Standards for Tolerances

This purpose of this section is to list some of the factors affecting tolerances and to indicate the magnitude of practical tolerances on the dimensions of molded solid rubber products. In general, the degree of reproducibility of dimensions depends upon the type of tooling used, the properties of the material, and the state of the art.

Note: Dimensional tolerances in a molded article are the allowable variations, plus and minus, from a nominal or mean dimension.

Discussion of Factors Affecting Tolerances

There are many factors involved in the manufacturing of molded solid rubber products which affect tolerances. Since these may be peculiar to the rubber industry, they are listed here.

Shrinkage

Shrinkage is defined as the difference between corresponding linear dimensions of the mold and of the molded part, both measurements being made at room temperature. All rubber materials exhibit some amount of shrinkage after molding when the part cools. However, shrinkage of the compound is also a variable in itself, and is affected by such things as rubber batch variance, cure time, temperature, pressure, post cure, and inserts, if any. The mold designer and the compounder must estimate the amount of shrinkage and incorporate this allowance into the mold cavity size. Even though the mold is built to anticipate shrinkage, there remains an inherent variability which must be covered by adequate dimensional tolerance. Shrinkage of rubber is a volume effect. Complex shapes in the molded product or the presence of inserts may restrict the lineal shrinkage in one direction and increase it in another. The skill of the rubber manufacturer is always aimed at minimizing these variables, but they cannot be eliminated entirely.

Mold Design

Molds can be designed and built to varying degrees of precision, but not at the same cost. As with any manufacturing process, the mold designer attempts to get the highest amount of precision and mold life per dollar of mold cost. With any type of mold, the mold builder must have some tolerance, and therefore, each cavity will have some variance from the others. Dimensional tolerances on the product must include allowances for this fact. The accuracy of the mold register must also be considered. This is the matching of the various plates of the mold that form the mold cavity. Most molded solid rubber products are made in two plate molds. More complicated molds require three or more plates. In multiple cavity molds especially, there will be some variation between cavities and plates causing a mismatch or out-of-register which manifests itself in the part. Register is usually controlled by sturdy dowel pins and bushings or by self registering cavities. For molds requiring high precision in dimensions and register, the design work and machining must be more precise and the cost of the molds will be greater than one with commercial requirements.

Trim and Finish

The objectives of trimming and finishing operations are to remove rubber material - such as flash, which is not part of the finished product. Often this is possible without affecting important dimensions, but in other instances, some material is removed from the part itself. Tumbling, for instance, will wear down the exposed surface of the part. Where thin lips or projections occur at a mold parting line, mechanical trimming may actually control the dimension.

Inserts

Most insert materials: metal, plastic, fabric, etc. have their own standard tolerances. When designing inserts for molding to rubber, other factors must be considered, however, such as fit in the mold cavities, location of the inserts with respect to other dimensions, proper hold spacing to match with mold pins, and the fact that inserts at room temperature must fit into a heated mold. In these matters, the rubber manufacturer can often be of service in advising on design features.

Distortion

Because rubber is a flexible material affected by temperature, distortion can occur when the part is removed from the mold by stretching over a core, or when it is packed for shipment. This distortion makes it difficult to measure the parts properly, some of the distortion can be minimized by storing the part as unstressed as possible for 24 hours at room temperature.

Environmental Storage Conditions

Temperature. Rubber, like other materials, changes in dimension with changes in temperature. It has a coefficient of expansion, which varies with different formulations. Compared to other materials the coefficient of expansion of rubber is high. To have agreement in the measurement of products that are critical or precise in dimension, it is necessary to specify a temperature at which the parts are to be measured and the time required to stabilize the part at that temperature.

Humidity. Some rubber materials absorb moisture. Hence the dimensions are affected by the amount of moisture in the product. For those products which have the property, additional tolerance must be provided in the dimensions. The effect may be minimized by stabilizing the product in an area of controlled humidity and temperature for a period not less than 24 hours.

Dimension Terminology

The following will provide a common terminology for use in discussing dimensions of molded rubber products, and for distinguishing various tolerance grouping:

Fixed Dimensions. (Dimensions not affected by flash thickness variation.) Definition-Parallel to mold parting line or the parting lines of major mold sections. In the case of a simple wheel, with half the wheel formed in each half of the mold and the flash line around the O.D., the O.D. and the hub diameter are fixed dimensions. Holes formed by solid pins will usually be included in the classification.

Closure Dimensions. (Dimensions affected by flash thickness variation). Definition-Vertical to the mold parting line or to parting lines of major mold sections.

In addition to the shrinkage, mold maker's tolerance, trim and finish, a number of other factors affect closure dimensions. Among these are flow characteristics of the raw stock, weight and shape of stock, types of flash grooves or other relief devices. These conditions all affect the degree of mold closure.

While closure dimensions are affected by flash thickness variation, they are not necessarily related to basic flash thickness. If a manufacturer plans to machine or die trim a product, the mold will be planned with an artificial flash, which would be thicker than if hand deflashing or tumble trim were to be employed. Thus products purchased from two sources could have different basic flash thickness at the parting line and yet meet drawing dimensions.

There is usually a logical place for the mold designer to locate the parting line for best dimensional control. If the product design limits this location, an alternate mold construction will be required, which may affect the tolerance control on the product, and may, in some cases, increase the cost of the mold.