Material Drying: Mythology or Methodology?

Key Highlights:

• Drying is essential for many materials - often even non-hygroscopic materials perform better having been through a dryer.
• Performance, cost of running and maintenance should be considered when choosing a dryer to suit your production requirements - i.e. small batch or long- production runs.
• By maximising efficiency with the correct selection of equipment production efficiency can be maintained, eliminating manufacturing problems caused by damp material.

In plastics processing plants material dryers are a standard piece of equipment as not correctly drying thermoplastics can cause aesthetic and mechanical defects triggered by the presence of water. 

Moisture can originate from within the plastic pellet of hygroscopic resins or condense on the surface of non-hygroscopic resins. If material is not dried correctly this can cause potential issues during processing and in the component post-processing. 

When cosmetic defects occur in non-hydroscopic resins, e.g. polyethylene or polypropylene, these can often be traced back to excess surface moisture deposited due to rapid changes in the temperature and humidity.  This can be resolved by allowing the raw material to come to equilibrium within the surrounding atmosphere where it is processed so that the surface moisture can evaporate, or it can be conventionally dried using a hot air dryer to remove the surface moisture.  This preheating 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. 

it is essential then that proper drying techniques are implemented, to prevent cosmetic problems and even chemical reactions, such as hydrolysis, where water molecules break chemical bonds within the polymer that can have an impact on the mechanical properties of the material. 

It is also important to be aware that some non-hygroscopic materials may still require drying due to the presence of additives. For instance, impact-modified acetal (POM) usually does not require drying, but if an impact modifier is added, then pre-drying becomes necessary. 

All material manufacturers will recommend an upper moisture limit for the optimum processing of materials, and this can be found on their material processing data sheets. 

It also needs to be remembered that over-drying material for too long or at too high temperatures can also cause potential problems. Over-drying and too long residence time in the drying process can cause colour changes and the subsequent scrapping of components. 

Drying Technology 

It was in the late 1950s that the desiccant dryer first emerged when the first generative dryer using silica gel as the desiccant was developed.  Further advancements were made with microprocessor controls being added and the dryer units becoming more compact.  The compressed air dryer offers a moisture removal membrane that is not a desiccant but a filter that blocks the water molecules, this type of dryer needs no regeneration, unlike desiccant dryers.  There is also challenge to desiccant drying with the vacuum-type dryers that are available. 

There are several drying options that are available to plastics processors to ensure that their raw material is in the optimum condition for processing today, these are: 

Hot Air Dryers - designed to remove surface moisture from no-hygroscopic material and to pre-heat material prior to processing. 

Twin Tower Central Desiccant Dryers - has two towers that are equally filled with hygroscopic materials. During operation, one tower is used to actively eliminate moisture from the compressed air routed through it while the other tower undergoes a reverse process where moisture is removed to revitalise the desiccant material. 

Rotor Wheel Dryers - designed to address energy issues presented by other types of drying. Rotor Dryers have a constant dew point. 

Vacuum Drying – designed to accelerate the plastic material drying process by using a vacuum to lower the boiling point of water from 100 °C to 56 °C drying materials in 20% less time than that required by a desiccant dryer. 

Compressed Air Driers - generally a method of drying employed for smaller throughput machines, the dryer is equipped with twin desiccant beds like that of a dehumidifying dryer. 

Silo Dehumidification Units – designed for bulk storage systems where a silo dehumidifier is used to remove moisture from the air inside the resin silo. The dehumidifier works by using a desiccant material to absorb moisture from the air. 

The Drying Process 

Proficiency is key in the drying process, and it involves four key elements: heat, airflow, dew point and drying time.  All play a critical role in achieving effective drying of material to maintain production efficiency and quality. 

The first drying parameter is heat, it is necessary to heat the pellet to release the moisture.  The second is the dew point, this being the temperature at which the air condenses, and moisture molecules can release from the exterior of the pellet. Thirdly, the drying parameters need to be known as it takes time to dry a plastic pellet to enable the water molecules to break away from the pellet surface.  Finally, the fourth parameter is airflow around the pellet which conveys low dew point heat or dry heated air to the pellets in the dryer.  The volume of this air must be enough to maintain the desired temperature in the dryer for the throughput of the material. 

Good drying procedures should take into account: 

• Temperature required to dry the material 
• Dew point of the air that surrounds the material in the dryer 
• Airflow around the material 
• Time required to achieve required dew point 
• Adherence to dryer maintenance requirements  
• Moisture analysis of material pre- and post-drying 

Correct drying procedures reduce potential manufacturing delays that are the result of moisture-related defects. 

Liaising with the equipment supplier when selecting equipment can streamline processes and sustain efficient production. Material consistency is paramount for producing quality products and correct drying ensures the material properties remain consistent during the process. Moisture-related challenges need to be fully addressed at the outset to prevent issues during manufacture. 

Incorrectly dried materials will impact the material in a number of ways, these being: 

• Poor surface finish, splay marks 
• Reduction in mechanical properties 
• Impact on electrical properties 
• Variation in impact strength and elongation to break 
• Reduced viscosity of material leading to flash 
• Short shots 
• Discolouration if over dried. 
• Increased power consumption 
• Poor melt performance and increased torque on IMM 

Technology 

When selecting a dryer, it is important to understand what the current cost per tonne is to dry your material with current equipment.  Understand your requirements and what the best system for your facility would be around your manufacturing methodology.  

Small batches through desiccant dryers is expensive conversely large throughputs through desiccant dryers bring the cost per tonne to dry material down significantly.  However, the cost of leaving a desiccant dryer on tick-over is an expensive practice. 

The cost of a conventional desiccant dryer versus adopting vacuum drying technology needs to be explored as generally the use of vacuum provides a number of revenue benefits e.g. faster drying, reduced maintenance etc., and is perceived as a modern and efficient process and there are several generations of these now being actively used in the industry. 

The drying process can also now be intelligent in the era of digitalisation with the addition of load cells and advanced control systems allowing the operator to monitor and manage the drying parameters at their fingertips.  Previously these drying parameters would need to be manually checked and monitored. 

Moretto through their R&D has challenged the design of the conventional dryer construction and developed a more efficient method of drying polymer. Their breakthrough was to redesign the material flow through the dryer hopper by adding a hollow ‘rocket-like projectile’ at the core of the dryer which enabled layering of the material, allowing a more consistent flow of material compared to a conventional desiccant dryer and reducing residency time.