ASTM D785: Rockwell Hardness Of Plastics And Electrical Insulating Materials

Introduction

This ASTM D 785 standard test method describes the two methods for determining the indentation hardness of plastics and plastic electrical insulating materials using the Rockwell hardness tester. The methods offer a trustworthy measurement method, assisting in evaluating the material’s characteristics and quality control in various applications.

Scope

The ASTM D 785 Standard test method describes how to record changes in depth to get Rockwell hardness numbers as an indenter is loaded with light to heavy loads (Procedure A). The Rockwell alpha (a) values represent the maximum indenter movement that can be known from the initial depth during load increase (Procedure B). The findings include symbols that show the indenter’s specifics.

Test Procedure

The ASTM D 785 Standard Test Method has two types of test procedures:

Procedure A:

The appropriate Rockwell hardness scale is chosen based on the test material, determined by indentation readings and material details. When switching ball indenters to ensure proper seating, the initial reading is ignored, and full main load pressure is applied to ensure appropriate indenters’ shoulder seating. 

Place the specimen on the anvil, and the capstan screw is adjusted so that the large pointer is within 65 divisions of B 30, or the “set” mark on the red scale, while the small pointer reads zero, applying a minor load of 10 kg. The knurled ring below the Capstan hand wheel helps fine-tune, with attention taken to prevent overshooting. After the minor load is released, the significant load is applied within 10 seconds and withdrawn after precisely 15 seconds. After eliminating the significant load for 15 seconds, measure the Rockwell hardness on the red scale closest to the full-scale division.

When a heavy load is applied, the count needle moves through zero on the red scale to record readings; after that, remove the load and subtract the passes.  A reading is marked as if there is no difference if there is a difference of 1, if there is a difference of 2, and as minus 100. Then, the overall indentation was assessed using pointer divisions within 15 seconds of the lever trip with the principal load. Use a less severe scale when there is excessive indentation. The manufacturers should maintain a minimum 6mm edge clearance to prevent damage to the plastic surface. Also, the manufacturers should avoid duplicate tests on the opposite face.

Procedure B:

This procedure uses an R scale, a 12.7 mm [12-in.] Indenter and a significant 60 kg load. Place a flat copper block on the anvil and raise the indenter to 12.7 mm. Then, the small pointer was aligned with the beginning point, and the large pointer was set to zero on the black scale to determine the machine’s “spring constant.” The technician should pull the load release lever to start the significant load. After accounting for the indenter spindle’s elastic deformation and the machine frame spring, the dial gauge provided the total indentation distance. Minor load iterations continued until a consistent deflection revealed solely the instrument’s spring effect. At this point, reset the dial to zero.

Place the test specimen on the anvil and apply a light weight of 10 kg on it. Adjust the zero setting to 10 seconds. Then, immediately following the zero setting, the significant load was applied. During 15 seconds of primary load application, the total divisions passed by the dial gauge pointer were counted and recorded. Ignore the digits on the traditional dial gauge in favor of counting the actual scale divisions that represented indentation. Consequently, it was wise to adopt the black scale.

Difference Between Procedure A and Procedure B:

The difference between Procedure A and Procedure B are as follows:

Test Specimen

ASTM D 785 A specimen used in a standard test procedure must be at least 6 mm [1/4 in.] thick. The specimen could be a piece cut from a sheet or molding. Ensure that the test specimen has parallel, flat surfaces to achieve proper seating on the anvil and prevent deflection that may be brought on by inadequate contact.

Data

Calculate the standard deviation (estimated) using the ASTM D785 Standard Test Method.

                  s = X2-nX2n-2

Where:

The total number of observations in the set is n, each observation’s value is X, the set’s arithmetic mean is X, and the set’s overall value is X.

 S stands for the estimated standard deviation.

Result

The ASTM D785 test results provide a Rockwell hardness value representing its indentation hardness. This value helps in material selection and quality decisions as it indicates the plastic material’s resistance to deformation.

Conclusion

The ASTM D785 Rockwell hardness test is a reliable means of determining the indentation hardness of plastics and electrical insulating materials. Procedures A and B can measure material response to specific loads, which can help assess suitability for applications where resistance to deformation is essential. Test results also mean industries can ensure product quality and performance through standardized material properties affecting durability and usability in final products.

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