Optimizing Energy Consumption & Drying Quality in Plastic Drying Process
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Optimizing Energy Consumption & Drying Quality in Plastic Drying Process

By 17/08/2022September 9th, 2022No Comments

Measuring Dew Point in Plastics Drying Process

At Dpstar, we understand plastics processing is a delicate process. There are many factors that play a role in successful plastics processing and when utilizing plastics processing equipment. A major factor, for instance, is dew point. Drying of plastic resin can be a key step in the injection molding process. If the plastic is not optimally dried during processing in injection molding machines, this has many negative effects on the end product. Excess moisture in plastics decreases product strength and causes a poor surface finish. On the other hand, excessive polymer drying wastes energy and lowers productivity as material delay in the dryer is needlessly prolonged.

The moisture content in plastic pellets has to be controlled as it’s an important aspect for the production of high-quality plastic parts. The right moisture content of polymer resins ensures trouble-free compounding and injection molding, smooth surfaces and ideal mechanical properties of the injection-molded parts. This is accomplished by keeping residual moisture levels within the limits set by resin manufacturers. The stable drying temperature is also evidence of a well-done drying, as well as maintaining the good operating conditions of the dryer over time (in heating elements, air ducts, filters, fans and drying material). Dew point measurement can be used to monitor a dryer’s performance or conditions in the hopper, assuring that the drying process remains within specification.

Dpstar offers the appropriate instruments for measuring the dew point in the plastic drying process. Vaisala’s dewpoint products are carefully designed to work perfectly in almost any application. Vaisala has a range of stable and reliable dew point transmitters both for OEM configurations as well as for end users. We are committed to providing our customers with the best air-drying solutions. Our services are aimed at helping our clients cost-effectively realize maximum industrial productivity.

What is Dew Point

How does the dew point impact the drying of plastics?

The dew point is the temperature at which the humidity in the air condenses and falls, and is dependent on the vapor pressure; a low vapor pressure around the material promotes moisture migration from the material into the outside air. A stable and low dew point temperature indicates the ideal condition for the material drying process. Most hygroscopic thermoplastic polymers may be dried at dew point temperatures close to -20 ° C. Lower dew point temperatures involve a significant amount of energy and are not reflected in significant increases in drying speed. The drying process does not happen instantly; it takes time. First, the temperature must be reached at which the water molecules may freely move. Then, it takes time for the diffusion process to take place through the material. The rate of diffusion is determined by the material as well as its internal humidity level; as it decreases, so will the rate of diffusion.

How does the dew point impact the drying of plastics?

The dew point is the temperature at which the humidity in the air condenses and falls, and is dependent on the vapor pressure; a low vapor pressure around the material promotes moisture migration from the material into the outside air. A stable and low dew point temperature indicates the ideal condition for the material drying process. Most hygroscopic thermoplastic polymers may be dried at dew point temperatures close to -20 ° C. Lower dew point temperatures involve a significant amount of energy and are not reflected in significant increases in drying speed. The drying process does not happen instantly; it takes time. First, the temperature must be reached at which the water molecules may freely move. Then, it takes time for the diffusion process to take place through the material. The rate of diffusion is determined by the material as well as its internal humidity level; as it decreases, so will the rate of diffusion.

Dry air flow (with a low dew point) will be responsible for removing moisture from the drying hopper. The flow is necessary to ensure that the temperature inside the hopper remains stable as recommended for the required time. If flow is not required, the temperature profile may decrease and drying may be less efficient. It is important to establish the optimal combination of the variables mentioned above, in such a way that the moisture conditions proposed for the process by the resin manufacturer are achieved. In addition, excessive energy consumption or even damage to the material (such as degradation or caking) is not produced.

Dry air flow (with a low dew point) will be responsible for removing moisture from the drying hopper. The flow is necessary to ensure that the temperature inside the hopper remains stable as recommended for the required time. If flow is not required, the temperature profile may decrease and drying may be less efficient. It is important to establish the optimal combination of the variables mentioned above, in such a way that the moisture conditions proposed for the process by the resin manufacturer are achieved. In addition, excessive energy consumption or even damage to the material (such as degradation or caking) is not produced.

Challenges in Plastic Drying

Moisture impacts the processability of almost all plastics, particularly hygroscopic equivalents like nylon (polyamide) and polybutylene adipate terephthalate (PBAT). Moisture content is regarded as a significant parameter, and therefore, accurate measurements are important for manufacturing these materials and for assuring their process quality. However, when these plastics are molded or processed with moisture content beyond the accepted range, structural or visual defects may occur, which are considered a major problem.

Lack of drying can be evidenced by defects such as:

Sink Marks
Discoloration
Irregular Moldings
Splay Marks
Bubbles
Brittleness

Challenges in Plastic Drying

Moisture impacts the processability of almost all plastics, particularly hygroscopic equivalents like nylon (polyamide) and polybutylene adipate terephthalate (PBAT). Moisture content is regarded as a significant parameter, and therefore, accurate measurements are important for manufacturing these materials and for assuring their process quality. However, when these plastics are molded or processed with moisture content beyond the accepted range, structural or visual defects may occur, which are considered a major problem.

Lack of drying can be evidenced by defects such as:

Sink Marks
Brittleness
Bubbles
Discoloration
Irregular Moldings
Splay Marks

Drying Of Plastic In General

Hygroscopic polymers commonly used as engineering plastics, such as polyethylene terephthalate (PET) and polyamides. They can and will absorb moisture from the atmosphere into the pellet to contain it in their internal structure. If the pellets are not dried before processing, the water will react with the molten polymer at the processing temperature. The result is a change to the molecular structure of the polymer. This chemical reaction is known as hydrolysis and is reducing the molecular weight of a molded part. It lowers plastic’s strength and can also affect its visual appearance.

Polyamide for example is one of the most hygroscopic plastic materials in common use. The moisture content of nylon molding resins is a particularly important parameter with a direct effect on molding, mechanical properties, viscosity of the melt and the appearance of the molded parts. All plastic producers have to realize the importance of proper drying for molding high quality products! To avoid hydrolysis, hygroscopic polymers require thorough drying prior to processing. Drying of hygroscopic materials is challenging because moisture is absorbed into pellets’ internal structure. Moist PET pellets can contain as much as 0.6 % water by weight. Depending on the applied process, PET must be dried down to as low as 0.003 % (30 ppm) residual moisture level prior to processing. The internal moisture can be removed by heating the raw material and simultaneously exposing it to dry air supplied from desiccant dryers.

Hygroscopic polymers commonly used as engineering plastics, such as polyethylene terephthalate (PET) and polyamides. They can and will absorb moisture from the atmosphere into the pellet to contain it in their internal structure. If the pellets are not dried before processing, the water will react with the molten polymer at the processing temperature. The result is a change to the molecular structure of the polymer. This chemical reaction is known as hydrolysis and is reducing the molecular weight of a molded part. It lowers plastic’s strength and can also affect its visual appearance.

Polyamide for example is one of the most hygroscopic plastic materials in common use. The moisture content of nylon molding resins is a particularly important parameter with a direct effect on molding, mechanical properties, viscosity of the melt and the appearance of the molded parts. All plastic producers have to realize the importance of proper drying for molding high quality products! To avoid hydrolysis, hygroscopic polymers require thorough drying prior to processing. Drying of hygroscopic materials is challenging because moisture is absorbed into pellets’ internal structure. Moist PET pellets can contain as much as 0.6 % water by weight. Depending on the applied process, PET must be dried down to as low as 0.003 % (30 ppm) residual moisture level prior to processing. The internal moisture can be removed by heating the raw material and simultaneously exposing it to dry air supplied from desiccant dryers.

How to Choose the Right Drying System

The control of the variables and the selection of the appropriate technology depend on the physical understanding of the process and the material to be processed. The presence of moisture in the materials usually generates appearance problems, sometimes making processing difficult. It may even lead to degradation of the resin, since the steam generated at high temperatures fractures the chains and modifies the properties, mainly in those materials that are produced by poly-condensation.

Drying of plastics by Hot-Air Systems

Basic plastics like for example PVC, Polystyrene or unfilled PE resp. PP are carrying moisture on the surface of their pellets as a result of temperature and humidity. These materials can normally be dried by using only hot air. Hot-Air dryers take the ambient air and heat it up to the recommended drying temperature for a given resin and let the air circulate through the resin hopper.

Drying of hygroscopic plastics by Desiccant-Dryers

All hygroscopic resins like ABS, PA, PC, PET, PMMA and other materials, can only be effectively dried by hot predried (desiccated) air. They work absolutely independent from the conditions of the ambient air as they are capable of producing dry enough air even for the most difficult-to-dry materials. The circulating air is passing a desiccant material like Silica gel or Molecular sieves. In a desiccant dryer’s closed air circulation system air can be dried down to a dewpoint range of -10 to -40 °C Td, and even down to -60 °C Td. Desiccant dryers produce hot dry air, which is blown through a duct to the hopper that contains the plastic pellets. This air removes moisture from the plastic material and the moist air is then circulated back to the dryer, where it is cooled. The desiccant absorbs the removed moisture and the air is reheated and directed back to the hopper.

Dryers usually consist of two desiccant filled towers with switching valves that direct the airflow to one tower at a time. One tower dries the air while the other is being regenerated by flushing the collected moisture to the ambient. Continuous dewpoint measurement can be used to optimise the regeneration interval of a desiccant dryer. Dewpoint measurement also help in the detection of potential dryer failures. Dewpoint is typically measured at the desiccant dryer’s outlet air duct. The desiccant-filled towers are set to switch at a set dewpoint value. Dewpoint controlled switching ensures consistent drying efficiency and product quality. Furthermore, energy consumption is minimized by the optimal regeneration intervals. By confirming the required operating performance of the dryer, users can optimize the drying time and save energy costs without compromising material quality. A much longer drying period under high temperature can also cause material degradation.

How to Choose the Right Drying System

The control of the variables and the selection of the appropriate technology depend on the physical understanding of the process and the material to be processed. The presence of moisture in the materials usually generates appearance problems, sometimes making processing difficult. It may even lead to degradation of the resin, since the steam generated at high temperatures fractures the chains and modifies the properties, mainly in those materials that are produced by poly-condensation.

Drying of plastics by Hot-Air Systems

Basic plastics like for example PVC, Polystyrene or unfilled PE resp. PP are carrying moisture on the surface of their pellets as a result of temperature and humidity. These materials can normally be dried by using only hot air. Hot-Air dryers take the ambient air and heat it up to the recommended drying temperature for a given resin and let the air circulate through the resin hopper.

Drying of hygroscopic plastics by Desiccant-Dryers

All hygroscopic resins like ABS, PA, PC, PET, PMMA and other materials, can only be effectively dried by hot predried (desiccated) air. They work absolutely independent from the conditions of the ambient air as they are capable of producing dry enough air even for the most difficult-to-dry materials. The circulating air is passing a desiccant material like Silica gel or Molecular sieves. In a desiccant dryer’s closed air circulation system air can be dried down to a dewpoint range of -10 to -40 °C Td, and even down to -60 °C Td. Desiccant dryers produce hot dry air, which is blown through a duct to the hopper that contains the plastic pellets. This air removes moisture from the plastic material and the moist air is then circulated back to the dryer, where it is cooled. The desiccant absorbs the removed moisture and the air is reheated and directed back to the hopper.

Dryers usually consist of two desiccant filled towers with switching valves that direct the airflow to one tower at a time. One tower dries the air while the other is being regenerated by flushing the collected moisture to the ambient. Continuous dewpoint measurement can be used to optimise the regeneration interval of a desiccant dryer. Dewpoint measurement also help in the detection of potential dryer failures. Dewpoint is typically measured at the desiccant dryer’s outlet air duct. The desiccant-filled towers are set to switch at a set dewpoint value. Dewpoint controlled switching ensures consistent drying efficiency and product quality. Furthermore, energy consumption is minimized by the optimal regeneration intervals. By confirming the required operating performance of the dryer, users can optimize the drying time and save energy costs without compromising material quality. A much longer drying period under high temperature can also cause material degradation.

Drying Solutions to Improve Your Process

Installing a Dewpoint Transmitter to a Desiccant Dryer

To measure and control the dewpoint of the air that is passed over the plastic material, the dewpoint sensor should be installed before the heater and the hopper. If the transmitter is installed after the heater, installation of the direct process is not always possible due to the high air temperature, and a sampling system is required. The sampling system cools and, if necessary, filters the air sample before it flows to the sensor. If the process operates in low or ambient pressure, a pump is required in the sampling system to draw air samples from the process.

Installing a Dewpoint Transmitter After the Hopper

For several hours, the temperature inside the hopper may exceed 300 °C. As a result, if dewpoint measurement from the hopper is required, a sample air flow should be extracted and cooled before contacting the dewpoint sensor. The air in the hopper may contain volatile compounds that evaporate from the plastic granules. Usually, the molecular size is relatively high so that it does not affect the measurement. Monitoring dewpoint at the hopper outlet in addition to measurement at the dryer allows the operator to optimize the drying time. The cycle is considered complete when the measured dewpoint has stabilised to a pre-defined low enough level. Prolonging material delay in the hopper does not improve drying. Monitoring the dew point of the air entering and exiting the hopper ensures that conditions for the drying process remain optimal for the entire duration of the drying cycle.

Drying Solutions to Improve Your Process

Installing a Dewpoint Transmitter to a Desiccant Dryer

To measure and control the dewpoint of the air that is passed over the plastic material, the dewpoint sensor should be installed before the heater and the hopper. If the transmitter is installed after the heater, installation of the direct process is not always possible due to the high air temperature, and a sampling system is required. The sampling system cools and, if necessary, filters the air sample before it flows to the sensor. If the process operates in low or ambient pressure, a pump is required in the sampling system to draw air samples from the process.

Installing a Dewpoint Transmitter After the Hopper

For several hours, the temperature inside the hopper may exceed 300 °C. As a result, if dewpoint measurement from the hopper is required, a sample air flow should be extracted and cooled before contacting the dewpoint sensor. The air in the hopper may contain volatile compounds that evaporate from the plastic granules. Usually, the molecular size is relatively high so that it does not affect the measurement. Monitoring dewpoint at the hopper outlet in addition to measurement at the dryer allows the operator to optimize the drying time. The cycle is considered complete when the measured dewpoint has stabilised to a pre-defined low enough level. Prolonging material delay in the hopper does not improve drying. Monitoring the dew point of the air entering and exiting the hopper ensures that conditions for the drying process remain optimal for the entire duration of the drying cycle.

Tips for Selecting a Dewpoint Transmitter for Plastics Drying

Select a transmitter with adequate measurement range to meet the dryer specification.

Check that the permanently installed transmitters can be checked on-site using a reference hand held.

Select a transmitter that tolerates the residual chemicals that may evaporate from the plastic materials.

Compare transmitters’ long-term measurement stability and select one with premium performance to minimize lifetime costs.

The transmitter should be easily removable for multipoint calibration when needed.

Select a transmitter with minimal maintenance needs to ensure reliable system optimization.

Tips for Selecting a Dewpoint Transmitter for Plastics Drying

Select a transmitter with adequate measurement range to meet the dryer specification.

Check that the permanently installed transmitters can be checked on-site using a reference hand held.

Select a transmitter that tolerates the residual chemicals that may evaporate from the plastic materials.

Compare transmitters’ long-term measurement stability and select one with premium performance to minimize lifetime costs.

The transmitter should be easily removable for multipoint calibration when needed.

Select a transmitter with minimal maintenance needs to ensure reliable system optimization.

Our Success Story

When manufacturing injection molded plastic products, correctly drying the raw material plastic pellets is crucial. The air used in the drying process is drier than the ambient air, so it is economical to recycle it back into the process using air dryers. Heating the air used in the drying process and regenerating the absorption dryers requires a lot of energy. Dew point measurement is key to optimizing energy consumption and drying quality. Our client’s state-of-the-art facilities have been equipped with Vaisala DRYCAP dew point measurement instruments for their drying units, with Vaisala technology also being relied upon to meet other measurement needs in their facilities. The company manufactures products for the medical, automotive and electronic industries, amongst others.

The production process starts with melting plastic pellets at a high temperature into a thermoplastic mass, which is then injected into molds. If the plastic pellets are too moist, visual and mechanical quality problems can both easily arise. To reach the correct level of dryness, plastic pellets are placed in a hopper and exposed to dry and warm supply air. The return air is cooled and dried during the regeneration process. Dew point measurement is essential at this stage to ensure that the air is suitably dry before it is heated and resupplied into the drying process. The optimal dew point to ensure the plastic pellets are correctly dried is -35 °C (-31 °F). The plastic dryers must operate with dew points below -40°F (-40°C) continuously for proper plastic drying. In order to be able to monitor the operation of plastic air dryers, our client uses two Vaisala DM70 Hand-Held Dew-point Meters. The DM70 measures dew point accurately in a wide measurement range, which is important in this application.  The plant is also equipped with a DMT143L Dew Point Transmitter to upgrade one of the dryers. To monitor a dehumidification unit, a Vaisala DMT340 Dew point Transmitter is used. Moreover, a HMT330 with a radiation shield is used to monitor outside conditions, measuring the wet bulb and enthalpy of outside air. This instrument allows the maximization of free cooling potential using outside air.

Vaisala products offer excellent accuracy and repeatability. Moreover, the ability to be interfaced with PCs is important. The instruments allow the recording and tracking of dew points and other measurements, which in turn allows to operate equipment more effectively and efficiently. Thanks to the Vaisala dew point measurement instruments, the company has achieved better overall profitability, product quality, and customer satisfaction.

Our Success Story

When manufacturing injection molded plastic products, correctly drying the raw material plastic pellets is crucial. The air used in the drying process is drier than the ambient air, so it is economical to recycle it back into the process using air dryers. Heating the air used in the drying process and regenerating the absorption dryers requires a lot of energy. Dew point measurement is key to optimizing energy consumption and drying quality. Our client’s state-of-the-art facilities have been equipped with Vaisala DRYCAP dew point measurement instruments for their drying units, with Vaisala technology also being relied upon to meet other measurement needs in their facilities. The company manufactures products for the medical, automotive and electronic industries, amongst others.

The production process starts with melting plastic pellets at a high temperature into a thermoplastic mass, which is then injected into molds. If the plastic pellets are too moist, visual and mechanical quality problems can both easily arise. To reach the correct level of dryness, plastic pellets are placed in a hopper and exposed to dry and warm supply air. The return air is cooled and dried during the regeneration process. Dew point measurement is essential at this stage to ensure that the air is suitably dry before it is heated and resupplied into the drying process. The optimal dew point to ensure the plastic pellets are correctly dried is -35 °C (-31 °F). The plastic dryers must operate with dew points below -40°F (-40°C) continuously for proper plastic drying. In order to be able to monitor the operation of plastic air dryers, our client uses two Vaisala DM70 Hand-Held Dew-point Meters. The DM70 measures dew point accurately in a wide measurement range, which is important in this application.  The plant is also equipped with a DMT143L Dew Point Transmitter to upgrade one of the dryers. To monitor a dehumidification unit, a Vaisala DMT340 Dew point Transmitter is used. Moreover, a HMT330 with a radiation shield is used to monitor outside conditions, measuring the wet bulb and enthalpy of outside air. This instrument allows the maximization of free cooling potential using outside air.

Vaisala products offer excellent accuracy and repeatability. Moreover, the ability to be interfaced with PCs is important. The instruments allow the recording and tracking of dew points and other measurements, which in turn allows to operate equipment more effectively and efficiently. Thanks to the Vaisala dew point measurement instruments, the company has achieved better overall profitability, product quality, and customer satisfaction.

Customer Benefits 

Accurate Measurements

Precise humidity or dew point measurement enables the ideal controlling of the dryers.

Intuitive User
Interface

Easy-to-use user interface, a clear graphical LCD display, and data-logging capability.

Reliable Measurements

The sensor withstands condensation and fully recovers from getting wet.

Optimizing Energy Consumption

More effective and accurate data collection from the production process.

Customer Benefits 

Accurate Measurements

Precise humidity or dew point measurement enables the ideal controlling of the dryers.

Intuitive User
Interface

Easy-to-use user interface, a clear graphical LCD display, and data-logging capability.

Reliable Measurements

The sensor withstands condensation and fully recovers from getting wet.

Optimizing Energy Consumption

More effective and accurate data collection from the production process.

Our Products Solutions

Vaisala DMT340 Dew Point & Temperature Meter Series 

Vaisala DMT143 & DMT143L (Long) Miniature Dew Point Transmitters 

Vaisala DM70 Handheld Dewpoint Meter 

Our Products Solutions

Vaisala DMT340 Dew Point & Temperature Meter Series 

Vaisala DMT143 & DMT143L (Long) Miniature Dew Point Transmitters 

Vaisala DM70 Handheld Dewpoint Meter 

Talk to an Expert

You will achieve your production goals with success if you use the right instruments from Vaisala, the industry leader in dewpoint and humidity sensing. Vaisala’s products are perfectly work in almost any application! Our measurement technology experts will help you find the right measuring device to measure and monitor your system’s dream point. Contact us now, we look forward to advising you!

Get In Touch With Our Experts Today!

Dpstar Group
No 35, Jalan OP ½, Pusat Perdagangan One Puchong,
Off Jalan Puchong, 47160 Puchong,
Selangor Darul Ehsan, Malaysia.
Email: [email protected]

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