Standards for Cross Sectional Tolerances

During vulcanization the tendency of the extrusion is to sag and flatten. The degree of change in shape is largely dependent upon the hardness or softness of the compound, the tensile strength or quality of the compound, the thickness or thinness of the cross sectional wall, the inner openings of the extrusion and the rate of vulcanization.

The tendency to distort during vulcanization can be minimized by the use of forms which generally add to the cost of manufacturing. This cost can sometimes be eliminated if contour conformity is not necessary for the finished extrusion. The degree of allowable collapse or sag in a cross section should be indicated on the drawing. Illustrations below show considerations when designing an extruded rubber part.


Tube May Collapse

Lip May Collapse
Tube Collapse Lip Collapse

The images below illustrates how one dimension is affected by the others and by shape variation. Dimension C is affected by the shape and tolerances of the other dimensions. The critical dimensions should be noted on the drawing.

Shape VariationsID and OD of Tube


Tolerances for tubing should be established on the I.D. or the O.D., and the wall thickness. To include tolerances on both I.D. and O.D. would conflict with the other tolerances.

Splicing

Seals Unlimited is well equipped to complete hot vulcanized splices as well as cold bonded splices. A hot vulcanized splice will utilize a rubber-based adhesive, and a custom fixture to produce the best quality bond possible. This process is typically more time consuming, however yields a splice that has many of the same properties of the rubber and is less noticeable. While no splice is as strong as the original material, a hot vulcanized splice is the most durable. Where splicing is a function to aide in installation and is not required to have longevity, a more cost effective method of cold bonding with a room temperature adhesive may be sufficient.

Storage

Rubber is a flexible material affected by temperature and humidity. Distortion can occur when the part is packed for shipment. This distortion makes it difficult to measure the parts property. Some distortion can be minimized by storing parts as unstressed as possible for 24 hours at room temperature.

Packaging

Distortion is common in transportation. It should not affect the shape of the part permanently if held for a reasonable period of time. To help prevent a permanent set, special packaging such as crates or custom boxes may be required. Since it is not always practical to ship in straight lengths, it may be necessary to remove the product when it arrives and store in straight lengths. Discuss your requirements with our sales representatives.