Molding is the rubber industry’s main way of transforming raw materials into usable products. While alternative methods like 3D rubber printing are gaining traction within the industry, they have yet to overtake the popularity of rubber molding (and likely never will).
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There are several different ways to
mold rubber, including injection molding, transfer molding and compression molding. The ideal type of rubber molding for your product depends entirely on your material and intended application.
What is rubber molding?
Rubber molding is the process of shaping a raw rubber material to a specific geometry to create a final product of some sort.
As stated, there are three different types of rubber molding. Each one involves shaping a raw rubber material into a final product using a mold of some sort. The mold is subjected to pressure and heat, which causes the rubber material inside to vulcanize, or cure.
1. ) What is Compression Molding?
Compression molding involves loading pre-formed rubber raw or unvulcanized materials into a mold cavity for curing. In the 1890s, Harvey Firestone produced the first modern rubber product in his home oven using compression molding. It’s the “original” method of rubber molding, and the process hasn’t changed much over the past 130+ years.
How it Works:
- Raw material is mixed, typically in 500-pound batches
- A rubber compound or mixed raw material is made into a “pre-form” in the shape of the end product
- Pre-forms are loaded into an open mold - typically into an closed space formed by two halves of the same mold
- The mold is closed
- The rubber takes the shape of the mold cavity and is cured through the application of heat and pressure
- The rubber is demolded, typically by hand, to reveal the end product
Pros of Rubber Compression Molding:
- Low tooling costs
- Fast lead times
- Lower waste
- Better at processing stiff, high durometer materials
Cons of Rubber Compression Molding:
- Manual demolding means slower cycle times
- Additional labor step to pre-form the material
Compression Molding Applications:
Rubber compression molding is less suitable when products require uniformity or have a low dimensional tolerance. It is, however, useful for small-run productions, especially prototyping and sampling.
Compression molding rubber can be cost-effective if one or more of the following is true:
- Compression molding tooling already exists.
- The quantity required is very low.
- The part’s cross-section is very large and it requires a long cure time.
2. ) What is Transfer Molding?
Transfer molding involves placing a preform into a molding chamber and forcing the raw material into a cavity to be cured. Like compression molding, transfer molding involves placing “pre-form” rubber materials into a mold. However, transfer molding involves more complex molds and tooling methods.
How it Works:
- Raw material is mixed, typically in 500-pound batches
- The pre-form material is loaded into a “pot” in the molding chamber.
- The pre-form material is forced into the party cavity via “sprues” by a “plunger.”
- The rubber takes the shape of the mold cavity and is cured through the application of heat and pressure
- The final part is demolded
Pros of Rubber Transfer Molding:
-Shorter processing time because you can use multiple cavities
-Fewer and simpler pre-forms (one pre-form can fill hundreds of cavities)
-Tighter dimension tolerance
-Easier to create colored rubber parts
-Higher accuracy and consistency in final part
Cons of Rubber Transfer Molding:
-Increased waste (This is the primary disadvantage; the flash pad or rubber left in the pot after transfer is typically cured and has to be recycled or thrown out.)
-Slightly higher tooling cost
Transfer Molding Applications:
Transfer molding is often used for creating parts that require higher accuracy with lower volume requirements. It is also used for:
- Bonding rubber to delicate metal parts
- Creating composite seals with insert molding
- Using mold designs involving multiple cavities.
3. ) What is Injection Molding?
The process of
injection rubber molding involves injecting molten polymer materials under high pressures and speeds into closed molds. It was invented as an extension of the plastic industry in the 1960s. In many cases, it is still the most efficient way to mold rubber.
How Rubber Injection Molding Works:
- Raw material is mixed, typically in 500-pound batches
- The raw material is stripped immediately after mixing - usually into continuous strips measuring approximately 1.25" wide and 0.375" thick
- The strip is fed into a screw on the injection molding machines, which charges a barrel as needed with a predefined amount of material
- The mold is closed
- The material in the barrel is injected into the mold cavities
- Heat and pressure are applied, vulcanizing the rubber
- The final part is demolded
Pros of Rubber Injection Molding
-High production rates
-Can be done with a variety of rubber materials, including silicone, neoprene, nitrile and EPDM
-Minimal waste
-Fast cycle and production time
-Complete elimination of time and labor involved in making pre-forms
-Complete elimination of operator placement of pre-forms
-Injection screw pre-heats the material before forcing it into the cavities, decreasing the viscosity of the material and allowing it to flow easily into the cavities
-Potential for decreased cure time due to pre-heating the rubber
-Potential for decreased cure time as a result of heat added during the screw charging
Cons of Rubber Injection Molding
-Increased setup and changeover costs
-Not ideal for lower production quantities
Injection Molding Applications:
Rubber injection molding is not suitable for prototyping because of the upfront costs involved in creating the mold. However, it is ideal for large-batch production of many common rubber products including electronic components, seals, gaskets and consumer products. Injection molded products are commonly found in the aerospace, medical and automotive industries.
Which type of rubber molding is right for your application?
The type of rubber molding right for your application depends entirely upon the material used, conditions present around the final product and batch size, among other factors.
At Custom Rubber Corp, we have been compression and transfer molding since our founding in 1956. We added our first injection molding machine in 1984. We have extensive experience in taking on existing tooling or determining the most cost-effective methods of production for your new part.
Our veteran staff are well-versed in all three types of rubber molding, with over 100 years combined experience in each one. We are highly skilled and experienced in every step of the process and are more than happy to discuss your unique rubber materials application and its requirements.
Click here to request a quote or ask Custom Rubber Corp. staff a question about the different types of rubber molding.