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Compression molding is the process wherein uncured rubber is placed into a pre-heated mold and then compressed at a specified pressure for a specific time to properly vulcanize (or cure) the material. Compression tools have to separate and close with each cycle so they are designed to open and close easily.
Compression molding is the most labor intensive rubber molding process. However, it is often the best choice due to program parameters such as tooling costs, production volume, and raw material properties.
In the transfer molding process the uncured rubber is drawn into a temporary chamber, heated, and then forced with pressure into a closed and preheated tool until vulcanization occurs. Transfer molds stay closed and static throughout the molding process allowing for higher tolerances and reduced flash lines.
By eliminating the hand placement of the material before vulcanization, transfer molding increases the production speed of the rubber molding process and is often used on large multi-cavity tools where quick and continuous cycle times are critical.
- Transfer Press Tool closed
- Sheet of material with flash after removal from tool
Injection molding is similar to transfer molding but is designed for higher production volumes and therefore utilizes a larger temporary chamber for raw material storage. Depending on the size of the item being manufactured and the size of the tool, it is possible to have more raw materials lost during the injection molding process in the runner tubes required to transfer the unprocessed matter to the mold cavity.
This type of molding process is utilized for all plastic manufacturing and some rubber manufacturing depending on the rubber compound being used. Increasing additives to the rubber base for desired chemical properties can adversely affect the flow properties and make injection molding impossible.
Injection molding utilizes lower labor content than compression or injection molding due to its automated processes. However, tooling for an injection molded press is often significantly more expensive than a compression tool which may also be cost prohibitive. Additionally, due to the tool design and the runners required to transfer the raw material, additional procedures may be required to de-flash the products.
Extrusion is the process by which objects of a fixed cross-sectional profile are created by pushing material through a die. This procedure can be done with plastic, rubber, and metal materials and can create parts of theoretically infinite length.
Machining is any process in which a piece of raw material is cut into a desired final shape and size by the controlled removal of unwanted material. The precise definition of machining has evolved as this technology has advanced. Traditional machining processes include turning, boring, drilling, milling, sawing, shaping, planning, reaming, and tapping. New machining technologies include the use of electrical discharge, electrochemical, and ultrasonic machining processes.
In many cases, a rough metal part is created through the extrusion process and then finished with specific machining.
Bonding / Over-molding
To provide a product with high structural integrity yet strong sealing capabilities, a combination of metal and an elastomer can be created. Joining these two materials requires three essential elements: the rubber material, a bonding agent, and the substrate.
Selection of these three components is dependant entirely on the application, environmental conditions, fluid compatibility, and more. A bond can be formed with almost any rubber compound with very few exceptions. Bonding agent selection depends on the type of rubber utilized, the modulus of the rubber, and the application. In most cases, the drying time required for the bonding agent is minimal and does not inhibit high volume production.
For rotary or high stress applications, utilizing an injection molding operation allows the greatest process controls and produces the best results. For basic or low volume applications, a compression process can produce high quality results as well.