SIST EN 27490:2000

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Dental gypsum-bonded casting investments for gold alloys (ISO 7490:1990)Gipsgebundene Einbettmassen für Goldlegierungen (ISO 7490:1990)Revetements a couler a liant-plâtre dentaire pour les alliages d'or (ISO 7490:1990)Dental gypsum-bonded casting investments for gold alloys (ISO 7490:1990)11.060.10Dental materialsICS:Ta slovenski standard je istoveten z:EN 27490:1991SIST EN 27490:2000en01-januar-2000SIST EN 27490:2000SLOVENSKI
STANDARD



SIST EN 27490:2000



SIST EN 27490:2000



SIST EN 27490:2000



INTERNATIONAL STANDARD ISO 7490 First edition 1990-06-15 Dental gypsum-bonded casting investments for gold alloys Reveternen ts a couler & Gian t-p& tre den taire pour les alliages d’or Reference number ISO 7490 : 1990 (El SIST EN 27490:2000



ISO 7490 : 1990 (El Contents Page Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii 1 Scope. 1 2 Normative references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3 Classification . 1 4 Requirements . 1 5 Sampiing . 2 6 Test conditions and preparation of test specimens . 2 7 Testmethods . 3 8 Information to be supplied by manufacturer . 10 9 Packaging . IO 10 Marking . 10 Annexes A Apparatus for measuring setting expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 B Apparatus for measuring thermal expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 0 ISO 1990 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronie or mechanical, including photocopying and microfilm, without Permission in writing from the publisher. International Organization for Standardization Case postale 56 l CH-121 1 Geneve 20 l Switzerland Printed in Switzerland ii SIST EN 27490:2000



ISO 7490 : 1990 (EI Foreword ISO (the International Organization for Standardization) is a worldwide federation of national Standards bodies (ISO member bedies). The work of preparing International Standards is normally carried out through ISO technical committees. Esch member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, govern- mental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. Draft International Standards adopted by the technical committees are circulated to the member bodies for approval before their acceptance as International Standards by the ISO Council. They are approved in accordance with ISO procedures requiring at least 75 % approval by the member bodies voting. International Standard ISO 7490 was prepared by Technical Committee ISO/TC 106, Dentistry. Annexes A and B of this International Standard are for information only. SIST EN 27490:2000



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INTERNATIONAL STANDARD ISO 7480 : 1990 (El Dental gypsum-bonded casting investments for gold alloys 1 Scope This International Standard establishes a classification of, and specifies requirements for, gypsum-bonded casting invest- ments together with the test methods to be used to determine compliance with these requirements. lt applies to gypsum-bonded investments suitable for casting dental gold alloy which have liquidus temperatures not higher than 1 080 OC. 2 Normative references The following Standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All Standards are subject to revision, and Parties to agreements based on this International Standard are encour- aged to investigate the possibility of applying the most recent editions of the Standards indicated below. Members of IEC and ISO maintain registers of currently valid International Stan- dards. ISO 1561: 1975, Dentalinlay casting wax. I SO 1562 : 1984, Dental cas ting gold alloys. ISO 3310-1: 1982, Test sieves - Technical requirements and testing - Part 7: Test sieves of metal wire clo th. ISO 7500- 1 : 1986, Metallic ma terials - Verifica tion of sta tic uniaxial testing macbines - Part 1: Tensile testing machines. 3 Classif ication For the purposes of this International Standard, gypsum- bonded investments are classified as follows: Type 1: Thermal expansion type, for casting inlays and crowns Type 2: Hygroscopic expansion type, for casting inlays and crowns Type 3: For casting complete and partial denture bases In its classification, a particular material need not be restricted to only one type. 4 Requirements 4.1 Material The investment shall be supplied as a powder consisting essen- tially of a mixture of Calcium sulfate hemihydrate and silica. The investment powder may also contain modifying agents. How- ever, it shall not contain extraneous material, and shall be dry, uniform and free from lumps. lf the instructions recommend the use of a mould liner (sec clause 81, the recommended lining material shall be available from the manufacturer. 4.2 Particle size The particle size distribution shall be such that, when the powder is tested .as described in 7.2.2, not more than 1 % (mlm) is retained on a 300 Pm test sieve, not more than 5 % (mlm) on a 150 Fm test sieve, and not more than 15 % (mlm) on a 75 Pm test sieve. 4.3 Unset investment When added to water in the proportions specified by the manufacturer, and mixed as directed in 6.3, the investment shall readily form a uniform Paste. The fluidity of types 1 and 2 investments shall be such that, when tested in accordance with 7.3.1, 2 min after the first con- tact of powder and water, the slump shall be not more than 40 mm high measured at its highest Point. The fluidity of type 3 investments shall be such that, when tested in accordance with 7.3.2, 2 min after the first contact of powder and water, the average diameter of the slump shall be not less than 60 mm. 4.4 Rate of setting 4.4.1 Working time 4.4.1 .l Low-Speed mixing ‘1 When the test described in 7.3.1.2 (types 1 and 2) or 7.3.2.2 (type 3) is repeated 5 min after the first contact of powder and 1) This requirement applies to mixes prepared by hand mixing (see 6.3.1) or in mechanical devices with a rotational frequency of less than 700 r/min. SIST EN 27490:2000



ISO 7490 : 1990 (El water, the mix shall still comply with the fluidity requirements specified in 4.3. No spatulation or Vibration of the mix shall be permitted other than that specified in 7.3.1 (types 1 and 2) or 7.3.2 (type 3). 4.4.1.2 High-Speed mixing, types 1 and 2’) When the test described in 7.3.1.2 is repeated 3 min after the first contact of powder and water, the mix shall still comply with the fluidity requirements specified in 4.3. No spatulation or Vibration of the mix shall be permitted other than that specified in 7.3.1. 4.4.2 Setting time When the mixed investment is tested in accordance with 7.4, the Vicat initial set shall not vary by more than 20 % from the setting time stated by the manufacturer (see clause 8). 4.5 Compressive strength The set investment, when tested in accordance with 7.5, shall comply with the requirements set out in table 1. Table 1 - Compressive strength I Type I Minimum compressive strength, MPa*), at room temperature I 2,3 2,3 5,6 1 *) 1 MPa = IO6 N/m2 1 4.6 Expansion The investment, when tested in accordance with 7.6, shall have a total linear expansion (setting and thermal combined) which camplies with the requirements set out in table 2. None of the thermal expansion curves produced according to 7.6.4.4 shall vary by more than + 0,l % expansion over the recommended casting range (see clause 81, and at least one of these curves shall agree, within 0,l % expansion, over the recommended casting range, with the thermal expansion cun/e provided by the manufacturer (see clause 8). Table 2 - Total expansion 1 ~~~~ ~~~~~~~ TYPe 1 Minimum linear expansion, 96 I 115 L5 L3 4.7 Behaviour during casting The investment mould, when tested in accordance with 7.7.1 (types 1 and 2) or 7.7.2 (type 31, shall not emit offensive fumes when heated, shall not contaminate alloy cast into it, and shall not Cause pitting, fins, rough sutfaces or voids in the casting. 5 Sampling The method of procurement and the amount of investment powder needed for testing shall be the subject of agreement between the interested Parties. Powder from broken or dam- aged Containers shall not be used for testing. If the powder is supplied in bulk, rather than in individual units, it shall be thoroughly mixed before testing and then stored in a moisture- proof Container. If a mould liner is recommended and is not supplied with the investment, an adequate amount of the recommended lining material shall be procured from the investment manufacturer. 6 Test conditions and preparation of test specimens 6.1 Test conditions All mixing and testing of the investment shall be carried out in a room free from draughts and under atmospheric conditions of 23 OC * 2 OC and (50 & 5) % relative humidity. 6.2 Materials and apparatus All apparatus and instruments used in mixing and testing shall be clean, dry and free from particles of gypsum. Before mixing and testing are begun, materials and apparatus shall be brought to ambient temperature. Irrespective of specimen size, all mixes shall be made with at least 100 g of powder. 6.3 Method of mixing Carry out the mixing according to the manufacturer’s instruc- tions (see clause 8). 6.3.1 Hand mixing 6.3.1.1 Apparatus 6.3.1.1.1 Clean scratch-free flexible mixing bowl, of at least 300 ml capacity. 6.3.1.1.2 Rigid spatula, with a blade at least 20 mm wide. 6.3.1 .1.3 Electric dental Vibrator. 6.3.1.2 Procedure Using the mixing bowl (6.3.1.1.1) and spatula (6.3.1.1.2), mix at least 100 g of powder with distilled water in the rec- ommended proportions (sec clause 8) according to the pro- cedure described below. 1) This requirement applies only to mixes prepared in mechanical devices with a rotational frequency of 1 400 r/min or higher. These devices include Provision for vacuum investing as well as mixing, and are suitable only for use with small casting rings (types 1 and 2 investments only). 2 SIST EN 27490:2000



ISO 7490 : 1990 (El Measure the mass of the powder and the mass distilled water with an accuracy of + 1 %. or volume of the Pour the water into the mixing bowl. Begin timing from the moment at which investment powder and water first make con- tact. Shake or sift the investment into the water over a period of 15 s, in such a way that entrapment of air is minimized. During the next 15 s, jar or vibrate the bowl. In this period ensure that the powder is completely wetted, using limited movements of the spatula if necessary, but avoiding any stirring movements. Then spatulate the powder and water for 30 s, using a circular stirring motion at a rate of approximately 3 r/s. Minimize the incorporation of air into the mix. During spatulation subject the bowl to appropriate Vibration by means of an electric dental Vibrator (6.3.1 .1.3). Complete the mixing procedure in 60s + 2s. Transfer the mixed testing a pparatus. investment i mmediately to the moulds or 7.2.2 Procedure Add 25 g of investment powder to 100 ml of 2-propanol (iso- propyl alcohol95 %) in a 250 ml conical flask. Stopper the flask and Shake for 2 min. Transfer the Suspension to the 300 (Im sieve (7.2.1 .l) nested on the 150 Fm sieve which is in turn nested on the 75 pm sieve, and wash with 2-propanol until the washings are clear. When the washings from the 300 Pm and the 150 Fm sieves are clear, the sieves may be separated to facilitate washing the residues on the 75 pm sieve. Dry the residues on the sieves at 40 OC rt 2 OC. Remove and weigh the residues, and report the mass of each residue as a percentage of the mass of the original Sample. CAUTION - When using 2-propanol keep away from flame, provide good Ventilation and avoid excessive inhalation of vapour. 7.3 Fluidity of the unset mix 7.3.1 Types 1 and 2 investments 6.3.2 Mechanical mixing 7.3.1 .l 6.3.2.1 Apparatus Mechanical mixing apparatus complying with the details specified by the investment manufacturer [see item d) in clause 81. 6.3.2.2 Procedure Using the mechanical mixing apparatus (6.3.2.1), mix at least 100 g of powder with distilled water in the recommended pro- Portion, for the time recommended by the investment manu- facturer [see item d) in clause 81. Transfer the mixed testing apparatus. investment immediately to the moulds or 7 Test methods 7.1 Visual inspection Visual inspection shall be used in determining compliance with requirements of 4.1 and clauses 9 and 10. 7.2 Particle size distribution Apparatus 7.3.1.1.1 Mould, as shown in figure 1. 7.3.1.1.2 Flat smooth glass or glazed plate, at least 80 mm Square. 7.3.1 .1.3 Electric dental Vibrator. 7.3.1.2 Procedure Make a mix of investment, following the appropriate method described in 6.3 and timing the test from the moment of first contact between powder and water. Stand the mould (7.3.1 .l .l) in the centre of the plate (7.3.1 .1.2) and fill it with the mix, applying sufficient Vibration to the plate so that the mould fills rapidly. The Vibration shall cease not later than 110 s from the Start of mixing. Place the plate and the filled mould on a Vibration-free surface. After 2 min * 1 s from the Start of mixing, lift the mould ver- tically away from the plate at a rate of approximately 10 mm/s and allow the mix to slump on the plate. When the investment has set sufficiently, of the slump at its highest Point. measure the heig ht lf in the first test the investment fails to meet the requirement specified in 4.3, repeat the test two more times and report the average of the three results as the height of the slump. 7.2.1 Apparatus 7.3.2 Type 3 investments 7.2.1.1 Test sieves, round, of diameter 200 mm and of the following nominal aperture sizes: 300 Pm, 150 Pm, and 75 Pm, complying with the requirements of ISO 3310-1. 7.2.1.2 Oven or drying cabinet, maintained at 40 OC + 2 OC. 7.3.2.1 Apparatus 7.3.2.1.1 Electric Vibrator with a substantially flat platform, to which is screwed or bolted a flat smooth glass or 3 SIST EN 27490:2000



ISO 7490 : 1990 (E) glazed plate at least 150 mm Square. The Vibrator shall operate on a 50 Hz or 60 Hz supply and shall be adjusted so that its plat- form vibrates with a peak-to-peak displacement of 0,2 mm. 7.3.2.1.2 Cylindrical mould, with an inside diameter of 25 mm + 0,5 mm and a height of 35 mm + 0,5 mm. 7.3.2.2 Procedure Make a mix of investment, following the appropriate method described in 6.3 and timing the test from the moment of first contact between powder and water. Place the mould (7.3.2.1.2) in the centre of the plate, and fill it with the mix, ap- plying sufficient Vibration to the plate so that the mould fills rapidly. The Vibration shall Start of mixing. cease not later than 110 s from the After 2 min + 1 s from the Start of mixing, switch on the Vibrator (7.3.2.1.1) and simultaneously lift the mould vertically away from the plate. Continue the Vibration for an additional period of 15 s + 1 s. When the investment has set sufficiently, measure the largest and smallest diameters of the slump and record the average of the two as the slump diameter. If in the first test the investment fails to meet the requirement specified in 4.3, repeat the test two more times and report the average of the three results as the slump diameter. Dimensions in millimetres; surface roughness value in micrometres Figure 1 - Slump test apparatus in brass or stainless steel 4 SIST EN 27490:2000



ISO 7490 : 1990 (El 7.4 Setting time 7.4.1 Apparatus 7.4.1.1 Vicat apparatus, consisting of a frame supporting a rod which is free to move up and down; an initial set needle, at least 35 mm long, of uniform circular Cross-section, 1 mm + 0,05 mm in diameter shall be attached to the lower end of the rod. The total mass of the rod and initial set needle shall be 300 g + 1 g. A pointer attached to the rod shall move over a scale, graduated in millimetres, indicating the depth of Penetration of the needle into the specimen. 7.4.1.2 Test specimen holder, consisting of a cylindrical mould and a detachable base, with the dimensions shown in figure 2. When the specimen holder is assembled, the base shall make a watertight seal with the mould. 7.4.1.3 Mould lining material, as supplied by the manu- facturer. 7.4.1.4 Water bath (for tests on type 2 investments), of area sufficient to accommodate the test specimen holder (7.4.1.2), with a depth of at least 35 mm and capable of being maintained at 23 OC + 2 OC. 7.4.2 Procedure: types 1 and 3 Line the mould with a Single piece of dry lining material (7.4.1.3). If the instructions (see clause 8) require wetting of the liner, immerse the lined mould in distilled water for 3 min, and then allow it to drain in air for 30 s. Attach the base. DO not allow any free water to collect in the bottom of the assembled specimen holder (7.4.1.2). Make a mix of investment, following the appropriate method described in 6.3 and timing the test from the moment of first contact between powder and water. Overfill the specimen holder with the mix and level the surface. Place the test specimen on the platform of the Vicat apparatus (7.4.1.1). A-A Drill and countersink Figure 2 - Vicat test specimen holder Dimensions in millimetres 3 holes equispaced polycarbonate disc on SIST EN 27490:2000



ISO 7490 : 1990 (E) When the glossy surface has completely disappeared from the mix, carefully lower the Vicat needle vertically until it touches the surface and release it gently, allowing it to sink into the mix under its own weight. Repeat this procedure at intervals of 30 s, wiping the needle clean after each Penetration and mov- ing the test specimen slightly so that the needle does not enter the same place twice. Avoid making any Penetration of the specimen closer than 5 mm to the Walls of the mould. it at 23 OC + 2 OC and not less than 95 % relative humidity. Prepare ten specimens from at least two mixes of investment. 7.5.3 Procedure: compressive strength at room temperature Test five specimens, made from at least two mixes of invest- ment. Start testing the specimens 2 h after making the last mix. Record the setting time as the total time from the Start of mix- Position each specimen between the loading platens of the ing to the time when the needle first fails to penetrate to within testing machine (7.5.1.4) so that the specimen will be loaded 5 mm of the base of the specimen holder. axially. Use no packing between specimen and platens. Record the mean of three such tests, rounded to the nearest 1 min, as the net result. If the mean falls exactly on a half minute, round it to the preceding whole minute. Apply a compressive forte increasing continuously at a rate of 5 kN/min & 2 kN/min until fracture occurs. compressive strength, k, in megapascalsl), using the following formula : For each specimen record the load at fracture and calculate the 7.4.3 Procedure: type 2 Follow the procedure outlined in 7.4.2, but perform the tests on test specimens placed in the water bath (7.4.1.4) on the plat- form of the Vicat apparatus. After the test specimen has been positioned in the empty bath, carefully pour distilled water at 23 OC + 2 OC into the bath until the level is at least 5 mm above the top of the test specimen. If the probable setting time of the material is unknown, Start Penetration tests 5 min after begin- ning the mix. 7.5 Compressive strength 7.5.1 Apparatus 7.5.1.1 Sectional mould, open at both ends, to produce one or more cylindrical specimens, each having a diameter of 20 mm * 0,05 mm and a length of 40 mm + 0,05 mm. 7.5.1.2 Two flat glass plates, large enough to close the ends of the mould. 7.5.1.3 Electric dental Vibrator. 7.5.1.4 Compression testing machine, complying with the machine class 1 requirements specified in ISO 7500-1 up to a forte of 20 kN. 7.5.2 Preparation of test specimens Lightly oil or grease the mould (7.5.1.1), or Spray with a suitable mould-release agent. Place the assembled mould on one of the glass plates (7.5.1.2). Make a mix of investment, following the appropriate method described in 6.3. Slightly overfill the mould with investment, applying suitable Vibration. As soon as the glossy surface has completely disappeared from the mix, apply the second glass plate (7.5.1.2) to the top of the mould and press it down until the surface of the mould is uniformly visible through the glass. When the specimen is strong enough to handle, remove it from the mould and store 4F k - = ,n n;dL where F is the applied load at fracture, in newtons; d is the diameter of the specimen, in millimetres ( = 20 mm). For all five results, calculate the mean, X, and the Standard deviation, a; calculate the coefficient of Variation, expressed as a percentage, using the following formula: IOOZ x If the coefficient of Variation is less than or equal to 15 %, report the mean; if the coefficient of Variation is greater than 15 %, carry out a further five tests and report the mean of all ten results. NOTE - If the testing machine is of the type in which Cross-head Speed rather than loading rate is controlled, it will be necessary to undertake preliminary trials to determine a Cross-head Speed which gives a loading rate within the limits specified in 7.5.3. These trials shall be made with a specimen in Position, which shall subsequently be discarded without further testing. A suitable starting Point is a cross- head Speed of 0,5 mm/min. 7.6 Expansion 7.6.1 Test specimen Both setting and thermal expansion shall be measured on the same specimen, which shall be 50 mm + 1 mm in length and of uniform Cross-section; the Cross-section may be circular, Square or an equilateral triangle. The Cross-sectional area shall be within the range from 78 mm* to 275 mm*. The total measuring pressure exerted on the specimen during setting or heating shall not exceed 2,4 kPa*). During setting, all sides of the specimen shall be in contact with the lining material sup- plied with the investment. If the manufacturer’s instructions recommend wetting the liner, this shall be done as described in 7.6.4. 1) 1 MPa = IO6 N/m2 2) 1 kPa = IO3 N/m2 6 SIST EN 27490:2000



ISO 7490 : 1990 (El 7.6.2 Test conditions 7.6.4.2 Preparation of lining material and apparatus Types 1 and 3 investments shall be allowed to set under con- ditions of 23 OC + 2 OC and not less than 95 % relative humidity. Type 2 investments shall be immersed in distilled water at 23 OC + 2 OC for the duration of setting. In all cases, one end of the specimen shall set against a heat- resistant alloy plate, 1 mm thick, of the same Cross-section as the specimen, which will form part of the specimen during the thermal expansion test and which will distribute the measuring forte of the thermal expansion apparatus uniformly over the Cross-sectional area of the specimen. During heating, the sides of the specimen should not contact the thermal expansion apparatus; the heat-resistant alloy plate is used to transfer specimen expansions to the moving Parts of the measuring apparatus. Frictional forces in the measuring System should be confined to the actual measuring device itself; it is important that these frictional forces be as small as possible. The temperature of the specimen shall be measured by a sheathed thermocouple the hot junction of which is in- serted to a depth of 15 mm in an axial recess drilled in the base of the specimen. 7.6.3 7.6.3.1 Apparatus Setting expansion apparatus Any apparatus tan be used which camplies with the general requirements set out in 7.6.1 and 7.6.2 and which tan be ad- justed to apply a measuring forte of 0,5 N + 0,l N to the specimen over a range of movement of 1 mm, and which produces a specimen that tan be used for the subsequent measurement of thermal expansion. The equipment shall be capable of measuring the specimen expansion with an accuracy of + 5 Fm. A suitable form of apparatus is described in annex A and illustrated in figures A.l, A.2 and A.3. 7.6.3.2 Thermal expansion apparatus Any form of thermal expansion apparatus may be used which will accept a specimen of the dimensions specified in 7.6.1 and which camplies with the other requirements specified in 7.6.1 and 7.6.2. The equipment shall be capable of measuring the specimen expansion with an accuracy of & 5 Pm. A suitable form of apparatus is described in annex B and illustrated in figure B.I. 7.6.4 Procedure 7.6.4.1 General The instructions specified in 7.6.4.2 to 7.6.4.5 apply specifically to the use of the setting expansion and thermal expansion ap- paratus described in annex A and annex B, respectively; they may have to be modified if some other form of apparatus is used. 1) An Office sponge or moistener is suitable. If the manufacturer’s instructions require wetting of the lining material, this shall be done as described below. mould Place a suitably sized piece of rubber spongel) in a Container, and add distilled water until the sponge is immersed to half its height. Lay each piece of lining material on the wet sponge until its upper surface is completely wetted. If the manufacturer’s instructions dry lining material. do not specify wetting , use Line the side Walls of the trough with two pieces of lining material and place the triangular plate vertically against the fixed end wall of the trough so that its spigot projects into the mould space, and its edges rest on the liners. 7.6.4.3 Procedure for determining setting expansion Make a mix of investment, following the appropriate method described in 6.3 and timing the test from the moment of first contact between powder and water. Pour or vibrate the mix into the trough up to the top edges of the two liners and the triangular plate. Then gently place the third piece of lining material on the upper surface of the specimen. For types 1 and 3 investments, place the lid in Position on the dry bath. For type 2 investments, pour sufficient distilled water at 23 OC * 2 OC ‘into the bath to cover the top surface of the specimen to a depth of at least 5 mm. Zero the recording apparatus or take an initial reading. Deter- mine the expansion of the specimen to the nearest 0,Ol mm over a period of 2 h; express the setting expansion as a percen- tage of the original length of the specimen, excluding the thick- ness of the end-plate (i.e. gauge length = 50 mm). 7.6.4.4 Procedure for determining thermal expansion After a period of 2 h, drain the water bath, free the set specimen from the trough and carefully remove the liners from its sides. The end of the specimen furthest from the triangular plate will become the base of the specimen during the thermal expansion test. Drill a hole in the centre of the base, at least 5 mm in diameter and 20 mm deep. Then Position the specimen vertically in the bottom of the dilatometer so that it fits over the specimen thermocouple and Stands free of the sides of the outer tube. Position the inner tube of the dilatometer so that its pointed end rests in the recess in the centre of the triangular plate. Place the heat shield over the upper end of the outer tube so that the inner tube Passes through its central hole without coming into contact with it, and then Position the expansion- measuring device so that it locates the inner tube in a vertical Position. Turn on the water cooling to the dilatometer head, and the exhaust System; zero the measuring System or take an initial reading. 7 SIST EN 27490:2000



ISO 7490 : 1990 (E) Raise the furnace temperature at a rate of about 5 OC/min. 7.7.1.1 Apparatus If the investment does not contain cristobalite (see note below), maintain this rate for the whole of the heating cycle. If it does contain cristobalite, slow the heating rate to approxi- mately 1 OC/min over the temperature range from 200 OC to 300 OC. In both cases raise the furnace temperature to the mid- Point of the manufacturer’s recommended casting range (see clause 8), and maintain at that temperature for the rec- ommended time. NOTE - To perform this test, it is necessary to know whether the investment contains cristobalite as a refractory filler. If this fact is not apparent from the name of the material, it tan be established by ex- amining the thermal expansion curve supplied by the manufacturer (sec clause 8). lf the investment contains cristobalite, it will show a well defined isothermal expansion at about 250 OC. During heating, if an X-Y recorder is being used, make a con- tinuous graph of thermal expansion; otherwise, make readings of expansion and temperature at intervals of 30 OC, and plot a thermal expansion curve. At the end of the heating cycle, determine the final expansion to the nearest 0,Ol mm; express the thermal expansion as a percentage of the original length of the unset speci- men, excluding the thickness of the end-plate (i.e. gauge length = 50 mm). 7.6.4.5 Expression of results Add the values for setting and thermal expansion and round the sum to the nearest 0,l % total expansion. Carry out three such measurements, and report the average of the three results as the total expansion of the investment. Check each thermal expansion curve to ens
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