Many engineered grade polymers absorb moisture, these resins are known as hygroscopic resins.
When moisture is not properly removed from the plastic resin, visible defects and structural deficiencies could occur such as:
Splay or striations
Bubbles in the part
Degradation of the polymer
Know your materials, if your material is hygroscopic you will need to dry the material prior to use. Examples of common plastic resin that drying are:
PC (Polycarbonate), PET, ABS, Nylon, Polyurethane, TPU
Know your minimum and maximum material throughput (lbs/hr)
Know your minimum and maximum temperatures your resins dry at
Know your resin’s highest residence time (Residence time is the time set by the resin’s supplier for the resin to reach optimal dryness)
How do you size your resin drying hopper?
Material throughput (lbs/hr) X residence time (hr) = ideal hopper size
e.g. 15lbs/hr. with a 4 hour residence time will require a 60lb hopper.
How do you want to get the resin in the drying hopper into your injection molding machine feed throat?
What are your typical runs before stopping or changing materials?
If your material change overs take less time than the resin’s residence time and happen frequently (twice a week or more) then we would recommend to have additional resin drying hoppers to pre-dry your next resin. Typical ROI will be seen within a year by reducing down time.
There are many different dryer / hopper combinations to suit your process flow, here are some options below:
Portable press side dual resin dryer and drying hopper ideal if you want to dry two different materials and have one on standby to eliminate downtime.
Machine feed throat mounted resin dryer with drying hopper
Central resin dryers and drying hoppers with portability. Conveys dried plastic resin to the machine’s feed throat via material conveying lines.
This article is meant to help those who have little to no understanding of drying plastic resins.
There are many different types of plastic resins, but they can all be split into two basic categories: hygroscopic or non-hygroscopic. The definition of hygroscopic is: of a substance tending to absorb moisture from the air. This means that some plastic resins/pellets absorb moisture just by being expo
sed to ambient air. (note that the surrounding “ambient” air we are exposed to is always considered moist/wet unless it is at 0 or negative F in dew point, this includes the air inside of material conveying lines and hoppers.)
The moisture that gets absorbed deep within a pellet can have negative effects to the quality of whatever product is being molded, and it then becomes necessary to dry the pellet before molding. Some of these “negative effects” are:
Splay or Steam typically visible streaks or bubbles in or on the end-product caused by the boiling of the moisture in the pellet that becomes gas/steam.
The loss of the strength in end products. The moisture in the plastic weakens the molecular bonds in the plastic, which can cause the end product to be brittle.
Inconsistencies in the injection molding or extrusion process caused by the fluctuations in viscosity of the resin which is a major factor in injection molding / extrusion. One major example of this is with Nylon, which will actually spit or drool out of the nozzle when purging if it is not dried properly.
How to Dry Hygroscopic Resins
There are 4 main factors that must be met and precisely controlled to dry hygroscopic resins.
One common misconception about drying material is that the “wetness” of resin is the topical surface moisture, but the actual moisture content of a plastic pellet is measured in parts per million (PPM) and refers to the moisture content within a pellet.
All hygroscopic resins have a specific temperature set point that they must be at to release the moisture within the pellet. It is important to know that if the material is below the predetermined temperature set point, even if it were to be left in a dryer all night it may l never release the moisture within the pellet (be dry). It is also important to note that raising the temperature above its specified setpoint is not an acceptable shortcut to drying it more quickly.
Air Flow & Dry Air
The next two factors work hand in hand, so they will be discussed together. Once the resin is at the right temperature to release the moisture it has absorbed, The released moisture must then be moved away. In order to both evenly distribute heat throughout the resin and move away the released moisture, there must be enough air flow, and the air must be dry. If the amount of air flowing through the resin is insufficient, or the dew point (the measurement of how dry air is, in either F. or C.) is not low enough, the moisture that is released from the resin will not get absorbed by the flow of air, and won’t get carried away. This will just result in the resin reabsorbing the moisture, and either take more time to dry, or just never get dried. The industry standard for how dry the air must be from a dryer is -40F., but it can range from -20F. to -80F depending on the type of desiccant being used, and the regeneration/life cycle of the desiccant. Note: For Most materials, having a -20F. dew point vs -50F. dew point will make no noticeable difference on how quickly the material is dried.
Residence time, the time required to dry a resin, in the real world, is constantly changing due to circumstances such as:
how wet is the material to begin with
how humid is the weather on the day you are drying the material
how dry do you actually need the material to be to achieve quality results
how many of the other 3 factors (mentioned above) are being met optimally
how efficient is your dryer (at meeting the other 3 factors of drying),
how much moisture is being regained during the process of getting the material from the drying hopper to the feed throat of your injection molding machine, extrusion line, or blow molding machine.
With all these considerations every type of resin has a predetermined residence time set by the manufacturer which is usually 4 to 6 hours of drying.
With that being said, once you have a properly sized dryer, you will need to follow these steps to start the molding/extrusion process.
Fill the drying hopper with your resin.
Set the dryer to the temperature specified by the resin’s manufacturer.
Let the material dry for the specified residence time (for example, 4 to 6 hours).
Convey the material from the dryer to your machine’s feed throat. (unless the dryer is directly on your machine’s feed throat, in which case, you are ready to start molding/extruding.)
Start molding! (note: for every x amount of material you are processing, you must add the same amount of material back into the drying hopper simultaneously, or you will not achieve the proper residence time of drying for your resin.
At the 2018 Plastec West show held at the Anaheim Convention Center, Hirate America, Inc. introduced the new Motan Luxor E A and Luxor EM A compact dryer with integrated conveying. This dryer is suitable for press-side drying of plastic resins for injection molding machines. Motan offers three sizes of drying bins – 60 liter, 100 liter and 150 liter.
Both the Luxor E A and Luxor EM A dryers with optional ETA plus energy saving technology are specifically designed to supply consistently low dew-point air to one drying bin, while reducing energy consumption. The Luxor E A dryer is designed for use with existing conveying systems or hopper loaders. Whereas the Luxor EM A offers a machine dedicated solution with integrated dry air conveying for up to two processing machines and the drying bin. As stated above there are three sizes with drying bin capacities of 60, 100 and 150 liters.
The LUXORnet controls utilize modern, flexible BUS-technology. As standard, the color touch control manages the dry air generator, one drying bin and with the EM version, material conveying to one drying bin and two processing machines.
The integration of dry air conveying, line purging and material proportioning provides maximum flexibility and productivity.
The METRO G system loaders are used to convey the plastic granulate quickly and cleanly to the processing machine or drying bin. Material is delivered exactly when needed, keeping machines running and avoiding costly downtime. Material wastage is eliminated and the work-space is kept clean and safe.
As an option electro-polished stainless steel METROMIX proportioning valve is attached to a METRO G material loader. You can easily adjust the percentage and number of material layers per conveying cycle at the dryer control. A large transparent access panel gives you easy access for cleaning.
For easy setup, pre-configured material feed line sets are available for one or two machines. The option of stainless steel vacuum and conveying pipework at the back of the drying bin, enables a simple and effective connection to the processing machines.
All Motan LUXORBINs are made of stainless steel and are completely insulated including the diffuser cone. The bins are mounted on solid frames and have a control box mounted at the front of the bin for optimum accessibility. The long-life solid state relays guarantee precise and reliable temperature control that does not damage thermally-sensitive materials.
The larger 100 and 150 liter drying bins are equipped with especially large clean-out doors fitted with a sight glass and easy-to-open quick release handles. They fit the shape of the bin in order to optimize material flow and simplify cleaning. In addition, they have a split hinged lid for easy access from above.
Perfect for moderate distance conveying applications, nothing beats the simplicity and reliability of a side-channel vacuum blower. The compact design allows the pump to be installed under the drying bin.
Dry air conveying is essential for hygroscopic materials. Conveying to the processing machine is carried out in a closed loop using dry, warm air. This maintains material temperature and eliminates re-absorption of moisture. As an option the material feed lines can be purged after every conveying cycle.
The cyclone dust filter offers you efficient filtering for small to medium throughput conveying. The cyclone separates the dust and a transparent dust collection bin with quick release clips makes for the quick and easy removal of fines.
The LUXORnet EM control offers user-friendly operation via a color graphic display with touch screen. The control incorporates comprehensive functions for operation and process control, recipe management, reporting for quality assurance, as well as enhanced service functions and trend charts. ETA plus airflow control with temperature adaptation is available as an option.
The LUXORnet control utilizes modern, flexible BUS technology. As standard, the control manages the dry air generator, one drying bin, as well as up to three material loaders.
LUXORnet technology stands out due to its modern, Ethernet-based open network architecture enabling you to connect LUXOR EM A dryers with a central WEBpanel this allows you to control all functions remotely and provides an overview of the entire production.
ETA plus technology
Optional ETA plus technology stands for innovative and energy-efficient drying technology which takes two individual factors into consideration. 1.) The process air-flow control adapts automatically the mass flow of air to the amount of material being dried as well as to seasonal and time-of-day related fluctuations. 2.) Once the mass air-flow has been reduced to its minimum value, the drying temperature adapts automatically to the amount of material being dried. This guarantees very gentle drying, especially when there is a large drop in material throughput rates.
Altogether, the combination of air flow control and temperature reduction offers the highest possible energy savings. Compared with conventional drying systems, energy savings of up to 64% are possible with ETA plus technology. Both functions can be either enabled or disabled dependent on your process requirements.
Having an extremely well engineering and reliable machinery for the plastics industry is a must these days and Hirate America partners with Motan, a German based company (one of the most well-known respected in the industry).
Please contact us if you have any projects you’d like to discuss with us.