There are many many sharp corners in every plastic product, what does a sharp corner mean? Sharp corners greatly increase the stress concentration during injection molding.it could lead to failure of plastic parts, especially for the high amount of stress concentration. So generally, as a mold or plastic produce designer, when he or she meet that kind of sharp corner, they need to be radiused, The stress concentration factor varies with radius, for a given thickness. Do you know how to evaluate it？we will discuss it in the next time.plastic part

The sharp corners should be avoided in the plastic parts, as it can impede from plastic melt flow, causing flaws on parts. Meanwhile, stress concentration can be easily generated in the sharp corners and further reduces the strength of parts. Therefore, adding radii in sharp corners is suggested. There are 3 design tips about adding radii.

1. The outside sharp corners

The design of outside sharp corners is as the figure.

Of course, not all outside sharp corners should have radii. For example, the rounded corners in the parting line could complicate the mold configuration, increase the mold cost, make the breakage, and impact the appearance. The sharp corner in parting line of the parts is suggested.

2. The sharp corners in the flow direction of plastic melt

The arrow direction in the figure is the flow direction of the plastic melt. In the original design, the sharp corner can easily cause trapped air during the injection molding, the local high temperature may degrade the parts, causing appearance defects. Adding radii is suggested to ensure the flow of plastic melt.

3. The sharp corners in the joints of wall

Stress concentration are likely generated in the sharp corners of parts. And sharp corners are easily appeared in the joints between the main wall and side wall of the parts, the joints between wall and ribs, and the joints between wall and pillars.
The relationship between the sharp corners and the stress concentration factor is shown in the below figure (T=Wall Thickness, R=Radii, p=the load borne by the part)

The inside radii in joints of part section should be 0.5T, and the outside radii should be 1.5T. This is to ensure the uniform wall thickness and reduce the stress concentration in part joints, as shown in the figure. Of course, the radii should not be too big, otherwise it may cause the thick wall, and generate shrinkage.

Sharp corners greatly increase the stress concentration.This high amount of stress concentration can often lead to failure of plastic parts.Sharp corners can come about in non-obvious places. Examples of this are a boss attached to a surface, or a strengthening rib. These corners need to be radiused just like all other corners. The stress concentration factor varies with radius, for a given thickness.

The stress concentration factor is quite high for R/T values lesss than 0.5. For values of R/T over 0.5 the stress concentration factor gets lower.The stress concentration factor is a multiplier factor, it increases the stress.

Actual Stress = Stress Concentration Factor K x Stress CalculatedThis is why it is recommended that inside radiuses be a minimum of 1 x thickness.
In addition to reducing stresses, fillet radiuses provide streamlined flow paths for the molten plastic resulting in easier fills.

Typically, at corners, the inside radius is 0.5 x material thickness and the outside radius is 1.5 x material thickness. A bigger radius should be used if part design will allow it.

Sharp corners greatly increase stress concentration, which, when high enough, can lead to part failure. Sharp corners often come about in non-obvious places, such as a boss attached to a surface, or a strengthening rib. The radius of sharp corners needs to be watched closely because the stress concentration factor varies with radius for a given thickness. As illustrated in the chart to the left, the stress concentration factor is high for R/T values less than 0.5, but for R/T values over 0.5 the concentration lowers. The stress concentration factor is a multiplier that greatly increases stress. It is recommended that an inside radius be a minimum of one times the thickness.

In addition to reducing stresses, the fillet radius provides a streamlined flow path for the molten plastic, resulting in an easier fill of the mold.

At corners, the suggested inside radius is 0.5 times the material thickness and the outside radius is 1.5 times the material thickness. A bigger radius should be used if part design allows.