Plastic Dryer
JIANGSU GET RECYCLING TECHNOLOGY CO,.LTD
Original from Europe from 2002, with over 160 plastic recycling projects currently in operation, G.E.T Recycling gives you a clear advice with a tailor made solution on the basis of your plastics and requirements. GET is one of your ideal partners in the field of recycling from beginning of negotiation to seeking for best solutions, and from machines manufacturing to after-sales service.
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GET is one of your ideal partners in the field of recycling from beginning of negotiation to seeking for best solutions, and from machines manufacturing to after-sales service.
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High efficiency at high capacities, high quality standard recycling plant at Reasonable investment, GET recycling equipment and lines support operational excellence and economic performance for each customer.
What is Plastic Dryer
Plastic Dryer 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.
How Plastic Drying is Done and Why It is Important
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.
Proper drying of plastic resin assures the best possible aesthetics and performance characteristics of a plastic product. Use this information as a guide to understanding the basics of dehumidifying plastic materials.
Dehumidifying of plastic resin is utilized to minimize or eliminate problems that may be caused by too much or too little moisture in a plastic material during processing. Although most processors accept the need to pre-dry resins, particularly highly hygroscopic ones, many do not understand the basics - or even the vocabulary of resin drying.
The extent to which moisture affects the quality of a molded or extruded part is determined by the specific plastic resin being processed and the intended purpose of the part. Regardless, it is highly probable that improper or incomplete drying of the raw material will create problems, whether during processing or when the product is in use, or worse - both.
Plastic Dryers, Moisture Measurement and Part Quality
Plastic dryers are used to remove moisture (water) that has been absorbed either onto the surface of plastic pallets or into the plastic pallets internal structure before they go into the injection molding machine to make parts.
There are 2 groups of plastics. The first group will only hold water on its surface (such as polypropylene) while the second group will absorb it into its internal structure (such as PET polyester).
Drying is required to ensure good quality parts are made consistently. Plastics with excess moisture levels will react when processed in the molding machine barrel and will produce by-products that will effect such things as impact strength in the finished molded part.
Plastics That Need To Be Dried Before Processing:
- SAN
- VC
- ABS
- PPO
- ACRYLIC
- ACETAL
- PPS
- POLYCARBONATE
- PET POLYESTER
- PEI
- POLYURETHANE
- NYLON
- PBT POLYESTER
Plastics 1 to 7 need to be dried for cosmetic reasons only. Excess moisture will cause bubbles, flow lines or surface defects in the molded part. However, their mechanical properties are not affected by moisture.
Excess moisture in the plastics 8 to 13 will affect the mechanical properties in the molded part. The part will have reduced impact and tensile strength but will not show any cosmetic defects. This fact is very important to know so that moulders do not rely upon visual checks to ensure the quality of the molded part.
Processing plastic pallets in an injection molding machine that have not been dried to the required level can lead to disastrous consequences in the field. Parts that look good are not necessarily strong enough to function properly. This means you are not just wasting time producing rejects but more importantly are likely to damage your company's reputation as a quality supplier.
Just because the plastic material is dried at the recommended temperature and length of time doesn't mean the material is dry enough to process. If the desiccant dryer has not been maintained properly then the plastic material may still have too much moisture and may need to be dried for a longer period of time.
That's why it is critical that the moisture content is measured before processing. This should be done on a daily basis so that moisture can be eliminated as a reason for any part quality issues.
Moisture Measurement Methods
There are 2 different systems of moisture measurement - massed based and sensor based instruments.
Sensor based (usually Karl Fischer) are the most accurate because they will only measure the moisture level in the pallets. However, the massed based systems will measure the moisture level along with other volatiles produced during the analysing process giving a false reading.
The appeal of massed based systems are their lower purchase price compared with sensor based and are easier to use. But these are not reasons to use this type of instruments because they do not give accurate and repeatable results.
Additional Comments
As an injection moulder you spend millions of dollars in equipment such as molding machines, moulds and chillers so there is no excuse not to spend a bit more on quality plastic dryers and moisture measuring equipment.
What's more, material cost is an injection moulders largest ongoing expense so it is critical that waste is eliminated.
Plastic Dryer - 4 Fundamental Drying Parameters
Heat
Heat is the driving force in drying. If you don't heat the pellet it will not release its moisture. Hygroscopic polymers have a strong attraction for water and the water molecules are bound to the polymer chains. Heat causes the molecules to move about more vigorously, weakening the forces that bind the water molecules to the polymer chains. Above certain temperatures, the force that binds the water molecules to the polymer chains are reduced permitting free movement of the molecules to aid the drying process. Non-hygroscopic resins do not adsorb moisture internally into the pellet, however, moisture can collect on the surface of the pellet. Applying heat becomes an important part of removing surface moisture when this happens.
Dew Point
Dew Point is the temperature at which moisture in the air begins to condense. The low vapor pressure (dew point) of the dry air surrounding the pellet causes the freed moisture molecules to migrate to the surface of the pellet.
Drying Time
Plastic pellets do not dry instantaneously. They must first be heated to allow the water molecules free movement. Then, there must be sufficient time for the water molecules to defuse to the surface of hygroscopic pellets or for surface moisture to evaporate from the surface of non-hygroscopic materials.
Airflow
Airflow carries heat or dry heated air to the material in the drying hopper. In the case of non-hygroscopic materials, you must force hot air over and around the pellets to remove surface moisture. With hygroscopic materials, you must force low dew point heated air over the material to make the molecules of moisture disengage from the polymer chains and move to the surface of the pellets where the airflow carries moisture away. The volume of dry air must be sufficient to develop and maintain the desired temperature profile within the drying hopper. If four hours drying time is required, you must maintain the drying temperature for the four-hour level within the drying hopper. If the volume of airflow is reduced, the temperature profile will be reduced.
Plastic Dryer - Maximizing Energy Efficiency in Resin Drying Systems
Some dryer manufacturers tout energy savings while others, who don't necessarily have testing facilities nor qualified personnel to conduct testing, try to convince processors that energy savings in a dryer is not worth talking about.
A look at energy costs of old vs. new dryers will quickly convince you otherwise. The trend of average electricity cost is UP so planning ahead to reduce your energy expenditures is a sensible thing to do. The larger the dryer - the greater the savings and the better the ROI.
The performance of any drying system is based on using the minimum heat to raise the temperature to the appropriate drying temperature while maintaining a constant material temperature at the throat of the injection molding machine or extruder. Also the regeneration system should minimize the energy used to heat the desiccant material while dedicating most of the energy to removing the moisture gained during the drying process.
Reducing Process Heat Loss
You must be cautious about how dryer manufacturers claim energy savings. Temperature Setback or Second Set Point are sometimes sold as energy-saving features but they are not. Most rely on "Temperature Setback" - what does that mean?
Temperature Setback lowers the process temperature in the dryer and thus the resin temperature entering the processing equipment. The result is the processing machine must make up for the lower heat in the resin by increasing the heat from shear and the band heaters. The processing machine ends up using more energy to overcome the shortcomings of the dryer, resulting in no energy savings, and contributing to an inferior end product or product consistency.
Reducing process heat loss to increase energy savings can also be accomplished through the use of VFD's (variable frequency drives) to control process blower speed. If the airflow rate and temperature rise are minimized, the process heating energy can be as low as necessary to heat the resin. The idea is to minimize the air required so that the minimum energy is used for process heating. The temperature of the resin is still raised to the appropriate temperature for the process machine but all of the energy remains in the drying hopper and a minimal amount of heat or energy is returned to the dryer.
To accomplish this effectively the temperature of the resin and the temperature leaving the drying hopper should be continually measured and the air flow rate adjusted so the return air temperature (the temperature returning to the dryer from the drying hopper) is only slightly higher than the temperature of the resin entering the drying hopper. This process is controlled by varying the speed of the blower with a Variable Frequency Drive (VFD) which changes the speed of the blower and thus the flow rate of the air. By minimizing the air flow rate, while maintaining the temperature of the resin, the process heat is maintained at the lowest possible level. This allows the required power to adapt to any changes in material rate, resin moisture and temperature of the resin.
Regeneration can account for as much as 35% of the total power used. To maximize energy savings, the power used in the regeneration of the desiccant must be minimal. Regeneration consists of heating the desiccant to a temperature at which it releases the moisture gained during the resin drying process. This involves raising the temperature of the desiccant to a point where the moisture retained in the desiccant dissipates.
There are two parts to this system - each with a specific task. We will demonstrate this using the desiccant wheel dryer as our model:
First, the speed (RPM) of the desiccant wheel is minimized which reduces the pounds per minute of desiccant media to be heated. This is important because heating of the desiccant media detracts from the primary goal of vaporizing moisture and heat is lost without accomplishing the primary goal of water removal. The wheel speed is controlled by a variable frequency drive (VFD) to no more than that required to adsorb the moisture from the return air. By controlling the speed of the wheel, the desiccant media is loaded to its maximum while maintaining a consistent dew point of less than - 40°F/C.
The second part is accomplished using a VFD on the regeneration air blower. The VFD minimizes the airflow to a point where water is desorbed from the molecular sieve but only minimal heat leaves the wheel during the regeneration process. As the ambient air is heated and passes through the wheel, the discharge temperature is constantly monitored and the VFD adjusts the air flow ensuring that minimal air flow is used to remove all of the moisture gained in drying but no excess air is used.
The temperature of the air exiting the top of the wheel is just enough to remove the moisture and carry it away but is kept at a constant temperature. This ensures that the wheel will remove all of the moisture from the resin regardless of the resin moisture level and will automatically adjust as the moisture changes due to seasonal variations or changes in the virgin/flake ratios.
Plastic Dryer - Consistency of Dried Materials
Consistency of Dried Materials
- Proper, uniform drying assists in the molding or extrusion process and aids in the prevention of defects.
Molding times and extrusion rates can be optimized and increased by:
- Uniform Drying to a consistent, low moisture level.
- Maintaining a constant drying temperature.
- Keeping your drying system in good working condition.
- Utilize dryers offering the latest, proven drying technology to avoid temperature or dew point spikes and other inconsistencies.
Our Factory
Original from Europe from 2002, with over 290 plastic recycling projects currently in operation, G.E.T Recycling gives you a clear advice with a tailor made solution on the basis of your plastics and requirements.GET is one of your ideal partners in the field of recycling from beginning of negotiation to seeking for best solutions, and from machines manufacturing to after-sales service.
