Injection Molding: A Dozen Common Drying Issues

Splay, brittleness, bubbles, and/or sticking problems, all are issues that can come from improper drying of hygroscopic materials.  The unfortunate thing is many people overlook the details when it comes to drying plastics.  If you ask the question "is the material dry?" the correct response should not be "I guess, it's been in the drying hopper for 3 hours".  The fact that the material was in the hopper for three hours is often meaningless since drying plastic relies on 4 things:
     1) Air temperature entering the drying hopper.
     2) Time that the material is in the hopper and exposed to warm dry air.
     3) Dew point of the air entering the hopper.
     4) Sufficient airflow within the hopper.
As you can see saying that the material is dry because it was in hopper for 3 hours is meaningless unless it was in the hopper with the correct conditions.

The following list is contains observations that I have made over the years while troubleshooting molding issues.  

1) Dryer setpoint is correct but the actual temperature reaching the plastic is too low.  Take a look and make sure you are controlling your dryer at the inlet to the hopper, otherwise your air temperature will be too low to dry effectively.  Also make sure that people have the correct set points for all of the materials you run, guessing doesn't cut it!

2) High dew point reading.  To dry efficiently the dew point of the air entering the hopper should be between -20 and -40F.  There are many potential root causes to high dew points including:  Remember that the dew point reading is a measuring the air and not the moisture content of the material.
     -Bad desiccant, either too old, or contaminated with fines or volatiles.
     -Regernation heaters not functioning correctly.
     -Leaking ambient air from poor seals or damaged hoses.
A quick test to check your desiccant is to take a cup of desiccant and add a small amount of water.  If the desiccant is good you will witness an exothermic reaction and feel the heat coming from the cup, bad desiccant won't react with the water and there will not be a temperature rise.

3) Not using an after cooler where required.  Desiccant works most effectively when the return air is between 120 and 150F.  If you are drying material at temperatures greater than 180F you should be using an after cooler to bring the return air temperature down to the optimal level.  Also don't use insulated hoses for the return air, since you want the return air to cool off.

4) No dew point meter available to check the dryer performance.  This is a very valuable tool that will help you determine if your dryer is working.  When using a portable dew point meter make sure you leave it on long enough to evaluate all of the desiccant beds.  You can purchase a chart recorder to document this information  over a time period.  Another good technique is to connect your dryer monitoring to a RJG eDART?.  You can use an 0-10v analog input module to connect the voltage output from a dew point meter or purchase the dew point unit that RJG sells.  You can easily monitor dryer temps and dew point on a continuous basis or set up a testing protocol to be conducted as scheduled.  This can allow you to have a predictive maintenance approach rather than reactive.

5) Lack of air flow.  This can be something as simple as having hoses that are crushed or excessively long restricting air flow or trying to use an undersized dryer for the application.  Always verify that the hoses are in good shape, no holes, crushed areas, etc.  Keep your filters clean to maximize performance, filters will plug more rapidly based on ambient air conditions as well as how dusty the material is.

6) Not accounting for higher then normal moisture content.  The perfect example of this trying to dry a nylon that has been sitting in open atmosphere during humid summer months.  You are going to need additional drying time to get the material to the desired level.  The standard recommended drying times are based on normal starting moisture content so it may not account for the extremes.

7) Not keeping your material dry.  After putting all this time and energy into drying material I have seen people load 30+ minutes worth of material into a machine hopper that was open to atmosphere.  Depending on the material and the conditions you have a limited time outside the drying hopper before the material is too wet to use.  The worst material I have documented for this issue was a TPU that we showed through a combination of Karl Fischer and loss on weight moisture analysis had 5 minutes in the open environment before it was too wet to use!

One shot hopper to limit material outside of drying hopper.

 8) Assuming that wet material will always create splay.  Polyesters are a great case where you may never see splay on the parts and the first indication of an issue is either parts sticking to the mold or parts cracking and crumbling due to degradation.

9) Allowing the material feed for the drying hopper to run dry.  This is typically a plant discipline issue with people not keeping the material supplied to the hopper.  This will significantly impact the drying residence time and will potentially allow wet material to be run in the machine.  

10) Improper hose setup.  First of all make sure that the supply is going to the inlet for the hopper not the outlet!  Also use an insulated hose for supply and keep it as short as possible.  Do not use insulated hose for the return air since you want the return air to be cooled to between 120 and 150F when it hits the desiccant.

11) Not understanding what is truly required to effectively dry plastic.  An overview class for all of your technicians and material handlers is time well spent.  Provide them with an understanding of why we dry and why the temperatures, times and dew point are so important.  

12) Not verifying material throughput to determine if the material has enough drying time.  If you are on the edge and you burn through a bunch of material purging during startup you may end up with a period of wet material.  

A lot of dryers have various options to help monitor performance including dew point monitoring, CFM monitoring, regeneration temperature monitoring, return air temperature monitoring, etc.  All of these make it easier to stay on top of the drying process.

These are some of the things I've seen over the years.  Drying is critical to our processes don't overlook the importance of the details.  

Steve Brammer