Plastic drying is critical to the process of molding or extruding any plastic part. Many defects, both cosmetic and functional will occur if the plastic resin is not adequately dried before processing.
Every type of plastic is affected by moisture and must pass through a drying process before it is molded or extruded. The drying requirements for the many types of plastics vary greatly, from materials such as polystyrene, which can be dried in a few hours at a low temperature, to PET, which may take up to 12 hours at a much higher temperature.
How plastic drying works
Basically there are three main methods of drying plastic to be used in injection molding or extrusion molding. These are in-the-machine, hot air and desiccant dryers.
Drying plastic in the machine is not commonly done because few machines have the degassing stage present in the plasticizing zone of the molding machine. Most processors use an external dryer instead.
Increasingly, companies are developing newer and better technologies to make the plastic drying process less expensive, more effective and greener.
A very good example of this newer technology is the desiccant wheel dryer. In this method the desiccant is much more effective, the electricity consumption is much less and the results are much better than the older, more traditional approaches.
Here is a good video showing how plastic is dried
Hot air dryers
The hot air dryer operates on a very simple principle. A heater/blower unit is mounted on the hopper together with a controller for the HB. The warm air is circulated through the resin, drawing moisture as it passes through the hopper, then it is released into the atmosphere. This process is repeated until the moisture content is reduced to specification.
Desiccant bed dryers
This type of plastic dryer uses a desiccant very similar to the little packets you find with consumer products. Air is forced through the plastic filled hopper and into a bed, or filter of desiccant material. The moisture is absorbed, the air heated and re-circulated back into the plastic resin. This cycle is repeated until the proper level of moisture is reached.
Why use plastic dryers?
The reason processors use dryers is simply because you must. Plastic either contains moisture or attracts it, and if molding is done without drying the results will be disastrous. In the same way that corn will simply not pop if it is too wet or dry, plastic will not mold properly and waste time and money.
Typical problems due to insufficient plastic drying
- Splay is occurs when water is present
- Plastic part failure is serious business and can result in legal action
- Silver streaks are a sure sign of wet plastic
- Voids may be caused by moisture in the resin
Main groups of plastics: Hygroscopic and Non-hydroscopic materials
Hygroscopic materials are those that absorb moisture within the pellets. This moisture forms a molecular bond with the polymer chains and requires heat to be released.
Non-hygroscopic materials do not absorb moisture, but rather collect the moisture on the surface of the pellets. This also must be removed using heat and time, though generally not as much time as hygroscopic plastic.
All plastics require drying and will not perform without proper processing. Choosing the right type of dryer is critical to the success of any plastic injection molding or extrusion business. Newer technologies promise greener, more efficient and reliable methods of drying both hygroscopic and non-hygroscopic materials.