When to Choose Injection Molding or Compression Molding for Silicone Parts

Updated on November 30, 2020

The medical field demands perfection at all levels. Since even the smallest imperfection on a medical device can have grave repercussions for a patient, it is vital for every part being used to make the grade. 

For silicon applications, medical-grade silicone manufacturers use injection or compression molding to create their customers’ components. Each method has unique strengths, and it is up to the engineers to decide which to use when filling the order.   

Injection Molding

Silicone is often the material of choice for medical devices because of its biocompatibility and durability. Injection molding works by adding an exact amount of liquid silicone rubber to a mold. The silicone is made of two parts, known as A and B. One of the parts contains the catalyzing agent responsible for curing the liquid after the introduction of heat.  

The liquid silicone containers are sealed until injected into the mold. Should there be any added pigments, it is also stored in a sealed container. This greatly reduces the risk of contamination to the ingredients while also reducing the amount of labor involved.

The silicone and pigment then get pumped from their containers, mixed, and injected into the mold. An injection press secures the mold while the silicone catalyzes and quickly cures in place. Once finished, the completed parts are removed, and the procedure repeats.

Injection Molding Applications

Due to how precise the process is, it is the preferred solution for quickly creating intricate parts. Despite the shortened cycle times, this is still the more expensive process when compared to compression molding. For this method to fully utilize its economic potential, it is best to apply it to the production of large volumes of parts.

Implantable medical devices, such as drug-eluting vascular stents, benefit greatly from this process. Owing to the closed-loop nature of injection molding and the low amount of physical handling, the risk of contamination is significantly reduced. As a result, the number of complications for patients with implanted parts decreases.  

An added benefit to using injection molding is the reduced amount of wasted materials. Unlike compression molding, which uses approximations, injection molding utilizes precise measurements of materials so that nothing goes to waste. This is not only better for the environment, but also reduces the amount of labor involved. 

Compression Molding   

This method requires the silicone rubber to be pre-catalyzed before being placed into the mold; otherwise, it will not be able to cure when exposed to heat. Unlike injection molding, if the silicone needs a pigment, then an extra step must come after. Using a two-mill roll, the pigment is mixed with the silicone. However, this additional step increases the contamination risk of the material, which is not ideal for medical parts. 

An approximate amount of the prepared silicone then gets pre-formed into the desired shape of the finished product. This step ensures the integrity of the finished product. After the silicone is pre-formed, it’s inserted directly into an open mold, where it is compressed and heated until it cures into its final form.

Compression Molding Applications  

Since it is the simpler of the two processes, compression molding is significantly cheaper than injection molding. Unfortunately, it is also the more labor-intensive process by a significant margin. This makes it more suitable for use in the creation of medical prototypes and lower volume production runs. 

Despite the increased risk of contamination with this method, there are still suitable applications for its use in manufacturing certain medical grade parts. Usually, this applies to parts that will not directly contact a patient but are not worth the injection molding process’s financial cost. Gaskets, washers, and seals are just a few of the possible uses for compression molding.    

So, Which Option Should You Choose?

There is no easy answer to the question of which molding process is the superior option. Both injection and compression molding hold essential places in manufacturing medical parts due to their unique advantages. It is best to consult an experienced manufacturer before making a final decision, as the type of silicone used will need to meet individual specifications depending on the intended product. The manufacturer will know which application is most appropriate for the specific task at hand.

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