Engineering an Elastomer Part
Lavelle Industries (www.lavelle.com has an image problem. Because of the amazing advances in plastics technology and the resulting explosive increases in their use in various products, most engineers in charge of product design and development have a working knowledge of the characteristics of different plastics and how they are processed, but have not had an opportunity to learn the vital facts about rubber.
Rubber technology has advanced so that natural rubber, which is still widely used, has been joined by a large array of synthetic materials called Elastomers which have rubber-like qualities. These elastomers have been developed in response to specific chemical, environmental or mechanical requirements. Elastomers range from inexpensive, limited use materials such as Polyisoprene and SBR; to compounds such as Neoprene, Nitrile or EPDM that meet a wide range of needs; to more exotic compounds such as Butyl, Hypalon, EPH, Silicone and Viton.
The major reasons for specifying an elastomeric part include the following:
Skid Resistance - This keeps a product from sliding around on all those new plastics.
Elasticity - The tendency for an elastomeric part to return to its original shape after it has been deformed.
Wide Variation in Hardness (Durometer) - The material used to make a part can be soft or hard. In several cases, Lavelle supplies the same part with the same material, but in different durometers to the same customer for two different applications.
Flexibility - A part can be stretched or deformed to fit a mating part. This, combined with the good coefficient of friction of most elastomeric surfaces, helps some parts to stay assembled without bonding or fastening.
Impact Resistance - The material can absorb a shock without cracking.
Chemical Resistance - Elastomeric compounds can be specially formulated to respond to a specific function, so there is no need to accept a "one-size-fits-all" material.
Resistance to Temperature Variation - Compounds can be tailor made to withstand temperatures to +550F, or down to -180F, or to accommodate most of the range in between.
Non-Abrasion/Non-Marking Qualities - Because of its non-abrasion characteristic and the fact that it can be formulated to be non-marking, an elastomeric part can be used as a foot or bumper to protect sensitive surface areas.
When one of the sales technicians at Lavelle is working with a design or manufacturing engineer on a new product or a variation of an existing part, determining the answers to questions about chemical, temperature, and mechanical requirements are essential to recommending the right elastomer. After the right elastomer has been identified, it is then necessary to determine the right formulation to meet all the functional and environmental requirements.
As with any other fabricated component, there is the essential question that must be answered first--What is the part supposed to do? This should be answered during the design phase of the product. The following should also be considered:
Always verify that the part you use will do what you want it to in the final product or sub-assembly. If you are going to use a standard part, then be sure to get samples. If you are going to use a custom part, then save the time you need, and eliminate the aggravation you do not need, by ordering prototypes before production tooling is built. The place to refine the part is in the prototype stage. At Lavelle, we have made it a policy to insist that design and material be verified through prototypes before a production mold is made. A number of customers have discovered that major changes were needed because of unforeseen factors. The lead time for prototypes can vary from a few days, as with Lavelle, to 8 weeks with some other companies.
- Does what I want the part to do add to the overall effectiveness and thereby the value of the final product?
- Can this function be accomplished by modifying an existing component that is already a part of the final product?
- Can several functions be combined in one part rather than having several parts?
- Can tooling costs be eliminated and lead times cut by using an existing, standard part in the design? Lavelle has helped numerous customers by making a standard part out of a different elastomer that better fit the functional requirements.
- Before going through JobShop.com or Thomas Register, is there a vendor already in the Engineering or Purchasing data base that produces a similar product that could answer questions and help with design of a customer elastomeric part?
- Are tolerances realistic for the design, material and function of the part? Remember that an elastomeric part can be formulated with a low or high degree of flexibility.
- Are critical dimensions identified?
- Has anything that might affect print tolerances been properly identified? For instance, will QC check dimensions using a CPK that actually reduces the allowable tolerances?
- Can the QC department check the parts? For example, if the ID of very flexible 8" ID X 1/16" wall O-ring is a critical dimension, then the vendor and customer should know how this will be checked prior to production.
- What other material does the elastomeric part come in contact with? Remember that most elastomeric compounds are processed using sulfur or peroxide, which may react with some plastics, metals, or coatings.
- Even if the function is "just" a rubber foot, never assume that there is a standard part somewhere out there that will meet your needs. Several times, Lavelle has had to "pull out all the stops" to provide a custom part for a customer because that perfect standard part did not exist.
- If you are unfamiliar with a material or process, there is no such thing as a stupid question! Take advantage of the expertise developed by a vendor over years of making elastomeric parts.
The differences between processing elastomers and processing plastic results in major differences in tooling and cycle times. In general, it takes 3 to 10 minutes for an elastomeric part to cure in the mold to the point where it can be removed, as opposed to seconds for most plastic parts. The size of the elastomeric part, the size of the mold, and the elastomer being used will all affect the cycle time. It is not unusual, therefore, to have molds used to make elastomeric parts with hundreds or thousands of cavities.
However, because elastomers flow differently than plastics do, most molds used to make elastomeric parts are far less expensive than molds for plastic parts. This is especially true in the compression and compression/ transfer molds where parts are made using a pre-weighed amount of uncured rubber.
In a compression mold, individual "slugs" of pre-weighed elastomer are placed in each cavity. The mold is placed in a press if it is not already bolted to the press, and the press is closed. As heat and pressure are applied, the elastomer liquefies and fills the cavity with any excess material flowing away from the cavities in a runner system.
In a compression/transfer mold, the mold is closed except for an open pot on the top of the mold. Inside the pot are small holes, or "sprues", that feed directly into the cavities, or into a runner system that feeds into the cavities. A slab of pre-weighed elastomer, enough to make all of the parts, is placed in the pot, and a ram is lowered into the pot. The combination of heat and the pressure from the ram causes the material to liquefy and "transfer" from the pot through the sprues and into the cavities. Any excess material remains in the pot.
The advancing technology of elastomers has also resulted in the introduction of a few liquid elastomers, such as liquid silicone. Parts made from these materials are made using LIM (Liquid Injection Molding) systems. The tooling is very similar in price and style to molds for plastic parts. These systems are very good when high precision and high quantity are both required. It should be remembered that the cost of these materials is higher than standard, solid elastomers.
There are also materials that combine a few of the desirable characteristics of rubber with the processability and reusability of plastic. These are variously called TPEs (ThermoPlastic Elastomers) and TPRs (ThermoPlastic Rubbers). Two of the more commonly known of these are Santoprene and Geolast. These materials can be processed in plastic injection tooling, and the scrap can be recycled and reused. This cannot be done with standard elastomers which are thermoset materials instead of thermoplastic materials. These types of parts are useful where the flexibility, non-abrasive/non-marking, or skid resistance are the only primary concerns.
It is because elastomers can be formulated in so many different ways that they remain popular in so many different applications, including the following partial list:
|Gaskets||Vibration Mounts||Sound Dampening||Caps|
New products and new technology insure that elastomers will remain a vital tool for the engineer.
Hypalon and Viton are registered trademarks of DuPont Dow Elastomers.
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