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When designing endless splices for extruded profiles, several
factors must be considered: compound durometer, cut length,
size of cross section, and (in the case of tubing) wall thickness.
Mold cavities are normally designed to the nominal dimension. If
the extrusion cross section is at its upper tolerance limit, mold
pressure marks will be visible on the surface. This phenomenon
is accentuated with the use of lower durometer compounds. If
the extrusion cross section is at its lower tolerance limit, the mold
cavity would have to be shimmed in order to attain the splicing
pressure necessary to create surface marking. The potential for
size differences at each end of a cut length increases with the
length of the cut. (One end may be at the upper tolerance limit,
and the other end may be at the lower tolerance limit.) Generally,
this gives the appearance of a step or mismatch. The illustration
below illustrates the phenomena of mold pressure marks and
apparent mismatches.
Standards for Spliced Extrusions
The manufacture of extrusions in circular or rectangular shaped
gaskets, or a combination of both can be accomplished by
means of butt or beveled vulcanized splices. The splice is
usually never as strong as the original material from which the
gasket is made. The stronger the splice is required to be, the
more difficult the labor operations. A pressure mark will appear
at the splice area due to the required holding pressure in the
mold.
The method of testing splices should be given serious
consideration. Doubling over, pinching, twisting, or bending
back a splice are not proper methods of testing. Splice strength
varies with the configuration of the cross section of the extrusion.
For this reason, splice strength is very difficult to define.
Pulling perpendicular to the plane of the splice is a sufficient test
in the testing of a splice. The gasket should be clamped in such a
way that the pull is evenly distributed over the splice.
Tolerances must be allowed in the length between splices.
These tolerances must be varied according to length between
splices and due to the method of making the splice. Tolerances
for commercial class splices are listed in the following table.
These tolerances are applicable for both conventional splices
and injection splices.
Spliced LengthTolerances
RMA Drawing Designation Class 2 (Commercial)
Inches Millimeters
Cross Section Tolerance Cross Section Tolerance
>0 - = 10 ± .250 >0 - = 250 ± 6.30
>10 - = 15 ± .280 >250 - = 400 ± 7.10
>15 - = 25 ± .315 >400 - = 630 ± 8.00
>25 - = 40 ± .355 >630 - = 1000 ± 9.00
>40 - = 63 ± .400 >1000 - = 1600 ± 10.00
>63 - = 100 ± .450 >1600 - = 2500 ± 11.20
>100 ± .500 >2500 ± 12.50
Design of Extruded Endless Splices
Testing Procedure