Joint Clearance and Fit Tolerance: How Much to Leave

Shear joint interference, the clearance either side of an energy director, and the fit at the locating faces are three dimensions often filled in with a habitual value. They decide more than they appear to.

Interference: both directions cost something

Too little and side-wall friction is insufficient, so melting fails to start or the fused band is discontinuous, and sealed parts leak first.

Too much and assembly becomes difficult, the side walls are held splayed as a permanent tensile stress, brittle resins crack outright, and melt is squeezed beyond the mating face so that less of it actually fuses.

The right value depends on the modulus and shrinkage of the resin and on the size of the part; there is no universal number. The handbook gives bands by part size as a starting point, but the real constraint is usually the next one.

Can the tool hold it

Shear joint interference is typically a few tenths of a millimetre, which places a demand on tool precision and moulding stability. In a multi-cavity tool, if cavity-to-cavity variation is the same order as the interference, the drawing value does not exist in production.

So before fixing the interference, ask what the cavity-to-cavity spread and batch shrinkage variation actually are. If the answer is "unknown", measure first and decide after. Drawing the ideal value and then rescuing it at the welding station is the most expensive route.

Clearance: give the melt somewhere to go

Space has to be left beside an energy director for the melt. Melt is conserved: the volume of the director determines how much is produced, and it must go somewhere. With nowhere to go it escapes as flash or forces the mating faces apart.

Size the flash trap from the director volume rather than drawing a groove by eye. Its position determines which way flash travels, and on cosmetic parts it should run inward.

Locating faces: locate first, then clamp

Many designs put location and sealing on the same face. The locating face then contacts first during assembly and holds the part, while the same face is expected to fuse during welding, and the two functions fight each other.

The stable approach is to separate them: give location its own features — a lead-in taper, pins, a step — to bring the halves into alignment, and let the weld face do nothing but fuse. Locating features fit loosely enough to absorb assembly variation; the weld face carries the interference fusion needs.

Three traps

The wall thickness changed and the joint did not. Director height follows wall thickness; the wall goes from 2mm to 3mm, the director stays, and weld quality degrades for reasons nobody can trace.

Corners copy the straight sections. Melt flows poorly into sharp corners and leaves discontinuities. Increase the radii and adjust the interference locally where necessary.

Large parts take values from small ones. The larger the part, the greater the absolute moulding deviation, so the joint needs more tolerance absorption. A shear joint is insensitive to height variation and suits large parts better than a butt-style joint.

The complete dimensional system for eight joint types, interference bands by part size, flash trap dimensions and a design-review checklist are in the Energy Director & Joint Design Guide on the downloads page.

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