Alexei Volkov
Thread wearing in plastics can be avoided by carefully considering thread size and pitch when designing parts, as plastic threads are more prone to stripping than metal ones. It is critical to apply the proper amount of torque to avoid damaging the part, as over-tightening can lead to stress cracking. Stress-sensitive plastics, such as amorphous thermoplastics, are more susceptible to cracking and breaking under load, so threaded fasteners must be carefully managed. To maintain structural integrity, skilled machinists are required to manage the liquid plastic flow during the insertion process. Additionally, designers should opt for materials like ABS, POM (Delrin), or nylon for internal threaded parts, while avoiding glass-filled and high-temp materials.
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What You'll Learn
Avoid over-tightening to prevent stress cracking
Over-tightening parts can lead to stress cracking in plastics. This is because when force is applied to plastic, it travels along the polymer chains, causing molecules to strain to remain in contact. The polymer chains eventually creep toward the pull and deform. With enough force and time, crazing or stress cracking occurs.
Stress-sensitive plastics are more likely to experience cracking and breaking under load. The majority of cases of stress cracking occur when threaded fasteners are accidentally over-torqued. Therefore, it is important to apply the proper amount of torque when tightening a fitting, fastener, or component. A torque wrench should be used to ensure the correct tightening technique.
Additionally, certain chemicals can speed up the process of stress cracking. Environmental stress cracking (ESC) occurs when the piping material is simultaneously subjected to mechanical stress and certain types of chemicals. For example, while CPVC may be able to withstand 99% of the chemicals in a facility's process fluid, the other 1% may cause the pipe to stress crack. Therefore, it is important to consider the mechanical stresses and chemicals that will affect a piping system's performance and address those factors during system design and installation.
Furthermore, internal stress can also cause cracking in plastics. Screws, metal inserts, and poorly designed snap-fits are all features that typically fail under stress cracking. Considerable changes in material thickness may also cause internal stress and cracking over time, especially in the case of amorphous plastics. Therefore, it is important to avoid features that add stress to the plastic manifold, such as press-in inserts (PIIs), which require mechanical force to place.
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Avoid using anaerobic compounds like Loctite
Thread wearing in plastics can be avoided by not using anaerobic compounds like Loctite. Loctite is a brand of anaerobic adhesives, also known as threadlockers, that are commonly used for fastener retention. While these adhesives offer several benefits, they can cause stress-cracking in plastics. This is especially true for stress-sensitive plastics, which are more prone to cracking and breaking under load.
Loctite offers a range of thread-locking products with strength ratings from light to permanent. The different colours of Loctite threadlockers indicate their strength, with purple being low strength, blue medium, and red high strength. However, the “high strength” label does not provide much information, as some epoxies may be stronger. Additionally, the viscosity of the adhesive can affect its performance, with more viscous adhesives often providing greater strength.
Anaerobic adhesives cure in the absence of oxygen and in the presence of metal ions, forming a tough cross-linked plastic with strong adhesion to metals. They are used to prevent loosening and corrosion of fasteners, enhance equipment reliability, and minimise aftermarket breakdown. However, they are not suitable for all assemblies. Anaerobic adhesives should not be used with thermoplastics, as they will soften the surface and cause cracking. They are also not recommended for assemblies that require repeated disassembly, as this can wear out the threads faster.
To avoid thread wearing in plastics, it is important to carefully consider thread size and pitch when designing parts. Additionally, the proper amount of torque must be applied to avoid damaging the part. For repeated assembly and disassembly, alternative methods should be considered. In the case of threaded holes, press-in inserts (PIIs) can be used, but they are not suitable for all plastics and have limited pull-out strength. Another option is to use soundwaves or higher temperatures to install metal inserts, creating a flush surface and design.
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Use ABS, POM, or nylon for internal threads
When designing threaded holes in plastic parts, it is important to consider the type of plastic used to avoid thread wearing. Internal threads in plastic parts can be challenging to design and manufacture, and the choice of plastic material plays a crucial role in the performance and longevity of the threads.
One key recommendation is to use ABS, POM (Delrin), or nylon for internal threads. These materials offer several advantages over other plastics. Firstly, they are not "sticky" like some engineering-grade polymers, especially those filled with glass or fiber. The "stickiness" of a material can make it difficult to remove inserts during the manufacturing process, hindering production efficiency.
ABS, POM, and nylon are also recommended because they are stress-insensitive plastics. Stress-sensitive plastics are more prone to cracking and breaking under load, especially when fasteners are over-tightened. By choosing stress-insensitive materials, you reduce the likelihood of thread wear and damage caused by stress cracking.
Additionally, these materials are suitable for creating internal threads using hand-loaded inserts, a technique employed by manufacturers to keep tooling costs low and speed up delivery times. It is worth noting that this method may not be ideal for deep perforations or products that require repeated disassembly, in which case alternative approaches should be considered.
In summary, selecting the right plastic material for internal threads is crucial to preventing thread wear. By choosing ABS, POM, or nylon, you can mitigate issues associated with "sticky" polymers, reduce the risk of stress cracking, and benefit from efficient manufacturing processes using hand-loaded inserts.
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Avoid press-in inserts for stress-sensitive plastics
Threaded inserts for plastics are widely used across various manufacturing sectors. They are designed to fortify the connection points within plastic assemblies. When selecting plastic options for inserts, thermoplastics and thermosets offer different advantages. Thermoplastics, which can be reshaped multiple times, work well with heat and ultrasonic inserts. On the other hand, thermosets, which are permanently rigid after moulding, require specialized inserts like moulded-in, press-in, or self-tapping inserts.
Press-in inserts (PIIs) are specialized inserts that require mechanical force to place. Due to this, they tend to add stress to the plastic manifold and should only be used in semi-crystalline plastics, such as fluoropolymers (e.g. Teflon) and thermosets (e.g. G10). They are not suitable for stress-sensitive plastics, as they can induce stress and cause cracking or breaking under load.
Stress-sensitive plastics require a good machining technique to thread them without inducing stress on the overall manifold. It is recommended to use a torque wrench for tightening and ensure a proper tightening technique to avoid over-tightening, which can lead to stress cracking. Additionally, avoid using anaerobic compounds like Loctite for fastener retention, as they can also cause stress cracking in plastics.
When constructing the manifold with inserts, the machinist needs to have some skill in managing the liquid plastic flow. Without this knowledge, the structural integrity of the inserts and the manifold could be compromised. It is important to ensure that the insert is a full diameter away from an edge to avoid sidewall bulge.
Self-tapping inserts are recommended for stress-sensitive plastics to avoid cracking. They are designed for post-mould installation and provide the best pull-out resistance. The external threads are thin-profiled to minimize stress on the plastic, paired with a relatively coarse pitch to maximize the plastic shear surface and prevent pull-out.
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Avoid locking or counter-rotating threads
When designing threaded holes in plastics, there are several factors to consider to avoid thread wear and failure. One critical aspect is understanding the behaviour of different plastic types under stress. Some plastics, such as amorphous thermoplastics, are stress-sensitive and prone to cracking and breaking under load. Therefore, it is essential to apply the correct amount of torque when tightening fittings, fasteners, or components to avoid over-tightening, which can lead to stress cracking.
Additionally, the choice of insert type and installation method is crucial. Staked inserts, which melt the plastic around them, require a specific distance from the edge to avoid sidewall bulging. Press-in inserts (PIIs), on the other hand, add stress to the plastic manifold and are recommended only for semi-crystalline plastics like fluoropolymers (Teflon) or thermosets (G10). Pipe threading is possible with PIIs, but they have limited pull-out strength and thread length options. Machinists should also be skilled in managing liquid plastic flow to maintain the structural integrity of the inserts and the manifold.
For internal threads, hand-loaded inserts are often used to keep costs low and speed up delivery. When using moulded parts with internal threads, materials like ABS, POM (Delrin), or nylon are recommended to avoid the "stickiness" associated with glass-filled and high-temp materials. It is also essential to avoid locking or counter-rotating threads in these moulded parts. To facilitate easy insert removal, designers should opt for an A2 or better surface finish and follow "best practice" rules, including draft angles of 1 degree or greater and consistent wall thicknesses.
Furthermore, threaded fasteners should not be used for repeated assembly and disassembly, as this can accelerate thread wear. When designing threaded holes, it is crucial to ensure a minimum of 3x thread-to-diameter engagement. Smaller threaded holes, such as those below #2 or 2mm, have limited pullout strength, and alternative options should be considered. Pipe threads are suitable for non-stress-sensitive plastics like Delrin. Using a torque wrench and proper tightening techniques can also help prevent thread wear and ensure the longevity of plastic parts.
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Frequently asked questions
Check the fabric composition on the clothing label. Common plastics include polyethylene terephthalate (Pet), polyamide, polyurethane, polyvinyl chloride (PVC), and polypropylene. Other plastic names include nylon, acrylic, elastane, Lycra, Spandex, Econyl, and Repreve.
Opt for natural fibres such as rayon, viscose, silk, wool, cashmere, and linen. If you must buy synthetic clothing, purchase second-hand items as most microplastic fibres wash out during the first few washes.
When designing plastic parts, carefully consider the thread size and pitch to avoid threads that strip easily. Avoid locking or counter-rotating threads and use draft angles of 1 degree or greater to assure easy part ejection.
Use a torque wrench and ensure proper tightening to avoid over-tightening, which can lead to stress cracking. Avoid using anaerobic compounds like Loctite, as they can cause stress-cracking in plastics.
Inserts should be a full diameter away from an edge to avoid sidewall bulge. Avoid press-in inserts (PIIs) as they add stress to the plastic manifold and are only suitable for semi-crystalline plastics.
