Factors that affect tolerances in moulded rubber parts

Moulded rubber part tolerances can be affected in various ways during manufacturing.

Design of a Mould

Moulded rubber tooling can be designed with varying degrees of precision, and more precise moulds are more expensive than less precise moulds. In addition, all moulds require some tolerances, and some cavities will differ. As a result, dimensional tolerances on the moulded rubber part should account for these minor variations. You also need to consider the accuracy of the mould register or the various mould plates that come together to form the mould cavity.

Most moulded rubber parts are made with two plate moulds, while more complicated rubber parts are made with three or more plate moulds. In multi-cavity moulds, some variation between plates and cavities is to be expected, which may result in mismatching or out-of-register manifestations in the part. These registering issues are typically handled and controlled by strong dowel pins and bushings or by self-registering cavities.

Inserts

Insert materials like metals, fabrics, and plastics frequently have their own tolerance standards. However, other factors that should be considered when designing moulded rubber parts with inserts include:

• How the inserts fit into the mould cavities.
• Proper hold spacing necessary to align the inserts with the mould pins.
• Insert placement in relation to other dimensions.
• Ensuring inserts at room temperature are fitted into a heated mould.

Finishing

Trimming and finishing operations remove excess rubber material or flash from the finished product. This process is frequently completed without affecting critical dimensions. However, the material can be removed from critical areas of the part itself in some cases during the tumbling process, as the process can wear down the finished and exposed surface of the moulded rubber parts. Dimensions in rubber parts with thin lips or projections in the mould parting line may be controlled by mechanical trimming.

Distortion

Because rubber is a flexible material affected by temperature, distortion can occur when the rubber part is removed from the mould by stretching it over a core. When the rubber part is packed for shipment, it may also experience minor distortion that can cause problems with rubber part measurements. This can be avoided by storing the rubber part in an unstressed state for 24 hours at room temperature after it has been moulded.

Shrinkage

At room temperature, shrinkage is the difference between two corresponding linear dimensions of the rubber mould and the moulded rubber part. After a rubber part is moulded and then exposed to cooler temperatures, all rubber materials will shrink to some extent. However, individual compound shrinkage rates are determined by the rubber compound itself. In addition, they are influenced by variables such as cure time, rubber batch variance, temperature, post-cure, pressure, and the presence of inserts.

Environmental storage conditions

Humidity — Certain rubber materials absorb moisture, and depending on the material used in the moulded rubber, some parts’ dimensions will change depending on the amount of moisture present in the rubber product. The effect of humidity on a rubber product can be minimised by stabilising the product in an area with controlled humidity for at least 24 hours.

Temperature — Temperature changes frequently cause changes in the dimensions of rubber materials. Depending on the environment’s temperature, different compounds will have different expansion coefficients. Therefore, the temperatures at which the rubber parts will be used, and the time required to stabilise the part at said temperature, should be measured in rubber products that need critical or precise dimensions.

Dimensional Terms

• Fixed dimensions are those dimensions parallel to the mould parting line or the parting lines of major mould sections and are not affected by flash thickness variation.
• Closure dimensions are affected by flash thickness variation and are vertical to the mould parting line or the parting lines of the major mould sections. In addition, the weight and shape of the raw material stock, the flow characteristics of that stock, and the type of flash grooves or other relief devices in the mould will all impact the closure dimensions. Furthermore, closure dimensions are not always related to basic flash thickness, despite being affected by variations in flash thickness. For example, when a moulded rubber part is machine or die trimmed, the mould itself will be designed with a thicker artificial flash than rubber parts designed for hand deflashing or tumbling. As a result, while the same rubber part from two different sources may meet drawing dimensions, the basic flash thickness at the parting line may differ depending on the moulding process and deflashing method.

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