From Limtech
The purpose of the section is to list some of the factors affecting tolerance. In general, the degree of reproducibility of dimensions depends upon the type of tooling and rubber used and the state of the art.
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Discussion of factors affecting tolerances
There are many factors involved in the manufacturing of molded 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 the molded part, both measurements being made at room temperature. All rubber material exhibit some about 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 material specification, cure, time, temperature, pressure, inserts and post cure. The mold designer and compounder must determine the amount of shrinkage for the selected compound 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 tin 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. 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 products 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. Register is usually controlled by dowel pins and bushings or by sel-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 mold 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 a part of the finished product. Often this is possible without affecting important dimensions, but in other instances, some material is removed form the part itself. Where thin lips or projections occur at a mold parting line, mechanical trimming may actually control the finished dimension.
Inserts
Most insert material (mental, plastic, fabric, etc.) have their own standard tolerances. When designing inserts for molding to rubber, other factors must be considered, 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 be of service in advising on design features.
Distortion
Because rubber is a flexible material, its shape can be affected by temperature. Distortion can occur when the part is removed from the mold 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 and must be heated to above room temperature to overcome this condition.
Environmental Storage Conditions
Temperature: Rubber, like other material, changes in dimension with changes in temperature. 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 time required to stabilize the part at that temperature.
Humidity
Some rubber material absorb moisture. Hence the dimensions are affected by the amount of moisture in the product. For those products which have this property, additional tolerance must be provided in the dimensions. The effects may be minimized by stabilizing the product in an area of controlled humidity and temperature for a period not less than 24 hours.
