Capillary rheometry uses a capillary rheometer to measure the viscosity of a polymer. The capillary rheometer melts the polymer inside a small barrel, and then a plunger forces the polymer melt through a small capillary.
The rheometer measures the amount of force required to push the polymer through the capillary. The shear stress on the melt equals the force divided by the surface area of the plunger.
The shear rate is a measure of how fast the material is being tested. The shear rate is determined by the rate of flow through the capillary, and the die geometry. The viscosity of the material is equal to the shear stress divided by the shear rate.
In capillary rheometry, the viscosity is usually determined at different temperatures and shear rates. When the viscosity data is graphed, it provides a good representation of how the material behaves during processing.
If capillary rheometry data can be obtained, it is a good method of comparing the flow characteristics of different resins. When comparing capillary rheometer data, try to compare the data at similar shear rates and temperatures.
Tuesday, October 6, 2009
Viscosity
Most injection moulding processes use a reciprocating screw injection moulding machine.
The reciprocating screw injection moulding machine uses a large screw inside a heated barrel to melt the plastics pellets, and convey the polymer melt. The screw typically turns counterclockwise, and the friction created by the screw pushing the material down the barrel causes shear heating.
Shear heating is responsible for most of the heat required to melt the plastics pellets. The rest of the heat is provided by the barrel heaters which enclose the barrel.
During injection, the reciprocating screw forces the polymer melt into the injection mould. As the melt is injected into the mould, the flow characteristics are dependent on the material’s viscosity.
Viscosity
The viscosity of the polymer is a measure of the material’s resistance to flow. A material which flows easily has a low viscosity, while a material with a higher viscosity does not flow as easily.
Most polymers are available in different grades, each grade has its own flow characteristics. Typically, materials with lower viscosity’s have lower molecular weight. These materials are easier to process, but typically have lower mechanical strength than the same polymer with a higher viscosity.
The viscosity of the polymer can be used to compare the flow characteristics of different polymers, or different grades of the same polymer.
Viscosity data can also be used to qualify a new material. You can compare a newer lot of material to a previously used material to determine whether or not the new material is the same.
Two of the most common methods of determining viscosity are capillary rheometry and melt flow indexing.
The reciprocating screw injection moulding machine uses a large screw inside a heated barrel to melt the plastics pellets, and convey the polymer melt. The screw typically turns counterclockwise, and the friction created by the screw pushing the material down the barrel causes shear heating.
Shear heating is responsible for most of the heat required to melt the plastics pellets. The rest of the heat is provided by the barrel heaters which enclose the barrel.
During injection, the reciprocating screw forces the polymer melt into the injection mould. As the melt is injected into the mould, the flow characteristics are dependent on the material’s viscosity.
Viscosity
The viscosity of the polymer is a measure of the material’s resistance to flow. A material which flows easily has a low viscosity, while a material with a higher viscosity does not flow as easily.
Most polymers are available in different grades, each grade has its own flow characteristics. Typically, materials with lower viscosity’s have lower molecular weight. These materials are easier to process, but typically have lower mechanical strength than the same polymer with a higher viscosity.
The viscosity of the polymer can be used to compare the flow characteristics of different polymers, or different grades of the same polymer.
Viscosity data can also be used to qualify a new material. You can compare a newer lot of material to a previously used material to determine whether or not the new material is the same.
Two of the most common methods of determining viscosity are capillary rheometry and melt flow indexing.
1.2.2 Polymer Classification
Most plastics materials used in the industry today fall into one of the three categories. These are the commodity, engineering, and specialty resins.
(i) Commodity Resins
Commodity resins are the least expensive and most commonly used polymers. These polymers are easy to produce and process. Commodity resins are materials such as polyolefins, polyvinyls, and polyureas.
Commodity resins typically have poor mechanical strength, and usually have only one useful property. For example, polystyrene has low mechanical strength, and poor impact and chemical resistance, but has excellent clarity.
Polyolefins are thermoplastics polymers such as polyethylene and polypropylene.
Vinylic polymers include thermoplastics such as PVC, polyvinyl alcohol, and polystyrene.
Phenolics are thermoset polymers, such as polyurea formaldehyde, and melamine formaldehyde.
(ii) Engineering Resins
Engineering resins are more expensive, and less commonly used than the commodity resins. Engineering polymers include thermoplastics materials such as nylon, polycarbonate, polyester, and PET, as well as themoset materials, such as certain polyureathanes.
Engineering polymers are more difficult to process and produce. Engineering resins are known for their good mechanical strength.
Each individual material usually has several particularly good properties. For example, polycarbonate has good mechanical strength, impact resistance, and clarity, but has poor chemical resistance, and fades in ultraviolet light.
(iii) Specialty Resins
Specialty resins are the most expensive, and least used type of polymer. These polymers are typically thermoplastics, and include materials such as PEEK, polysulfone, liquid crystal polymers, and flouropolymers. These polymers are difficult to process and produce.
Specialty resins are known for their high heat resistance, and each specialty resin has one or two excellent properties. For example, PEEK has very high heat resistance and mechanical strength, but is very expensive, and difficult to process.
Many specialty resins also have to be annealed after processing. Annealing involves the heat treating of the produced parts to reduce any stress within the part, which increases the long term part performance.
Your material supplier should be able to tell you whether or not your specialty material should be annealed, as well as provide the required annealing times and temperatures.
(i) Commodity Resins
Commodity resins are the least expensive and most commonly used polymers. These polymers are easy to produce and process. Commodity resins are materials such as polyolefins, polyvinyls, and polyureas.
Commodity resins typically have poor mechanical strength, and usually have only one useful property. For example, polystyrene has low mechanical strength, and poor impact and chemical resistance, but has excellent clarity.
Polyolefins are thermoplastics polymers such as polyethylene and polypropylene.
Vinylic polymers include thermoplastics such as PVC, polyvinyl alcohol, and polystyrene.
Phenolics are thermoset polymers, such as polyurea formaldehyde, and melamine formaldehyde.
(ii) Engineering Resins
Engineering resins are more expensive, and less commonly used than the commodity resins. Engineering polymers include thermoplastics materials such as nylon, polycarbonate, polyester, and PET, as well as themoset materials, such as certain polyureathanes.
Engineering polymers are more difficult to process and produce. Engineering resins are known for their good mechanical strength.
Each individual material usually has several particularly good properties. For example, polycarbonate has good mechanical strength, impact resistance, and clarity, but has poor chemical resistance, and fades in ultraviolet light.
(iii) Specialty Resins
Specialty resins are the most expensive, and least used type of polymer. These polymers are typically thermoplastics, and include materials such as PEEK, polysulfone, liquid crystal polymers, and flouropolymers. These polymers are difficult to process and produce.
Specialty resins are known for their high heat resistance, and each specialty resin has one or two excellent properties. For example, PEEK has very high heat resistance and mechanical strength, but is very expensive, and difficult to process.
Many specialty resins also have to be annealed after processing. Annealing involves the heat treating of the produced parts to reduce any stress within the part, which increases the long term part performance.
Your material supplier should be able to tell you whether or not your specialty material should be annealed, as well as provide the required annealing times and temperatures.
1.2.1 Types of Polymers
The two types of polymers commonly used in the plastics industry are thermoplastics and thermosets.
(i) Thermoplastics
Thermoplastics are the most widely used type of polymer. Thermoplastics polymers are comprised of many long polymeric chains. The entanglement of these polymeric chains is what gives thermoplastics polymers much of their strength.
Thermoplastics are typically melt processed. Melt processing thermoplastics involves heating the polymer until the polymer chains become untangled. Melt processing only changes the physical structure of the material.
After the polymer has been melted, the polymer is forced into a mould or die. This is where the shape of the final product is determined. The melted polymer is cooled and the polymeric chains are re-entangled, giving the rigidity to the once fluid plastics material.
Thermoplastics parts can be ground up and re-processed. Therefore, most thermoplastics parts are recyclable.
(ii) Thermosets
Thermosets, such as phenolics, are not as commonly used as thermoplastics materials. Thermosets are polymers comprised of one large polymer matrix. The interconnection of the polymer matrix gives the thermoset polymer it’s strength.
Thermosets are typically poured or injected into a heated mould or die. The heat from the mould or form causes the thermoset to cure and crosslink.
When thermosets cure and crosslink, both the chemical and physical structure of the polymer has changed. Thermosets cannot be melted and reprocessed, and therefore are not recyclable. However, some scrap may be ground up and used as filler for other polymers.
(i) Thermoplastics
Thermoplastics are the most widely used type of polymer. Thermoplastics polymers are comprised of many long polymeric chains. The entanglement of these polymeric chains is what gives thermoplastics polymers much of their strength.
Thermoplastics are typically melt processed. Melt processing thermoplastics involves heating the polymer until the polymer chains become untangled. Melt processing only changes the physical structure of the material.
After the polymer has been melted, the polymer is forced into a mould or die. This is where the shape of the final product is determined. The melted polymer is cooled and the polymeric chains are re-entangled, giving the rigidity to the once fluid plastics material.
Thermoplastics parts can be ground up and re-processed. Therefore, most thermoplastics parts are recyclable.
(ii) Thermosets
Thermosets, such as phenolics, are not as commonly used as thermoplastics materials. Thermosets are polymers comprised of one large polymer matrix. The interconnection of the polymer matrix gives the thermoset polymer it’s strength.
Thermosets are typically poured or injected into a heated mould or die. The heat from the mould or form causes the thermoset to cure and crosslink.
When thermosets cure and crosslink, both the chemical and physical structure of the polymer has changed. Thermosets cannot be melted and reprocessed, and therefore are not recyclable. However, some scrap may be ground up and used as filler for other polymers.
1.0 Understanding Plastics Materials
1.1 DEFINITION
Although the terms polymer, plastics, and resin are not technically the same, they are used interchangeably in the plastics processing industry. The word polymer can be broken down into two parts, ‘poly’ meaning many, and ‘mer’ meaning unit.
Although there are many different types and classifications of plastics, all polymers share three common factors.
Polymers are organic in nature, have high molecular weights, and have the ability to change shape. Any substance which contains carbon molecules is considered to be organic. Most polymers are comprised mainly of carbon and hydrogen.
Polymers have high molecular weights because they are made up of many large molecules. For example, water molecules have a molecular weight of 18 while polyethylene can have a molecular weight of over one million.
All polymers have the ability to change shape. This property allows the material to be processed into a useable product. An example of polymers changing shape is injection moulding. In injection moulding, the solid polymer pellets are melted and then moulded into the shape of the final part.
1.2 POLYMERISATION
Polymerisation is the process by which polymers are made. The process of polymerisation involves converting many single organic units into long polymer chains or one large polymer matrix.
Some polymers, called copolymers, are long chains comprised of two or more polymer chains. An example of a copolymer is ABS
.
Although the terms polymer, plastics, and resin are not technically the same, they are used interchangeably in the plastics processing industry. The word polymer can be broken down into two parts, ‘poly’ meaning many, and ‘mer’ meaning unit.
Although there are many different types and classifications of plastics, all polymers share three common factors.
Polymers are organic in nature, have high molecular weights, and have the ability to change shape. Any substance which contains carbon molecules is considered to be organic. Most polymers are comprised mainly of carbon and hydrogen.
Polymers have high molecular weights because they are made up of many large molecules. For example, water molecules have a molecular weight of 18 while polyethylene can have a molecular weight of over one million.
All polymers have the ability to change shape. This property allows the material to be processed into a useable product. An example of polymers changing shape is injection moulding. In injection moulding, the solid polymer pellets are melted and then moulded into the shape of the final part.
1.2 POLYMERISATION
Polymerisation is the process by which polymers are made. The process of polymerisation involves converting many single organic units into long polymer chains or one large polymer matrix.
Some polymers, called copolymers, are long chains comprised of two or more polymer chains. An example of a copolymer is ABS
.
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