140 ABSTRACTS OF PAPERS

QUANTITATIVE TESTS ON FINISHED MOULDINGS

By W. D. OWEN, Associate Member.

[ABSTRACT of a Measurements Section paper (based on Report Ref. B/T40 of the British Electrical and Allied Industries Research Association) whichwas published in December, 1947, in Part II of the Journal.]

Moulded insulating materials made from a given base by heatand pressure differ appreciably according to the type of mouldand the moulding technique used. To avoid anomalies from thiscause when comparing alternative materials, manufacturers ofelectrical equipment have adopted a standard test and test-specimen for each of the salient properties. The values obtainedin this manner are, by common consent, deemed to be those ofthe materials. It is recognized that these values may be higheror lower than those manifested by commercial mouldings, andthat it is sometimes necessary to know the actual values. Sinceit is usually not possible to apply the standard tests to non-standard test-specimens, a new set of tests and test-specimens isrequired, the latter being cut from commercial mouldings. Atthe request of the B.S.I. anjnvestigation was put in hand by theE.R.A. to explore how this might be done.

The whole of the investigation work was carried out at theUniversity of London, under the direction of Prof. W. John, byB. C. Fleming-Williams, Z. Rogowsky and D. J. Strong, whosubmitted a number of interim reports. These were consideredby a Technical Committee of the E.R.A. and subsequentlycollated by B. Shearman, who prepared an omnibus reportpublished by the E.R.A. as Ref. B/T40—1944, and by the B.S.I.as B.S. 1330: 1946. In this omnibus report the suggestion wasmade that the proposed tests "should be tried out under industrialconditions and revised in the light of experience." The presentpaper describes the work in an abridged and modified form andwas submitted as a basis for discussion.

With the object of determining the state of the art, a study wasfirst made of the published work of previous investigators and aquestionnaire was addressed to persons and organizations knownto be interested. These led to a useful bibliography and to anexhaustive study of the German Dynstat—a machine for applyingdynamic and static loads to miniature specimens.

The investigation began with a metrical examination of severalhundred commercial mouldings in order to ascertain whatpercentage of them yielded test-specimens of specified dimensions.The results are given in a series of statistical curves. Usefulinformation is also given on the preparation of specimens fromfinished mouldings.

The properties dealt with in current Standard Specificationsare tabulated, and a group of standard test-specimens is illustratedin order to facilitate comparisons. It is noted that only 9 of the17 principal properties are covered by the new technique, namely

Mr. Owen is with the British Electrical and Allied Industries Research Association.

resistance to impact and slow bend (using the Dynstat), crushingstrength, electric strength, surface resistivity, plastic yield,resistance to heat, specific gravity and water absorption.

For each of these an appropriate method of test is describedusing miniature test-specimens cut from commercial mouldingsas distinct from standard mouldings specially designed for thepurpose. In several cases a test-jig or machine had to bedevised. Some of the machines made up for the investigationare described and illustrated.

Work done on the Continent, previously, had led to the generaladoption of the Schopper Dynstat for impact and slow bend.This was fully exploited in this investigation as a time-savingexpedient, and consequently is featured in the report despite theconclusion that a simpler machine is recommended.

Owing to the system of grading by yield temperature used bythe electrical industry in this country (see Journal I.E.E., 1937,81, p. 553), the property known as plastic yield or heat stabilityis regarded as an important one. For this reason, two methodsof test were explored, one using a small specimen loaded as abeam, the other using a sensitive penetrometer. The relativemerits of each are discussed.

The paper includes a short dissertation on resistance tocrushing, with particular reference to the effect of the size of thespecimen and the effect of the position in the moulding fromwhich the specimen was cut.

Electric strength is determined by means of a ball-and-planetest, the specimen being a disc 8 mm in diameter and 1 mm thick,fixed to an anti-flashover shield suitable for a maximum voltageof 14 kV, determined by experience.

Provision is made for the determination of surface resistivityby setting up a potential difference between two Aquadagelectrodes drawn on the surface of the moulding with an ordinarysteel pen.

Heat resistance is an ambiguous term because there are severaldifferent heat hazards, ranging from the electric arc to radiantheat. Flame is convenient for test purposes but is not realistic.The investigators found the V.D.E. test a reasonably goodcompromise. This involves "contact with glowing hot body"—in this case a glow-bar heated to 950° C—and leads to a groupof five distinguishable grades.

Neither specific gravity nor water absorption presented anydifficulty.

The results are summarized in Table 1, which gives particularsof the degree of correlation with standard tests.

ABSTRACTS OF PAPERS 141

TABLE 1

Summary of Results

Property tested

Impactstrength

Cross-breakingstrength

Crushingstrength

Electricstrength

Surfaceresistivity

Resistance toheat (glow-bar test)

Plastic yield

Specificgravity

Water absorp-tion

Means or apparatus used

Dynstat with modifiedpendulum

Dynstat (unmodified)

Simple beam-typemachine giving loadup to 4"5 kg

Electrodes are a 2\ mmsteel ball and a brassplate respectively. Aspecial anti-flashovershield of ebonite issealed to the specimen

Aquadag line electrodesapplied with specialpen. Metal contactfingers and sensitiveballistic galvanometer

Glow-bar apparatus

Three-point load appar-atus

Vicat needle apparatus

Direct weighing andcomputation of di-mensions, or weighingin air and in liquid

Immersion and constanthumidity vessels.

Size of miniature specimen

Notched:—10 x 10 x 2 mm

Unnotched:—1 0 x 1 0 x 2 mm to10 X 5 x 1 mm

15 x 10 x 4 mm to10 x 5 x l-6mm

Recommended size:—10 X 5 x 2 mm

3 mm cube or cylinder

Disc 8 mm diameter andup to 1 mm thick

Surface of actual mould-ing between line elec-trodes 25 mm longand 2 mm apart

10 x 15 x 1 mm

5 x 10 x 1-5 mmor 10 x 10 X 1 mm

Disc 8 mm diameter,1 mm thick

Immaterial

Strip about 2 cm x 1 cmx 0-15 mm cut ascylindrical shavingfrom moulding, orpowder filed frommoulding

Percentage ofmouldings fromwhich specimenis obtainable

63%

63%97%

66%

63%

83%

85%

100%

Over 80%

85%70%

85%

100%

Nearly allmouldings

100%

Remarks

Striking distance shouldbe 1 • 75 timesspecimenthickness

Variation is largeAverage of a number of

tests should be taken

Tests may be made underoil, using shield, ifdesired

Surface resistivity varieswith gap width be-tween electrodes, andvariation differs fordifferent materials

Classificationintoclearlydefined groups reason-ably satisfactory

Short-time tests (tem-perature raised at50 deg C per hour)gave rough indicationof yield temperature.Long-time tests (stressapplied at constanttemperature) for moreaccurate work

Tests were made by im-mersion, by exposureto known relative hu-midity, and by expo-sure of powder toknown relative hu-midity

Correlation of results withstandard tests

No correlation expected orobserved

Cross-breaking strength canbe very approximately re-lated to all the dimen-sions. Hence approxi-mate correlation can beobtained

Approximate correlation isobtainedforgeometricallysimilar specimens usingformula Y = y + nylv', ifY=M30, and n = A/48,where h = height ofspecimen in mm

No correlation expected orobserved

Reasonable correlationwithin limits of experi-mental error

Since B.S. documents giveno classification, no cor-relation was possible

Reasonable correlation inshort-time tests, correla-tion to within 1 deg C inlong-time tests

Fair correlation in short-time tests, no correlationin long-time tests

Satisfactory correlation ob-tained

No clear correlation by anymethod

(Correlation not expected)