Last week, we wrote about the 3 main types of molding that we do at Stern Rubber Company: compression, transfer and injection. Last week we discussed compression, and this week, we will cover transfer molding.
Transfer molding is similar to compression molding, as they are run the same presses. These presses are run at high hydraulic pressure, and have electrically heated platens that are run in the 300 to 400 degrees Fahrenheit range. The difference is all in the tooling, and not in the press. Similar to compression, it is good for low to medium volume parts, and can be used for high volume parts, if the part is small. Transfer and compression molding are both good for rubber to metal bonding.
Here are a few links to learn more about the presses that we use for compression and transfer molding, from some of the suppliers where we purchase presses.
Like compression, these types of molds are made from steel or aluminum. The shape of the part is cut into the mold. The main difference between compression tooling and transfer tooling, is that in transfer, the rubber is fed, or “transferred” through small feed holes from a “pot” into each cavity.
In transfer molding, you can place one large piece of rubber material into the pot, instead of having to load rubber in each of many cavities. A simple part that is molded compression can be made on a 2 plate mold. Half of the part cavity is cut into each half of the mold. With transfer, you still have the 2 cavity plates, but in the top plate, you have to add the feel holes, and on top of the top plate, we add the pot, and aluminum plunger, or lid that forces the rubber through the feed holes and into the cavities.
The tool is held closed for 5 to 15 minutes depending on the thickness of the part. When the cure time is complete, the tool is opened, the flash pad is removed, and the part is removed.
The advantages are very similar to compression molding as they both include lower cost tooling, and are both good for small production runs, as the set up time is normally very low, as the tool normally just has to be slid into the press and heated, and it is ready to run. There is also no wasted material for set up, as you load the rubber into the tool, and do not have to use rubber to clean out the injection unit, like we do with an injection tool. As in compression molding, transfer molding is also a good way to produce large parts that take a significant amount of material, or a long cure time to manufacture. The one advantage that transfer does have over compression, is that it is much faster to load the material, as discussed above. This also reduces the material prep time. Transfer also usually allows for less flash than compression, and therefore we can usually allow tighter tolerances than compression.
Some of the disadvantages include slower process times compared to injection, and greater waste of material, even when compared to compression. With transfer, we have the waste of the material that stays in the pot and feeds. The tolerances that can be held are still not as tight as injection molded parts.
If you have want to learn more about this process, or think you have a project that will be a good fit for compression molding, please contact us at email@example.com.