Plastics have transformed the healthcare landscape. Today, everything from syringes and catheters to sophisticated implantable devices are molded rather than machined. This shift isn’t limited to a single manufacturing method – the medical industry relies on a spectrum of specialized injection‑molding techniques to meet diverse performance requirements. Understanding the capabilities of each molding technique helps engineers choose the right process for their project.
This article examines the major types of injection molding used in medical device manufacturing.
Conventional Thermoplastic Injection Molding for Medical Devices
The workhorse of plastic manufacturing is thermoplastic injection molding, which melts resin pellets and injects them into a mold at high pressure. After the part cools, it is ejected and the cycle repeats.
Thermoplastic injection molding produces parts with superior surface finish and is ideal for prototypes and full production runs. Because the process is highly automated and repeatable, it delivers consistent parts with tight tolerances. It is the default choice for enclosures, diagnostic housings, instrument handles, and other high‑volume components in the medical industry.
Medical‑grade thermoplastics such as polypropylene (PP), polycarbonate (PC), and high‑performance polymers like PEEK can be molded to meet FDA and ISO 13485 requirements.
Micro Medical Injection Molding
As devices become smaller and more complex, manufacturers turn to micro injection molding. This method is modeled after conventional molding but on a much smaller scale. Traditional injection molding machines exert thousands of pounds of clamping force, whereas micro injection machines use less than a hundred pounds, allowing them to create parts less than a fraction of an inch across. Micro molding can produce features measured in microns and parts with dimensions smaller than half an inch. Because plastics behave differently at this scale, tool design and material selection are critical.
These tiny components include connectors for implantable pacemakers, hearing aids, microfluidic chips, and drug‑delivery systems. Micro molds are significantly smaller and thinner than conventional tools, allowing greater temperature control and faster cycle times. Because tolerances are measured in microns, micro molding often involves specialized machines and in‑process metrology.
For designers new to medical micro molding, KS Group’s article on what sets medical device injection molding companies apart discusses the importance of material selection and design for manufacturability at miniature scales.
Insert Molding
Insert molding integrates metal or other pre‑formed components into a plastic part. The process places a metal insert into the mold cavity and then injects molten plastic around it, forming a single piece. This approach differs from conventional molding, where the part is entirely plastic. Insert molding is ideal when a component needs the strength of metal and the versatility of plastic, such as threaded inserts, electrical connectors, or sensor housings.
Engineers can use a variety of thermoplastic and elastomeric materials. For example, silicone elastomers are prized for their thermal stability and biocompatibility, making them suitable for prosthetics and surgical instruments. A key advantage of insert molding is design flexibility. It allows complex shapes and multi‑function components – electrical connectors can be built into plastic casings, and catheter tips and surgical instrument handles are often produced via insert molding. By combining multiple components into one, insert molding reduces assembly steps and lowers costs.
Overmolding and Two‑Shot Injection Molding
When a medical device requires multiple materials or textures, manufacturers use overmolding or two‑shot injection molding. Overmolding is a variation of conventional molding in which a soft thermoplastic elastomer is molded over a rigid substrate, creating a single integrated product. A good example is a plastic surgical instrument that has a rubberized grip. The process involves molding the substrate first, then injecting the overmold material onto it. Material compatibility is critical; substrates often use ABS, PP, or PEEK, while overmolds use flexible TPEs or TPUs.
Medical overmolding improves device performance in several ways. Benefits include enhanced durability, better ergonomics, and user comfort through cushioned grips, and the ability to incorporate soft-touch surfaces on surgical instruments, diagnostic devices, and wearable sensors. Overmolding can be performed through insert molding or two‑shot molding; the choice depends on design complexity and volume.
Two‑shot injection molding – also called dual‑shot or 2K molding – takes multi‑material integration a step further. In this process, two different polymers or colors are injected into the same mold during a single cycle. The first material forms the substrate, after which the mold rotates or shifts to align the part for the second injection. A specialized machine with two injection units ensures precise control over each material. Because both shots occur in one machine, the process is highly automated and produces a strong chemical bond between materials, eliminating assembly.
Two‑shot molding is ideal for high‑volume production of complex, ergonomic medical tools where a soft grip must seamlessly bond to a rigid body. It offers higher consistency and faster cycle times than overmolding but requires more expensive tooling and stricter material compatibility. Designers should weigh the part’s complexity, production volume, and budget when choosing between two‑shot and overmolding. For more insights on manufacturing techniques, explore KS Group’s comparison of plastic injection molding vs. alternative methods.
Liquid Silicone Molding (LIM) and Micro Molding
Another specialized technique is Liquid Silicone Molding (LIM), which uses liquid silicone rubber (LSR). LIM injects liquid silicone into a pre‑made mold and uses heat to cure and solidify the shape. Silicone offers durability, strength, and flexibility – LIM parts can stretch up to six times their length without breaking. Silicone also boasts fire‑retardant properties and excellent biocompatibility, making it suitable for implantable devices. Components produced via LIM maintain tight tolerances and are highly accurate. Typical medical applications include catheters, seals, gaskets, respiratory masks, and elastomeric valves.
Within LIM, micro molding is used for components requiring extremely small shot volumes (often under six grams) and tight tolerances. This variant is common in minimally invasive surgical tools, valves, micro‑sensors, and microfluidic devices. Silicone’s ability to bond to metal inserts also supports silicone‑to‑metal bonding/insert molding, enabling durable hybrid components. Learn more about biocompatible materials for medical injection molding on the KS Group blog.
Gas-Assisted Injection Molding
For larger components or parts with thick ribs, gas-assisted injection molding offers efficiency and material savings. Gas‑assisted molding injects a small amount of molten plastic into the mold, followed by high‑pressure nitrogen gas. The gas flows into thicker sections and displaces the plastic outward, forming a hollow core and pushing molten material toward the cavity walls. This process reduces sink marks and warping, improves surface quality, and allows for complex, lightweight parts. Because the gas core acts as a heat insulator, cycle times are shorter and tooling experiences less wear and tear.
For medical devices, gas‑assisted molding is valuable when designing lightweight yet strong housings, ergonomic handles, or large panels that would otherwise require thick, heavy walls. The technique allows manufacturers to produce precise, intricate components for medical devices. The specialized equipment and mold design, however, make gas‑assisted molding most cost‑effective for high‑volume production.
Reaction Injection Molding (RIM)
Reaction Injection Molding differs from traditional injection molding because it uses low‑viscosity liquid polymers that react and cure inside the mold. RIM is a low‑pressure, low‑temperature thermoset process in which two liquid polymers are impinged together and then injected into a mold to polymerize and cure. RIM offers numerous advantages: lower tooling costs, complete design freedom, high strength‑to‑weight ratio, elimination of secondary operations, absence of sink marks, and reduced part weight. Because it operates at lower pressures and temperatures, engineers can produce large or thick-walled parts without expensive molds.
RIM is well‑suited for lower‑volume production and large, complex housings, such as casings for diagnostic equipment, imaging machines, and laboratory devices. The process is also used in healthcare, medical devices, and laboratory/diagnostic equipment. With continuing innovations in polyurethane chemistry, RIM parts can achieve a wide range of mechanical properties and aesthetics.
Choosing the Right Plastic Injection Molding Technique
Selecting the appropriate injection‑molding process depends on the part’s size, material requirements, production volume, and performance goals. Conventional thermoplastic molding remains the default for many medical housings and disposables, while micro molding serves miniaturized sensors and implantable devices. Insert molding integrates metal or electronics into plastic parts, and overmolding provides ergonomic grips and soft‑touch surfaces.
Two‑shot molding combines multiple materials in a single cycle for high‑volume, complex parts. Liquid silicone rubber molding offers unparalleled flexibility and biocompatibility, especially for micro‑scaled components. Gas‑assisted molding produces lightweight, hollow parts with excellent surface quality, and reaction injection molding enables large, low‑volume parts with low tooling costs.
Manufacturers like MOS Plastics provide all of these capabilities under one roof, operating ISO‑certified clean rooms and partnering with OEMs from design through production. When selecting a process, engineers should also consider material selection and sustainability.
By understanding the strengths of each injection‑molding technique, medical device developers can design products that meet clinical performance requirements, regulatory standards, and cost targets. As micro‑technology advances and healthcare demands more sophisticated devices, the right molding partner and process will be essential to bringing innovations to market.
Ready to explore injection molding for your medical device project? Contact KS Group’s medical injection molding experts to discuss your requirements and discover how our advanced manufacturing capabilities can bring your vision to life.