When the ideas of correct and uniform wall thickness are put into practice the result is a plastics part composed of relatively thin
surfaces. The way in which these surfaces are joined is equally vital to the quality of a moulded part.
Walls usually meet at right angles, at the corners of a box for example. Where the box walls meet the base, the angle will generally be slightly more than 90 degrees because of a draft angle on the walls. The easiest way, and the worst, to join the walls is to bring them together with sharp corners inside and out. This causes two problems.
The first difficulty is that the increase in thickness at the corner breaks the rule of uniform wall thickness. The maximum thickness at a sharp corner is about 1.4
times the nominal wall thickness. The result is a longer cooling time accompanied by a risk of sink marks and warping due to differential shrinkage.
The other problem is even more serious.
Sharp corners concentrate stress and greatly increase the risk of the part failing in service. This is true for all materials and especially so for plastics. Plastics are said to be notch-sensitive because of their marked tendency to break at sharp corners. This happens because the stress concentration at the corner is sufficient to initiate a microscopic crack which spreads right through the wall to cause total failure of the part. Sharp internal corners and notches are the single most common cause of mechanical failure in moulded parts.
The answer is to radius the internal corner, but what size should the radius be? Most walls approximate to a classical cantilever structure so it is possible to calculate stress concentration factors for a range of wall thicknesses and radii. The resulting graph shows that the stress concentration increases very sharply when the ratio of radius to wall thickness falls below 0.4. So the internal radius (r) should be at least half the wall thickness (t) and preferably be in the range 0.6 to 0.75 times wall thickness.
If the inner corner is radiussed and the outer corner left sharp, there is still a thick point at the corner. For an internal radius of 0.6t, the maximum thickness increases to about 1.7 times the wall thickness. We can put this right by adding a radius to the outside corner as well. The outside radius should be equal to the inside radius plus the wall thickness. This results in a constant wall thickness
DESIGNER’S NOTEBOOK
Avoid sharp internal corners.
Internal radii should be at least 0.5 and preferably 0.6 to 0.75 times the wall thickness.
Keep corner wall thickness as close as possible to the nominal wall thickness. Ideally, external radii should be equal to the internal radii plus the wall thickness.
DESIGN GUIDES for PLASTICS
Clive Maier, Econology Ltd
pdg
Plastics Design Group - Plastics Consultancy Network
British Plastics Federation
