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Fusible alloy tooling
elastomers and the chemical effects to changes in the molecular structure by ageing, oxidation and similar processes. In general, physical effects are most important at normal or low temperatures a n d / o r short times, whilst chemical effects are most important at high temperatures and/or long times. Unfortunately, creep and stress relaxation are very time and temperature dependent and they are seriously affected by the media with which the elastomer is in contact. A number of different methods of measuring creep and stress relaxation have been developed, but the results obtained from one method may be difficult to compare directly with those from another. In practice, correlation between creep and stress relaxation is rarely attempted. The choice of test usually depends upon the application, s o m e methods being more relevant to certain conditions than others. Simulated service tests are again to be r e c o m m e n d e d whenever practicable. In addition, extreme caution should be exercised before attempting to extrapolate results obtained by short time tests to considerably longer periods, or using results obtained at higher temperatures, as accelerated tests to provide information for lower temperatures. Creep is usually defined as the increase in deformation which has occurred after a specified time interval. It should be noted that this definition differs from that commonly used for metals and plastics: in their case, creep is the percentage deformation relative to the original thickness of the piece - ie the percentage strain at any given time. At normal or low temperatures and/or short times, the physical process is dominant and creep is found to vary directly with time plotted on a logarithmic scale. At high temperatures a n d / o r long times, chemical processes are dominant and chemical creep is often found to vary directly with time. In the absence of chemical effects, the longer the relaxation processes continue, the slower they become.
For example, the amount of creep occurring in a decade of time, from say one to 10 minutes after loading, can be the same as the amount in the much longer decade from one to 10 weeks afterloading. In such cases, it is usual to give test results as creep rate", which is defined as the ratio of creeps to the c o m m o n logarithm of the time interval over which it was measured, expressed as per cent per decade. There is not at present (1988) an ISO standard covering creep, but BS 903: Part A15 has been submitted to ISO for consideration as an international standard. This standard is restricted to tests in which the elastomer is deformed in compression or in shear. In practice, rubbers are seldom used where they are continuously deformed in tension. Creep varies considerably with the composition and types of elastomer, but, in general, it has been established that the higher the initial strain, the higher the creep. Creep also increases with temperature. Creep is further dependent upon the type of strain: it is greater under tension strain than under an equal shear strain and it is greater under shear strain than under an equal compression strain. It is greater under dynamic loading than under static loading. Nnally, creep increases with the amount of filler present. Chemical effects can cause creep to occur more rapidly than short term tests predict: high humidity, for example, can double the creep rate. Oxidation also has an effect, but this can be reduced by the use of suitable antioxidants. However, large rubber components, such as bridge bearings, are usually protected from oxidation, simply because their bulk is able to prevent easy diffusion of oxygen to the interior. In engineering applications, creep is most rapid during the first few weeks under load, but should not exceed 20% (for 701RHD) of the initial deformation for this period. Allowance can be made for this in the design. Thereafter, in correctly c o m p o u n d e d rubbers, only a further 5-10% increase in deformation should occur over a period of many years.
Fusible alloy tooling Press & Shear: Elfiott House, Victoria Road, London NWIO 6NY. Tel: 01-965 6535
Abstract Improvements in the process and uses as described
Introduction Many of the applications for fusible alloys involve the making of moulds or patterns and in addition, there are two well-reported examples in the past five years of the use of fusible metals as removable cores in the moulding of hollow plastic articles. The first of these relates to a hollow and injection mouldable tennis racquet moulded from carbon-reinforced nylon. The fusible alloys are widely used in both ferrous and non-ferrous foundries for
MATERIALS& DESIGN Vol. 10 No. 5 SEPTEMBER/OCTOBER1989
making patterns. The composition of the white metals used for this purpose varies considerably and includes 50%Sn/50%Zn, Wood's metal, type metals and other leadrich Pb/Sn/Sb alloys. Much improved results can be obtained with bismuth-containing alloys, since bismuth not only lowers the melting point of the metal, but also reduces contraction during solidification and so facilitates the reproduction of patterns to maintain close dimensional tolerances.
Main ram platen Anchor pl mounted I tooling st Blunkhold platen ~
Die-plate end spacing collars: casting semi position Air pressure
Jewellta bath
Halting elements
4 Blankholder plate mounted Cooling wntel Air pressure
Figs 1 - 4
The process sequence for producing tooling for the deep drawing process from low melting-point alloys. temperatures, the fusible alloy is approximately as hard as brass and six to 10 stampings can be produced before rechilling is necessary. Another method of increasing the life of fusible alloy dies is by plating.
Forming blocks for aircraft parts and dies for short runs on light-gauge sheet materials have been achieved by casting against wood or metal pattens. A related application is the making of forming blocks for automobile metal panels produced by hammering; similar operations can be used for removing dents from sheet metal parts. Work at the International Tin Research Institute has shown that the compression fatigue resistance of eutectic bismuth-tin alloy can be raised by adding tin-antimony intermetallic compound to the molten alloy. Subsequently it was shown that separate additions of tin and antimony were also effective in improving the mechanical properties. Adding 7.5% antimony and 7.5% tin to the bismuth/43% tin alloy raised from 150 to 275 the number of compression cycles required to give 5% permanent deformation. These strengthened alloys have potential for use as dies for short run pressings. Another interesting technique in this field is the chilling of fusible alloy dies in liquid nitrogen; at these low
Dualfor~ processes Of the many processes using low-melting point alloys in this area, one of the most remarkable is the Dualform deep drawing press - because it allows low-cost production for small volumes of pressed sheet metal panels using tooling manufactured on the press itself. The process involves the use of tooling cast from a low melting point alloy: tooling may be manufactured in situ on a Dualform press or it can be made on a purpose-built casting rig for use in a compatible existing press. In either case, the basic process is identical and allows manufacture of a complete male and female die set in no more than a single shift. For small volumes, this provides a way of avoiding high costs and long lead times, yet with greater flexibility than
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MATERIALS & DESIGN Vol. 10 No. 5 SEPTEMBER/OCTOBER 1989
obtainable with epoxy resin dies. The bed of the Dualform press or casting rig incorporates a bath of Jewelite BP low melting point laloy (155°C) from which the tooling is manufactured (Fig 1). The bath of alloy is melted by builtin electric heaters. The alloy properties are constant throughout successive re-meltings and it is therefore infinitely re-usable. An important feature of the sample is that it is perforated at the bottom, so that immersion in the molten alloy causes it to fill up with liquid metal (Fig 2). The ram of the press is then lowered to immerse into the melt an anchor plate, with tee-bolts screwed into it, and the liquid
MATERIALS & DESIGN Vol. 10 No. 5 SEPTEMBER/OCTOBER 1989
metal is allowed to solidify, assisted by the application of cooling water (Fig 3). Raising the ram detaches the alloy within the sample former, producing the male die (the punch). Removal of the former leaves the female die in the bath (Fig 4). Once smoothed, and with the addition of a 25-50mm thick blankholder plate, the dies may be used for production runs of up to 6000 panels from l m m thickness deep-drawing steel. Tool life is related to the die shape and the material being pressed, but Dualform has been used successfully with titanium, aluminium and steel up to 3.5mm thickness.
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For example, the amount of creep occurring in a decade of time, from say one to 10 minutes after loading, can be the same as the amount in the much longer decade from one to 10 weeks afterloading. In such cases, it is usual to give test results as creep rate", which is defined as the ratio of creeps to the c o m m o n logarithm of the time interval over which it was measured, expressed as per cent per decade. There is not at present (1988) an ISO standard covering creep, but BS 903: Part A15 has been submitted to ISO for consideration as an international standard. This standard is restricted to tests in which the elastomer is deformed in compression or in shear. In practice, rubbers are seldom used where they are continuously deformed in tension. Creep varies considerably with the composition and types of elastomer, but, in general, it has been established that the higher the initial strain, the higher the creep. Creep also increases with temperature. Creep is further dependent upon the type of strain: it is greater under tension strain than under an equal shear strain and it is greater under shear strain than under an equal compression strain. It is greater under dynamic loading than under static loading. Nnally, creep increases with the amount of filler present. Chemical effects can cause creep to occur more rapidly than short term tests predict: high humidity, for example, can double the creep rate. Oxidation also has an effect, but this can be reduced by the use of suitable antioxidants. However, large rubber components, such as bridge bearings, are usually protected from oxidation, simply because their bulk is able to prevent easy diffusion of oxygen to the interior. In engineering applications, creep is most rapid during the first few weeks under load, but should not exceed 20% (for 701RHD) of the initial deformation for this period. Allowance can be made for this in the design. Thereafter, in correctly c o m p o u n d e d rubbers, only a further 5-10% increase in deformation should occur over a period of many years.
Fusible alloy tooling Press & Shear: Elfiott House, Victoria Road, London NWIO 6NY. Tel: 01-965 6535
Abstract Improvements in the process and uses as described
Introduction Many of the applications for fusible alloys involve the making of moulds or patterns and in addition, there are two well-reported examples in the past five years of the use of fusible metals as removable cores in the moulding of hollow plastic articles. The first of these relates to a hollow and injection mouldable tennis racquet moulded from carbon-reinforced nylon. The fusible alloys are widely used in both ferrous and non-ferrous foundries for
MATERIALS& DESIGN Vol. 10 No. 5 SEPTEMBER/OCTOBER1989
making patterns. The composition of the white metals used for this purpose varies considerably and includes 50%Sn/50%Zn, Wood's metal, type metals and other leadrich Pb/Sn/Sb alloys. Much improved results can be obtained with bismuth-containing alloys, since bismuth not only lowers the melting point of the metal, but also reduces contraction during solidification and so facilitates the reproduction of patterns to maintain close dimensional tolerances.
Main ram platen Anchor pl mounted I tooling st Blunkhold platen ~
Die-plate end spacing collars: casting semi position Air pressure
Jewellta bath
Halting elements
4 Blankholder plate mounted Cooling wntel Air pressure
Figs 1 - 4
The process sequence for producing tooling for the deep drawing process from low melting-point alloys. temperatures, the fusible alloy is approximately as hard as brass and six to 10 stampings can be produced before rechilling is necessary. Another method of increasing the life of fusible alloy dies is by plating.
Forming blocks for aircraft parts and dies for short runs on light-gauge sheet materials have been achieved by casting against wood or metal pattens. A related application is the making of forming blocks for automobile metal panels produced by hammering; similar operations can be used for removing dents from sheet metal parts. Work at the International Tin Research Institute has shown that the compression fatigue resistance of eutectic bismuth-tin alloy can be raised by adding tin-antimony intermetallic compound to the molten alloy. Subsequently it was shown that separate additions of tin and antimony were also effective in improving the mechanical properties. Adding 7.5% antimony and 7.5% tin to the bismuth/43% tin alloy raised from 150 to 275 the number of compression cycles required to give 5% permanent deformation. These strengthened alloys have potential for use as dies for short run pressings. Another interesting technique in this field is the chilling of fusible alloy dies in liquid nitrogen; at these low
Dualfor~ processes Of the many processes using low-melting point alloys in this area, one of the most remarkable is the Dualform deep drawing press - because it allows low-cost production for small volumes of pressed sheet metal panels using tooling manufactured on the press itself. The process involves the use of tooling cast from a low melting point alloy: tooling may be manufactured in situ on a Dualform press or it can be made on a purpose-built casting rig for use in a compatible existing press. In either case, the basic process is identical and allows manufacture of a complete male and female die set in no more than a single shift. For small volumes, this provides a way of avoiding high costs and long lead times, yet with greater flexibility than
266
MATERIALS & DESIGN Vol. 10 No. 5 SEPTEMBER/OCTOBER 1989
obtainable with epoxy resin dies. The bed of the Dualform press or casting rig incorporates a bath of Jewelite BP low melting point laloy (155°C) from which the tooling is manufactured (Fig 1). The bath of alloy is melted by builtin electric heaters. The alloy properties are constant throughout successive re-meltings and it is therefore infinitely re-usable. An important feature of the sample is that it is perforated at the bottom, so that immersion in the molten alloy causes it to fill up with liquid metal (Fig 2). The ram of the press is then lowered to immerse into the melt an anchor plate, with tee-bolts screwed into it, and the liquid
MATERIALS & DESIGN Vol. 10 No. 5 SEPTEMBER/OCTOBER 1989
metal is allowed to solidify, assisted by the application of cooling water (Fig 3). Raising the ram detaches the alloy within the sample former, producing the male die (the punch). Removal of the former leaves the female die in the bath (Fig 4). Once smoothed, and with the addition of a 25-50mm thick blankholder plate, the dies may be used for production runs of up to 6000 panels from l m m thickness deep-drawing steel. Tool life is related to the die shape and the material being pressed, but Dualform has been used successfully with titanium, aluminium and steel up to 3.5mm thickness.
267