Definition:  Over; 1) in excess of, more than, 2) too great   Dry; 1) free from moisture, or excess moisture,   

Though the term is used many times in industry to provide the excuse for why the material (resin) is not processing correctly it may or may not be the case for a particular mold.  The subject itself is fairly complex, yet simple.

The basics of drying are to take the moisture level down to a point where any moisture still in the plastic resin well not adversely hinder the products performance both physical and visual. This means that we apply heat and air flow for a period of time to effetely remove said moisture. (a previous article describe the basic process)

It can be stated that moisture in the resin can aid flow of the resin within a tool and or process. Examples are polycarbonate that is wet tends to flow easier, PET and PBT flow easier and Nylons tend to flow easier to name a few.  The critical point in processing is to be below a level of moisture in the material so as not to result in an issue. If too much moisture is involved we can get hydrolysis which breaks down the molecular chains, and issues result, splay, flash and overpacking to name a few.

Maximum moisture content prior to processing:

Polycarbonate:              0.02%

Nylon 6/6:                    0.2%

PET:                             0.02%

                                                                                                       Figure 1

Drying for extended periods of time and or drying to lower levels than the maximum allowed can result in a stiffer flowing material. The graph above is an example of this for nylon 66.  It can be seen that drying to 0.07% level creates a stiffer flowing material. The question is does your machine have the capabilities to compensate for this?

Another example was a PET/PC blend that was used in manufacture of a part. This particular molder had been molding this part for years, but after an issue of splay and flash found their dryers' desiccant had gone bad, they replaced said desiccant and found that they could not fill the part. Upon leaving the lid off the dryer for a period of time we than started up the process and were able to make parts that met all specifications. What we did was allow moisture to be reabsorbed into the material, which than allowed it to flow into the complex path of their tool. Not necessary a good practice but one which shows that moisture can be reabsorbed into a material, if moisture content is the issue.

We did not test the material for moisture content, and most material manufactures well give a maximum moisture content but not necessary a minimum level.

So the normal statement a processor would use is that material is overdried. Which we can debate, but it could be that though the material is dried to a lower moisture level than needed, one of issues is that the viscosity change in our material is now too great for the equipment to compensate for.

It is important to talk with the resin supplier and get both upper and lower limits to moisture content, along with residence time in a dryer. Again drying is a function of temperature, time, airflow and dryness of our air. If we use too big of a dryer, or leave for extended periods of time in the dryer, check with the manufactures for their  recommended temperatures to use for those conditions.

To be continued:

SL Silvey