Medical Plastic Failure: Why It Happens and What OEMs Can Do About It
Improper material choices are a common cause of failure in medical devices. This can lead to serious consequences for OEMs, including expensive warranty claims, legal disputes and product recalls.
Brand and market perception can also be adversely affected if customers and end users associate a company with poor quality components. Combined, these consequences can have a significant impact on an OEM’s bottom line.
To avoid these problems, it is essential to identify common types of failures and their underlying material causes. By having a better understanding of a plastic’s strengths and weaknesses, design engineers and manufacturers can avoid future problems through appropriate use and application of the material. Ultimately, this will allow OEMs to deliver the best products to their customers.
Types of Medical Plastic Failure
There are three general types of plastic failure: mechanical, thermal and chemical.
Mechanical failure occurs when a product is exposed to external forces that are greater than the product is designed to handle. This ultimately causes the product or component to deform, crack, or break when the yield strength of the material is exceeded. When it comes to mechanical stress, failure isn’t always sudden. Repeated exposure to the same inappropriate force or condition over time will weaken the part little by little until it finally fails.
Thermal failure occurs as a result of exposure to extreme temperatures. Very high temperatures can cause the plastic part to melt and warp, while extremely cold temperatures can cause the plastic part to become brittle, making it prone to cracking and shattering. Plastics can also deteriorate as a result of overheating during processing, which may lead to color change or property reduction. Long term heat aging can also lead to reduction of properties at lower temperatures.
Chemical failure affects the strength, flexibility, appearance, and weight of plastic components. This happens in one of three ways: chemical attack on the plastic, physical changes such as softening and swelling, and stress cracking. Chemicals that do not normally affect the properties of an unstressed thermoplastic may cause stress cracking under thermal or mechanical stress such as constant internal pressure or frequent thermal or mechanical stress cycles.
Causes of Medical Plastic Failure
There are four key areas to look at when determining the cause of plastic failure. Although failures don't always fit neatly into a single area, we will look at each category assuming there is no influence from any other factors.
Medical device recalls due to material failure can often be traced back to the material selection process. Manufacturers must take into account a range of factors when selecting materials, but perhaps most important is the intended use.
By carefully considering a material’s mechanical, thermal, environmental, electrical, and chemical aspects, manufacturers can select a material that will serve its appropriate function.
Best Practices for Using Material Data Sheets
One of the most significant contributors to poor material selection is an overreliance by engineers on material data sheets. This is because the properties listed on datasheets are short-term, single point measurements, and there is no consideration of time, temperature, environment or chemical contact.
Rather than being used as the sole source for engineering design or final material selection, data sheets can instead be used to compare similar grades of plastics from different suppliers. However, they should not be used to compare significantly different materials as they may fail in completely different ways.
Polymer suppliers provide material data sheets with properties derived from standardized testing on ideal test specimens processed and tested under ideal laboratory conditions. ASTM or ISO test conditions are usually 23°C and 50% relative humidity, which rarely corresponds to the product service conditions you have in mind.
That’s why the disclaimer on every data sheet states that the properties are typical for the material — they should not to be used for specifications or design.
Improving Material Selection
Serious product failures can result from selecting the wrong polymer. This is especially true in applications involving chemical attack and environmental stress cracking.
Always ask your material manufacturer for advice during your material selection process to benefit from their experience and materials expertise. Chances are that your supplier has additional data to help you properly evaluate the performance capabilities of the material for your application.
After material selection, design is another contributor when it comes to medical plastic failure. Not only does a product need to be designed with its function in mind, it must also consider the properties of the chosen polymer.
There are no absolute rules for designing plastic components, and design criteria changes from polymer to polymer and application to application. Since different materials respond differently, the way a manufacturer approaches design is heavily contingent upon the product’s composition.
Tips for Improving the Design Process
You'll want to consider several things on the front end of your design process that will save time and money during production. The following list is by no means exhaustive, but it will serve to point you in the right direction.
Uniform wall thickness: uniform wall thickness is a key consideration for plastics design. Uneven wall thicknesses can restrict material flow and greatly increases the likelihood of sink marks, warpage, voids, and molded-in stress. If wall thickness must be uneven, design smooth transitions that taper over some distance. Part size and material flow will determine the minimum wall thickness allowed for your plastic injected component.
Avoid sharp corners: Sharp corners are a leading cause of part failure. Sharp edges, such as corners of a square hole, will produce a part with high levels of molded-in stresses, which compromises toughness and strength. Adding radii to sharp corners redistributes the stress more evenly. Corner radii should be as large as possible to allow material flow and improved strength distribution.
Gate location: Gate location can make or break a part design, yet many CAD designers leave that detail to the mold designer. This is a big mistake. Along with choosing the wrong gate size and type, a less-than-ideal gate location is the root cause of many molding difficulties. For a part with both thick and thin areas, the gate should be located in the thick areas. Resin flowing through the thick area retains heat long enough to continue flowing and fill the adjoining thin areas. Where the weld lines are, where the sink marks or voids will be, and how the part warps — all of these are determined by where the molten plastic is injected into the part cavity.
Draft angles: Draft is the angling of otherwise vertical walls in order to make it much easier for a part to release cleanly from a mold. Draft angles greatly reduce friction between the finished, cooled part and the side of the mold. The lack of an appropriate draft will make the removal of plastic parts almost impossible.
Ribs: Ribs are a great way to add strength and stiffness to a molded part while keeping material consumption to a minimum. The rib thickness should never exceed the wall thickness of the part. Thick or deep ribs can cause sink marks and filling problems respectively. This can be resolved by using multiple thinner or shorter ribs instead. Other features like gussets and bosses are used in conjunction with ribs to maximize structural support.
The manufacturing process is crucial when it comes to preventing plastic failure. Even the best plastic designs with good material selection can fail due to a disregard for processing procedures and guidelines provided by material manufacturers. This is often driven by the need to achieve reduced cycle times, higher production yield or better looking parts.
Optimizing the Manufacturing Process
Material processing: How the material is processed needs to be taken into account. Data sheet properties are derived from testing injection molded tensile bars. If your process is different than that, the properties may shift. Be sure to test your material the way it will be processed for your application at the temperatures, pressures and humidity to which it will be exposed.
Drying resin: Drying resin before processing is necessary to remove moisture and ensure maximum polymer performance. Nearly all polymers absorb moisture. How much any given polymer can absorb depends upon the chemistry of the polymer and the atmospheric conditions. Desiccant dryers are most commonly used, but other options like hot-air, vacuum and compressed-air dryers are available. Polymer manufacturers provide specific recommendations for drying times, temperature and the moisture content (dew point) of air being supplied to the dryer.
Material testing: While some materials are dried solely to optimize surface appearance, many polymers suffer irreversible structural damage due to hydrolysis. This is a chemical reaction that breaks long polymer chains into shorter fragments, reducing strength and other properties. When it comes to judging the physical properties of a molded part, you cannot rely on visual inspections. Some molded parts with poor surface aesthetics can be structurally sound; some molded parts may look fine on the outside but they are structurally weak and have concentrated areas of molded-in stress. Because of this, it’s important to test the molded part under exacting conditions to ensure that it can stand up to its intended use without any failures or issues.
Other considerations: There are a host of other factors like mold temperature, fill balance, fill time, injection pressure and cavity pressure that must be optimized during the manufacturing process to help protect the product against process-induced flaws that could lead to various kinds of failure. On-site assistance with molding trials and production runs should be a consideration when selecting your materials supplier.
Although rough handling and misuse certainly contribute to premature part failure, more often failure is the result of not fully understanding the end-use requirements. For example, the actual load is greater than expected load, or the protocol for disinfection and sterilization is not compatible with the specified material, causing the polymer to degrade.
Disinfectants and Medical Polymer Failure
One of the biggest causes of plastics failure in healthcare related to service conditions is environmental stress cracking. In an effort to curtail hospital acquired infections (HAIs), hospitals and healthcare providers have started using stronger disinfectants, and using them more aggressively.
This has led to an industry-wide problem related to medical equipment housings, which are cracking within months of being used in the healthcare environment. Subsequent recalls and repairs can be costly in terms of equipment downtime.
Only a handful of specialty polymers for healthcare have the chemical resistance to stand up to these new procedures, and they are quickly replacing lower performing polymers, such as polycarbonate (PC), acrylonitrile butadiene styrene (ABS) and polybutylene terephthalate (PBT), that are being pushed past their performance limits.
Evaluating Polymers for Environmental Stress Cracking Resistance
Environmental stress cracking (ESC) is a leading cause of plastic failure in healthcare for medical devices and components that come in contact with cleaners, disinfectants, and other chemical agents.
A chemical attack is considered ESC when the chemical would not have reacted with the plastic in an unstressed state. ESC commonly manifests as thin spider web-like cracks called crazing around areas where the stresses are concentrated.
An in-depth understanding of how the plastic part will be used (and potentially misused) is always the first step. This includes the use of different disinfectants throughout the day to clean the same surface area, a high concentration of a disinfectant because mixing directions were not followed properly, or a component being used for something other than its intended use.
Multiple factors must be considered when evaluating chemical compatibility, including: Type and concentration of the reagent • Exposure temperature and time • Residual or applied stress on the fabricated part.
Stress can be caused by an external load applied during use or by residual internal stress in the molded part due to processing or part geometry, such as sharp corners. Evaluation of the finished device should consider how stress and reagent interaction could lead to cracking.
ASTM D543 is a standardized test method used to evaluate the resistance of plastics to chemical reagents. Testing covers changes in weight, dimensions, appearance, and strength properties based on various exposure times, strain conditions and elevated temperatures. Test data can be used to screen materials, but it does not replace testing the material and device under the design criteria and conditions that best simulate the intended use.
Reduce Medical Plastic Failure with Solvay
For over 30 years, Solvay has been a leading supplier of specialty polymers for healthcare. We have conducted extensive testing to evaluate the chemical resistance of our polymers to hospital disinfectants most commonly used today.
In response to the industry-wide problem with cracked polymer housings, we recently published a report on the chemical resistance of specialty polymers for medical equipment housings. The study compares the environmental stress cracking resistance of a range of Solvay’s sulfone polymers and high-performance polyamides to PC/ABS and PC/PBT, two incumbent materials being used in equipment housings.
Contact us for a free material consultation and we’ll show you how the right material can reduce your costs and your customer’s downtime through more durable, longer-lasting plastic components for medical devices and equipment.