Process Insights

Medical Molding With the Silicone Professionals

Industries are constantly changing and evolving, becoming saturated as time goes on, making it important to find a niche market. Such is the case for Sil-Pro, who found their niche as a “one-stop shop” medical silicone parts manufacturer. The company often works with medical design engineers from the part design stage to create the necessary molding, production, and inspection process until the part is fully produced and tested. The majority of parts are implant components for use in pacemakers, defibrillators, and drug pumps.

“We’re somewhat unique because we help with part design, machine the molds, shoot the test molds, inspect, and test the parts, then actually shoot the molds to produce the final parts, all under one roof,” says Kevin Carver, president of Sil-Pro. “A medical design engineer walks in our door with an idea and walks out a few weeks later with working, fully tested parts. In medical manufacturing, the key is control and accuracy. Having everything in-house allows us to do that perfectly.”


Lee Carver, Sil-Pro founder and father of Kevin Carver, started his career in rubber and plastics manufacturing more than 44 years ago. In 1998, Carver decided to apply his years of experience to the area of silicone medical components in Delano, Minn. It wasn’t until a year later that an employee was hired, but still no customers were to be found.

“It’s funny, looking back now, but I remember pulling friends and family in to pose as workers, making it appear as if the shop looked busy to potential customers,” says Kevin. “We never meant to deceive anyone into believing that we could handle a larger capacity than what we actually could, but looking in at our shop at the time, no one would know just how much production power we could truly handle. Once production started, it was all growth from that point on, and just a few years later we were up to 90 employees.”

Buying a Mold Shop

“The parts we produce are complex, made from a material most moldmakers will never deal with, and often have to be delivered within nearly impossible deadlines,” says Kevin. “To optimize production, it was best to control all aspects.”

To best handle the process, Sil-Pro purchased one of the mold shops they had been working with and brought their people and equipment in-house.

“I’ve been in a lot of mold shops, but never one that has a clean-room attached to it,” says Chris Tellers, Sil-Pro’s mold shop manager. “This speaks to just how different Sil-Pro is. Our product is very unusual for most mold builders. We’re dealing with medical-grade silicone, a very different material than plastic in both mold design and accuracy requirements.”

Medical-grade silicone has a high potential for flashing during the molding process, sometimes forcing moldmakers to create shut-offs accurate down to one ten-thousandth of an inch. In addition, the material does not flow like plastic, creating venting and surface finish concerns.

“When the surface finish isn’t just right on a car’s bumper, it’s very different than when it’s not right on a part that prevents fluids from rushing out of your heart during surgery,” comments Tellers. “There’s no room for error in this type of molding, so the machines we use have to be up to the task and produce flawlessly.”

Because Sil-Pro’s entire production process is under one roof, they have full control over quality and lead-times without making compromises. And because everything is interrelated, different internal departments aren’t fighting each other for profit, making the final product paramount. This production style has also enabled Sil-Pro to quote turnaround times for parts at four weeks or less, even producing prototype parts in as little as two weeks.

“For most industries, two weeks is lightning fast,” says Kevin. “But in medical moldmaking, faster is always better. We’re aiming for turnaround in as little as three days when necessary, though we can never sacrifice quality. We began by building automated part production systems at our facility to speed production. When we saw how valuable that investment was, we realized that spending money on technology quickly pays off, so we began to invest in other areas.”

Machines Added

When acquiring the mold shop, Sil-Pro obtained several commodity machine tools and a Makino S56 vertical machining center. The S56 provided significantly better accuracies, more consistency, and faster cycle times than the commodity machines. It was the first indicator as to what was possible in machine tool accuracy and speed, and led to purchases of additional Makinos, including a V33, V22 Graphite, and an EDGE2 Ram EDM.

“We discovered that you get what you pay for when it comes to machine tools,” says Tellers.

As an added benefit, Sil-Pro’s machinists have realized the programming and operating benefits of the Pro5 control on their machines. “We have experience with many different brands of machines, yet all agree that the Pro 5 control adds flexibility and functionality,” comments Tellers.

Soon after installation, the new machines were running unattended, quickly and consistently producing accurate molds. Sil-Pro also benefited from decreased cycle times of 25 to 40 percent due to the elimination of flashing problems, while still obtaining the needed surface finishes of 4 to 8 Ra.

The Makino machines have become Sil-Pro’s workhorses, providing the speed and finish required for this type of mold manufacturing.

The company utilizes high-speed machining and hardmilling whenever possible, but for mold features that cannot be milled, an EDGE2 Sinker EDM allows Sil-Pro to produce sharp corners and deep ribs.

The Mold-Making Process

“Most of the molds we create are for medical surgical seals, and the nature of a seal itself requires accuracy,” says Kevin. “If it leaks in any way, it’s no longer a seal. These molds have to be consistently perfect.”

The process requires tooling sizes as small as 0.1 mm and geometries within 0.0005 inches inside cavities.

“Another challenge we face is that many of the parts we produce are tubular in nature, creating a unique degree of difficulty,” adds Tellers. “These parts demand extremely tight radiuses, which require accuracies within five ten-thousandths, sometimes even smaller. Shut-offs for these parts can be as strict as two ten-thousandths.”

All molds produced at Sil-Pro require high finish qualities, with finished parts that often require no hand-polishing. By machining molds in only one operation, Sil-Pro can achieve the strict and demanding time frames their customers request.

After a mold is designed, machined and CMM verified, it’s moved to the injection molding facility.

Silicone Medical Implants

Silicone has become widely used in the medical industry for implants, such as the tubing and seals Sil-Pro produces.

Silicone’s usage in medical implants is due to its composition as a sturdy yet flexible material. It is resilient and is unaffected by harsh temperatures and many chemicals of the body. The flexibility of silicone makes its use in joint implants an easy fit and reduces the odds for implant fractures.

Why Silicone Injection Molding?

In its hardened state, silicone can be molded using transfer molding, a process that can result in less material consistency, control, and increased lead-time from longer vulcanization times. For these reasons, injection molding is popular for the production of silicone parts.

“Due to this material’s composition as high-purity platinum-cured silicone, the liquid injection process proved to retain high stability even in extremely high temperatures,” says Kevin. “This process is an ideal choice for part production. And since silicone molding doesn’t require mold release or surface preparation before injection, it saves us some labor.”

Although silicone molding has many benefits, shops that work with silicone often struggle with the material’s high flash rate during injecting. This requires shut-offs to be accurate within 0.0002 inches to build in shrinkage needed to prevent part imperfections.

“Many machine tools don’t give you the confidence to produce silicone molds accurately enough to prevent hours of hand-finishing. With the Makinos, we are able to cut right to the numbers and achieve the tight shut-offs needed for silicone injection molding,” says Tellers. “The machines have proved to be fully reliable and consistent, even when running unattended.”

Out of the Mold Shop

“These parts cannot be contaminated,” says Kevin. “When the mold comes out of the mold shop and is cleaned, it won’t be touched with bare hands until we’re done shooting the final parts.”

Workers in the injection molding room wear head-to-toe clean-room gear, and the room is sealed off from the rest of the facility, in accordance with FDA regulations. The molds are shot either in a one-stage setup for prototyping or in a fully automated production cell for full-run parts.

“If it’s for a prototype, the customer typically needs a handful of parts, so we don’t go to the expense or time of creating an automated cell,” explains Kevin. “Once the prototype is checked and any final changes are made to the mold, we often build an automated setup to allow continuous, high-speed production of the parts and to verify quality and consistency, as is required for this type of work.”

Sil-Pro has outsourced the building of a few automation cells, but has begun building cells in-house to control the process from start to finish.

“These machines will automatically shoot the silicone, pull the parts, perform any needed secondary operations such as slitting, then inspect the parts,” adds Tellers. They machine many of the components for the automation cells, utilizing their machinists’ programming and manufacturing abilities.

Once the parts are produced, they move to an inspection clean-room connected to the injection molding room. Every part is examined by a trained inspector using a microscope and any required measurement device before being packaged and shipped out to the customer.

“Our real responsibility to the customer is the part, not the mold, but the mold is an essential component of the process,” explains Kevin.

“We must ensure that the parts meet all the required guidelines and are perfect every time. Makino equipment allows our mold shop to do that.”


“With a growing number of mold shops jumping into medical manufacturing, we feel we offer a unique solution,” says Kevin. “There’s no doubt we will continue to invest in advanced technologies to keep our competitive edge, though expanding into more industries is also an option as we become a stronger force in manufacturing. No matter what, we will continue to be a one-stop shop for silicone medical injection molds.”

Delano, Minnesota
Phone (763) 972.9206