Background:

Impact Resistance is the resistance of a plastic to fracture by shock loading. The ability of a plastic film to withstand an impact load depends on its toughness. There are different impact testing techniques that are used to determine the impact resistance of plastic materials. In general, they can be broken into two categories, one being free falling weight and the other being pendulum impact.

Method Falling Dart Pendulum Impact
Standard ASTM D1709 ASTM D3420
Definition The test specimen is clamped in a pneumatic ring at the base of the drop tower. The dart is clamped at the dropping height and then released to drop onto the centre of the test specimen. The drop weight and the test result (pass/fail) are recorded. Data are analysed using the ‘Bruceton Staircase’ method.

From these tests the impact failure mass (g) – the point at which 50% of the test specimens will fail under the impact – is calculated.

The energy necessary to burst and penetrate the centre of a specimen, mounted between two plates with a circular aperture, is measured by the loss in mechanical work-capacity due to the expenditure of kinetic energy by a pendulum, the rounded probe of which passes through the test specimen.
Method Method A and B with different dart diameter and falling height Method A and B with different test sample diameter
Expression of Impact Strength Gram (The energy is expressed in terms of the weight of the missile falling from a specified height which results in 50% failure of specimens tested, subsequently, the impact strength can be calculated as the energy required to cause a break) Joule (the energy required to rupture the film is indicated on a scale)

Target Industries/Applications:

Both methods are used to measure the impact resistance of plastic film (though in different mechanism).

i) Packaging Materials:

It is widely used in plastic industry doing the packaging material as one of the biggest challenges for plastic bag producers is developing advanced technologies that reduce costs and increase impact resistance of the packaging material that may be subject to impacts during these manual movements.

ii) QA/QC on Finished Plastic Product:

Fabricating process can influence the impact strength. The flow behaviours of the material during the filling mold, specimen geometry, impact properties of the unoriented material are important in governing the impact behaviour. Therefore the impact strength of the finished product may need to be tested in the usual QA/QC.

Correlation of both methods:

A study was conducted in which four films made from two resins (polypropylene and linear low-density polyethylene), with two film thicknesses for each resin, were impacted using Test Methods D1709 (Method A), Test Method D3420 (Procedures A and B). Differences in results between Test Methods D1709 and D3420 are observed (see table below).

Film comparison

Yasuda Falling Dart Machine

Yasuda Falling Dart Machine

 

Yasuda Impact Pendulum Machine

Yasuda Impact Pendulum Machine