Brittleness Of Workpieces

Workpiece brittleness is caused most often by a loss of molecular weight in the polymer material. A reduction in molecular weight results in a reduction of the mechanical properties of the material, such as tensile strengh, elongation, and impact strength. The lists causes and solutions for brittleness of Workpieces.

Cause:

1. Brittleness from material degradation can be caused if materials are not dried sufficiently. This applies specifically to hygroscopic materials. In this

case,a chemical reaction called hydrolysis takes place when materials are melted with moisture present. When hydrolysis occurs, a loss in molecular weight results in a reduction in physical properties.

2. Excessive use of regrind can cause part brittleness since regrind has dy been exposed to heat and as a result suffers a loss in molecular weight. 

3. Melt temperatures may be higher than recommended process temperatures. In this case, melt temperatures at the middle of the recommended melt temperature range should be used

4. The molecular weight reduction is a result of material sitting in the barrel for long periods of time, referred to as residence time

5. The bSR should fall between 30 and 65%. If this falls well below 30% this will indicate a residence time problem that can lead to brittleness

6. High screw speeds and back pressure can"overwork"the material

resulting in an increase in melt temperature

7. Workpiece design can play a major role in object brittleness behavior

Solutions:

1. The barrel-to-shot ratio(BSR) is a measurement, in terms of percentage, used in determining residence time

2. A reduction in nozzle temperature will reduce melt temperature avoiding brittleness from occurring

3. Reducing screw speed and lowering back pressure can prevent resinmelt temperature

4. Increasing wall thickness, rib designs, avoidance of sharp corners, and addition of radii can reduce the chance of brittle parts

The gate location affects the orientation of the polymer molecules. Polymer materials exhibit two types of orientation behavior: isotropic and anisotropic. In isotropic behavior, mechanical strength varies little in the machine and cross-machine direction. This condition provides flexibility in gate location In anisotropic behavior, mechanical strength is higher in the machine direction than in the cross-machine direction. In this case, gate location may become more critical.