In October of 2011, the Association for Rubber Products Manufacturers (ARPM) was formed when the non-tire members of the Rubber Manufacturers Association (RMA) split off to create a trade association that more closely aligned with their needs. Upon its creation, the ARPM took over the publishing, sale, and editing rights to many of the former RMA publications. The most widely used of these publications is
The Rubber Handbook. The ARPM rubber handbook provides standardized drawing designations for specific design requirements of molded rubber products. The information is used to set up specifications for molded rubber products. Drawing specifications and accompanied detail are of paramount importance, whereas the ASTM D 2000 call out determines the qualities of the rubber necessary, the ARPM ensures the finished product is feasible, effective and cost effective. Among the most used metrics are dimensional tolerances, finish, and flash extension.
Dimensional Tolerance There are many factors that affect dimensional tolerance, creating a high precision rubber product requires extensive preparation, most rubber products are made to a commercial tolerance, a balance of precision and cost management. 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. Shrinkage is a variable which is affected by such factors as material specification, cure time, temperature, pressure, inserts, and post cure. The mold designer and compounder's task is to define how much shrinkage there is and incorporate this into the design of the mold cavity. Complex shapes or the presence of an insert can restrict shrinkage in one direction yet increase it in the other. Higher grade designs require a more precise cut tooling than commercial requirements. Inserts must be factored into the planning and design of the mold. 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 shipped. To reduce distortion proper storing is important. Environmental storage conditions are also very important factors of maintaining dimensional tolerance precision. Rubber has a high coefficient of expansion in relation to temperature. It also absorbs moisture, hence minimizing higher temperatures and humidity will allow a product to properly “set. “A1”: is the highest precision classification for dimensional tolerance and requires the most expensive molds, each with fewer cavities per mold, and costly in-process controls and inspection procedures. “A2”: the precision class is not as strict, but the molds must be kept in excellent working condition at all times. “A3”: the commercial class, is the most common and is used with most products “A4”: the basic class, usually specifications are second priority to cost. Some basic dimensional terminology: fixed dimensions are not effected by flash thickness, closure dimensions are affected by flash thickness. Measurements of dimensions shall be made on parts conditioned at room temperature for at least 24 hours after the molding operation. These are made by one or more of the following methods:
- Coordinate Measuring Machine (CMM)
- Dial micrometer
- Suitable optical measuring device
- Fixed gauges appropriate to the dimensions being measured
- Any other method agreed between customer and supplier.
There are four relative dimensions that need to be accounted for in the measurement and design specification process.
- Concentricity: the relationship between two or more circles which have a common center.
- Squareness: the quality of being at an angle of 90* degrees such as “surface must be square with axis,” a non ground rubber product should not have more than 2* degrees deviation from 90 degrees.
- Flatness: the flatness of a surface on a part that is deviating from a true plane or straight edge, i.e. how convex or concave a surface is from being perfectly flat
- Parallelism: the relationship of surfaces in different planes, the surfaces must be equidistant from each other at all points when measured at 90* degrees to the planes.
Finish The quality of the finish and appearance of a molded product is determined by a few factors. The machined finish of the mold has a considerable effect on the surface finish or appearance of a rubber product. A highly polished steel mold, free of imperfections and tool marks can create the best finish. This type of mold is naturally more expensive to construct and maintain and is generally not required unless surface finish is of paramount importance. The commercial type mold is a machined steel mold and conforms to good machine shop practice. Imperfections are not polished; however, a overall appearance of the rubber surface will depend largely on the other factors. The type of rubber material used can greatly affect the appearance of the product. Some compounds have a bright glossy surface while others may be dull as molded or become dulled very easily during handling or storage. Many rubber products have antiozonants added which age and “bleed out” with time resulting in a colored or waxy surface often referred to as “bloom.” Certain rubber compounds can be removed from the mold with the use of little or no mold release lubricant. The flash removal method also has an affect on the final appearance of the product, for example, hand trimming will ordinarily have no effect, while tumbling may result in a dull surface. There are four classifications on the level of tool finish quality. “F1”: is a smooth, polished and uniform finish completely free of tool marks, dents, nicks and scratches, as produced from a highly polished steel mold. Where specified, product must have a surface finish of 10 micro-inches or better. “F2”: A uniform finish as produced from a polished steel mold. A surface finish of 32 micro-inches or better, tool marks do not need to be polished out. “F3”: Surfaces of the mold will not conform to good machine shop practice and no micro-inch finish will be specified. This is “commercial finish” “F4”: A Satin finish There are a variety of ways to improve the visual appeal of a rubber product beyond a smoother and more polished appearance. Some rubber parts have a texture etched or engraved into the mold. Textures can improve the appearance of the part and may be easier to apply than a high polish.
Flash Flash is excess rubber on a molded product resulting from cavity overflow and is common to most rubber molding operations. Flash has two dimensions: extension and thickness. Flash extension is the film of rubber projecting from the part along the parting line of the mold. Flash thickness is measured perpendicularly to the mold parting line. The rubber manufacturer and consumer must agree on which areas of a part a parting line is permissible and which areas should not have a parting line. The designer generally need not specify how flash is to be removed, but should specify the amount of flash extension which can be tolerated without impairing the product function or appearance. “Flash-less” products are available; however, come at a higher cost for tooling and equipment and require close scrutiny during production. Rubber flash adhering tightly to inserts is generally acceptable, if it is needed to be removed, wire brushing, abrasive belts, or spot facing can eliminate the flash. Some factors that affect flash is the flash location, parting lines must be located to facilitate part removal from the mold cavity after curing. Flash thickness serves as an aid to part removal, flash extension usually needs to be removed during the finishing process. There are many methods in removing flash.
- Buffing: an 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, creates a shearing or pinching action removing the excess flash.
- Machine trim: Flash is removed by passing the rubber part through machines geared with devices customarily adapted to a particular product which can shear, saw or skive the flash away
- Tumble trim: There are two basic types of tumble trimming. Dry tumbling is most effective for higher durometer “hard” compounds and is done at room temperature. Carbon dioxide or nitrogen tumbling is used to freeze the products making the compound brittle so the flash will break more readily. All tumbling operations have an effect on the products finish.
- Mechanical deflashing: Modern deflashing machines utilize an abrasive medium, tumbling, and a refrigerant for quick freezing.
- Pull trim or tear trim: Pulling the excess/tearing the flash by hand results in a sawtooth or irregular appearance and is limited to certain compounds and product designs
- Hand trim: flash is removed by an expedient method such as knives, scissors, razor blades or skiving knives.
The methods of designating flash are as follows: Extension: The symbol “T” followed by the maximum millimeter or inches length of the extension. The standard flash tolerances are: T.000 T.016 T.032 T.064 T.080.Thickness, location and standards may all be designated in the drawing specifications by the an agreement between designer and manufacturer. We have touched upon the importance of specifying rubber dimensional tolerance, finish and flash. For more information and an in depth look on other drawing specifications, read the RMA Rubber Handbook which also specifies extruded, lathe cut and cellular products.
Note: All references to the RMA standards, such as RMA T.036 flash, RMA F1 tool finish, RMA Class III dimensional tolerances, translate to ARPM T.036, ARPM F1, ARPM Class III.