Several moulding parameters directly influence the injection of plastics into the mould, including the following:
· Injection speed
· Melt cushion
· Injection pressure
· Injection time
3.4.1 Injection Speed
Injection speed is a key processing parameter.
· For older machines
Older machines have only a simple flow control valve that regulates the amount of hydraulic oil going to the injection cylinder piston. Opening the valve will allow more oil to enter the piston at a greater rate and thus the plastics is injected faster.
With materials that flow with some difficulty it is recommended to use full injection velocity- (75% to 100% of the available injection velocity.) For moulding plastics which flow more readily it is recommended to start at 50% of the potential injection velocity and slowly work up as needed.
· For newer machines
More advanced machines control multiple stages of the injection velocity to more accurately control the process. Later model injection moulding machines allow you to better control your process and, as a result, will give you less part-to-part variation and better part performance.
3.4.2 Melt Cushion
The melt cushion is the material at the front of the screw when the screw is in the forward position. Always injection mould with a melt cushion of 1/8” to 1/4” to allow the part to pack out evenly. A pressure loss can result if the cushion is too high, and the parts will not mould consistently.
3.4.3 Injection Pressure
Pressure is created by a resistance to flow. As hydraulics are controlled by this property, injection pressure settings can be developed.
To establish first stage injection pressure, raise the pressure to a point where the part fills out without any packing. The screw moves forward and stops as it reaches the melt cushion.
At this point, second stage pressure is implemented to allow the cushion to pack out the part. Older machines usually have a combination hydraulic pump that includes a high-volume, low-pressure pump and a low-volume, high-pressure pump.
The high-volume pump allows for a fast, steady, forward movement of the screw, and the low-volume pump handles the packing of the part. As moulding machines are designed around their hydraulics, the high-volume pump consumes more energy than the low-volume pump. Therefore, it is more efficient to switch to the low-volume pump as soon as possible to reduce the amount of energy consumption. As a result, more energy is conserved by reducing the first stage time than by reducing the cooling time.
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The more advanced machines control multiple pressure settings and therefore more accurately control the moulding process and final moulded part quality.
3.4.4 Injection Time
Injection time is the amount of time that the screw remains forward. By controlling the rate of hydraulic oil to the injection cylinder piston, injection time is controlled.
Assuming a machine has only a first and second stage:
· The first stage is the amount of time it takes to fill the part.
· The second stage is the time required for the gate to freeze off.
In a multiple stage injection, the last stage is usually the packing stage because the sprue, runner, and parts are full. At this point, you will notice that the part weight remains constant from shot to shot.
Saturday, October 31, 2009
Reciprocating screw injection moulding
The reciprocating screw injection moulding machine is the most common machine
found in today’s injection moulding operations.
This machine uses a reciprocating screw that rotates counterclockwise while moving backward and forward. This rotating action causes a shearing of the plastics pellets between the barrel and screw by creating the friction that melts the material. Seventy to ninety percent of the heat needed to melt the plastics comes from this shearing action.
The reciprocating screw injection moulding machine follows this sequence of operation:
(i) As the screw rotates it transfers the melted material to the front of screw.
(ii) As the melted plastics builds up in front of the screw, it forces the screw to move back toward the rear of the injection unit.
(iii) The screw stops turning once it reaches the preset distance or shot size.
(iv) The screw moves forward, acting as an injection ram and forcing the melted plastics material into the closed mould.
found in today’s injection moulding operations.
This machine uses a reciprocating screw that rotates counterclockwise while moving backward and forward. This rotating action causes a shearing of the plastics pellets between the barrel and screw by creating the friction that melts the material. Seventy to ninety percent of the heat needed to melt the plastics comes from this shearing action.
The reciprocating screw injection moulding machine follows this sequence of operation:
(i) As the screw rotates it transfers the melted material to the front of screw.
(ii) As the melted plastics builds up in front of the screw, it forces the screw to move back toward the rear of the injection unit.
(iii) The screw stops turning once it reaches the preset distance or shot size.
(iv) The screw moves forward, acting as an injection ram and forcing the melted plastics material into the closed mould.
Polymer Processing
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.
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.
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.
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.
Understanding Plastics Materials
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.
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.
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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