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Inconsistent Dryer Dewpoint Readings…

I was asked this question recently…

Kevin
We were encountering complications with the drying of nylon. We have attached a dewpoint meter to our desiccant dryer and the measurements range erratically between -37 and -10°C while operating. The desiccant regeneration is set to 3200 second. There seems to be plenty of suction on the return line from the hopper to the unit. Could you recommend What is your interpretation of the data and how do you think it will affect the performance of our equipment?
note: Kevin attached photos of the dryer and the dewpoint measurement graph showing an erratic fluctuations with a slight incline at the regeneration period.
My Response
You should consider shorter bed times and/or replacing the desiccant bed if it is older.
It would be worthwhile to check all the seals, connections, and filters to see if there is a leak causing moisture from the atmosphere to leak into the hopper.
Additional Thoughts
If you are not getting adequate support and guidance from the dryer manufacturer on this matter, I strongly suggest considering another brand.
-Andy

More on Venting…

I have received a few follow-up emails on venting, and would like to add the following comments and links for information…

Alternate Venting Techniques
Ejection – You should always consider venting your moving components such as ejector pins, ejector blades, lifters, stripper plates, and slides are all great components to vent. Since these move each cycle, they usually act as self-cleaning vents. Components such as vented ejector pins are already available commercially through many suppliers.
Porous Mold Steel – As long as you can vent to atmosphere, porous steels can be a great option for improving venting. These materials allow air to actually vent through the steel. For some design guidelines for such materials, please feel free to visit this helpful site:
Vacuum venting – To improve the efficiency of venting, many molders are turning to ‘negative cavity pressure’ techniques. These systems help draw the air out of the mold cavity faster allowing you to inject faster and maintain a more repeatable process. For a design guideline for vacuum venting, please feel free to visit this helful site:
-Andy

You Can’t Over-Vent…

I received this commentary form one of our frequent blog contributors…

Nathan
I expect you to give a very strong message on tool venting in your Plastic Training and Technology Blog. Basically, many of the Moulders are not aware of venting. What should be the best way to make them understand?? Even though they agree, they still have a strong concern on venting which may cause flash on the product.
My Response
The best rule of thumb is ‘You can never over-vent your tool’.
Since the melted polymer will not flash if the vents are of appropriate depth, the number of vents has no affect on the likeliness of flash.
When I am involved in a tool design or revision, my motto is to vent everything… runners, cavities, parting lines, ejector pins, slides, and lifters. Drawing a vacuum on the mold or incorporating porous materials such as Porcerex are very helpful options. 
Additional Thoughts
Poor venting often contributes to defects such as burning, poor surface finish, short shot, and part hang-up. This also restricts your ability to increase injection velocity to help correct many defects including poor surface finish, short shot, sinks, runner flash, voids, large dimensions, and warpage.
Essentially, inadequate venting restricts your ability to develop and maintain a good Scientific, or Decoupled, molding process.
-Andy.

Material on the Screw…

Someone asked me this during a classroom session last week…

Participant
How long can material remain stuck to the screw?

My Response
Indefinitely. Essentially, if the material adheres to the screw, it can degrade and become baked onto the surface of the screw. I have seen this with materials and additives such as colorants. This is one of the reasons you should take great care of the polymer during screw recovery to promote smooth material conveyance and reduce excessive back-flow and stagnation.
Additional Thoughts
Although there are many advances in purging compounds over the years, the most effective method of cleaning the screw is still to actually remove the screw and manually clean the screw surface.
-Andy

Process Outputs for Blow Molding…

I was recently asked this interesting question via email…

Technician
I hear all this discussion about process outputs for injection molding. Is this also important for blow molding?

My Response
Yes, process outputs are important parameters to document.  These could include parameters  such as melt temperature, coolant temperature, part weight, screw RPM, and flash weight can all be useful.
In the extrusion-blow process, the outputs from both the injection and blow stages can be very helpful in both setup and troubleshooting.
-Andy

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