 8 times, is that the same as drying all at once for 8 hours?
A.Drying effects are cumulative. Those additives that are lost in the drying process cannot be added back to the plastic. However, the lost moisture can be reabsorbed. So if you decide to use a material that was dried one hour at a time for eight times, you will still need to dry it for the recommended time.
Q.Are there any plans for testing other materials, or is there a known source for finding this type of information on other materials?
A.Since there are a variety of additives and multiple grades of materials, it is difficult to test all the possible materials. At this time there is no source to find such data. The best source is the material supplier’s datasheet.
Q.If I get resin pre-dried and in sealed bags, are there any conditions (other than obvious damage to the seal) that would negate the affect of having dried material.
A.No. However do check to see if it says it will need to be dried after ‘x amount of time after packaging’.
Q.Can repeated drying cause a cumulative effect of degradation? Do you have any suggestions for choosing the best dryer for my resins given the different types and claims from manufacturers.
A.Repeated drying in the particular grade of PBT that was tested caused degradation. However each material and its additives will act differently and should be individually tested. Unfortunately, I am unable to name any dryer companies here.
Q.Is yellowing of nylon a sign of degradation or what? What benefits do you think there is of vacuum drying nylon? Would vacuum drying eliminate the yellowing of nylon?
A.Yellowing of nylon, is not necessarily a sign of degradation. If the yellowing is caused because of oxidation – vacuum drying will help.
Q.Can monitoring of viscosity (via injection or hydraulic pressure) be an effective tool for detecting drying problems?
A.Monitoring viscosity is a great tool to detect overdrying, especially in Nylon. However, there are many other reasons why the viscosity could change and so this should be used with caution.
Q.Are PBT 4 and PBT 12 lines reversed on TGA curve (slide 20 from Suhas’s Presentation for reference)?
A.No, they seemed to be reversed. The difference is between the two is small and could be the variation error.
Q.Does the drying time depend on the size of the polymer granule?
A.No. The absorption of the moisture continues until the moisture reaches the saturation point for that particular plastic. For example, the saturation point for a nylon is between 3 and 4%. Regardless of the size of the granule, it will always absorb the same amount. During the drying process, the rate of reaction of breaking the secondary water bonds is again the same, regardless of the size of the granule. We are, however talking of the normal size pellet received from the manufacturer for melt processing. A thick block of plastic will take more time to dry than a sheet of plastic made of the same material.
Watch Suhas’s Drying & Over Drying Plastics webinar [insert link] and download presentation slides [insert link].
About the Author
|Suhas Kulkarni FIMMTECH||Suhas Kulkarni is the President of FIMMTECH, a consulting firm that specializes in services related to injection molding. He earned his Masters in Plastics Engineering from the University of Massachusetts, Lowell and a Bachelors in Polymer Engineering from the University of Poona, India. He has 18 years of experience as a process engineer.His main area of expertise is Scientific Processing for Injection Molding. Based on his experience, he has developed a custom software called Nautilus, that aids the complete process development routine to production release.He also teaches a plastics and molding course at the University of California, San Diego and is a contract faculty at the University of Massachusetts at Lowell.
The company website is www.fimmtech.com and Suhas can be reached at 760-525-9053.
Check out Suhas’s Free Global Resource for Injection Mold Processing at Injection Molding Online
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