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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.


Medical Silicone Molding Company FDA Ready

Is Your Medical Silicone Molding Company FDA Ready?

As a medical device original equipment manufacturer (OEM), you already know that staying in compliance with the United States Food & Drug Administration (FDA) regulations requires being on the ready for unannounced visits from FDA inspectors.

However, with today’s increased regulatory scrutiny on the whole medical device supply chain, your company isn’t the only one that needs to stand ready. Your medical silicone molding supplier needs to be ready as well.

Day-to-day compliance to 21 CFR Part 820 is not just having “i’s dotted and the t’s crossed” on paperwork. Due to today’s advanced structure of global supply chain management and converging quality standards, it becomes increasingly important to have all plants compliant as a bare necessity.

In this article, we’ll focus on four ways to encourage your medical silicone molding supplier to become FDA ready; that is, if they aren’t already (but like ProMed, they already should be).

Fostering a Culture of Compliance

Your medical silicone molding partner needs to actively and continuously foster a culture of compliance throughout their entire organization. This includes training and empowering employees to speak up when they notice noncompliance. This helps the entire organization quickly find and resolve potential compliance issues before an FDA auditor might. A real culture of compliance motivates all employees to follow the required regulatory requirements applicable to their daily work, and thus minimizes the likelihood employees will cut corners, ultimately reducing your noncompliance risk.

A true culture of compliance extends beyond just following the regulatory rules. Encouraging personal development & growth, and ensuring that each employee has the skill sets necessary to perform their jobs, must also be a priority for your supplier. Like so many other things, the proof is in the procedural output, which naturally leads us to the next critical area: Training.

Training

Digital and paper proof of hands-on training, procedure training meetings, and even online training must be consistently and accurately recorded. Especially records for the training required at regular intervals, like annual safety training or gowning procedures for cleanroom operations. It must also be triggered from the introduction of a new manufacturing process, or the production of a new customer product.

All employees must also familiarize themselves with their quality management system (QMS) documentation in order to implement the policies and procedures in their work. FDA and even customer scheduled auditors can, and do, quiz those on the shop floor about their company’s SOPs, quality policies, and FDA’s quality requirements.

Meeting Standards

Of course, employees are much more likely to accurately recall both their company’s quality policies, and the applicable procedures that adhere to FDA rules, if all procedures and processes are written and performed with FDA inspections in mind. This is one key reason why medical device OEM’s and medical suppliers achieve an ISO 13485 certification: if the companies can demonstrate that they consistently are performing and conforming to the FDA’s Part 820 requirements, and that they strive to continuously improve their processes to achieve and even exceed ISO 13485 standards, it virtually guarantees their compliance & re-certification. And it also means that if they stay vigilant, they shouldn’t really have to worry about an FDA auditor stopping by unannounced.

As part of your own supplier quality performance program, you should ensure your medical silicone molding vendors all have these practices as a priority.

Internal and Customer Audits

In addition to your silicone molding vendor’s own internal audits, and those of its ISO registrars (if they’re ISO certified), your audits can also help keep this critical supplier on their toes and compliant, minimizing your own regulatory risk as well as ensuring the quality and safety of your products.

Internal, registrar, and customer audits force suppliers to maintain and demonstrate regulatory and quality compliance in practice (and not merely on paper). As much as reasonable, these audits should mimic the regulatory ones. If an FDA auditor were standing over the shoulder of each of your silicone molding vendor’s employees, what would they see?

Wouldn’t you want to find out before the FDA does?

The Cost of Not Being FDA Ready

If your medical silicone molding supplier isn’t ready for an FDA audit, it’s very likely that significant negative findings will result, or worse, that supplier could fail the audit and be shut down, leaving you without crucial components. There is a serious business cost to FDA non-compliance.

ProMed’s customers need not worry about that. As a medical silicone molding company passionately focused on the quality of our people, processes, and products, we are always FDA ready. Which means we are always ready to partner with you.


Thermoplastic & Silicone Use for Medical Molded Components

High quality medical components—whether for implants, instruments, or IV bags—must be safe and durable, because lives literally depend on them. Low toxicity, high biocompatibility, chemical inertness, and the ability to repeatedly withstand sterilization environments (like gamma rays, steam, or EtO) are all requirements of the materials that long-life medical components are made of. The ability to safely reuse the same medical devices reduces the cost of medical care, while re-use procedures put in place protect not only the healthcare professional, but also the patient’s health and safety.

For disposable, single-use medical parts where repeated sterilization isn’t a requirement, the toxicity and biocompatibility requirements, however, still apply. In addition, those single-use parts must be cost-effective for the manufacturer and affordable for the consumer.

For both categories of medical components, plastics and elastomers (rubbers) are the materials of choice. Since medical grade thermoplastics (which harden when cooled down to near ambient temperature) and silicones (like LSR which permanently sets when heated) have a lot to offer designers of new molded medical components, we’ll be discussing them in this post.

Silicone: Safe and Versatile

Let’s begin with silicone, which has many chemical and mechanical properties well suited for medical molded components:

Very chemically inert: Medical grade silicone resists attack from disinfecting chemicals and biochemical interaction. Medical grade silicones have excellent biocompatibility.

Strong, flexible, and durable: Silicones have high tear and tensile strength, great elongation, and low compression set, even over a wide temperature range. They’re high elasticity and flexibility is a great match for applications such as feeding tubes and seals for peristaltic IV drug delivery pumps.

Sticky when it needs to be: Although silicone has a low surface energy (and is thus used in applications that need to repel liquids), there are formulations of Liquid Silicone Rubber (LSR) that are self-adhesive and can stick to other plastics without priming. Overmolding silicone to specific thermoplastics is a common occurrence for durable medical devices that need extra grip capabilities for the doctor/nurse.

Silicone is permeable and thus makes a great matrix for pharmaceutical delivery in drug-eluting implants.

Wide range of available durometers: from 0 Shore A to 80 Shore A. This customizability makes it great for applications like clinical and surgical instrument grips, gaskets and o-ring seals.

There’s a Great (Medical) Future in Plastics

All of these features are why medical grade silicones have been widely used for decades, and will continue to be considered, in spite of LSR’s higher cost compared to some other resins.

But they are not the only game in town when it comes to molded medical components. Thermoplastics are also popular choices, especially for niche use. With so many different polymers (and varying molecular weights of each polymer), this family of plastics exhibits a wide gamut of thermal, chemical, and mechanical properties. A few of these are worth mentioning here:

Polysulfone (PS): This thermoplastic elastomer (TPE) has excellent resistance to both hydrolysis and heat, and thus can be sterilized by steam and autoclaving. PS has great biocompatibility, and can be thermoformed by injection molding and extruding.

Polyether ether ketone (PEEK): PEEK maintains its excellent chemical resistance and mechanical stability at high temperatures (and thus can be sterilized by heat and disinfected by chemical agents). Like both silicone and PS, PEEK can be molded, albeit at very high temperatures (PEEK melts at about 343°C). Since PEEK resists biodegradation, it’s a good candidate for implantable medical devices.

Medical device OEM’s can choose from many polymers for their next innovative, life-saving product. Although silicones continue to dominate (particularly in implantable devices), some thermoplastics have long been chosen for use in healthcare.

ProMed’s extensive LSR and thermoplastic expertise and manufacturing capabilities can take your molded medical product from concept to completion, just as we have for so many other global OEMs.