Creep, or cold flow, is the tendency of material to deform permanently over time under a static load. Creep is one of many factors we measure to understand the lifecycle of a product.
If your design needs to maintain integrity under persistently high temperatures and load, you need creep-resistant material. Typically, these parts and products are:
- Structural components (beams, columns, hanging supports)
- Hydrostatic pressure vessels (pipes, tanks, valves)
- Joints and interference fittings (over-moldings, press fits, snap fits, mechanical fasteners)
Designers and engineers must pay special attention to creep-resistance in polymeric materials due to their molecular composition and tendency to deform with time. The properties of molded plastics change with time under constant pressure, temperature, and load more than they do with metals.
It’s important to note, however, that creep may not always be a bad thing. In some applications, a certain level of creep is required to relieve tensile strength, for example, in the case of seals. This is what makes plastics a great choice, because you can choose a material that offers just the right amount of creep-resistance for your needs.
At Xcentric, we understand the consequences of creep failure and can guide you through material selection and design to ensure they are minimized.
Stages of Creep
Creep occurs in stages, and you can measure how long a part or product will last based on how it progresses.
First, there’s an almost immediate strain when the first exposure to high heat or load occurs. After this, the deformation continues even under the same amount of thermal or mechanical pressure. The material goes through this phase at a relatively rapid rate.
This is when creep enters a steady state. It begins when the primary creep rate decreases, a phase called strain hardening. The material stays in this secondary stage for longer and undergoes a linear progression during this time. Engineers study this stage (and other relevant factors such as stress rate) to estimate how long a part or product will last until it fails.
This is the stage at which there’s a rapid onset of creep before the part or product ultimately fails. There are microstructural changes including grain boundary shifts and internal cracks, among others.
It is important to conduct a creep test, in which constant stress is applied to a material sample, to plot the lifespan of a part or product.
When Is Creep-Resistance Critical?
Creep, and more importantly the incorrect estimation of its rate of progression, can have catastrophic consequences in certain industries. Imagine if the steam turbines at a power plant failed before they were supposed to be replaced!
Heavy industries like aerospace, automotive, semiconductor, petrochemicals, and nuclear energy all rely on parts and components made of creep-resistant material.
Creep-resistance is crucial when:
- Parts under load are frequently exposed to elevated or extreme temperatures.
- There is an applied load for extended periods of time.
- Parts are subjected to heavy load stress, such as stackable storage boxes.
- Tight tolerances are critical to the successful performance of the part; even minimal creep failure can affect the repeatability and specificity of many parts or products.
Top 5 Creep-Resistant Plastics
There are many reasons to choose high-performance plastics over metals, including:
- Lower material cost
- Part weight reduction
- Reduced production time
- Color options
- Optical clarity
- Wider range of part decorating options for aesthetic purposes
All the better if they can deliver the mechanical performance of metals, especially creep-resistance. These high-performing plastics qualify:
- PEEK (PolyEtherEtherKetone) is a thermoplastic in the PAEK (PolyArylEtherKetone) family. It is widely used in the production of aerospace and automotive components, medical devices, machinery parts, semiconductors, and electrical and electronic machinery. Tests show that PEEK is more durable than many other polymers and even some metals. It can withstand high loading at elevated temperatures without permanent deformation.
- ECTFE (Ethylene ChloroTrifluoroEthylene) is a fluoropolymer with excellent creep-resistance. It is strong, stiff, and abrasion resistant. Due to its high purity, it is widely used as linings for tanks, vessels, and reactors in the shipping industry, in piping systems, and in chemical storage tanks.
- PAI (polyamide-imide), especially bearing grade PAI, is strong enough to be used in high-temperature conditions. Its low tendency for thermal expansion makes it especially suitable for parts that require tight tolerances. PAI is commonly found in semiconductor machinery parts, aerospace components, and pump and valve parts.
At Xcentric, we are well-stocked with high-performing plastics with varying creep-resistance and other mechanical properties. Our team can take you through the selection process to find the plastic with the optimum creep-resistance for your needs.
Stress Relaxation in Elastomeric Parts
Elastomeric parts experience creep differently as they would also be affected by constant shock loading. Elastomers are polymers with elastic properties, like rubber. Elastomers can be stretched up to 200% of their original length and return to their original form and length when released. They’re often used when parts need to change shape as pressure is applied, such as in seals, gaskets, and springs.
These parts are sometimes required to revert when the pressure is released, for example, in aircraft door and window seals.
As with all plastics, they gradually deform in harsh operating conditions.
Two types of creep affect elastomeric parts:
- Reversible creep relaxation. This is when the deformation recovers when the stress is reduced or removed. This is a good thing in dynamic applications, when stress is applied and removed multiple times, as with pot lid seals. This type of creep occurs throughout the life of the elastomer part or product.
- Irreversible viscous flow. This is a permanent deformation that leads to ultimate part or product failure.
How quickly an elastomer part or product experiences ultimate creep failure depends on several factors:
- The extent of dynamic use
- Frequency of expansions and contractions
- Load pressure
Despite its bounce-back characteristic, at some point, the material will degrade, and the part will become unusable. This is called stress relaxation. When this happens in the case of pot lid seals, for example, steam or liquids can leak. In the case of aircraft door seals, the consequences are much worse.
Design Tips for Minimizing Creep
Nothing lasts forever, but with a suitable material, good design, and best-practice manufacturing processes, you can ensure your part lasts for as long as you need it to.
Here’s what you can do at the design stage:
- Select a semi-crystalline polymer such as PA6, PA66, PP, or PEEK. Use materials with fillers like glass or carbon fiber to reduce the amount of segmental motion between the molecular chains.
- Use semi-crystalline resins with bigger grain size to resist creep better.
- Design components in such a way to minimize long-term loading.
- Use materials with higher melting points. Determine the working temperature of the part or product to shortlist a material that can withstand that allowable range.
- Speak to your injection molding partner about creep test data.
Good Molding Practices Can Improve Creep-Resistance
Simply put, creep can be a problem, and one that affects all materials. While it’s a relatively bigger concern with plastics, in many cases the benefits of using high-performance polymeric materials outweigh those of metals.
In addition, creep can be anticipated and then managed all the way from the material selection stage through to design and production. The team of engineer centric experts at Xcentric can guide you every step of the way.
For more advice on minimizing creep and how we can work with you to produce high quality and long-lasting parts, submit your project and our team will be happy to provide expert advice.
Founded in 1996, Xcentric Mold & Engineering is an innovator of on-demand digital manufacturing and continues to lead advances in injection molding and rapid prototyping. We know what it takes to deliver a high quality product on time and on budget. Xcentric is engineered to be nimble, employs a team of experts in injection molding, and takes an engineer centric approach to everything we do. Tens of thousands of product developers and engineers across North America trust Xcentric to bring their products to life.
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