Foamed plastics:
Any plastic material possessing cellular structure and density lower than the original solid material is known as foamed plastic. Their density ranges from 0.5 to 40 or 60 pounds per cubic feet (Pcf). They are also known as sponged, expanded or cellular plastics.
Syntactic foam:
Special type of plastic foam using hollow spheres made of phenolic resin, clay, glass or other hollow particles. These spheres (micro spheres) are dispersed throughout a matrix of either epoxy or polyester resin which when cured resembles a conventional foam plastic.
Foam density:
Weight of foam in pounds per cubic feet (Pcf) i.e. 6 pound foam have a density of 6 Pcf. Density range for foam is 2-20 Pcf. Density range for syntactic foam is 10-60 Pcf, depending on the type and the percentage of the micro spheres used in the resin.
Closed cell foam (Unicellular):
Foam in which the greatest proportions of the cells are not connected together by open passage ways are known as closed cell foam. Such a material (in a flexible grade) when squeezed under water, absorbs water in the surface cells and a few cells under the surface. They are used for floatation devices and similar items. They are not used satisfactorily for sound absorption.
Open cell foam (interconnecting cells):
Foam in which the greatest proportions of the cells are connected together by open passage ways are known as open cell foam. Such a material (in a flexible grade) when squeezed under water, absorbs water throughout its mass. They are good sound absorbing material.
Compression resistance:
For flexible foams it is the force required to compress up to 25% of one square inch of the flexible foam.
For rigid foam it is the force in per square inch required to cause permanent crushing. (Compression resistance is also called the compressive strength).
Compression set:
It is exclusively applied to flexible foams and/or solid rubber like material. It is the percentage of permanent compression in a sample after a definite sized specimen has been compressed for a given period of time at a given temperature. Common test procedures employs one inche2 sample compressed to 25% for 22 hours at 700 C, and then measured 20 minutes after removal from the oven and the compression test fixtures.
K Factor:
It refers to the heat insulating properties of the material. It indicates the thermal conductivity of the foam in BTUs per hour, per square feet, per inch of thickness under a thermal difference of 10 C. K Factor range for foam is 0.15 to 0.35 at room temperature. K Factor is directly proportional to the temperature. K Factor is inversely proportional to the insulating quality. R-Factor indicates the resistance of the material to the transmission of heat. K Factor is inverse of R-Factor.
Blowing agent:
Material that causes any given plastic to foam is known as blowing agent.
Types of blowing agents are:
a) Gas: It is introduced into the molten or liquid plastic material.
b) Chemicals: they are incorporated in the polymer. It decomposes to liberate gas, at a given temperature.
In both the cases if the gas is evenly dispersed, it expands to form the cells in plastic. Different ways to bring about cell formation depend on the gas being used; the CBA; the type of resin; and the particular process being used.
Monday, February 28, 2011
Foam Process- Significance
Foam processes:
Importance of foamed plastics material in plastic industry:
Versatility of foamed material:
1. Foamed materials are versatile in many forms in which the materials are used to produce the foams including the production of various shapes by casting, extruding, injection moulding, thermoforming and RIM.
2. Any plastic material, whether thermoplastic or a thermoset, may be produced in a cellular form (foam form).
3. The solid parts made by using foam in place of plastic and /or metal, the part may be improved and/or made more cheaply, simply because of savings in the material quantity and weight.
4. The parts made by using foam in place of a solid plastics part may be lighter and/or stiffer.
General production methods:
Plastics foams are principally made by following different methods:
1. Incorporating a chemical blowing agent (CBA or foaming agent) into the polymer to form a gas by decomposition at a given elevated temperature:
a) Finely powdered CBA is either evenly dispersed in a liquid resin or is mixed with moulding pellets.
b) From azo compounds (organic material), usually nitrogen gas is liberated.
c) CBAs decompose at temperatures ranging from 1100—2800C
d) The CBA should match the melting point or the processing temperature of the polymer.
e) Typical CBA is azobisformamide (ABFA), also called azodicarbonamide.
2. A gas usually N2 is injected into the molten or partially cured resin either in the barrel of an extruder or injection press or into a large mass in an autoclave. When the pressure is decreased the gas is expanded to form the cellular structure.
3. A bifunctional material (isocyanate) may be combined with polyester or other liquid polymer. During the polymerisation reaction to form a solid polymer, these bifunctional materials also reacts to liberate gas which forms the cells.
4. Volatilization of a low boiling liquid by applying heat. This heat required for the volatilization may be externally supplied or may be liberated by an exothermic reaction. Fluorocarbons (Freons) are commonly used liquids. PUR foams are usually made by this method.
5. Whipping air into a colloidal resin suspension and then gelling the porous mass. Foamed latex rubber is made by this method.
6. Incorporating a non chemical, gas-liberating agent into the resin mix which releases a gas when heated. Gas adsorbed on finely divided carbon can be used for the purpose.
7. Expanding the small beads of thermoplastic resin by heating an internally contained blowing agent. Expansion of Polystyrene beads used to make cups, packaging, mannequin heads etc is made by this method.
Applications of foamed plastics:
Flexible foams:
In very soft cushioning material used in upholstery, clothing inter layer, automobile seats, vibration absorbers etc, as electrical, thermal and acoustical insulator.
Semi rigid foams:
For floatation devices, marine bumpers, special electrical insulation on TV cables,packaging etc.
Rigid foams:
In production of airplane parts, boats, electronic encapsulation and in many furniture applications where wood was formerly used.
Importance of foamed plastics material in plastic industry:
Versatility of foamed material:
1. Foamed materials are versatile in many forms in which the materials are used to produce the foams including the production of various shapes by casting, extruding, injection moulding, thermoforming and RIM.
2. Any plastic material, whether thermoplastic or a thermoset, may be produced in a cellular form (foam form).
3. The solid parts made by using foam in place of plastic and /or metal, the part may be improved and/or made more cheaply, simply because of savings in the material quantity and weight.
4. The parts made by using foam in place of a solid plastics part may be lighter and/or stiffer.
General production methods:
Plastics foams are principally made by following different methods:
1. Incorporating a chemical blowing agent (CBA or foaming agent) into the polymer to form a gas by decomposition at a given elevated temperature:
a) Finely powdered CBA is either evenly dispersed in a liquid resin or is mixed with moulding pellets.
b) From azo compounds (organic material), usually nitrogen gas is liberated.
c) CBAs decompose at temperatures ranging from 1100—2800C
d) The CBA should match the melting point or the processing temperature of the polymer.
e) Typical CBA is azobisformamide (ABFA), also called azodicarbonamide.
2. A gas usually N2 is injected into the molten or partially cured resin either in the barrel of an extruder or injection press or into a large mass in an autoclave. When the pressure is decreased the gas is expanded to form the cellular structure.
3. A bifunctional material (isocyanate) may be combined with polyester or other liquid polymer. During the polymerisation reaction to form a solid polymer, these bifunctional materials also reacts to liberate gas which forms the cells.
4. Volatilization of a low boiling liquid by applying heat. This heat required for the volatilization may be externally supplied or may be liberated by an exothermic reaction. Fluorocarbons (Freons) are commonly used liquids. PUR foams are usually made by this method.
5. Whipping air into a colloidal resin suspension and then gelling the porous mass. Foamed latex rubber is made by this method.
6. Incorporating a non chemical, gas-liberating agent into the resin mix which releases a gas when heated. Gas adsorbed on finely divided carbon can be used for the purpose.
7. Expanding the small beads of thermoplastic resin by heating an internally contained blowing agent. Expansion of Polystyrene beads used to make cups, packaging, mannequin heads etc is made by this method.
Applications of foamed plastics:
Flexible foams:
In very soft cushioning material used in upholstery, clothing inter layer, automobile seats, vibration absorbers etc, as electrical, thermal and acoustical insulator.
Semi rigid foams:
For floatation devices, marine bumpers, special electrical insulation on TV cables,packaging etc.
Rigid foams:
In production of airplane parts, boats, electronic encapsulation and in many furniture applications where wood was formerly used.
Definitions TQM
Product
A good idea, method, information, object, or service that is the end result of a process and should serve as a need satisfier. It is usually a bundle of real and elusive attributes (benefits, features, functions, uses) that a seller offers to a buyer for purchase.
Property/characteristic:
A distinguishing feature or attribute of an item,product, person, phenomenon, etc., usually divided into three categories: (1) physical, (2) functional, and (3) operational.
Defect:
Shortcoming that prevents a product from being complete, desirable, effective, safe, or of merit, or makes it to malfunction or fail in its purpose.
Quality:
The totality of features and characteristics of a product or service that bear on its ability to satisfy stated or implied needs.
Quality level:
The amount of replication of set Standards. A general indication of the extent of departure from the ideal/standard (usually a numerical value). It indicates either the degree of conformity or non-conformity of quality. It is used in a comparative sense.
Quality policy:
Intentions, direction, and aims regarding quality of its products and processes defined by the Top management. The quality policy forms one element of the corporate policy and is authorized by the Top management.
Quality Management:
All management activities and functions involved in determination of quality policy and its implementation through quality planning, quality assurance and quality control.
Quality Assurance:
All those planned and systematic actions necessary to provide sufficient confidence that a product or service will satisfy given requirements for quality. QA frameworks include
(1) testing of procured material for its conformance to established quality, performance, safety, and reliability standards,
(2) certification and rating of suppliers,
(3) determination of adequate technical requirement of inputs and outputs,
(4) proper receipt, storage, and issue of material,
(5) evaluation of the process to establish required corrective response,
(6) audit of the process quality, and
(7) audit of the final output for conformance to (a) technical (b) reliability, (c) maintainability, and (d) performance requirements.
Quality Control:
The operational techniques and activities that are used to fulfill the requirements of quality. QC involves operational techniques and activities aimed both at monitoring a process and at eliminating causes of unsatisfactory performance at relevant stages of the quality management.
Quality system:
Collective organizational activities, incentives, plans, policies, procedures, processes, resources, responsibilities, and the infrastructure required in formulating and implementing a total quality management (TQM) approach.
Total quality management (TQM):
A long-term success of an organization requires progressive changes in the attitudes, practices, structures, and systems. TQM involves everyone in the organization, and covers every function like administration, communications, distribution, manufacturing, marketing, planning, training, etc. This term has several meanings
(1) building quality into products and practices right from the beginning and doing things correct right from the very first time,
(2) understanding of the changing needs of the internal and external customers, and stakeholders, and satisfying them in a cost effective manner,
(3) commitment and direct involvement of highest-level executives in setting quality goals and policies, allocation of resources, and monitoring of results,
(4) realization that transforming an organization means fundamental changes in basic beliefs and practices and that this transformation is everyone's job,
(5) instituting leadership in place of mere supervision so that every individual performs in the best possible manner to improve quality and productivity, thereby continually reducing total cost,
(6) instituting flexible programs for training and education, and providing meaningful measures of performance that guide the self-improvement efforts of everyone involved, and
(7) eliminating barriers between people and departments so that they work as teams to achieve common objectives.
Product quality:
To ensure the quality of products, it is necessary to identify the losses in quality and then find methods to control the process and to improve the product. If a problem is due to poor quality of raw materials or ingredients, can be discussed with suppliers and if necessary, the processor should introduce appropriate testing methods with tolerance limits that are agreed with the supplier. If a problem is due to a processing condition, such as the time or temperature of heating, the process control is improved by better staff training, use of thermometers etc. All changes should be monitored to make sure that they are effective and details of the changes should be recorded in a Production Workbook. Such procedures are intended to control the parts of the process that significantly affect product quality and therefore help the processor to employ staff where they are most effective.
A good idea, method, information, object, or service that is the end result of a process and should serve as a need satisfier. It is usually a bundle of real and elusive attributes (benefits, features, functions, uses) that a seller offers to a buyer for purchase.
Property/characteristic:
A distinguishing feature or attribute of an item,product, person, phenomenon, etc., usually divided into three categories: (1) physical, (2) functional, and (3) operational.
Defect:
Shortcoming that prevents a product from being complete, desirable, effective, safe, or of merit, or makes it to malfunction or fail in its purpose.
Quality:
The totality of features and characteristics of a product or service that bear on its ability to satisfy stated or implied needs.
Quality level:
The amount of replication of set Standards. A general indication of the extent of departure from the ideal/standard (usually a numerical value). It indicates either the degree of conformity or non-conformity of quality. It is used in a comparative sense.
Quality policy:
Intentions, direction, and aims regarding quality of its products and processes defined by the Top management. The quality policy forms one element of the corporate policy and is authorized by the Top management.
Quality Management:
All management activities and functions involved in determination of quality policy and its implementation through quality planning, quality assurance and quality control.
Quality Assurance:
All those planned and systematic actions necessary to provide sufficient confidence that a product or service will satisfy given requirements for quality. QA frameworks include
(1) testing of procured material for its conformance to established quality, performance, safety, and reliability standards,
(2) certification and rating of suppliers,
(3) determination of adequate technical requirement of inputs and outputs,
(4) proper receipt, storage, and issue of material,
(5) evaluation of the process to establish required corrective response,
(6) audit of the process quality, and
(7) audit of the final output for conformance to (a) technical (b) reliability, (c) maintainability, and (d) performance requirements.
Quality Control:
The operational techniques and activities that are used to fulfill the requirements of quality. QC involves operational techniques and activities aimed both at monitoring a process and at eliminating causes of unsatisfactory performance at relevant stages of the quality management.
Quality system:
Collective organizational activities, incentives, plans, policies, procedures, processes, resources, responsibilities, and the infrastructure required in formulating and implementing a total quality management (TQM) approach.
Total quality management (TQM):
A long-term success of an organization requires progressive changes in the attitudes, practices, structures, and systems. TQM involves everyone in the organization, and covers every function like administration, communications, distribution, manufacturing, marketing, planning, training, etc. This term has several meanings
(1) building quality into products and practices right from the beginning and doing things correct right from the very first time,
(2) understanding of the changing needs of the internal and external customers, and stakeholders, and satisfying them in a cost effective manner,
(3) commitment and direct involvement of highest-level executives in setting quality goals and policies, allocation of resources, and monitoring of results,
(4) realization that transforming an organization means fundamental changes in basic beliefs and practices and that this transformation is everyone's job,
(5) instituting leadership in place of mere supervision so that every individual performs in the best possible manner to improve quality and productivity, thereby continually reducing total cost,
(6) instituting flexible programs for training and education, and providing meaningful measures of performance that guide the self-improvement efforts of everyone involved, and
(7) eliminating barriers between people and departments so that they work as teams to achieve common objectives.
Product quality:
To ensure the quality of products, it is necessary to identify the losses in quality and then find methods to control the process and to improve the product. If a problem is due to poor quality of raw materials or ingredients, can be discussed with suppliers and if necessary, the processor should introduce appropriate testing methods with tolerance limits that are agreed with the supplier. If a problem is due to a processing condition, such as the time or temperature of heating, the process control is improved by better staff training, use of thermometers etc. All changes should be monitored to make sure that they are effective and details of the changes should be recorded in a Production Workbook. Such procedures are intended to control the parts of the process that significantly affect product quality and therefore help the processor to employ staff where they are most effective.
Tuesday, February 22, 2011
Decorating Techniques of plastics
DECORATING of plastics is required in the articles manufactured from any plastic/material, whether a thermoplastic or a thermoset, for aesthetic or functional purposes. Functional purposes include improved resistance to wear, scratching, light or heat etc.
COLOURING:
Integral colouring:
The coloured parts can be obtained by getting coloured parts from the polymer manufacturer. Here the manufacturer mixes the colourants like organic or inorganic pigments and/or transparent dyestuffs, with the resin during fabrication. The colourants can be mixed using hot rollers or Banburry mixers or sometimes it is mixed when the resins are in a liquid or pasty state.
Dry powder colouring: (In-house colouring):
Here, the uncoloured, natural materials are mixed with powdered colourants in a tumbling barrel.
Advantages: This system allows the Moulder to stock a large amount of crystal pellets (uncoloured) and small stocks of many colours.
Disadvantages:
1. The parts either produced by extrusion or ram injection moulding machine do not always have the highest quality—streaks and non uniform colouration is common.
2. The dust produced by the process requires a separate mixing room.
3. Relatively high amount of labour is required for operation.
4. The process is used for the production of low cost parts.
Colour Concentrates/Masterbatch: Here 50-80% of pigment is dispersed in a polymer carrier and then converted into small pellets. The carrier can be any of the thermoplastic resin either produced as a stock concentrate/Masterbatch by the concentrate/Masterbatch manufacture or as a custom concentrate/Masterbatch for the Moulder. In both the cases, the pigment is thoroughly dispersed in the resin using hot rollers, Banburry mixers etc.
Advantages:
1. The Moulder has to stock the natural material in large quantities and small quantities of colour concentrate/Masterbatch.
2. Dust problem is eliminated because the pellets are dust free and automatic colour blenders are used to mix the virgin material and colourants.
Functions:
1. The addition of pigments imparts opacity, though it is not its function because opacity can be increased by increasing the amount of carbon black and titanium dioxide in a colourant.
2. The colour concentrate/Masterbatch imparts the colour to the resin.
3. The concentrates/Masterbatch now also contain fire retardants, blowing agents, UV absorbers, anti oxidants and other additives in addition to the colourants. Before using these multifunctional concentrates/Masterbatch, it should be checked whether the carrier resin in the concentrate/Masterbatch is compatible with the matrix resin to be coloured e.g. LDPE matrix carrier resin is compatible with other Olefinics like HDPE or PP and lower melting range of LDPE assures thorough melting and dispersion in the matrix while LDPE carrier is not compatible with PS, Acrylics etc. matrix.
Universal colour concentrates/Masterbatches are also available, which can be blended in with most of the resins. However they also should be checked for compatibility before using.
Terms used in the colourant system:
1. Let-Down ratio: It is the number of Kg. of natural resin which can be satisfactorily coloured by 1 Kg. of the colourant.
Factors Affecting:
a) L/D ratio of the press or extruder will determine the Let Down ratio that is possible with any given concentrate. The higher the L/D/ ratio, the greater will be the Let Down ratio.
b) Speed of the screw: Too fast a screw speed will lower the Let Down ratio.
c) The temperature profile of the barrel: Slight increase in the temperature at the feed section can result in a higher Let Down ratio.
2. Metamerism: It is the phenomenon exhibited by the two surfaces that appea to be of same colour when seen under the same light source, but have different colour when seen under different light source. To prevent this, the viewing light source should be specified when colours matching other colours are specified.
Liquid colour concentrates:
These can be metered directly in to the throat of the moulding press or extruder. Their Let Down ratio is 100:1. Their viscosity is approximately equal to that if a heavy oil. Usually the metering pump, which is mounted on a liquid reservoir, is tied directly with the screw rotation i.e. it pumps only on the return stroke of the injection screw, and when it is used on an extruder, it can be driven by the extruder screw to compensate for the fluctuations in the screw speed.
Advantages:
1. Power requirement is reduced due to the lubricating properties of the liquid carrier.
2. The change in the physical properties of the matrix resin is negligible because only a small quantity of carrier is to be absorbed by the resin.
3. It can also incorporate blowing agents, UV absorbers etc.
4. Though the cost/Kg. of the liquid colourants is considerably higher than the pellet concentrate/Masterbatch, the ultimate cost is much lowered due to the higher Let Down ratio.
Colouring of thermosets:
Basic problems in colouring of thermosets:
1. Thermosets undergo a chemical reaction during the moulding operation.
2. They are sometimes used at elevated temperatures.
3. Some of thermosets have a tendency of yellowing with age or from exposure to UV radiation from the Sun.
Colouring of various thermosets:
Moulding powder like Phenolics, Ureas, Melamines, Epoxies and Alkyds can be produced in coloured state by the raw material manufacturer. All other, except Phenolics can be made in various colours and most of them are available in stock colours from the manufacturer. The Phenolics are usually available only in brown, black or dark red, green or blue colours because of the natural tan or brown colour of the polymer and tendency of yellowing with age.
Liquid thermoset polymers like Epoxies, Polyesters and Urethanes are usually coloured by the paste colourants which can be added directly to the liquid resin.
The paste colourants are made by grinding the pigments into the appropriate carrier; by the companies specialized in the work because the simple mixing of the powdered pigment in the resin by the Moulder usually do not result in a uniform dispersion.
Selection criteria of colourant system:
Following factors should be considered while selecting a colourant system for a liquid resin system:
1. The colourant should be stable at the moulding temperature or at the service temperature of the part.
2. The colorant should not affect the polymerisation reaction.
3. The colourant should not affect the physical properties of the final part e.g. amount of colourant should not reduce the tensile and tear strength of the Urethane foam.
4. Inorganic pigments must be used as colourants for outdoor use. This is particularly important with Polyesters which are frequently exposed to the weather.
COLOURING:
Integral colouring:
The coloured parts can be obtained by getting coloured parts from the polymer manufacturer. Here the manufacturer mixes the colourants like organic or inorganic pigments and/or transparent dyestuffs, with the resin during fabrication. The colourants can be mixed using hot rollers or Banburry mixers or sometimes it is mixed when the resins are in a liquid or pasty state.
Dry powder colouring: (In-house colouring):
Here, the uncoloured, natural materials are mixed with powdered colourants in a tumbling barrel.
Advantages: This system allows the Moulder to stock a large amount of crystal pellets (uncoloured) and small stocks of many colours.
Disadvantages:
1. The parts either produced by extrusion or ram injection moulding machine do not always have the highest quality—streaks and non uniform colouration is common.
2. The dust produced by the process requires a separate mixing room.
3. Relatively high amount of labour is required for operation.
4. The process is used for the production of low cost parts.
Colour Concentrates/Masterbatch: Here 50-80% of pigment is dispersed in a polymer carrier and then converted into small pellets. The carrier can be any of the thermoplastic resin either produced as a stock concentrate/Masterbatch by the concentrate/Masterbatch manufacture or as a custom concentrate/Masterbatch for the Moulder. In both the cases, the pigment is thoroughly dispersed in the resin using hot rollers, Banburry mixers etc.
Advantages:
1. The Moulder has to stock the natural material in large quantities and small quantities of colour concentrate/Masterbatch.
2. Dust problem is eliminated because the pellets are dust free and automatic colour blenders are used to mix the virgin material and colourants.
Functions:
1. The addition of pigments imparts opacity, though it is not its function because opacity can be increased by increasing the amount of carbon black and titanium dioxide in a colourant.
2. The colour concentrate/Masterbatch imparts the colour to the resin.
3. The concentrates/Masterbatch now also contain fire retardants, blowing agents, UV absorbers, anti oxidants and other additives in addition to the colourants. Before using these multifunctional concentrates/Masterbatch, it should be checked whether the carrier resin in the concentrate/Masterbatch is compatible with the matrix resin to be coloured e.g. LDPE matrix carrier resin is compatible with other Olefinics like HDPE or PP and lower melting range of LDPE assures thorough melting and dispersion in the matrix while LDPE carrier is not compatible with PS, Acrylics etc. matrix.
Universal colour concentrates/Masterbatches are also available, which can be blended in with most of the resins. However they also should be checked for compatibility before using.
Terms used in the colourant system:
1. Let-Down ratio: It is the number of Kg. of natural resin which can be satisfactorily coloured by 1 Kg. of the colourant.
Factors Affecting:
a) L/D ratio of the press or extruder will determine the Let Down ratio that is possible with any given concentrate. The higher the L/D/ ratio, the greater will be the Let Down ratio.
b) Speed of the screw: Too fast a screw speed will lower the Let Down ratio.
c) The temperature profile of the barrel: Slight increase in the temperature at the feed section can result in a higher Let Down ratio.
2. Metamerism: It is the phenomenon exhibited by the two surfaces that appea to be of same colour when seen under the same light source, but have different colour when seen under different light source. To prevent this, the viewing light source should be specified when colours matching other colours are specified.
Liquid colour concentrates:
These can be metered directly in to the throat of the moulding press or extruder. Their Let Down ratio is 100:1. Their viscosity is approximately equal to that if a heavy oil. Usually the metering pump, which is mounted on a liquid reservoir, is tied directly with the screw rotation i.e. it pumps only on the return stroke of the injection screw, and when it is used on an extruder, it can be driven by the extruder screw to compensate for the fluctuations in the screw speed.
Advantages:
1. Power requirement is reduced due to the lubricating properties of the liquid carrier.
2. The change in the physical properties of the matrix resin is negligible because only a small quantity of carrier is to be absorbed by the resin.
3. It can also incorporate blowing agents, UV absorbers etc.
4. Though the cost/Kg. of the liquid colourants is considerably higher than the pellet concentrate/Masterbatch, the ultimate cost is much lowered due to the higher Let Down ratio.
Colouring of thermosets:
Basic problems in colouring of thermosets:
1. Thermosets undergo a chemical reaction during the moulding operation.
2. They are sometimes used at elevated temperatures.
3. Some of thermosets have a tendency of yellowing with age or from exposure to UV radiation from the Sun.
Colouring of various thermosets:
Moulding powder like Phenolics, Ureas, Melamines, Epoxies and Alkyds can be produced in coloured state by the raw material manufacturer. All other, except Phenolics can be made in various colours and most of them are available in stock colours from the manufacturer. The Phenolics are usually available only in brown, black or dark red, green or blue colours because of the natural tan or brown colour of the polymer and tendency of yellowing with age.
Liquid thermoset polymers like Epoxies, Polyesters and Urethanes are usually coloured by the paste colourants which can be added directly to the liquid resin.
The paste colourants are made by grinding the pigments into the appropriate carrier; by the companies specialized in the work because the simple mixing of the powdered pigment in the resin by the Moulder usually do not result in a uniform dispersion.
Selection criteria of colourant system:
Following factors should be considered while selecting a colourant system for a liquid resin system:
1. The colourant should be stable at the moulding temperature or at the service temperature of the part.
2. The colorant should not affect the polymerisation reaction.
3. The colourant should not affect the physical properties of the final part e.g. amount of colourant should not reduce the tensile and tear strength of the Urethane foam.
4. Inorganic pigments must be used as colourants for outdoor use. This is particularly important with Polyesters which are frequently exposed to the weather.
Monday, February 21, 2011
Rotational moulding Process
The rotoforming process involves five steps
1. Loading. 2. Moulding. 3. Cooling. 4. Unloading.5. Finishing.
1. Loading: the mould is charged (or loaded) with required amount of powder or liquid. This can be done by weighing the raw material so that uniform parts are produced. The weight can be calculated from the volume of product and the density of the material used.
2. Moulding: It contains two simultaneous processes: a) rotation and b) heating
a) Rotation: the mould is rotated in two axes. This biaxial rotation can be done by using a mould on the end of an offset arm or using a series of moulds on the end of a centre swing arm, which has an effect of a radius arm powered from a central hub. In each case the ratio 1:4 is maintained between the speed of rotation of the major axis and the minor axis. The typical speed being 3-10 rpm for the major axis and 12-40rpm for the minor axis. However the actual speeds must be determined experimentally and it depends on the part size, geometry, material, heating rate etc.
b) Heating: It is accomplished either by hot air—by placing mould assembly in an oven or in some cases by a hot liquid—either oil or molten salt. The oven temperature can be 200-3700C or even higher for some material. The major requirement of the oven is to furnish the hot air in large quantities at right temperature and with an even air flow having minimum hot or cold spots. The types of heating are:
i) Liquid heating: Though considerably faster has a disadvantage of requiring thicker moulds and more perfect parting line closure. The liquid used is usually molten salt, which may lead to corrosion problems with some materials unless precautions are taken to wash the mould after each use.
ii) High frequency RF Heaters: They are the 3rd heating technique in use (after hot air and liquid heating). It is very efficient but quite costly.
iii) Infrared heating: Though it is fast and efficient method, it has its own distinguished limitations. Its use is confined to simple shapes and single mould because of the “shadowing effect” of one mould on the other which causes very uneven heating.
In general the heating units in the mould should cause the resin to melt. The heating period may range from 1-10 minutes depending on the part, the wall thickness and the type of the resin being used. Here the gravitational force (and not the centrifugal force) distributes the material uniformly over the entire inside surface of the mould.
3. Cooling: After heating period (i.e. the moulding is completed) the mould assembly is moved to the cooling station, where cooling is done by means of a cold water spray. The spray is sometimes combined with cold air for greater flexibility. The moulds must be rotated during the cooling stage to prevent distortion in the moulded part. Sometimes too rapid cooling can cause warpage in the finished parts. However, as the heating time usually determines the time for all other operations, cooling can almost always be adjusted satisfactorily with little difficulty.
4. Unloading: After cooling, the mould assembly is moved to the loading station where loading and unloading takes place. For low volume production, a simple, single-spindle, rotating mould on a reciprocating unit is used for loading, moulding and cooling. For high production, a multiple spindle unit is used where one or more spindles undergo different stage of the process. With such an assembly, there are no interruptions at any time during the cycle, but the times (intervals) must be coordinated so that each step is completed in same time. This can be done by adjusting the size of the oven or cooling chamber, the rate of heat input etc.
5. Finishing: The rotationally moulded parts require very slight finishing only at the mould parting line. For olefins the surface can be treated by flame or electrical discharge method to promote ink and paint adhesion. The use of pigment in resins should be kept to a minimum (0.1-1.0%) in order not to affect the mechanical properties of the finished parts.
Automatic loading device: For small production runs, filling is usually done manually. But for large production runs fillings are usually done on weight basis because the apparent density of a powder may vary with the mesh.
Mould design: Introduction of inert gas like N2 or CO2 to replace air must be made possible by redesigning mould, so that the resin can be heated to the highest temperature and the danger of oxidation, discolouration, odours etc is minimized. The physical properties of the final part can be improved, compared to the parts in an air atmosphere because the thermal and oxidative degradation is reduced. Also the mould must be manifold to allow introduction of coolant like cool air followed by a water spray. Internal cooling can reduce the cooling time and in the case of crystalline material like linear PE can result in improved physical properties by lowering the crystalline content, thus improving the toughness of the final product.
Sunday, February 13, 2011
Materials for rotational moulding
Materials for Rotational moulding:
1. PVC plastisols: Originally thick, viscous liquid PVC plastisols were used with the process and the process was known as slush moulding. These vinyl dispersions mainly made toys, beach balls, artificial flowers and similar objects. In this process the liquid was placed in a closed mould which was then rotated and heated until the plastisol was fused.
2. Today almost any thermoplastic can be used for rotoforming due to the advent of powdered PE and a wide variety of products can be manufactured by this process. Now parts are also manufactured from cellulosics, nylon, polycarbonates, acetal, styrenes etc. However LDPE and HDPE is the most widely used material. Nowadays crosslinked PE with improved strength, environmental resistances and toughness is employed.
3. The use of thermosets in rotoforming is still in early stage. A resin grade which will not become brittle or a technique which can add fibrous reinforcements uniformly should be developed in order to popularize the rotoforming process as the heating stage may be entirely eliminated or the parts may be removed from the mould and be post cured thus making the moulds free for the next cycle during the curing stage of the present cycle.
4. In composite moulding, two resins are used to form one part. One resin melts and forms the outer skin of the part. The second resin then melts, fuses to the 1st and forms the inner skin. A variation of this process is the production of foamed parts in which two or three resins are used. The first resin will melt and form the outer skin. Then the foaming resin will melt and foamed. The third resin then melts and fuses to the foam to form the inner skin. This process is used in the production of small boats, and similar parts.
1. PVC plastisols: Originally thick, viscous liquid PVC plastisols were used with the process and the process was known as slush moulding. These vinyl dispersions mainly made toys, beach balls, artificial flowers and similar objects. In this process the liquid was placed in a closed mould which was then rotated and heated until the plastisol was fused.
2. Today almost any thermoplastic can be used for rotoforming due to the advent of powdered PE and a wide variety of products can be manufactured by this process. Now parts are also manufactured from cellulosics, nylon, polycarbonates, acetal, styrenes etc. However LDPE and HDPE is the most widely used material. Nowadays crosslinked PE with improved strength, environmental resistances and toughness is employed.
3. The use of thermosets in rotoforming is still in early stage. A resin grade which will not become brittle or a technique which can add fibrous reinforcements uniformly should be developed in order to popularize the rotoforming process as the heating stage may be entirely eliminated or the parts may be removed from the mould and be post cured thus making the moulds free for the next cycle during the curing stage of the present cycle.
4. In composite moulding, two resins are used to form one part. One resin melts and forms the outer skin of the part. The second resin then melts, fuses to the 1st and forms the inner skin. A variation of this process is the production of foamed parts in which two or three resins are used. The first resin will melt and form the outer skin. Then the foaming resin will melt and foamed. The third resin then melts and fuses to the foam to form the inner skin. This process is used in the production of small boats, and similar parts.
Monday, February 7, 2011
rotational moulding -1
Definition: When finely ground powders (Usually thermoplstics) are heated in a rotating mould until it is melted or fused, the molten resin will form a uniform coating or lining on the inner surface of the mould, which when allowed to cool, is removed as a finished part.
Significance/Advantages
1. The tooling required is usually very simple and relatively inexpensive.
2. The process is well suited for making very large and/or very complex parts with single or double walls.
3. Strain free parts are produced.
4. Parts with square corners are thicker at corners.
5. Parts are usually made with very uniform wall thickness except at the square corners.
6. The parts re free of weld lines, sprue marks, ejection marks etc.
7. Very little or no scrap is produced.
8. Hollow parts are easily produced with simple tooling.
9. Usually no secondary operations are required.
10. Changes in wall thickness can be easily made without employing new tooling or modification of the starting raw material.
11. Relatively unskilled labours can be employed.
Disadvantages/Limitations:
1. Material costs are relatively high because most materials are produced as pallets and then reduced to fine powder.
2. The process is not suitable for production of parts with wall thickness less than 0.03”.
3. Large production runs of small parts are not suitable.
Applications:
Rotoforming offers a different technique for producing large complex plastic parts. It is well suited for prototype production of large parts like boats (length up to 12ft. approximately), missile containers, gasoline tanks, toys, artificial flowers, water tanks.
Significance/Advantages
1. The tooling required is usually very simple and relatively inexpensive.
2. The process is well suited for making very large and/or very complex parts with single or double walls.
3. Strain free parts are produced.
4. Parts with square corners are thicker at corners.
5. Parts are usually made with very uniform wall thickness except at the square corners.
6. The parts re free of weld lines, sprue marks, ejection marks etc.
7. Very little or no scrap is produced.
8. Hollow parts are easily produced with simple tooling.
9. Usually no secondary operations are required.
10. Changes in wall thickness can be easily made without employing new tooling or modification of the starting raw material.
11. Relatively unskilled labours can be employed.
Disadvantages/Limitations:
1. Material costs are relatively high because most materials are produced as pallets and then reduced to fine powder.
2. The process is not suitable for production of parts with wall thickness less than 0.03”.
3. Large production runs of small parts are not suitable.
Applications:
Rotoforming offers a different technique for producing large complex plastic parts. It is well suited for prototype production of large parts like boats (length up to 12ft. approximately), missile containers, gasoline tanks, toys, artificial flowers, water tanks.
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