Several attempts have been taken, to assess the best and most efficient drying method for plas- tic granules. It is an important issue, because according to one industry expert, William F. Thorne, senior process development engineer at Bayer, "Drying is the biggest source of material-related problems."
Failure to reach the correct humidity levels in plastic granulates processing is known to be a fre- quent cause of deterioration. If you attempt to process raw materials that have too much moisture content, you could find a number of ultimately expensive issues, such as product quality in terms of strength and appearance. Typically, evident in two different ways:
- Polymer breakdown from hydrolysis, oxidation etc.
- Splay marks, usually appearing as silver streaks along the flow pattern, however depend ing on the type and color of material it may represent as small bubbles caused by trapped moisture.
The failure tolerances are small, as hydroscopic variations above only a fraction of a percent can be detrimental to the performance of the finished item. The permissible moisture levels vary according to choice of polymer resin. All within a narrow band of tolerances, as described in the Handbook of Industrial Drying, Fourth Edition, edited by Arun S. Mujumdar, table 47.1
At Labotek we too have asked ourselves what category of dryer work the best and is most cost effective. Obviously, you would probably think that our answer to that question would be biased. Or at best, conducted under laboratory conditions, which rarely represent the actual conditions of a factory working environment.
However, most industry sources agree, that drying is the biggest source of material-related prob- lems. Particularly in the summer heat during periods with variations in air humidity. In this article, we take a look at different drying methods, and the issues relating to each of them, as to discuss the pros and cons of the different possibilities.
Most raw plastic material looks dry. However, everything is not always what it seems. Hygroscopic materials such as Polycarbonate, Nylon and PET to name just three, attract moisture from the sur- rounding air. So, although it is manufactured "dry" by the time it gets to the processor it will have some moisture content. During processing a chemical reaction (hydrolysis) takes place making the long polymer chains shorter. Long polymer chains are required to make good product. Short chains result in poor quality moldings.
Equally, over-drying of material, for too long or at too higher temperatures is also potentially disas- trous. When color pigments are being used, over drying and too long residence time in the drying process can cause shifts in the color, potentially causing it to darken.
In this article, we are looking at six commonly used methods to ensure that raw material is in the optimum condition for processing.
Compressed Air Dryers
Compressed Air Dryers, compact and light in weight these Dryers are designed to fit to the throat of the processing machine, although a free standing frame can be employed.
Generally, a method of drying employed for smaller throughput machines, as the cost of generating compressed air is prohibitive for drying of larger volumes of resin.
The Compressed Air Dryer is similarly equipped with twin desiccant beds as the Dehumidifying Dry- er.
The main difference between the two is that air flow is generated by compressed air instead of electrically driven mechanical blowers. In some instances, a membrane is added to the Dryer.
Dehumidifying Dryers are designed to eliminate moisture in the plastic material before processing. Air is forced through a desiccant bed to make it extremely dry. This air is then heated to a specified temperature and fed into a Drying Hopper containing the material to be dried.
The hot dry air draws the moisture out of the material; the saturated air is then fed out of the Drying Hopper and back through the desiccant bed again to remove the moisture before the cycle begins again. This desiccant has the ability to hold moisture before it needs to be regenerated (dried out).
Typically, plastics processors use regenerative or twin tower dryers and in this case, when the des- iccant bed has reached its moisture retaining capacity the airflow is automatically switched to the second desiccant bed to allow the drying process to continue. At the same time, the first desiccant bed is regenerated by heating to remove moisture, allowed to cool and it is then ready to absorb moisture again. This technology is well proven and acknowledged for its durability. The desiccant beads named "molecular sieves" normally lasts for more than a decade, when protected against contamination.
Moreover, Dehumidifying Dryers can provide a more cost effective ventilation process, when de- creasing the air pressure by using larger diameter ducting. And thus providing a drying result that exceeds the processors expectations.
Recent development in Dehumidifying Dryer technology combines the advantages gained from both loss-in-weight hopper technology and an internal heat recovery process, proving this method to be the most energy efficient and cost effective drying method.
The Honeycomb Dryer has a matrix canister that is sending dry air to the resin hopper, providing a steady flow of dried air. Those dryers operate efficiently at their touted -10° to -40°F dew point on initial start-up, but rapidly lose effectiveness as the desiccant material canister absorbs moisture and becomes saturated.
Their effectiveness is further reduced when moisture laden air is recirculated back from the plas- tics being dried through the desiccant bed which, in fact if hot, can force moisture back into the material being dried.
Thus, the introduction of time cycles and moisture monitors are needed to determine when the air flow should be routed to a fresh dry desiccant tank. The wet desiccant tank or cartridge requires regeneration which simply means to dry or bake out the moisture at extremely high temperatures.
This air flow change and desiccant bead canister regeneration requires automatic or manual cy- cling to achieve effective continuous material drying. Therefore, desiccant bead dryers, due to their size and related equipment, including multiple bead canisters, are expensive to purchase, costly to operate and, in most cases, require constant attention.
Hot Air Dryers
Hot Air Dryers are designed to remove surface moisture from no-hygroscopic material and to pre- heat material prior to processing. Non-hygroscopic materials are polymers which do not adsorb moisture within the granules, but, hold the moisture on the granule surface, trapping a relatively small amount of humidity. In some instances, Hot Air Dryers are also used for hydroscopic mate- rials, such as certain polyolefins whose additives don't retain moisture, and even ABS for non-criti- cal applications.
However, the processing of plastic materials with the presence of water moisture, may seriously affect the process and hence, the final product quality. A Hot Air Dryer unit has a blower that sucks in ambient air and blows it into a process heating chamber to heat it up to a set working tempera- ture. This heated process air, is then blown into a drying hopper containing the non-hygroscopic polymer, where, the hot air flows from the bottom to the top of the hopper, the hot air passes over the surface of the granules, heating them up and removing the moisture from the granule surface and evaporating it in the hot air stream.
The return air is filtered through an air cartridge paper filter. After successfully removing the sur- face moisture, the moist laden air can either be totally exhausted into the atmosphere or partially returned into the incoming ambient air with consequent energy savings.
The pre heating is particularly useful in circumstances where material has been stored outside in the cold and is brought into more warm and humid conditions for processing. Hot air dryers usually comprise a well-insulated hopper with an attached blower and heater. Hot air is blown through the material granules and the wet air is dispelled into the atmosphere.
Rotary Wheel Dryers
Rotary Wheel Dryers have been designed to address energy and other revenue draining issues pre- sented by other types of drying. Rotor Dryers have a constant dew point whereas in twin desiccant dryers the dew point starts very low but increases as the desiccant bed saturates. When the air- flow is switched to the second desiccant bed the dew point starts low again.
Dew point control is a design feature of Rotor Dryers which allows the user to select a dew point suitable for the material to be processed and maintain this.
Rotor Dryers utilize a rotating wheel which continuously puts dry desiccant into service due to its ability to regenerate moisture saturated desiccant in the same rotational cycle. These systems are often used for the low capacity drying system up to 300m3/h, since the cost of running at higher airflows, exceed the dehumidifying dryers. The lifetime of the rotor unit itself has proven to be shorter than seen in the desiccant systems and quite often the rotor must be replaced after only a few years of service.
Vacuum Dryers accelerate the plastic material drying process by using a vacuum to lower the boil- ing point of water from 100 °C to 56 °C. The moisture is rapidly extracted from the heated material. Leaving it in a condition to be molded or extruded.
Typically, these Vacuum Dryers will dry materials faster than a desiccant dryer and the use of vac- uum provides both process and revenue benefits, when compared with conventional drying methods.
This method of drying does not use desiccant at all and as such not only saves the expense but also saves the time required to maintain and replace desiccant. The low drying times offered by these machines reduces the risk of material degradation from exposing material to prolonged ele- vated drying times.
The method however, cannot be used for all resin types and due to the acceleration of the drying process using vacuum, there is the unpleasant side effect, that some resins are unable to release their true moisture content and residual moisture levels will be too high to process the resin as desired.
In conclusion, the choice of drying method depends on several factors that all need to be taken into consideration, when choosing the best drying method:
- The chemical structure and consistency of the plastics material
- The level of moisture absorption of the material
- The environment in which the production is taking place
- The variety of temperature and humidity at the production facility
- The service level and cleaning processes of the drying equipment
- The technical sophistication of the operatives of the equipment
- The type and quality of the finished application
- The color and consistency requirements to the finished items
- The production volume and expected exchange frequency of dies
- The expected return on investment for the production line
- The timeframe for payback on investment
At Labotek we have been improving drying of plastic granule for more than sixty years in most corners of the world. It is our experience, that the right choice depends on several factors, such as those mentioned in this article. Therefore, the best choice of drying process has to be based on an analysis of all these factors.
If, however, we should point towards one method that we find stands out, it would be Dehumidify- ing Drying, when combined with "loss-in-weight" gravimetric drying hoppers and internal heat re- covery. Such solutions have proven to be highly reliable and very cost effective. Moreover, the add- ed advantages of heat recycling make this technology the most environmentally friendly choice. But the right choice obviously does depend on the task at hand.
Our team of experts are always keen to explore the best solutions for you. So contact us now for a chat regarding your possibilities, and a dialogue about the best solution for your needs.
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