This page is a guide for design engineers, purchasing departments, and quality control departments.
Rubber Development's Engineering Supplement
MOLDED SOLID RUBBER PRODUCTS
This online handbook is offered as a guide to design engineers, purchasing departments and quality control departments of the users of rubber and rubberlike elastomer products.
This handbook is published by Rubber Development, Inc. in conjunction with the molded and extruded rubber manufacturing industries including an expert form the "Rubber Products Handbook"; fourth edition; December, 1984 published by the Rubber Manufacturers Association, 1400 K Street, NW, Washington, D.C. 20005. The purpose of this publication is to provide a uniform method of stating these requirements in a manner readily recognized as a "standard" whereby these various degrees of accuracy and acceptability can be described by general nomenclature.
As you know quality levels and engineering standards both have a direct relationship to the price of a product. For this reason it is important that both are applied with careful consideration in the design phase.
We look forward to working with you to establish the specific level of quality and value that you require and deserve.
MOLDED RUBBER PRODUCTS
PURPOSE AND SCOPE
The purpose of this section is to provide a method for standardizing drawing designations for specific design requirements of molded rubber products. Information set forth on the pages that follow should be helpful to the design engineer in setting up realistic specifications for milled rubber products.
The use of proper symbols by designers in specifying on drawings exactly what is required is a matter of paramount importance. Proper use of these symbols by both product designer and rubber manufacturers will result in a common understanding of the requirements which must be engineered into molded rubber products. To assure a uniform method for use on drawings and in specifications. The symbols on the following pages have been standardized by the Rubber Manufacturers Association for use in the molded rubber field.
Although rubber manufacturers can produce products to high standards of precision, they welcome the opportunity to suggest modifications which would reduce costs. The purchasers of milled rubber products can assist to this end by furnishing the manufacturers with details about the application of their parts.
The scope of this section presents to the user the tolerances and standards the rubber manufacturers are normally able to maintain.
NOTE: Where the term "Rubber" is used in this section, it is intended to include the more common synthetic elastomers as will as natural rubber. Text reference is to material commonly measured with a Shore "A" durometer. The applicability of this information to thermoplastic rubbers has not been determined.
STANDARDS FOR TOLERANCES
FACTORS AFFECTING TOLERANCES
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 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.
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.
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.
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.
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.
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.
Drawing designation "A1" is the tightest tolerance classification and indicates a high precision rubber product. Such products require expensive molds, fewer cavities per mold, costly in-process controls and inspection procedures. It is desirable that the exact method of measurement be agreed upon between rubber manufacturer and customer, as errors in measurement may be large in the relation to the tolerance. Some materials, particularly those requiring post curing, do not lend themselves to drawing designation "A1" tolerances.
Standard Dimensional Tolerance Table - Molded Rubber Products
Drawing Designation "A1" High Precision
Sizes Sizes (In) Fixed Closure Fixed Closure (mm)
0-.40 0.004 0.005 0-10 0.10 0.13
.40-.63 0.005 0.006 10-16 0.13 0.16
.63-1.00 0.006 0.008 16-25 0.16 0.20
1.00-1.60 0.008 0.01 25-40 0.20 0.25
1.60-2.50 0.01 0.013 40-63 0.25 0.32
2.50-4.00 0.013 0.016 63-100 0.32 0.40
4.00-6.30 0.016 0.02 100-160 0.40 0.50
Drawing designation "A2" tolerances indicate a precision product. Molds must be precision machined and kept in good repair. While measurement methods may be simpler than with Drawing Designation "A1", careful inspection will usually be required. Standard Dimensional Tolerance Table - Molded Rubber Products
Drawing Designation "A2" Precision
Sizes Sizes Fixed Closure Fixed Closure (In) (mm)
0-.40 0.006 0.008 0-10 0.16 0.2 .40-10-0.008 0.01 0.2 0.25 .63 16
.63-16-0.01 0.013 0.25 0.32 1.00 25
1.00-25-0.013 0.016 0.32 0.4 1.60 40
1.60-40-0.016 0.02 0.4 0.5 2.50 63
2.50-63-0.02 0.025 0.5 0.63 4.00 100
4.00-100-0.025 0.032 0.63 0.8 6.30 160
Drawing designation "A3" tolerances indicate a "commercial" product and will normally
be used for most products.
Standard Dimensional Tolerance Table - Molded Rubber Products Drawing Designation "A3" Commercial
Sizes Sizes Fixed Closure Fixed Closure (In) (mm)
0-.40 0.008 0.013 0-10 0.2 0.32 .40-10-0.01 0.016 0.25 0.4 .63 16
.63-16-0.013 0.02 0.32 0.5 1.00 25
1.00-25-0.016 0.025 0.4 0.63 1.60 40
1.60-40-0.02 0.032 0.5 0.8 2.50 63
2.50-63-0.025 0.04 0.63 1 4.00 100
4.00-100-0.032 0.05 0.8 1.25 6.30 160
Drawing designation "A4" tolerances apply to products where dimensional control is non-critical and secondary to cost.
Standard Dimensional Tolerance Table - Molded Rubber Products
Drawing Designation "A4" Non Critical
Sizes Sizes Fixed Closure Fixed Closure (In) (mm)
0-.40 0.013 0.032 0-10 0.32 0.8
.40-10-0.016 0.036 0.4 0.9 .63 16
.63-16-0.02 0.04 0.5 1 1.00 25
1.00-25-0.025 0.045 0.63 1.12 1.60 40
1.60-40-0.032 0.05 0.8 1.25 2.50 63
2.50-63-0.04 0.056 1 1.4 4.00 100
4.00-100-0.05 0.063 1.25 1.6 6.30 160
Measurement of Dimensions
Conditioning of Parts: Measurement of dimensions shall be made on parts conditioned at least 24 hours after the molding operation. Measurements shall be completed within 60 days after shipment or before the part or put into use, whichever is the shorter time. Care shall be taken to insure that the parts are not subjected to adverse storage conditions.
In the case of referee measurement, particularly on Drawing Designation "A1" tolerances or for materials known to be sensitive to variations in temperature or relative humidity, the parts in question should be conditioned for a minimum of 24 ours at
Methods of Measurement: Depending upon the characteristics of the dimension to be
measurements may be used.
A. A dial micrometer with a plunger size and loading as agreed upon by the customer and
the rubber manufacturer.
B. A suitable optical measuring device.
C. Fixed gauges appropriate to the dimensions being measured.
Under no circumstances should the part be distorted during measurement. On dimensions which are difficult to measure or which have unusually close tolerances, the exact method of measurement should be agreed upon in advance by the rubber manufacturer and the customer.
General Information: Relative dimensions such as concentricity, squareness, flatness, parallelism, or location of one or more inserts in the product are dimensions described in relation to some other dimension. Since it is impossible to foresee the many potential designs of all
molded products in which relative dimensions will be specified, it is impractical to assign standard drawing tolerance designations to these dimensions. The design engineer should recognize that the more precise the requirement, the more expensive the product. He must allow the rubber manufacturer to use support pins, lugs, chaplet pins, or ledges in the mold to provide positive location and registration of the insert or inserts in the mold cavity. With this in mind, it is suggested that the design engineer discuss relative dimensional tolerances on all products directly with the rubber manufacturer.
STANDARDS FOR FINISH AND APPEARANCE
The purpose of this section is to list and discuss some of the factors that have an effect on the finish and appearance of molded products and to present standards covering four classes of finish.
FACTORS AFFECTING FINISH AND
Machined Finish of Mold
The machined finish of the mold has considerable effect on the surface finish or appearance of a rubber product.
The best finish can be obtained from a highly polished steel mold, free from all tool marks or other imperfections. Naturally, this type of mold is quite expensive to construct and maintain and is not generally required unless surface finish is of paramount importance from either an appearance or functional standpoint. In addition, it may be desirable in some cases to chrome plate the mold in order to maintain the required surface finish under production conditions.
The commercial type mold is a machined steel mold made to conform to good machine shop practice. Machine tool marks will not ordinarily be polished out of this type of mold. It should be noted that regardless of how highly the mold itself is polished, the appearance of the rubber surface will depend to a large extent upon the factors outlined in the following paragraphs.
Type of Rubber Material Used
The type of rubber material used can greatly affect the appearance of the rubber product. Some compounds lend themselves to a bright glossy surface while others may be dull as molded or become dulled very easily during handling or storage. Also, there are some rubber compounds to which antiozonants are added to impede attack from ozone. As these compounds age, the antiozonants "bleed out," giving the product a colored or waxy surface. This is a common practice and the product should not be considered imperfect or defective in any way. This or other specification requirements may make it impossible to produce a product with a glossy surface.
Mold Release Used
There are certain compounds that can be removed from the mold with the use of little or no mold release lubricant, while others require the use of considerable quantity of mold release lubricant. The latter may have the appearance of being oily.
If the surface of the rubber product is to be bonded to other materials in its application or is to be painted, the designer should designate this on the drawing so that the manufacturer may use a mold release lubricant that will not impair adhesion quality
Flash Removal Method
Some of the many methods used to remove flash from rubber parts may affect the appearance of the finished product. As an example, hand trimming will ordinarily have no effect, while tumbling may result in a dull surface.
Method of Designation of Finish
The symbol "F" followed with an appropriate number selected from table 5 shall be used to designate the type of finish required.
An arc enclosing the actual are included by this designation and a leader to the finish number designates the type of finish desired. The use of a finish symbol on the surface does not preclude the possibility that other surfaces may require different finishes. However, the use of a standard notation is desirable wherever possible to eliminate the repetition of finish symbols and maintain simplicity. Always permit "Commercial Finish" (F-3) whenever possible.
A smooth, polished and uniform finish completely free of tool marks, dents, nicks and F1 scratches, as produced from a highly polished steel mold. Finish of 10 microns.
F2 A uniform finish as produced from a polished steel mold. Finish of 32 microns.
Surfaces of the mold will conform to good machine shop practice. No micron finish is F3 specified. A "Commercial Finish"
F4 Satin Finish
STANDARDS FOR FLASH
It is the purpose of this section to list and discuss many of the factors that have an effect on the amount of flash, to describe the basic methods by which flash can be removed, and furnish the means by which the designer can designate on the product drawing the flash location and flash variation permissible.
Flash is excess rubber on a molded product. It results from cavity overflow and is common to most molding operations. Flash has two dimensions - Extension and Thickness. (B) Flash Extension
Flash extension is the film of rubber projecting from the part along the parting line of the mold. (C) Flash Thickness
Flash thickness is measured perpendicular to the mold parting line. Variations in flash thickness are normally included in closure tolerances.
A method for designation permissible flash extension and thickness on a molded product will result in better understanding between rubber manufacturer and consumer and benefit both. This method must permit the designation of a surface where no parting line is permissible. It must also designate areas where a parting line is permissible and define the amount of flash
extension tolerable in such areas. The designer, without specific rubber processing knowledge, should be able to specify flash extension limits in any given area on his drawing. Use of RMA Drawing Designation provided in this section will provide this capability, however, the designer should not specify the amount of flash extension which can be tolerated without impairing product function or appearance. A method designating areas permitting flash and describing flash extension tolerance will result in the following benefits:
(A) Avoid errors in mold design concerning parting line location.
(B) Uniformity in appearance and function of molded products supplied by more that one source. (C) Simplification of inspection procedures.
(D) Reduce over-finishing or under-finishing products.
Molding techniques have been developed to produce "flashless" products. The mold parting line, depending on location on the product, is barely discernible with no measurable thickness or extension. Initial cost and maintenance of this tooling and equipment is high and very close manufacturing control is required.
In instances where flash extension is not a problem or where it is otherwise advantageous, parts are shipped as molded with no flash removal necessary.
Methods for removing flash from products with metal or other inserts are approximately the same as the methods for non-inserted rubber products. Rubber flash adhering tightly to inserts is generally acceptable. If it must be removed, it is done by mechanical means such as wire brushing, abrasive belts or spot facing. If adhered rubber flash is not permissible, is should be so specified on the drawing.
Flash removal is an important cost factor in producing finished molded rubber products. Cost conscious designers will permit the widest possible latitude in flash thickness, flash extension, and in location of flash consistent with adequate function and appearance of the product.
FACTORS TO BE CONSIDERED ON SETTING STANDARDS ON
Parting lines (flash lines) must be located to facilitate part removal from the mold cavity after curing.
Flash thickness is determined in the molding operation and may vary with mold design, closing pressure, with weight and shape of preform, and type of compound used - and many lesser factors. Normal variations in flash thickness have been taken into account in the tables set up for closer tolerance, and will receive no further consideration.
There are many methods by which flash extension on rubber products can be removed. The particular method selected will be determined by the degree of flash extension permitted as well as by the flash location, flash thickness, and other factors. Some of the more common methods of flash removal are as follows:
A moving abrasive surface material is applied to the rubber part to remove excess rubber
by abrasive action.
; Die Trim
A cutting tool, shaped to the contour of the molded product at the parting line, is applied
with a force perpendicular to the flash extension and against either a flat plate or a fitted
shape. This creates a shearing or pinching action removing the excess flash. Die trim can
be done with a hand or machine mounted die. Machine mounted dies are often used for
multiple trimming of small uniformly shaped products from multi-cavity molds.
; Machine Trim
Flash is removed by passing the rubber part through machine mounted rotating or
reciprocating cutting tools. These devices are customarily adapted to a particular product
and may shear, saw, or skive the flash away.
; Tumble Trim
There are two basic types of tumble trimming. Both utilize a rotating barrel or drum in
which the heavier rubber sections strike the thinner and more fragile flash breaking it
free. Dry tumbling at room temperature is most effective with the higher durometer
"hard" compounds. The other type of tumbling utilizes carbon dioxide or other refrigerant
to freeze the molded parts, thus making the compound more brittle so the flash will
break more readily. Any tumbling operation will have an effect on surface finish.
; Mechanical Deflashing
Modern deflashing machines utilize an abrasive medium, tumbling, and a refrigerant for
quick freezing. The time and temperature is closely controlled while the parts are
agitated in an enclosed barrel. Refrigerant (usually carbon dioxide or nitrogen) is
metered into the deflashing chamber while the parts are being impinged with a
mechanically agitated abrasive medium. The flash, being thin, freezes first and is broken
away by the abrasive medium and the tumbling action before the heavier rubber part
itself has lost its resiliency. Some loss of surface finish may be expected and some of the
abrasive medium may adhere to the molded parts.
; Pull Trim or Tear Trim
A very thin flash extension is molded immediately adjacent to the part and a thicker flash
is molded adjacent to the thin flash but farther from the part. When the flash is pulled
from the molded part, it separates at its thinnest point adjacent to the molded part. This
Method of Designation of Flash
The symbol "T" with a notation in thousandths of an inch for the maximum extension shall be used. Example: T .032 (.032 in. maximum extension permitted.) IF METRIC DIMENSION THE DRAWING DESIGNATION WILL BE FOLLOWED BY mm INDICATING mm OF FLASH PERMITTED. Example T .80mm (.80mm maximum extension permitted.)
The flash thickness may be specified following the extension limit if it is critical to the function of the part. Closure tolerances will apply as in tables 1, 2, 3, and 4.
An arc enclosing the actual area included by this designation and a leader to the trim symbol, designates the maximum allowable flash extension and thickness thus enclosed. If no flash can be tolerated in a given area, the symbol "T" .000 is used.
The designer may indicate on his drawing any amount of maximum flash extension permissible. However, as a matter of simplicity, a progression of flash extension Drawing Designations is suggested in Table 6. Only those areas requiring such a designation should be specified. The use of a standard note can frequently be used with no further notation.