Standards for Tolerances

 

Introduction

The purpose of this section is to list some of the factors affecting tolerances. In general, the degree of reproducibility of dimensions depends upon the type of tooling and rubber used, and the state of the art.

 

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 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 for the selected compound and incorporate this allowance into the mild 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. With any type of mold, the mold builder must have some tolerance, and therefore, each cavity will have some variance form 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 mild that form the mold cavity. Register is usually controlled by 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 a part of the finished product. Often this is possible without affecting important dimensions, but in other instances, some material if removed form the part itself. Where thin lips or projections occur at a mold parting line, mechanical trimming may control the finished dimension.

 

Inserts

Most insert material (metal, 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 hole 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 is 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. Some rubber will crystallize (stiffen) when stored at low temperature and must be heated to above room temperature to overcome this condition.

 

Environmental Storage Conditions

Temperature: Rubber, like other materials, 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 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 this property, additional tolerance must be provided in the dimensions. The effect may be minimized by stabilizing the product in an area for 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 discussing dimensions of molded rubber products, and for distinguishing various tolerance groupings:

Fixed Dimension: Dimensions not affected by flash thickness variation. (Mold Closure) See figure #1.

Closure Dimension: Dimensions affected by flash thickness variation. (Mold Closure) See Figure #1.

In addition to the shrinkage, mold maker's tolerance, trim and finish, a number of other factors affected closure dimensions. Among these are flow characteristics of the raw stock, weight, shape of perform and molding process.

 

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 have a built- in flash, which will 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 need drawing dimensions.

 

There is usually a logical place for the mold designer to locate the parting line for best dimensional control and part removal. 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.

 

Registration Dimension: Dimensions affected by 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 self- registering cavities.

 

 

TOLERANCE TABLES

The tables of page 5 are presented as a guide in selecting tolerances.

When applying tolerances the following rules should be kept in mind.

(1) Fixed dimensions tolerances apply individually to each fixed dimension by its own size.

(2) Closure dimension tolerances are determined by the largest closure dimension and this single tolerance is used for all other closure dimensions.

(3) Fixed and closure dimensions for a given table do not necessarily go together, and can be split between tables.

(4) Tolerances not shown should be determined in consultation with the rubber manufacturer.

(5) Care should be taken in applying standard tolerances to products having wide sectional variations.

Click on the buttons below to open any of the four tolerance tables in a separate window.