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Taking the guesswork out of the drying process

Source:The Conair Group Release Date:2011-12-06 763
Plastics & Rubber
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By Jamie Jamison, The Conair Group

Proper drying is the key to ensuring quality in parts made of engineering thermoplastics. Considering how costly these materials are, it is easy to see why it is so important to have good data about what is actually happening in the drying hopper and to use that information to gain tight control over that process. A patented device called the Drying Monitor measures temperature at several different levels in the drying hopper, documenting whether or not conditions are right for proper drying. 

Why dry
Producers of hygroscopic materials — companies such as Bayer, Eastman, SABIC (GE Plastics), DuPont, BASF and others — have spent millions of dollars over the years analysing the conditions that result in optimum performance in their materials. They have a huge vested interest in getting this right because, if a moulder or extruder can't make good product, they're going to be asking the resin company why their material is failing to perform up to spec. These materials companies will tell you that the vast majority of customer complaints about resin performance are rooted in improper (and, therefore, incomplete) drying.
When an incompletely dried polymer is run, several problems are encountered. For one, high levels of moisture in or on the surface of the pellets will be superheated and turned to steam during melt processing. In extrusion, this may cause surging, spitting, sputtering and a foamy melt at the die. In injection moulding, it can cause over or under packing or flashing, and in extreme cases could cause the barrel to crack or explode due to the high pressures of trapped steam.  Cosmetic problems also occur especially in injection moulding where excessive moisture can cause bubbles on or in the part, sink marks, splay and surface defects. In extrusion, moisture escaping as steam through the die may cause a rough and scaly surface on the extrudate. Another concern is structural and physical problems.
A small amount of moisture can cause hydrolytic degradation, which in turn can change the polymer's melt viscosity, molecular weight and mechanical strength.  This may be the most dangerous problem because it often escapes detection with the naked eye and can lead to long-term liability issues that could have been avoided.
There are a few fundamental factors that govern the drying process.  One factor is polymer temperature where the temperature of the polymer determines the rate at which water molecules move through a hygroscopic polymer. In most dryers, a stream of low-dew-point (dry) air provides this low-vapor-pressure environment. Moisture is removed from the air by a molecular sieve desiccant and then the air is heated to lower its relative humidity, increasing its ability to hold moisture.
The movement of hot dry air through the drying hopper is the mechanism by which the polymer is heated and also how the moisture is conveyed away from the polymer and out of the drying hopper. By regulating air flow, processors can control the temperature of the resin in the hopper to ensure proper drying whist preventing degradation due to overheating and saving energy. 
Plastic pellets do not dry instantaneously. It takes time to raise the temperature of the pellets and, once they are subjected to lower vapor pressure conditions, it takes more time for moisture to diffuse and migrate to the surface.

What goes on in the drying hopper
As long as the drying process is under control, there is an assurance that the material being delivered to the moulding machine or extruder has been properly conditioned and good parts will result.
If the material being dried requires 4 hours under proper drying conditions, and 100 lb (45 kg) per hour is being processed, then there is aAir Jordan III 3 Shoes

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