Shrinkage rate Calculation

In Moulding process the rubber is Cured at high temperature and Pressure, the resulting Moulded parts are smaller in size than the actual.

The Difference in the thermal expansion of the mold material result in shrink rate

Shrink Rate= ( Cavity dimension- Part Length)/ Cavity dimension)

(Typical units in/in)

This shrinkage value is applied to the desired finished part size to “scale up” the cavity. The resulting cavity dimension is calculated as:

Cavity Dimension= finished part length/ (1- Shrink rate)

For example:

Shrinkage rate testing was performed on a 70 Shore A Nitrile (Buna-N), Ethylene-Propylene (EP), Fluorosilicone (FVMQ), and 75 Shore A Fluorocarbon (FKM). 

The Fluorosilicone and Fluorocarbon materials undergo a secondary vulcanization process called post-curing.

During the post-curing process, the molded parts were baked in a ventilated oven at a specific time and temperature recommended by the material manufacturer, or at a temperature greater than the service temperature of the final application.

This was done to drive off any residual volatiles (processing additives, curing catalyst) resulting from the initial vulcanization process. These volatiles must be eliminated to mitigate potential outgassing that can be a source of contamination in food, medical, electronic or other sensitive applications.

An important note — post-curing also promotes further cross-linking of the elastomer’s polymer chains, which results in improved physical properties like tensile strength, elongation, compression set resistance, chemical resistance, and, for this focus, additional shrinkage

Shrinkage During the Manufacture of Molded Articles

An article molded from a rubber compound always ends up smaller than the mold in which it was vulcanized. The difference between the dimensions of the finished article and the mold, measured at room temperature and expressed in per cent is called the degree of shrinkage.

Since the dimensions of the molded articles have to be very often within

very close tolerances, it is important to know the degree of shrinkage so

that it can be considered during the design of the mold.

On the one hand the shrinkage of the rubber during cooling can be welcomed. Had the rubber and the mold, the same coefficient of expansion, removal of the part from the mold would be problematic.On the other hand, this phenomenon complicates the manufacture of

articles of correct dimensions.

Basic Principle. 

The degree of shrinkage is primarily determined by the

difference of the coefficient of expansion between the vulcanizate and the

mold material as well as by the vulcanisation temperature. The rubber 

part which fills the mold completely during vulcanisation contracts 

during cooling to room temperature more than the mold because it has

a considerably higher coefficient of expansion than the latter. Thus the

degree of shrinkage increases with increasing difference between the

coefficients of expansion of the rubber compound and the mold material

as well as between vulcanisation and room temperature. The difficulties 

during the calculation of the degree of shrinkage arise mostly from the

fact that the coefficient of expansion of vulcanizates changes with the

compound composition. In spite of this, degrees of shrinkage can be

determined approximately without preparing sample moldings.

Determination of the Degree of Shrinkage. Aside from the influence

of the vulcanisation temperature the elastomer content of the compound

is of importance for the degree of expansion and thus the degree of

shrinkage. The coefficient of expansion is the higher, the higher the

elastomer content of the compound. The coefficients of expansion of the

filler are of the same order of magnitude as those of the mold materials

because of which essentially only the elastomer content of the compound

is considered during the calculation.

Acetone soluble chemicals and additives like accelerators, antioxidants,

plasticizers, resins, waxes and sulfur, also factices and reclaim have,

in contrast, a coefficient of expansion similar to that of the rubber so

that these substances are considered as part of the elastomer during the

calculation of the degree of shrinkage.

The vulcanisation time and the mastication time during the preparation

of the compound are practically without influence on the degree of

shrinkage.