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The Introduction of Common Engineering Plastics in the Medical Field

Date: Sep 07 2020 | By:LC Rapid
Tag:Medical, Materials, Plastic Injection Molding,

This article mainly introduces medical engineering plastics commonly used, which are composed of materials that can be easily machined into shapes. These plastics tend to cost more relative to weight because most materials are lost due to the debris during processing.

Some of the more accessible materials that suitable for processing are listed below, which are sold in grades that meet FDA standards. These materials are listed together with the plastic supplier as precision engineering plastics. Please consult your supplier to ensure that the materials you order meet FDA standards and your intended use. Not all plastics listed here have a USP VI rating. However, if the application does not involve patient contact or the liquid for patient contact flow through, this is not a problem.

We also listed some high-performance engineering plastics. These are suitable for applications with special requirements. Compared with general-purpose plastics, the cost of these plastics can be very high.

1. Acrylonitrile-butadiene styrene (ABS)

The terpolymer is made of SAN (styrene-acrylonitrile) and butadiene synthetic rubber. SAN (styrene-acrylonitrile) gives ABS hardness and surface finish, butadiene gives it toughness. Plastics are usually available to make 4-inch thick plates and 6-inch diameter rods, which can be easily bonded and laminated to form thicker plates and components. Due to its reasonable cost and easy processing, it is a popular material for manufacturing computer numerical control (CNC) prototypes.

2. Acrylic resin (PMMA)

Acrylic resin is actually one of the earliest medical device plastics, which is still commonly used in the molding of anaplastic restorations. *Acrylic acid is essentially polymethyl methacrylate (PMMA).

Acrylic resin is firm, clear, machinable and adhesive. A common way to bond acrylic acid is to bond it with a chloromethane solvent. Acrylic acid has almost unlimited kinds of rods, sheet and plate shapes, and various colors. Acrylic resins are particularly suitable for light pipes and optical applications.

Acrylic resin for signage and display can be used for benchmark tests and prototypes; however, must be careful of the medical grade version before using it in any clinical trials. Commercial grade acrylic resins may contain UV resistance, flame retardants, impact modifiers and other chemicals, making them unsuitable for clinical use.


3. Polyvinyl chloride (PVC)

PVC comes in two forms, rigid and flexible, depending on whether or not plasticizers are added. PVC is usually used for water pipes.

The main disadvantages of PVC are poor weather resistance, relatively low impact strength, and the weight of the thermoplastic sheet is quite high (specific gravity 1.35). It is easily scratched or damaged, and has a relatively low thermal deformation point (160).

Unplasticized PVC is produced in two main formulations: Type I (corrosion resistance) and Type II (high impact). Type I PVC is the most commonly used PVC, but in applications requiring higher impact strength than that provided by Type I, Type II has better impact resistance and corrosion resistance slightly reduced. In applications requiring high-temperature formulations, polyvinylidene fluoride (PVDF) for high-purity applications can be used at approximately 280°F.

4. Polycarbonate (PC)

Polycarbonate (PC) is the toughest transparent plastic and is very useful for prototype medical devices, especially if UV curing bonding is to be used. PC is easy to combine, which has several forms of rod, plate and sheet.

Although more than a dozen performance characteristics of PC can be used alone or in combination, seven are the most commonly relied upon. PC has high impact strength, transparent water transparency, good creep resistance, wide operating temperature range, dimensional stability, wear resistance, hardness and rigidity, despite being ductile.

PC is easily discolored by radiation sterilization, but radiation stability grades are available.

5. Polypropylene (PP)

PP is a light-weight, low-cost polyolefin plastic with low melting point, so it is very suitable for thermal forming and food packaging. PP is flammable, so if you need fire resistance, looking for flame retardant (FR) grades. PP is resistant to bending, commonly known as "100-fold glue". For applications that require bending, PP can be used.

6. Polyethylene (PE)

Polyethylene (PE) is a common material used in food packaging and processing. Ultra-high molecular weight polyethylene (UHMWPE) has high wear resistance, low friction coefficient, self-lubricity, surface non-adhesion and excellent chemical fatigue resistance. It also maintains high performance at extremely low temperatures (for example, liquid nitrogen, -259°C). UHMWPE starts to soften and loses its abrasion resistance around 185°F.

It is not recommended for performing precision tolerance application in these environments, since UHMWPE has a relatively high expansion and contraction rate when temperature changes.

PE may be difficult to bond due to its high surface energy, non-adhesive surface. Components are easiest to fit together with fasteners, interference or snaps. Loctite Corporation produces cyanoacrylate adhesives (CYA) (LoctitePrism surface-insensitive CYA and primer) for bonding these types of plastics.

UHMWPE also gets great success in orthopedic implants, which is the most commonly used material in the acetabular cup duringtotal hip replacement arthroplasty and the tibial plateau component during total knee replacement arthroplasty. It is suitable for highly polished cobalt-chromium alloys. *Please note that the materials suitable for orthopedic implants are special materials, not industrial versions. Medical grade UHMWPE is sold under the trade name Lennite by Westlake Plastics (Lenni, PA).

7. Polyformaldehyde (POM)

DuPont's Delrin is one of the best-known POMs, the name most designers use for the plastic. POM is synthesized from formaldehyde. POM was originally developed in the early 1950s as a tough, heat-resistant non-ferrous metal substitute. It is a tough plastic with a low coefficient of friction and high strength.

Delrin and similar POM are difficult to bond, and mechanical assembly is best. Delrin is commonly used for machined medical device prototypes and closed fixtures. Its high process-ability makes it ideal for prototyping machining equipment that requires strength, chemical resistance, and  materials that meet FDA standards.

One disadvantage of Delrin is its sensitivity to radiation sterilization, which tends to make POM brittle. Snap-fit, plastic spring mechanism and thin section under load may break with radiation sterilization. If sterilizing B POM components, please consider using EtO, Steris or autoclaves, depending on whether the device contains any sensitive components, such as electronic devices.

8. Nylon (polyamide, PA)

Nylon is available in 6/6 and 6/12 formulations. Nylon is tough and heat resistant. The identifiers 6/6 and 6/12 refer to the number of carbon atoms in the polymer chain, and 6/12 is a long-chain nylon with higher heat resistance. Nylon is not as processable as ABS or Delrin (POM) because it tends to leave sticky chips on the edges of parts that may need to be deburred.

Nylon 6, the most common is cast nylon, which was developed by DuPont before World War II. However, it was not until 1956, with the discovery of compounds (co-catalysts and accelerators) that cast nylon became commercially viable. With this new technology, the polymerization speed is greatly increased, and the steps required to achieve polymerization are reduced.

Due to fewer processing restrictions, cast nylon 6 provides one of the largest array sizes and custom shapes of any thermoplastic. Castings include bars, tubes and plates. Their size ranges from 1 pound to 400 pounds.

9. Fluorinated ethylene propylene (FEP)

Fluorinated ethylene propylene (FEP) has all the desirable properties of tetrafluoroethylene (TFE) (polytetrafluoroethylene [PTFE]), but has a lower survival temperature of 200°C (392°F). Unlike PTFE, FEP can be injection molded and extruded into bars, tubes and special profiles by conventional methods, bars up to 4.5 inches and plates up to 2 inches are available, which becomes a design and processing advantage over PTFE. The performance of FEP under radiation sterilization is slightly better than that of PTFE.

High-performance engineering plastics

Note: The availability of these high-performance engineering plastics (special orders) may be more restricted. Some can also be quite expensive.

10. Polyetherimide (PEI)

Ultem 1000 is a thermoplastic polyetherimide high-heat polymer, designed by General Electric Company for injection molding. Through the development of new extrusion technology, manufacturers such as A. L. Hyde, Gehr and Ensinger produce various models and sizes of Ultem 1000. Ultem 1000 combines excellent processability and has cost saving advantages compared to PES, PEEK and Kapton in high heat applications (continuous use up to 340°F). Ultem is autoclavable.

11. Polyetheretherketone (PEEK)

Polyetheretherketone (PEEK) is a trademark of Victrex plc (UK), which is a crystalline high-temperature thermoplastic with excellent heat and chemical resistance as well as excellent wear resistance and dynamic fatigue resistance. It is recommended for electrical components that require high continuous operating temperature (480°F), exposing to flames and emit extremely low emissions of toxic smoke.

PEEK meets Underwriters Laboratories (UL) 94 V-0 requirements with 0.080 inches. The product has extremely strong resistance to gamma radiation, even beyond that of polystyrene. The only common solvent that can attack PEEK is concentrated sulfuric acid. PEEK has excellent hydrolysis resistance and can operate in steam up to 500°F.

12. Polytetrafluoroethylene (PTFE)

TFE or PTFE (polytetrafluoroethylene), commonly known as Teflon, is one of the three fluorocarbon resins in the fluorocarbon class, which is composed entirely of fluorine and carbon. The other resins in this group, also known as Teflon, are perfluorooctane alkyl fluorocarbons (PFA) and FEP.

The forces that bind fluorine and carbon together provide one of the strongest known chemical bonds among closely symmetrically arranged atoms. The result of the bond strength in this kind with chain configuration is a relatively dense, chemically inert, and thermally stable polymer.

TFE resists heat and almost all chemicals. It is insoluble in all organic matter with the exception of a few foreign species. Its electrical performance is very good. Although it has high impact strength, compared with other engineering thermoplastics, it has lower wear resistance, tensile strength and creep resistance.

TFE has the lowest dielectric constant and lowest dissipation factor of all solid materials. TFE is almost unattractive to different molecules because of its strong chemical connection, which results in a friction coefficient as low as 0.05. Although PTFE has a low coefficient of friction, it is not suitable for load-bearing orthopedic applications due to its low creep resistance and low wear properties. Sir John Charnley discovered this problem in his pioneering work on total hip replacement in the late 1950s.

Note: Fluoropolymers (TFE and PTFE) are sensitive to radiation sterilization. PTFE is obviously difficult to bond, however, chemical or plasma etching can be used to create a cohesive surface.

13. Polysulfone and polyphenylsulfone (PSF or PSU for short)

Polysulfone was originally developed by BP Amoco, currently manufactured by Solvay under the trade name Udel, while polyphenylsulfone is sold under the trade name Radel. Polysulfone, designed for FDA-approved equipment, is a tough, rigid, high-strength transparent (light amber) thermoplastic that can maintain its properties in a wide temperature range from -150°F to 300°F, passing all USP Class VI (biological) tests, which meets the drinking water standards of the National Sanitation Foundation, up to 180°F.

Polysulfone has very high dimensional stability. After exposure to boiling water or air at 300°F, the linear dimensional change is usually 1% or less one-tenth. Polysulfone has high resistance to inorganic acids, alkalis and salt solutions. Even at high temperatures under moderate stress levels, it has good resistance to detergents and hydrocarbon oils. Polysulfone is not resistant to polar organic solvents such as ketones, chlorinated hydrocarbons and aromatic hydrocarbons.Radel is used for instrument trays that require high heat resistance and high impact strength, and for hospital autoclave tray applications. Polysulfone engineering resin combines high strength and long-term resistance to repeated steam sterilization. These polymers have proven to be substitutes for stainless steel and glass. Medical grade polysulfone is biologically inert, has a unique long life in the sterilization process, can be transparent or opaque, and is resistant to most common hospital chemicals.

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