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A number in parentheses indicates the year of last reapproval. A superscript epsilon indicates an editorial change since the last revision or reapproval. Differences in mold shrinkage may also be observed among the three specimen geometries described in this test method. The values given in parentheses are given for information only.
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. Referenced Documents 2.
Terminology 3. Current edition approved Nov. Published December Originally approved in Last previous edition approved in as D - Summary of Test Method 4. Signicance and Use 5. Factors such as mold and melt temperature, ll times, and packing conditions are known to affect shrinkage signicantly. Adherence to the specied mold design see 7. Factors, such as the amount of material in charge, cooling time, and pressure application are known to affect shrinkage signicantly.
Sample Preparation 6. For example, thermoplastics, which absorb moisture must be dried before molding. The preparation given to the material prior to molding shall be recorded and reported. Apparatus 7. Pressure transducers for monitoring the pressure in at least one cavity are mandatory for Type D2.
Pressure transducers are recommended, but not mandatory for specimens Type A and Type B. NOTE 2—Although scribe marks are not required, if they are used for injection or compression molded specimens for the measurement of shrinkage, the scribe marks shall be 1. NOTE 3—If the injection machines of appropriate capacity are not available, the requirements of 7. Test Specimen 8.
Orientation effects due to ow direction do not generally pertain to compression molding. The thickness shall be 3. The mold shall have at one end a gate 6. NOTE 4—Although this specimen may be used to determine mold shrinkage in both the ow and cross ow directions, the lling pattern does not produce uniform ow lengths and orientation. Consequently, there may be signicant differences when measuring the specimen at different points around the circumference. Values would not be expected to be in agreement with those obtained using the specimens described in 8.
Conditioning 9. Procedure Record these values as l and w, respectively. Three percent warp is dened as 3 mm depth deection, positive or negative, per mm in length.
The temperature of the heating cylinder and the mold shall be maintained at a point which, on a cycle selected, will produce temperature within the range recommended by the material molding standard. Begin with a short shot to ensure the ow front is straight and not radial and that the ow is laminar and does not exhibit melt fracture jetting.
Collect samples after the machine is at equilibrium. Do not modify or alter the edges used for the measurement of dimensions.
It is recommended to cool specimens in a horizontal position at room temperature by placing them on a material of low thermal conductivity to minimize warpage. Calculation and Report Report mold shrinkage in both ow and cross direction to two signicant gures.
Precision and Bias Each material was supplied in granular form to each of the testing laboratories by a single supplier. Each test result is the average of ve individual determinations from successive injection molding cycles. Each laboratory obtained one test result for each material. The results, including suggested cavity hold pressure for various materials, are shown in Appendix X1.
Request RR: D Poorer precision larger values of Sr and r would be expected if the same operator were to shutdown and then restart the injection molding machine on the same day with the same mold, material and operating set points.
Repeatability under such circumstances was not evaluated. The data in Tables should not be rigorously applied to acceptance or rejected of material, as those data are specic to the round robin and may not be representative of other lots, conditions, materials, or laboratories. Users of this test method should apply the principles outlined in Practice E to generate data specic to their laboratory and materials, or between specic laboratories.
The principles of It would be incorrect to assume values form Tables for any new material. It is the intent of this method to control and document as many of these variables as possible.
There are no recognized standards by which to estimate the bias of this test method. Keywords Intermediate values may also be used, however. NOTE X2. Thus the mass of material injected into the cavity is not clearly dened and the orientation of the material near the gate will be perturbed. Due to the inertia of the injection-molding machine, the effective change-over time is longer than its nominal value.
The correct change-over point shall therefore be adjusted individually for each value of the injection speed and for each material under test. The decrease in the cavity pressure at hold to zero indicates that the material in the gate has solidied sufficiently to stop ow into the cavity.
As the cooling rate of the material is proportional to the square of the reciprocal of the thickness, the minimum cooling time for the cavity can be expected to be close to 1. At point R, the value of the cav ity pressure at hold is recorded. The minimum hold time can be read from the decrease of the cavity pressure to zero.
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ASTM D955-08 PDF
ASTM D955 - 08(2014)