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Top 5 Design Considerations for Medical Injection Molded Parts

For medical device OEMs, the design phase is when a general concept and a list of requirements becomes a detailed plan for creating a potentially life-saving product. While there are several fabrication methods available for making medical devices, injection molding continues to be the dominant method. Its advantages of low price per part, high production volumes, compatibility with many different FDA-approved materials, and its ability to maintain tight tolerances, produces consistent results.

Injection molding is a supremely flexible process, but there are a few constraints and requirements that need to be incorporated into the design of any parts made by that process. For medical injection molded parts in particular, we’ve identified the five most important of these design considerations, which we’ve listed below along with our advice for part design success.

Part Function: What is it Supposed to Do?

All five of these design considerations are interrelated—design choices in one area constrain your options in the other four—but the primary driver of the part design process is ultimately the intended end-use of the medical injection molded part.

Are you designing a device meant to remain implanted in a patient for several years, or is the part a knob or button on a monitoring device or life-supporting equipment? Perhaps it’s a patient connected device that is disposable? Each of those uses implies specific operating temperatures, chemical exposure, and applied stresses over the lifetime of a product. Everything from material choice to the shape of the part is determined by this, so having a well-defined list of requirements at the beginning of the design process will not only help your team, but also assist your medical injection molding contract manufacturer with their DFM review and subsequent mold design.

Will the Part Need to be Repeatedly Sterilized or will it be Disposable?

Single-use products meant to be incinerated give you more leeway with material choice than those that will need to repeatedly withstand the abuse from the sterilization method(s) chosen. Devices that will be steam sterilized will need to be made out of materials that not only have a high melting temperature, but are also highly resistant to both heat and hydrolysis. On the other hand, ethylene oxide (EtO) sterilization requires excellent chemical resistance. UV, gamma, and e-beam methods limit your choices to other materials. Finally, only a handful of niche materials are suitable for devices which could potentially be sterilized by multiple methods over their lifetime.

What will it be Made of?

From liquid silicone rubber (LSR) and thermoplastic elastomers (TPEs) to polysulfone and PEEK, the choices of rubbers and resins are almost endless. With that wide selection of materials come wide ranges of durometers, opacities, biocompatibilities, lubricities, and resistances to heat, steam, radiation, chemicals, tearing, and wear.

With overmolding, design engineers aren’t restricted to just one material. A stiff thermoplastic component can be overmolded with soft silicone rubber grip, a popular combination for the product to be mechanically strong yet comfortable to hold.

How Easy is the Part to Actually Mold?

In order to consistently make high quality parts without exorbitantly expensive revisions, your part design needs to incorporate features such as adequate draft angles, consistent wall thicknesses, and generous radii for perpendicular features such as walls, bosses, and ribs.

Furthermore, for parts made with thermoplastics, really thick walls should be eliminated via core-outs. This not only helps prevent sink marks and warping, but also reduces the cost per part since less cycle time is required to fill and cool large volumes, not to mention the material cost savings from the reduction in resin used to pack the mold.

Price of the Finished Part

Ultimately, medical injection molded parts must be price competitive with competing products already on the market, and affordable enough to provide compelling value over the lifetime of the product.

Closely tied to the price per part, is the production volume expected for the tooling. If you are making millions of parts with a single mold, it’s easier to justify more expensive mold materials (like hardened steel) and features like hot runners for thermoplastic molds and cold decks for LSR.  Multi-cavity molds may require a larger upfront investment, but also pay for themselves in the long run due to time and material savings.

Having processed many medical injection molded parts from initial concept to finished product, ProMed’s medical device design expertise can help your engineering team avoid common pitfalls, improve your product, and ensure consistent, high quality results.

ProMed Molding. Medical Molding

5 Molding Processes From ProMed Molded Products

From surgical instrument handles and catheters, to drug-eluting implants and stents, different jobs require different devices to be made. Different devices in turn, require different fabrication processes. In this article, we’ll discuss five different molding processes within our expertise here at ProMed, and explain the niches that each of them fill.

Silicone Injection Molding

Liquid Injection Molding (LIM) is a process in which liquid silicone rubber (LSR) is injected into a heated mold under pressure, completely filling the cavity before curing into a solid part which is then ejected from the mold. This process is virtually identical to the injection molding of thermoplastic polymers, except for the fact that LSR is a thermoset polymer. This means that the mold must be heated (not cooled as with thermoplastic resins) so that the mold permanently cures the silicone in a process called vulcanization. Once cured, the part will not melt back into a liquid. This flipping of temperature zones also means that LIM utilizes “cold decks”, not “hot runners” in order to conserve material and reduce cycle times.

Injection molding is a natural fit for making medical devices out of silicone because LSR’s low viscosity allows the mold cavities to be filled quicker and at lower pressures. LIM’s short molding cycles produce cost-effective parts in medium- or high-volume production runs, making it a popular choice for our OEM customers.

Transfer molding

Transfer molding is a process that’s similar to injection molding, and uses many of the same elements: a heated mold cavity, sprue channels, and an external actuator that pushes the molten material into the mold. In transfer molding, an open chamber (called the pot) is filled with the material to be molded (which can start as either a solid or liquid). Then, a plunger pushes on this material and squeezes it into the mold, which is connected to the pot via channels.

Transfer molding typically uses higher pressures than injection molding does to fill the mold. Another difference is the fact that the mold casting material may begin the process as a solid, in contrast to both LIM and thermoplastic injection molding.

Whereas LIM is the preferred process for LSR, transfer molding (along with compression molding, explained below) is commonly used for a different type of silicone called high consistency rubber (HCR). HCR’s higher viscosity makes that particular silicone unsuitable for injection molding.

Compression molding

If the pot in the transfer molding process were removed, and the top half of the heated mold took the place of the plunger, the result would be compression molding. Unlike both injection molding and transfer molding where the molded material is forced into the cavity, compression molding forces the heated cavity onto the material.

Like transfer molding and LIM, thermoset elastomers like silicone are used as the molding material. The heat for the vulcanization is provided by the mold and usually a preheating of the material as well.

Suitable for high-volume production, compression molding excels at fabricating large parts at low cost and with less waste compared to other methods (as there is no runner system or gates to trim off). One disadvantage of compression molding is that the process doesn’t accommodate undercuts in the parts, as any undercuts make ejecting the cured part very difficult.

Insert Molding & Overmolding

Parts made by the three molding processes described above need not be a single material all the way through. It’s often very desirable to make a composite product which has a plastic or silicone layer molded over some or all of a piece of a different material. Silicone gripping surfaces on steel surgical instruments are just one example of such overmolding.

Creating overmolded parts is typically a two-shot (or more) process—essentially a separate molding process for each layer. OEMs must carefully check the material compatibility of the materials they wish to combine because not all combinations of elastomers, thermoplastics, and metals are possible. On the whole, though there are few obstacles, leaving the OEM’s design team’s creativity as the limiting factor.

Insert molding also involves combining premade parts with molded rubber or plastics, but is a one-shot process. This is because the inserts are usually metal parts like threaded studs, which are machined rather than molded.

Both overmolding and insert molding are great for joining parts to moldable materials without using adhesives or mechanical fasteners.

RTV Casting

The last molding process on our list is actually an indirect molding (i.e. casting) process, since it’s a method to make molds that then make the actual parts. With that aside, room temperature vulcanizing (RTV) silicone casting definitely deserves to be on any list of processes applicable to silicone molded medical devices.

A cost-effective way to produce small volumes of parts, RTV casting reproduces surface textures and other fine details. Furthermore, since silicones feature great chemical and heat resistance, RTV molds can be used to cast materials like low melting point metals (e.g. zinc and pewter), epoxies, waxes, and gypsum—all without needing a mold release agent.

The team at ProMed specializes in molding medical devices, including the five methods we touched on here. Whether you’re in the market for micro-molded implantable devices, or an RTV casting for new design concept, the professionals at ProMed have you covered.

Silicone Injection Molding Advancements. industrial prototyping

Silicone Injection Molding Advancements

Given its excellent biocompatibility, heat resistance, and durability, silicone is an extremely popular material for medical devices (including implantable ones). Injection molding is just one way parts can be manufactured out of silicone, and in this article, we’ll discuss a few key advances that have brought Silicone Injection Molding industry to the mature state it is in today.

In this post, we’ll focus on silicone liquid injection molding (LIM), which uses a liquid silicone rubber (LSR), a thermosetting elastomer, which is injected into a heated mold and vulcanizes (cures) into the shape of the desired part. Since the viscosity of LSR is low, it’s a natural fit for injection molding, as the LSR can quickly fill the mold without excessive pressure. In addition to sharing silicone’s intrinsic biocompatibility and wide temperature range, LSR features a wide gamut of available hardness: from 5 to 80 Shore A.

These are just a few of the technologies that have advanced LIM, and propelled it into applications all the way from cookware to implantable medical devices:

Cold Decks Produce Hot Results

A “cold deck” is a cooled (usually by circulating water) section of a LIM mold that prevents the silicone from curing until it reaches and fills the hot mold. Thus the LSR remains liquid, and no material is lost to a solid sprue and runner system. This is the idea behind “hot runners” for thermoplastic resins (which keep the resin inside the runner system hot so that the molten resin doesn’t solidify). Cold decks also reduce cycle time since there is no attached sprue and runner to remove from the part after curing. This helps eliminate what is often a manual step. For high volume production runs, the reduced material waste and shorter cycle times provided by the cold deck can more than pay for that higher initial investment.

Self-Sticking Silicone

In a previous article titled ‘Thermoplastic & Silicone Use for Medical Molded Components‘ we discussed the self-adhering property of some LSR formulations. Continued innovations by silicone material suppliers have resulted in a wider selection of these self-adhering silicones. By eliminating a time-consuming (and often hazardous) priming operation, these LSR formulations improve machine operator safety as well as reduce total cycle times. With the hardness range of these formulations increasing to anywhere from 5 to 70 Shore A, LSR is satisfying the growing demand for softer, self-adhering silicones that meet regulatory standards for biocompatibility, and thus can be used in medical devices.

Overmolding is Outperforming

Advances in overmolding have also played a key role. This includes the use of High temperature thermoplastic substrates (like PEEK and polysulfone). Since both of these polymers have exceptionally high melting temperatures, the molds can be run hotter, curing the silicone faster, reducing cycle time, minimizing price per part, and increasing annual part yield.

Precise Control Yields Production Consistency

Not all of the innovation is happening in materials. As with so many other industrial processes, precision control, advanced sensors, and automation have improved the consistency and quality of parts made by LIM. By combining servo-electric motors, valve timing, intra-cavity pressure monitoring and precise control over pressure, flow rate, & temperature, integrated automation has led to more consistent results for OEMs and their Contract Manufacturing partners.

Precise Control Yields Production Consistency

Lastly, simulation and the application of CAD and CAE to LIM tooling and process parameters have taken hold in the LIM industry, just as it has in thermoplastic injection molding. Simulation early on in tool design can determine thermal behaviors in steel molds before any expense is made into machining them, enabling quick and comprehensive design for manufacturing (DFM) review and optimization of elements like the heating system and mold cavity entry points. Simulation can catch design mistakes early in the process, saving OEMs time and money.

ProMed’s expertise in silicone injection molding can guide your team’s silicone medical device concept from initial design to product delivery, leveraging these and many other technical advances.

Injection Molding and Its Application to Drug Delivery

Injection Molding and Its Application to Drug Delivery

Injection molding, a manufacturing method used for making everything from car parts to kids’ toys, is also utilized to make life-saving medical devices, including those inserted or implanted into patients’ bodies. Catheters, balloons, and feeding tubes are all made possible and affordable when biocompatible materials combine with injection molding.

As we have discussed before in an earlier article, the material of choice for implantable medical devices is often medical grade silicone. Its range of available durometers, extreme chemical inertness and biocompatibility, excellent tear and heat resistance make it ideal for parts that need to remain in the human body for extended periods of time.

Furthermore, the low viscosity of liquid silicone rubber (LSR) make that elastomer ideal for injection molding (and therefore mass producing) implantable medical devices, making life-saving advances in medical technology more affordable for patients.

Polymers Delivering Doses: Drug-Eluting Implantable Devices

But those advances don’t stop with opening up arteries or providing ports into and out of the body. Increasingly, injection molded implantable medical devices are being used to deliver steady, long-term doses of hormones, cancer drugs and other active pharmaceutical agents (APIs). Injection molding of medical devices is extending its impact into drug delivery.

Drug-eluting medical implants offer several advantages over both pills and injections when it comes to drug delivery. Perhaps the most important clinical benefit is the larger amount of time the API dose is within therapeutic window—the range of concentrations within the body low enough not to be toxic, but high enough to be effective. Both pills and injections produce API concentrations that rapidly rise and then exponentially decay as the body dilutes, metabolizes, and/or excretes the pharmaceutical compounds. By contrast, drug-eluting implants can slowly and steadily release the API at a controllable, optimal rate within the therapeutic window.

These implants are able to do so because the matrix of the device is loaded with the API before they are molded. Silicones, because of the relatively low temperatures at which they can be injection molded and vulcanized and their ability to be compounded with various APIs, are optimal for this application because the injection molding process is less likely to degrade the drug.

Molded medical implants can also provide site specific administration of a drug, and therefore achieve local concentrations of an API that would be above the therapeutic window if present systematically. This enables lower total doses, reduces side effects, and has a greater therapeutic effect.

A third benefit of drug delivery via an implantable device is much greater patient compliance. Since the implant can continually release the drug within the body for several months, there are no daily doses for the patient to forget.

Peering Beyond Silicone

Even as medical grade silicone finds wide use in drug-eluting implantable devices, an exciting new frontier is opening up: expanding beyond silicone into synthetic biodegradable polymers. Such polymers open the door to drug-eluting implants which slowly and safely dissolve away inside the patient’s body, releasing the loaded therapeutic as they do so. These implants don’t need to be removed at the end of the treatment period. Another benefit is the potential to slowly release difficult to deliver API’s, because the therapeutic is released as the polymer encasing the API particles dissolves away, much like an oral pill. This results in steady release rate over time even for these difficult molecules.

Although this new breed of drug-eluting implants won’t be made with silicone, they in all likelihood will still be made via injection molding. Although technical questions still remain—like which polymers in this class have low enough melting points to be molded without significantly degrading any compounded API—injection molding’s ability to produce high volumes at low price per part while at the same time maintaining tight dimensional tolerances, will surely play a key role in this new drug delivery technology.

ProMed Molding. Medical Molding

The Advantages of Medical Grade Silicone for Implantable Devices

From catheters and stents to pacemakers, implantable medical devices help extend and improve the quality of patients’ lives every day. Continued use inside the human body, however, demands many requirements on both the overall design of the device, and the material(s) it’s made of. This is why not all plastics or elastomers are suitable for implantable devices.

The Multiple Requirements of Implantable Devices

For starters, the material must have long-term mechanical stability under the conditions inside the human body. Flaking, cracking, pitting, shearing or otherwise disintegrating could prove deadly for a patient. Materials for implantable devices must also have very low to no toxicity, therefore they must not leech out unintended compounds that could disrupt the complex biochemistry of the patient. The material must be very chemically inert. The material must also have the proper flexibility for its role, so that it can bend along with the surrounding tissues, instead of impacting or puncturing nearby organs.

The material must be resistant to whatever sterilization method is to be used for the device before it is implanted into the patient. Polymers susceptible to hydrolysis (chemical breakdown caused by water or steam) can’t be used for devices that will be sterilized by steam. Plastics with low melting points are also unsuitable in devices that are autoclaved. Of course, materials that can withstand many different sterilization processes (e.g. ethylene oxide, gamma ray, autoclaving) have a large advantage, and play a huge role in the medical industry.

With the rise of drug delivery via implantable devices, another increasingly important requirement is the ability of the material to be loaded with pharmaceutical agents that slowly release them over time. This method of drug delivery has many advantages over pills and injections, one of them being increased time within the therapeutic window.

Finally, implantable medical devices are, after all, still devices which need to be made economically and consistently in order to be safe, effective, and viable in the market. Therefore, materials that are easily molded or extruded in ways that don’t compromise all the other requirements above, can be considered great candidates for these types of products.

Why Medical Grade Silicone is a Medical Device Mainstay

One material that excels at such demands is silicone, which is why medical grade silicone is often the material of choice for implantable devices.

Due to its extreme chemical inertness, durability, stability, wide operating temperature range, and low toxicity, silicones find their way into many applications in consumer goods and in industry (like sealants and lubricants). Medical grade silicone—specific silicone formulations that have been extensively tested for human biocompatibility—brings these benefits to implantable devices.

Medical grade silicone’s exceptional chemical resistance and high heat tolerance make it perfectly suited for all the major sterilization methods used today. Being pliable and soft, it’s also great for prolonged contact with delicate internal tissues and skin.

As an added benefit, it can be can be compounded with various pharmaceuticals, and released at a steady, controlled rate once implanted. The fact that medical grade silicone doesn’t require high temperatures or pressures for injection molding or extruding, makes it a very attractive material for device making since the cycle time per part will be short, making the end product inexpensive to make. If high volume production isn’t necessary, silicone can be compression molded, further adding to its manufacturing versatility.

Here at ProMed Molded Products/Prototypes/Pharma, we have extensive experience with manufacturing implantable devices with medical grade silicone.

By utilizing our technical expertise and robust medical device manufacturing capabilities, we propel our customers’ ideas from design through prototype to full production by delivering medical molding for life.

Minneapolis-St. Paul: Connecting to the Future

ProMed is honored to be feature in the New York Times' Minneapolis-St. Paul: Connecting to the Future.

"THE MINNEAPOLIS-ST. PAUL REGION spends much of the winter in a deep freeze, but its economy burns hot all year, recently outpacing national averages in unemployment, income and wage growth. It’s also home to Fortune 500 companies, research institutions and cultures from around the world."


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Manufacturers Alliance Article "Molding for Life"

ProMed Molded Products was published in the Manufacturers Alliance August newsletter. The article features ProMed's history and purpose.

ProMed has adopted the tag line “molding for life”, affirming pride in producing medical components and devices used to save or improve lives of patients worldwide. Although silicone molded products are used in many different ways, ProMed chose to focus on medical component manufacturing to make a difference and have a long lasting impact on the users of their products. In producing approximately 15 million components and devices every year, a ProMed customer states they “help improve another life every 3 seconds”. This really resonates with the ProMed organization, so “molding for life” seems to fit the way ProMed thinks and how they perform their jobs every day.

Read the full "Molding for Life" article.

Medical Design Briefs Article "Designing for Success with Molded Silicone Components"

Designing a silicone component can be a challenge when trying to balance design for manufacturability and the optimum design for end use. ProMed's New Product Development Tech Center Manager, Jason Nelson, was featured in a Medical Design Briefs article that discusses ways to set up new projects for success and covers ideas for material selection, dimension and tolerance, and critical feature selection. See the Designing for Success with Molded Silicone Components article on Medical Design Briefs' website.