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General Drying Questions

Question:

I have read your classification of hygroscopic and non-hygroscopic resins and I am interested in physical dimension that tells me how fast moisture is absorbed or removed by a specific resin and how much moisture can be absorbed by them.

Do you know such a quantity? Where can I find specific values for different materials, especially polyurethan foams?

I would also like to know more about the necessary testing procedure.

 

Response: 

There is not a physical dimension that determines moisture adsorption rate or the total amount that can be held by a resin. There are a number of things that enter into the discussion. Please go to this link for a detailed explanation.

http://www.novatec.com/servicecenter/article/drying-factors.html 

 

Question:

What is the best kind of dryer for PETE regrind drying for running preforms.

Response:

A lot depends on the source of the regrind and the way it’s being combined prior to the Injection Machine. 

Dryers today are much more energy efficient than dryers in the past and have a number of features that make them easy to use.  Either dual bed or wheel dryers perform well, but wheel dryers have a much more consistent dew point and temperature that lends it well to preform manufacturing (where the machine (often Husky) requires very consistent temperature to the machine throat for the best preform production).  I’ve included a Data Sheet on Novatec’s IntelliPET dryer that is specifically designed for preform production and consistency.  It includes variable frequency drives so that the power varies in relationship to the material rate and thus has lower energy use than older models.

If the regrind is preform regrind it can generally be added in the processing up to 10-15% without much change in the hardware.  If the regrind is re-pelletized, it can be used much like virgin resin but may have a slightly lower IV than the virgin.  If the regrind is PCR (post-consumer) flake, there is more to consider.  The PCR has already been through a heat cycle and has both a lower IV and a tendency to produce a considerable amount of volatiles that exit in the drying process and tend to contaminate the desiccant materials.  This requires an additional treatment step on the hopper’s return air to limit and desiccant contamination.

Question:

I am extruding PC sheets for hard luggage case; I have problem of breaking shells after dropping; please kindly help me to find out the problem and suggest any solutions? I am using 70% resin and 30% regrind.

Response: 

We do have a gentleman with PC processing experience, and he suggested the following:

1. Resin isn't properly dried. Being its colored PC, they wouldn’t see the air bubbles and thermoforming usually doesn’t show the kind of surface splay you with injection molding. Sheet is thermoformed too hot.

2. The regrind is poor quality...could be a 2nd, 3rd, 4th generation of regrind and the PC has somewhat degraded from the multiple heat histories.

3. A contaminate (other resin) in the regrind that becomes a weak point compared to PC resin.

4 . If the luggage has holes drilled for hinges etc, if the holes are impropriety drilled, would be a weak spot. If the luggage blank is saw cut to trim, could cause micro-cracks

5. If they "clean" the luggage after processing, the cleaning agent may not be compatible and cause micro-stress cracking.

6. Would like to see a photo of a broken one. Are they seeing brittle cracks (like what one would see with say polystyrene)...or ductile tears as cracks.

8. The drop test is too severe.

 If you'd like to send photos, we can have him take a look and offer any other appropriate insights.

Question:
How often should desiccant be changed out assuming we run 24/7? 

Response:  
I have seen desiccant running well in service for over 10 years but the supplier guideline is 3-5 years. Desiccant normally fails due to poor filter maintenance or, in cases of high levels of volatiles, off-gassing from material. The best way to preserve desiccant is to change filters on a regular basis and always turn the dryer off when changing filters. There is no reason to change desiccant that is providing good dew point and airflow performance.

Don Rainville: Energy Consultant, Novatec, Inc.

Question:
How do we minimize the amount of dust entering the dryer either through resin or regrind?

Response: 
My first recommendation is to install a good cyclone. Second make sure that your filter maintenance is good and that your filter changing procedure prevents dust from getting past the filter housing. It’s also possible to reduce dust by installing several outlets in the top of the hopper. Multiple outlets balance the airflow in the hopper and helps reduce the velocity at each outlet. You can also weld a drop tube around the loader throat which contains the material causing it to flow slowly into the hopper at a steady state.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
What material would you use to insulate a 4,500lb Hopper?                 

Response: 
Most companies use standard fiberglass insulation which has proven to have the best cost benefit ratio. There are many other materials available but they are much more expensive and perform marginally better.  The goal is to limit the external skin temperature to a point where it is warm to the touch but will not burn you.  Also, don’t limit this to just the hopper.  Some of the biggest savings we’ve seen is improper insulation of the heater boxes and piping and hopper cones.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
If a single blower supplies multiple hoppers is there any opportunity for improving airflow control?  Each hopper has independent return air.

Response: 
Yes it can be done but it is probably too expensive for the return on investment. You can put individual dampers on the inlet of each hopper to control airflow. If you add airflow sensors you will start spending more money but achieve better control.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
How can a single Central Dryer, dry several different materials?

Response: 
A Central Dryer produces -40˚ dew point process air but has no process heater. The dry air is piped to multiple drying hoppers, each having its own heater/blower. These hoppers can contain different resins in different colors. The heater/blower on each hopper is adjusted to match the drying temperature of the material in that hopper.
The throughput of the central dryer is matched to the number of drying hoppers being served. A central drying system may consist of more than one central dryer, each serving multiple dryers.
A central material conveying system typically feeds material to the drying hoppers and then, on to processing machines.
A Quick Selection Manifold can be used to shuttle materials from any given material source to any given hopper so material changes occur at the drying hopper, reducing the time required for changeover.
Central Drying Systems reduce:

  • Energy costs
  • Labor costs
  • Material losses
  • Safety issues

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
Can a dual bed desiccant dryer be retrofitted with a desiccant wheel?

Response:
The use of a desiccant wheel requires a new dryer.  If you have a dryer that works well now, it may be best to continue to use it and look at things like water flow control, cycle timing and insulation to improve the energy of the system.  However, when you look at a dryer replacement or a new line, the opportunity becomes very attractive.

Don Rainville: Energy Consultant, Novatec, Inc.

Question:
We have a desiccant wheel dryer with no dew point monitor.  Running a PET dryer at 320 degrees, should this give us dry material?  We dry the material for 4.5 hours?

Response:
It’s not just time and temperature but also air flow rate and hopper design that needs to be considered.  If the air flow is correct and the hopper is well designed to provide uniform air and material flow, PET processing usually does well with 4-6 hours at 320-350° F.

Don Rainville: Energy Consultant, Novatec, Inc.

Question:
We pre-dry all of our materials in a separate area out back and seem to have different molding problems throughout the year.  Sometimes things run perfect for long periods of time, and then for no apparent reason, we start getting streaks, bubbles and other problems.  What gives?

Response: 
There are several possible reasons for the problems you are having. It could be that the problems are related to the condition of your incoming material.  In colder and dryer times of the year, when the air outside is dry, the material is already fairly dry. But, when the weather is humid, the material is more humid and your dryer must work harder to remove its accumulated moisture [see drying parameters]. An older, or outmoded dryer will have a more difficult time providing uniform drying results.  Check how old your desiccant is and replace it if it is more than 2 years old.  Make sure all heaters are operating at full capacity and, of course, the blowers of your dryer must be operating correctly. Make sure the process and regeneration filters are clean because dirty filters can reduce the airflow, which has a direct effect on the efficiency of the dryer.

Jeff Rodway...Service Manager, Novatec, Inc.

Question:
We have a central loading system circa 1998. All make up air from the dryer supply trunk line to the collector plenums have been disconnected. Make up air ports on the collectors are open to atmosphere. Ports on the dry air trunk are plugged. We run negative pressure in the hoppers by adjusting the dampers on the dry air supply and return lines. Q. How negative must we be on the hoppers to pull air through the hopper back to the dryer? At present, we are pulling atmospheric air into the drying hopper.

Response:
Typically, drying hoppers operate at a positive pressure on the inlet and at a very slight vacuum to near neutral at the outlet. By restricting the inlet enough that the hopper operates at a vacuum, you may be limiting the drying air to the point that good drying may not occur and atmospheric air enters the hopper even adversely affecting drying further.
Additionally, the vacuum in the hopper can lead to difficulties in the hopper discharging properly into the convey lines and limit the available capacity if the convey pumps.
Is the dryer connected to more than one hopper? If so, I recommend that the primary restriction occurs at the hopper outlet rather than the inlet.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
Currently using PVC durometer 65 bubles in the injected outsole.

Response:
Although PVC is typically not hydroscopic, the presence of moisture on the surface can lead to inconsistent processing and the requirement for adjustments in the Injection Molding Machine.  
The use of a Hot Air Dryer will remove the surface moisture and keep the PVC at a consistent temperature such that machine process parameters do not require as much monitoring and result in a more consistent product.
There are also IMM parameters that can lead to bubbles in the PVC.  Please contact your machine vendor for suggestions that they may have.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
I'm trying to explain to a client the finer points of resin drying and can not get a good answer to a question related to the return air temperature of a dryer and how it affects desiccant performance. How does high return air temperature experienced running PET affect the desiccant's ability to adsorb more moisture?

Response:
The optimum performance of a desiccant occurs at near ambient temperatures.  Temperatures in excess of this lower the desiccant capacity and require that the air be cooled before going to the desiccant.  This cooling of the air, for better adsorption, means that you have heated the air unnecessarily and thrown that heat into cooling water which requires more process heat to be re-added.  
Novatec’s Power Guard systems continuously monitor the return air temperature and adjust the process air flow for optimum drying and the minimum energy usage through a patented process that also adjusts the set point based on resin temperature.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
The dispute is about Cactus dryers. I will not use them because they only use ambient air passed over heaters. I have heard they do not work well unless it is only surface moisture. I cannot find any data that supports their drying claims. Am I currect in my assumption that they cannot dry ie PU weel enough or be consistent in an extrusion tubing process.

Response:
The Cactus is typical of hot air dryers with the exception that it uses compressed air instead of blower power to move the air. Because of this, it will be slightly lower in dew point, at times, if your compressed air is very dry. However, it will not be more consistent than your plant Relative Humidity. This can be and issue, especially in the summer.

When drying PU, it is usually important to control both the temperature and resin moisture to get consistent product flexibility and strength.

There are other dryers that use compressed air and a membrane and develop very consistent processing of PU. The NovaDrier is a good example of these and it uses far less power and compressed air than any other of its kind. The dew point and temperature are consistent, year round.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
I’m following the instruction manual, and everything seems to be working correctly but my PC is not drying completely. I’m using a compressed air dryer and I was told that it would dry most materials under most conditions.

Response:
We’ve heard this story before. Unfortunately, you never know when the problem will crop up. Compressed air dryers only produce a dew point of about -10 degrees, under the best of circumstances, but summer temperatures or high humidity can push the dew point up to +10 or +20 degrees. That’s when you notice defects in your molded products.

There are also other problems with regular compressed air dryers but let’s concentrate on your current problem.

First, I think you should call the manufacturer of your dryer and see if there is any way to return it, depending on how old it is and what your warranty says. If that is not possible, ask if they have a membrane option. If you buy the membrane option and install it, you will get -40˚dew point process air and that should solve the current problem.

Note that regular compressed air dryers consume a lot of compressed air and when you add an optional membrane, the compressed air usage goes even higher. Next time you buy a dryer, consider one with a built-in membrane to save lots of energy and be guaranteed that you can dry all resins, regardless of the time of year.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
How do I know whether desiccant beads are still good?

Response:

  • Visual inspection - Color should be creamy, eggshell white in color.  If it is brown or black, replace the desiccant..
  • Crush test - Take a few pellets and put between your fingertips.  If you can crush the pellets, replace the desiccant
  • Water test - After the desiccant has been regenerated and cooled off, take a hand full of desiccant and put into a glass or metal container.  Add enough water to cover the beads.  You should get a reaction between the beads and the water - a bubbling action and some heat. If you don’t get this reaction – replace the desiccant.

Poor drying could also be due to the heater elements not producing the proper heat or drawing the proper amperage. Another cause could be air leaks or obstructions in the air hoses

Jeff Rodway: Technical Service Manager, Novatec, Inc.

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