Time to read: 5 min
Injection molding for medical devices can use plastics, thermoplastic elastomers (TPE), silicones, metals, or composite materials. There are many types of injection moldable plastics, but medical device designers often choose polypropylene, polyethylene, polystyrene, or polyetheretherketone (PEEK), depending on the application. Silicone elastomers also offer a choice of materials and combine chemical stability with support for sterilization. Metals provide strength and can resist microbial growth while also supporting medical injection molding.
In addition to conventional plastic injection molding, there are six main types of injection molding used in medical devices:
- Thin wall injection molding
- Gas-assisted injection molding
- Overmolding
- LSR injection molding
- HCR injection molding
- Metal injection molding
This article describes each of these methods, and if you’re looking for information about the best way to design injection molded parts for medical devices, check out our design guide!
Download Fictiv’s Injection Molding Design Guide
Thin Wall Injection Molding
Thin wall injection molding produces thin, lightweight parts for reduced material costs and faster cycle times. As a medical molding process, it’s typically used for small, complex parts that require a high degree of precision. The thinness of the wall is important because the size of the part limits how thin the wall can be. Typically, injection molded parts with a wall thickness of less than 1mm are considered to be thin-walled.
Because thin wall injection molding can provide significant cost savings, it’s often used with high-volume production. Potential applications for thin wall injection molding include small enclosures for medical devices. And Fictiv offers cost-effective steel molds for medical device prototype development and low-volume manufacturing — an ideal solution for medical device designers looking for an alternative to 3D printing to ensure that the material used for functional samples can be used for high-volume production.
Gas-Assisted Injection Molding
Gas-assisted injection molding is a low-pressure plastic injection molding process that heats plastic, injects it into the mold, then injects pressurized nitrogen or carbon dioxide gas. In turn, the gas pushes the molten plastic into the mold’s extremities and produces thicker walls with hollow sections. When molding is complete, the gas is vented and the parts are removed. Typically, this process is used for plastic parts that have large surface areas, and require detailed textures and superior surface quality.
Like thin wall injection molding, gas-assisted injection molding reduces the amount of plastic required, decreases cycle times, and creates significant material cost savings. Gas-assist plastic injection molding also improves part appearance by reducing sink marks, blemishes, warpage, and distortion — the gas packs out the resin removing blemishes as the plastic solidifies. Also, because molding pressures are lower, there’s less residual stress on parts and less wear on molds.
Overmolding
Overmolding is a form of injection molding that molds a polymer over a substrate. It eliminates the need for assembly after molding. Overmolding is used to produce handles on surgical instruments, but it can also be used in medical products without any metal parts — like silicones or thermoplastic elastomers (TPE) overmolded onto thermoplastic substrates.
Overmolding isn’t just about layering one material on top of the other, however. Chemical bonds form at the molecular level and are a function of how well the overmold material wets the substrate. Mechanical bonds depend on the physical geometry at the interface of the substrate and the overmold. Two-shot molding, or double-shot molding, is a two-step continuous process that first molds a substrate, then over molds that substrate with a different material. Pick-and-place molding is used mainly for prototyping and for low-volume production.
LSR Injection Molding
LSR injection molding is used with liquid silicone rubber (LSR), a two-component elastomer that requires curing — a chemical process that crosslinks polymer chains — to achieve the material’s end-use properties. Typically, the components for LSR injection molding come in a 5-gallon pail or a 55-gallon drum. They are poured into a static mixer and fed into the injection unit, where additional mixing occurs. The injection screw then pushes forward the shot, or the amount of material, that’s needed to fill the mold.
The two halves of the mold remain clamped during curing, a process that averages 30 seconds with LSRs. By contrast, cure times for HCR, or high-consistency silicone rubber (see the next section) last five minutes or longer, especially if the rubber uses a peroxide curing agent. Injection-molded LSR can also be platinum-cured for greater optical clarity, tensile strength, and tear strength. Importantly, platinum-cured silicones have fewer extractables, aka compounds that can leach out of the material. With higher purity and faster cycle times, platinum-cured LSRs are used in high-volume medical applications like spacers for asthma inhalers.
HCR Injection Molding
HCR injection molding is used with high-consistency silicone rubber (HCR), or heat-cured silicone rubber. Typically, this injection molding process involves heating a gum-like raw material and injecting it into a mold under high pressure. The injection screw preheats the gum rubber to reduce its viscosity, or resistance to flow. The silicone then fills the mold cavity completely, and the preheating at the beginning of the molding cycle helps to fill the cavity more rapidly.
HCR’s higher viscosity makes this material more challenging to process, and HCR injection molding is generally more complex than LSR injection molding. Still, HCR can achieve better overall properties than LSR rubber. Depending on a part’s geometry, however, high shear conditions in the mold can cause shrink rates to vary — which underscores the importance of proper tool design and an experienced manufacturing partner. Applications for HCR silicone injection molding include gaskets and O-rings for medical devices.
Metal Injection Molding
Metal injection molding (MIM) mixes powdered metal with a thermoplastic binder and heats this material so that the metal grains bond to the thermoplastic. The material is then heated again and injected into a mold to create the part. After the part is ejected, the thermoplastic binder is removed. The next step is sintering, a heat treatment process that subjects the part to high temperature and pressure. Sintering compacts any loose material, and this fusion results in a net-shape or near-net-shape part. Depending on the injection molded product, post-sintering operations may be required.
MIM supports the high-volume production of medical parts with complex shapes in various sizes, and with superior strength and an excellent surface finish. This form of medical injection molding also provides a cost-effective alternative to CNC machining for parts made of difficult-to-machine metals such as stainless steels, cobalt-chromium alloys, and titanium alloys. Importantly, the dimensions and end-use properties of metal injection molded parts are comparable to those of wrought and cast materials. Applications include surgical instruments and implants, both of which use stainless steel.
Complex Medical Parts at Ridiculous Speeds
Do you need medical injection molding for prototyping, low-volume, or high-volume production? A good plan and skilled production partner are critical to avoid the added injection molding costs and risks that come with inadequate drying or improper processing methods.
Fictiv has the global manufacturing network and production experts you need to get quality medical injection molded parts, no matter how complex your designs. And we have the skills and know-how to support you from part design through prototyping and production.
Create your free Fictiv account and request an injection molding quote today — we mold complex parts at ridiculous speeds!
