Ethan Clarke
Plastic is a versatile material used across sectors, including building, packaging, industrial machinery, electrical, electronics, transportation, and consumer products. Creating threading in plastic is a complex process that requires careful consideration of thread size and pitch to avoid easily stripped threads. Various methods for creating threads in plastic include thread-forming, thread-cutting, and using self-tapping screws or metal inserts. CNC machines can be used for precise plastic threading, offering controlled feed rates and spindle revolutions for fine threading. The choice of plastic material and type of threading are also critical factors in the threading process.
| Characteristics | Values |
|---|---|
| Threading Purpose | To join different parts of an item together to create a finished product |
| Threading Benefits | Durability, reliability, versatility, leak prevention, customisation |
| Plastic Types | HDPE, LDPE, ABS, acrylic, engineered plastics, POM-C (Ertacetal) |
| Thread Types | Internal, External |
| Thread Design Considerations | Thread size, pitch, parting line, undercutting, thread form, depth, helix angle |
| Threading Techniques | Thread cutting, thread forming, self-tapping, CNC threading |
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What You'll Learn
Threading for injection-molded plastic parts
When designing injection-molded plastic parts, it is important to consider the two types of forces that come into play: pull-out force and torque-out force. Pull-out force refers to the force required to pull out a screw from a hole without unscrewing it, which should be as high as possible to prevent brute force removal. On the other hand, torque-out force is the torque needed to remove a screw by turning it after tightening, which should be minimized while still allowing sufficient tightening.
There are different methods for creating threads on injection-molded plastic parts. One approach is to use a removable threaded core or cavity, where the core is placed inside the mould tool, and the polymer is injected to create the thread. This method is suitable for both internal and external threads. For external threads, the mould tool can be designed with half of the thread in the core and the other half in the cavity, allowing for easy removal without the need for a removable core.
Another method is to mould the part without threads and then CNC machine the threads afterward. This approach offers flexibility in thread size and polymer type, and it eliminates the need for a threaded insert. Additionally, overmoulding a metal threaded insert can be considered, where the insert remains inside the moulding.
It is worth noting that the parting line, which is the lengthwise separation of the mould into two equal halves, should be considered when designing external threads. Internal threads can be created using hand-loaded inserts to keep tooling costs low and speed up delivery times.
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Thread-forming vs thread-cutting screws
Threaded parts made of plastic can fail more easily than metal ones, so it's important to get the threading right. When creating threads in plastic, you can use either thread-forming or thread-cutting screws. Both are types of self-tapping screws, which don't require a pilot hole.
Thread-forming screws displace material during installation. They are used when large stresses are needed to increase resistance to loosening. Because no material is removed, the mating part creates a fit with zero clearance. They usually do not need lock washers or other types of locking devices to prevent loosening.
Thread-cutting screws, on the other hand, remove material during installation. They have sharp surfaces and are typically installed using a power tool such as an electric drill. You'll see material coming out of the hole when installing this type of screw. They have a lower torque needed for insertion compared to thread-forming screws.
Thread-cutting screws are rarely used in plastic encasements, possibly because there is nowhere for the chips to go when the screw is purely cutting, and most plastics will flow nicely under the right conditions. Thread-forming screws are also generally used for lower-quality tools, while higher-quality tools use a nut that is molded in or a loose nut and bolt.
When using outside threads, more material is needed for a thicker section. The thickest threaded projection will require more plastic and a longer molding cycle.
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Plastic thread profiles and common failures
Threading in plastics is a complex process that requires careful consideration of various factors to avoid common failures. One of the primary challenges in creating threading in plastic is that screw threads were originally designed for metal parts, which have different physical properties and strengths. As a result, plastic parts designed with metal thread specifications are more prone to stripping and failing. To mitigate this issue, it is recommended that internal threads in plastic be no smaller than 0.3 inches (7.6 mm) in diameter, and that the coarsest pitch possible be used.
Common plastic thread profiles include the American Standard 60-degree sharp thread, tapered pipe thread, and buttress threads. The American Standard, or machine screw, is the most common type, featuring sharp corners at the roots and crests of the threads. However, these sharp corners act as stress concentrators in moulded plastic, weakening the thread. A better alternative is the British Whitworth thread, which provides radiuses at the roots and crests, distributing stress over a broader area and increasing strength.
The buttress thread is another favourable option, offering resistance to shearing due to its thicker cross-section and wider spacing between threads. This type of thread is commonly found on blow-moulded plastic bottles and threaded bottle caps. However, the wide profile of the buttress thread limits the number of turns that can fit in a given length. Additionally, when using outside threads, more material is required for a thicker section, necessitating a longer moulding cycle to allow for adequate cooling and shrinkage.
To avoid common failures, it is crucial to ensure perfect threading in plastic parts. Poor design can lead to thread failure, causing parts to fall apart, resulting in inconvenience, accidents, and even deaths. Flashing, or the formation of fine, feathery sections sticking out from the finished part, is another common flaw in injection-moulded plastic parts. This occurs when plastic escapes from the mould cavity during the manufacturing process.
When connecting plastic to metal, it is recommended to use plastic as the male thread (external) and metal as the female thread (internal). Additionally, threads with radiused crests and roots, a thicker cross-section, and wider spacing between threads are more likely to cause the plastic part to break or fail, rather than the screw or bolt.
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Designing threaded holes
Threaded holes are a crucial aspect of plastic manifold design and other custom plastic products. They provide versatility, leak prevention, and customization. When designing threaded holes, it is important to consider thread size and pitch to prevent stripping, a common issue with plastic parts. Protolabs recommends internal threads be no smaller than 0.3 inches (7.6 mm) in diameter, and to use the coarsest pitch possible.
The parting line is another important consideration for external threads. To avoid undercutting, a side-action mold can be used, although slight flashing may occur where mold sections meet. Additionally, the location of the threads relative to the parting line of the mold should be considered. Threads located outside the parting line may require a longer molding cycle and more material, but eliminate the need for an unscrewing mechanism.
The choice between machined threaded, inserts, and helicoils is also significant. Machined threading offers versatility in thread length, size, and material choices, making it a cost-efficient option. However, press-in inserts (PIIs) should only be used in semi-crystalline plastics due to the added stress they place on the plastic manifold. Helicoils are recommended for products that need to be regularly assembled and disassembled, and they work well with thermoplastics and thermosets.
When cutting threads into holes in plastic, a tap wrench or a hand drill can be used. For sizes larger than 1/4", a tap wrench is preferable due to the high torque required, while a hand drill works best for a range of smaller sizes. Chrome-plated taps are a good option as they do not rust and have a lower friction coefficient. For harder materials, gradual cutting is recommended, but for plastic, lighter passes will cause melting.
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Plastic threading services
Plastic threading is a complex process that involves creating helical or spiral grooves on plastic surfaces to interlock two or more parts. Ehren-Haus Industries, Inc. offers plastic threading and CNC threading services for custom plastic parts and industrial applications. Their manufacturing facility is equipped with advanced CNC machines, allowing them to offer in-house plastic manufacturing and plastic component threading services.
The process of plastic threading differs based on the type of plastic material, product, and threading. For example, thread cutting and thread forming are two different methods of creating threads in plastic. Thread cutting involves using a tapping tool or a single-tip cutting tool to form the grooves, while thread forming uses a screw to create the thread. The choice between these methods depends on various factors, such as the need for disassembly, strength requirements, and cost.
When designing threaded holes in plastic, several critical considerations must be made, including material selection, thread sizing, and techniques to prevent stress cracking. Different thread sizes, depths, and types cater to specialized requirements, enhancing functionality. It is important to select the appropriate type of screw for the plastic being used, as the wrong choice can lead to irreparable damage. Self-tapping screws, for instance, are suitable for semi-crystalline plastics like PET but can cause over-torque and strip other plastic resins.
Additionally, the location of the threads is an important consideration. Threads located outside the parting line of the mold can eliminate the need for an unscrewing mechanism, reducing costs. However, this may require redesigning and the use of more material for thicker sections. On the other hand, internal threads can be created using hand-loaded inserts to keep tooling costs low and speed up delivery time.
Overall, plastic threading services play a vital role in joining different parts of plastic items to create durable and reliable products. With the wide usage of plastic across industries, including packaging, industrial machinery, and transportation, plastic threading services are in high demand to ensure optimal performance and safety.
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Frequently asked questions
Plastic threading is the process of forming helical or spiral grooves on plastic surfaces to interlock two or more parts.
There are two main types of plastic threading: thread-forming and thread-cutting. Thread-forming is a process of creating threads on the plastic surface without removing material, while thread-cutting involves cutting and forming the grooves.
Plastic threading provides versatility, leak prevention, and customization. It allows for the attachment of components, fittings, and accessories, enabling customized applications. Proper threading ensures secure and tight connections, reducing leak risks.
