ASTM D955-89(1996)
(Test Method)Standard Test Method of Measuring Shrinkage from Mold Dimensions of Thermoplastics
Standard Test Method of Measuring Shrinkage from Mold Dimensions of Thermoplastics
SCOPE
1.1 This test method is intended to measure shrinkage from mold cavity to molded dimensions of thermoplastics when molded by compression or injection processes with specified process conditions.
1.2 This test method covers initial shrinkage measurements. The method also accommodates shrinkage at 24 and 48 h, which may be critical for some materials.
1.3 This method will give comparable data based on standard specimens and can not predict absolute values in actual molded parts with varying flow paths, wall thicknesses, pressure gradients and process conditions. Differences in mold shrinkage may also be observed between the three specimen geometries described in this test method.
1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are given for information only.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
Note 1--This test method is technically identical to ISO 294-4 where Type D2 specimens are used except that pressure transducers are an option in this test method and required in ISO 294-4.
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NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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Designation: D 955 – 89 (Reapproved 1996) An American National Standard
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Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Measuring Shrinkage from Mold Dimensions of Molded
Plastics
This standard is issued under the fixed designation D 955; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope D 4066 Specification for Nylon Injection and Extrusion
Materials
1.1 This test method is intended to measure batch-to-batch
D 4181 Specification for Acetal (POM) Molding and Extru-
uniformity in initial shrinkage from mold to molded dimen-
sion Materials
sions of either thermoplastic or thermosetting materials when
D 4549 Specification for Polystrene Molding and Extrusion
molded by compression, injection, or transfer under specified
Materials (PS)
conditions.
D 4976 Specification for Polyethylene Plastics Molding and
1.2 This test method does not provide for the measurement
Extrusion Materials
of shrinkages that may occur as molded materials age, after the
E 691 Practice for Conducting an Interlaboratory Study to
first 48 h out of the mold.
Determine the Precision of a Test Method
1.3 The values stated in SI units are to be regarded as the
standard. The values given in parentheses are for information
3. Terminology
only.
3.1 General—Definitions of terms applying to this test
1.4 This standard does not purport to address all of the
method appear in Terminology D 883.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Significance and Use
priate safety and health practices and determine the applica-
4.1 Compression Molding—In compression molding, the
bility of regulatory limitations prior to use.
difference between the dimensions of a mold and of the molded
2. Referenced Documents article produced therein from a given material may vary
according to the design and operation of the mold. It is
2.1 ASTM Standards:
probable that shrinkage will approach a minimum where
D 618 Practice for Conditioning Plastics and Electrical
2 design and operation are such that a maximum of material is
Insulating Materials for Testing
forced solidly into the mold cavity or some part of it, or where
D 647 Practice for Design of Molds for Test Specimens of
2 the molded article is hardened to a maximum while still under
Plastic Molding Materials
pressure, particularly by cooling. In contrast, shrinkages may
D 788 Specification for Poly(Methyl Methacrylate)
2 be much higher where the charge must flow in the mold cavity
(PMMA) Molding and Extrusion Compounds
but does not receive and transmit enough pressure to be forced
D 796 Practice for Compression Molding Test Specimens of
2 firmly into all its recesses, or where the molded article is not
Phenolic Molding Compounds
2 fully hardened when discharged. The plasticity of the material
D 883 Terminology Relating to Plastics
used may affect shrinkage insofar as it affects the retention and
D 956 Practice for Compression Molding Specimens of
2 compression of the charge.
Amino Molding Compounds
4.2 Injection Molding—In injection molding, as in com-
D 1896 Practice for Transfer Molding Test Specimens of
2 pression molding, the difference between the dimensions of the
Thermosetting Compounds
mold and of the molded article produced therein from a given
D 1897 Practice for Injection Molding Test Specimens of
2 material may vary according to the design and operation of the
Thermoplastic Molding and Extrusion Materials
2 mold. The difference may vary with the type and size of
D 1898 Practice for Sampling of Plastics
molding machine, the thickness of molded sections, the degree
D 3419 Practice for In-Line Screw-Injection Molding Test
3 and direction of flow or movement of material in the mold, the
Specimens from Thermosetting Compounds
size of the nozzle, sprue, runner, and gate, the cycle on which
the machine is operated, the temperature of the mold, and the
This test method is under the jurisdiction of ASTM Committee D-20 on Plastics
length of time that follow-up pressure is maintained. As in the
and is the direct responsibility of Subcommittee D20.09 on Specimen Preparation.
Current edition approved Nov. 24, 1989. Published January 1989. Originally
published as D 955 – 48 T. Last previous edition D 955 – 88.
2 4
Annual Book of ASTM Standards, Vol 08.01. Annual Book of ASTM Standards, Vol 08.03.
3 5
Annual Book of ASTM Standards, Vol 08.02. Annual Book of ASTM Standards, Vol 14.02.
D 955
case of compression molding, shrinkages will approach a one half to three fourths of its rated shot capacity at melt
minimum where design and operation are such that a maximum temperatures recommended by the material supplier. Different
of material is forced solidly into the mold cavity and where the screw and barrel configurations are required for thermoplastic
molded article is hardened to a maximum while still under and thermoset materials.
pressure as a result of the use of a runner, sprue, and nozzle of
NOTE 2—If injection machines of appropriate capacity are not avail-
proper size, along with proper dwell. As in compression
able, the requirement of 6.5 may be met in machines of larger capacities
molding, shrinkages may be much higher where the charge
by providing test molds with multiple cavities to be filled from a common
must flow in the mold cavity but does not receive and transmit sprue, so that the total weight of the shot, including sprue and runner will
fall within the specified limits.
enough pressure to be forced firmly into all of the recesses of
the mold. The plasticity of the material used may affect
6.6 Transfer Press—A suitable hydraulic press that will
shrinkage indirectly, in that the more readily plasticized mate-
deliver a pressure of 70 to 140 MPa (10 000 to 20 000 psi) on
rial will require a lower molding temperature.
the material in the pot of the die or the cylinder of the press.
4.3 Transfer Molding—In transfer molding, as in compres-
6.7 Balance—A balance for weighing compression-molding
sion or injection molding, the difference between the dimen-
charges.
sions of the mold and of the molded article produced therein
6.8 Measuring Tools—Measuring tools (micrometers, ver-
from a given material may vary according to the design and
nier calipers, etc.) accurate to 0.02 mm (0.001 in.) for measur-
operation of the mold. It is affected by the size and temperature
ing the molds and the test specimens.
of the pot or cylinder and the pressure on it, as well as on mold
7. Test Specimens
temperature and molding cycle. Direction of flow is not as
important a factor as might be expected, although it can have
7.1 Compression-Molding Materials—For mold shrinkages
some bearing on results.
of compression-molding materials, the test specimens shall be
1 1
bars, 12.7 by 12.7 by 127 mm ( ⁄2 by ⁄2 by 5 in.), or a disk 3.2
5. Sample Preparation
mm ( ⁄8 in.) in thickness and 102 mm (4 in.) in diameter, made
5.1 Some materials require special treatment before they are
in a positive mold in such a way as to minimize lateral
molded. For example, thermoplastics which absorb moisture
movement of the plastic during the molding.
must be dried before molding. Materials to be tested shall be
7.2 Injection-Molding Materials—For mold shrinkage of
prepared for molding in accordance with the manufacturer’s
injection-molding materials, specimens of two types shall be
recommendations. The preparation given to the material prior
1 1
used: (1) bars 12.7 by 3.2 by 127 mm ( ⁄2 by ⁄8 by 5 in.), gated
to molding shall be recorded and reported.
at the end to provide flow throughout the entire length, shall be
used for measurements of shrinkage in the direction of flow,
6. Apparatus
and (2) disks, 3.2 mm ( ⁄8 in.) in thickness and 102 mm (4 in.)
6.1 Compression Mold—A single bar, single-cavity positive
in diameter, gated radially at a single point in the edge, shall be
mold having a cavity cross-section 12.7 by 127 mm ( ⁄2 by 5
used for measurements of shrinkages of diameters parallel and
in.) and a loading depth sufficient to obtain a molded bar 12.7
perpendicular to the flow.
mm ( ⁄2 in.) in thickness, or for diametral shrinkage, a single
7.3 Transfer-Molding Materials—For shrinkage of transfer-
cavity positive 102-mm (4-in.) disk mold. Both shall conform
molding materials, specimens 12.7 by 12.7 by 127 mm ( ⁄2 by
to Practice D 647.
⁄2 by 5 in.), gated at the end or at the top near one end, so as
6.2 Injection Mold—For shrinkage parallel to flow, an
to provide flow throughout their entire length, shall be used for
impact bar mold having a cavity 12.7 by 127 mm ( ⁄2 by 5 in.).
1 measurement of shrinkage in the direction of flow. A disk
The thickness shall be 3.2 mm ( ⁄8 in.), unless otherwise agreed
specimen 3.2 mm ( ⁄8 in.) in thickness and 102 mm (4 in.) in
upon by the seller and the purchaser. The mold shall have at
1 1 diameter gated radially at a single point in the edge, shall be
one end a gate 6.4 mm ( ⁄4 in.) in width by 3.2 mm ( ⁄8 in.) in
used for measurements of shrinkages of diameters parallel and
depth (Note 1). For diametral shrinkage, where shrinkage both
perpendicular to the flow.
parallel to flow and transverse to flow are to be measured, the
mold shall have a cavity 102 mm (4 in.) in diameter by 3.2 mm
8. Conditioning
( ⁄8 in.) in thickness with a gate, placed radially at the edge,
8.1 Conditioning—Conditioning of molded specimens shall
1 1
12.7 mm ( ⁄2 in.) in width by 3.2 mm ( ⁄8 in.) in depth.
be done in the Standard Laboratory Atmosphere, 4.2, of
NOTE 1—If, for any reason, a test specimen of thickness greater than
Practice D 618 for various lengths of time as discussed in 9.3.
3.2 mm ( ⁄8 in.) is agreed upon, the depth of the gate may be greater than
8.2 Test Conditions—Conduct tests in the standard labora-
3.2 mm ( ⁄8 in.) and must be reported.
tory atmosphere of 23 6 2°C (73.4 6 3.6°F) and 50 6 5
6.3 Transfer Mold—An impact bar mold having a cavity
percent relative humidity, unless otherwise specified in the test
1 1
12.7 by 12.7 by 127 mm ( ⁄2 by ⁄2 by 5 in.) and having either
methods or in this specification. In cases of disagreement, the
an end gate or top gate at one end 6.4 by 1.52 mm ( ⁄8 by 0.060
tolerances shall be 61°C (61.8°F) and 62 percent relative
in.) in depth.
humidity.
6.4 Compression Press—A suitable hydraulic press that will
9. Procedure
deliver a pressure of 20 to 35 MPa (3000 to 5000 psi) to the
material in the mold. 9.1 Measure the length of the cavity of the bar mold, or the
6.5 Injection Press—A suitable injection-molding machine diameter of the cavity of the disk mold parallel and perpen-
that will fill the test molds when it is operated in the range from dicular to the flow, to the nearest 0.02 mm (0.001 in.). Make
D 955
the measurements at standard laboratory temperature as de- should be such that the entire charge can be forced into the die
fined in 4.1 of Practice D 618. before premature curing can take place, which would be
evidenced by roughness and poor finish.
9.2 Mold at least five sound test specimens from the sample
to be tested, under such conditions of pressure, temperature,
NOTE 3—Soft flow epoxy molding materials require a press suitable to
time, etc., as the manufacturer and the purchaser may agree are
deliver 4 to 14 MPa (500 to 2000 psi) to the material in the pot. See also
suitable for the material. In the absence of other definite
Practice D 1896.
recommendations, the following are suggested as suitable
9.2.6 Thermosetting Materials Molded by Injection—The
molding procedures for plastics of various types:
molding machine used shall be such that the injection pressure
9.2.1 Thermoplastics Molded by Compression—For ther-
can be controlled between 70 and 140 MPa (10 000 to 20 000
moplastics, such as cellulose acetate, load the mold evenly at
psi). The capacity shall be adequate to produce a shot within
room temperature with the requisite quantity of granular
one half to three fourths of the rated capacity. The cylinder and
material, also at room temperature, and place the mold in a
mold heating systems shall be capable of heating the shot and
hydraulic press such that a pressure of 20 to 35 MPa (3000 to
the mold to the temperatures required by the material specifi-
5000 psi) on the projected area of the mold cavity can be
cation. The molding should be carried out in accordance with
applied and the mold can be heated to 121 to 129°C (250 to
Practice D 3419.
265°F) in 5 to 10 min. Apply heat and pressure, within these
9.3 After molding, allow the specimens to cool in the
ranges, to mold the specimen. Cool the mold to room tempera-
Standard Laboratory Atmosphere prescribed in 4.2 of Methods
ture under pressure and discharge the specimen. The rate of
D 618 before being measured. Standard shrinkage measure-
heating, the maximum temperature reached, and the rate of
ments shall be made to determine the “48-h” or “normal” mold
cooling do not seem critical, but the temperature of the mold at
shrinkage. Shrinkage occurring during shorter periods of time
the time of discharge should be definitely controlled.
designated “initial” molding shrinkage or “24-h shrinkage” as
9.2.2 Thermoplastics Molded by Injection—Molding of
defined below are optional and may be made upon agreement
thermoplastic materials should be carried out in accordance
between the buyer and the seller. The period of storage for
with Practice D 1897. The molding machine used should be
“initial molding shrinkage” for specimens 3.2 mm ( ⁄8 in.) in
such that it is operated without exceeding one half to three 1
thickness shall be from 1 to 2 h; for specimens 6.4 mm ( ⁄4 in.)
fourths of its rated shot capacity. The temperature of the
in thickness, 2 to 4 h; and for specimens 12.7 mm ( ⁄2 in.) in
heating cylinder should be maintained at a point which will on
thickness, from 4 to 6 h. Measure the length or diameter of
a cycle selected, produce a melt at a temperature within the
each specimen at Standard Room Temperature to the nearest
range recommended by the manufacturer of the material. The
0.02 mm (0.001 in.) and then return the specimens to storage in
mold should be maintained at a temperature recommended by
the Standard Laboratory Atmosphere. Measure the specimens
the material manufacturer and should be maintained within
again not less than 16 nor more than 24 h after molding, in
3°C (5°F
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