Flexural Strength

The Flexural Strength of injection molding materials

The Flexural Strength of injection molding materials

What is Flexural Strength?

Flexural Strength, also known as Bending Strength, refers to the maximum stress that an object can withstand before Bending and breaking, the ability of the material to resist bending without breaking. The main purpose is to determine the bending resistance of plastics. The unit can be Kg/cm2 or MPa. Generally, the Flexural Strength and bending modulus of thermosetting plastics are higher than that of thermoplastic plastics. 

The test element of Flexural Strength

1. Mechanical properties: 

Various mechanical properties of a material when subjected to external forces.

2. Strain: 

Deformation is caused when external forces act upon a material. The following formula expresses the strain size: strain (ε) = change the length (△L)/ initial length (L). 

3. Tensile stress

The internal stress of a material when it is stretched.

4. compressive stress:

The internal stress of a material when it is compressed. 

5. Shearstress: 

The internal stress of sliding parallel parts of material is subjected to shearing force. 

6. Compressive Strength: 

The compression load is applied on the specimen to the value of load per unit area when the specimen is destroyed. 

Test method for bending Strength

The test method for bending Strength is usually ASTM D790 or ISO 178. ASTM generally uses 3.2mm(thick) x 12.7mm(wide) x 125mm(long) (0.125″ x 0.5″ x 5.0″) plastic strip samples. In contrast, ISO uses 4mm x 10mm x 80mm strip samples. During the test, the sample is placed on two fixed anvils with a certain distance and then pressed at the center of the two fulcra with a point (three-point test method) from top to bottom at a speed of 15 mm/min until the sample breaks or exceeds a certain limit.

Two points at a fixed distance can also be applied to the mean center of the two fulcrums, known as the four-point test. But generally, adopt the three-point test method. 

Please note: the picture is only schematic; according to the regulations, the point of contact with the plastic sample should be made radius to avoid stress concentration. 

It is usually required to calculate the bending stress of the test sample under the bending strain of 5% without breaking. 

Flexural Modulus

Within the elastic limit, the proportion of bending deformation caused by the compression of the object under stress is called bending modulus. 

At the beginning of the test, the Stress and Strain of plastic samples are approximately linearly proportional, and the bending modulus is taken as a reference.

The following are flexural strength and flexural modulus of commonly plastics used in injection molding

Materials Type Flexural Strength Flexural Modulus
ABS 75 MPa 2.5 GPa
ABS + 30% Glass Fiber 120 MPa 7 GPa
Acetal Copolymer 85 MPa 2.5 GPa
Acetal Copolymer + 30% Glass Fiber 150 MPa 7.5 GPa
Acyrlic 100 MPa 3 GPa
Nylon 6 85 MPa 2.3 GPa
Polyamide-imide 175 MPa 5 GPa
Polycarbonate 90 MPa 2.3 GPa
PE, HDPE 40 MPa 0.8 GPa
PET 80 MPa 1 GPa
Polyimide 85 MPa 2.5 GPa
Polyimide + Glass Fiber 140 MPa 3 GPa
Polypropylene 40 MPa 1.5 GPa
Polystyrene 70 MPa 2.5 GPa
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