Threaded Holes & Hardware Options
Understanding threaded holes is a crucial step in achieving optimal performance and versatility within plastic manifolds, bonded or not. Here is what our experts have gathered about threaded holes, which are most suitable for which applications, and other matters of functionality.
Why Threaded Holes Matter in Plastic Manifold Design
While many parts of the manifold are integral to its function, these kinds of holes matter for three reasons.
While each product, device, or assembly is going to have its differences from one another, threaded holes mark key differences in their applications. Designers utilize threaded holes to attach particular components, fittings, and/or other accessories to the manifold to customize its usage. Without these critical aspects, operations in fluid and gas (for example) would not hold up to the demanding tasks.
Properly threaded holes ensure secure and tight connections that lower leak risks. In harsh use cases, this aspect is a non-negotiable.
These also offer flexible options to customize a manifold to meet certain specialized requirements for more niche products and assemblies. Some manifolds need different sizes, depths, or thread types for optimal performance.
How To Choose The Right Threading
The right threading matters. Make sure that you're choosing the right one the first time. We have the following points to consider before making a decision.
Select The Right Material For The Job
This is one of the most important steps before proceeding after the final design. If you're having trouble figuring out which is best, our Materials Selector Tool can help you find the right resin or material to get the job done right.
Choose The Right Thread Size & Pitch
This crucial step works to ensure a leak-free, tight, and secure connection to the other parts of your assembly. Ensure that you have a grasp on what your product needs to maintain peak performance.
Do you know your industry's standards? Familiarize yourself with that as well to understand compatibility with your expected fittings and accessories. Need some help? Feel free to book a consultation with our staff for some guidance.
Stress cracking from over tightened threads
We'll cover this more later, but be aware that applying the proper amount of torque on a fitting, fastener, or component is critical to avoid damaging the part. Threading that has been cracked from over torquing can easily be spotted. It will be cracked at the deepest thread of the fitting and the rest will look alright. Do not use Loctite products for fastener retention. Anaerobic compounds attack and stress crack plastics.
About Stress-Sensitive Plastics
In the materials world, we commonly refer to two types of plastics: stress-sensitive and not stress-sensitive. A stress-sensitive plastic will be more likely to experience cracking and breaking under load. Majority of cases are with threaded fasteners where others have accidently over-torqued. Not stress-sensitive can take more force than their more sensitive siblings.
Amorphous thermoplastics are often more stress sensitive than semi-crystalline and fully-crystalline thermoplastics. Most amorphous thermoplastics are identified by their clarity while semi-crystalline and fully-crystalline thermoplastics are opaque.
To assist in further understanding, we have categorized those plastics here.
NOT Stress-Sensitive Plastics
*These plastics, while non-stress sensitive, are amorphous
More about different Threaded Hole Types & Hardware
When speaking to threaded holes within plastic manifolds, our experts have stated it's important to know the differences between machined threaded, inserts, and helicoils within an overall design in order to achieve and optimal product.
For applications where the assembly happens only once, fully machined threads are a great choice for several reasons. Many often cite the versatility of this option over others. Machined threading offers maximum flexibility on thread length and thread size as well as material choices. There are few resins that do not take machined threading well. Moreover, designers have more free range with machine threading as it makes more room for special threading if the product requires.
Also, out of the other possible options, they usually come out as one of the most cost efficient options. It often does not require extensive work to create such holes within a device compared to other techniques.
We do want to be clear about a few matters here. Stress sensitive plastics require good machining technique. It does require a certain amount of skill to thread these without inducing stress on the overall manifold. We also do not recommend machined threading for repeated assembly/disassembly as this can wear out the threads that much faster. We recommend a different approach for those that need that feature.
As with most techniques, we have some design suggestions to bear in mind when considering machined threading as an option.
- The threads should have a minimum 3x thread-to-diameter engagement.
- Any threaded holds smaller than #2 or 2mm threads have limited pullout strength. It's best to consider other options.
- Pipe threads are ok in non-stress sensitive plastics such as Delrin.
Use a torque wrench for tightening and ensure proper tightening technique. Here is our recommended chart for reference about the recommended tightening torque in plastics without inserts.
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When it comes to self-tapping screws, designers choose this option when a product needs to have a few key features. If a product needs to have good holding power with thread cutting or forming choices, then self-tapping screws could be the answer. This also applies if one needs a relatively inexpensive option to connect components within an assembly.
However, like everything else, these do have their disadvantages. With these screws, they can create a good amount of stress when applied or used constantly. For that reason, we recommend that designers use these only in semi-crystalline plastics, such as PET. If used with other plastic resins, one could over-torque and strip the plastic, causing irreparable damage to a point of needing replacing.
Lastly, unlike its machined counterparts, these cannot be removed once installed. They cannot be re-inserted once they have been placed. If choosing this option, ensure that you have it right the first time.
Inserts & Hardware
Metal inserts find use in applications where a designer plans that a product will be repeatedly disassembled or need maximum pull out strength. With metal inserts, our operators usually perform this task either by staking (ultrasonic or thermal) or pressing. If the product needs an insert but space is constrained, we recommend selecting Helicoils, as they are more compact and ready to serve in that function.
Inserts can offer some advantages that neither machined threading or self-tapping screws can. For example, unlike self-tapping screws, inserts work well with stress-sensitive plastics (e.g. SMPs and ER polymers) and excellent pullout strength. We also recommend these for threads below #2 or 2mm, as they can accommodate the smaller sizes without much issue and offer pipe threading as an option.
Some barriers do stand regarding inserts. For one, manufacturers can only offer limited choices for thread length. They are not meant for deep perforation, and designers should consider other options if their product needs this feature. Additionally, when constructing the manifold with inserts, the machinist needs to have some skill as they need to know how to manage the liquid plastic flow. Without this knowledge the inserts and the manifold could suffer threats to their structural integrity.
Methods For Inserting Metal Threading into Plastic Manifolds
Using soundwaves instead of heat, machinists install inserts this route to avoid high temperatures that could alter the component's integrity. This method works great for high quantity orders.
Using higher temperatures, machinists heat the metal inserts hot enough to melt the inserts immediate area within a manifold. That insert then melts and "glues" itself into the manifold, creating a flush surface and design. This method works better for smaller quantities.
Pressing often implies a more mechanical way of embedding metal inserts into plastic manifolds using force.
For inserts in plastic manifolds, we recommend two different materials for use here: stainless steel and brass. While these cover the common thread sizes, each has their advantages and disadvantages independent of those thread sizes. These need careful consideration from the designers to decide which would be best suited to the product.
Stainless steel inserts feature chemical resistance, which has designers' favor over their brass counterparts. Usually, these steel inserts resist many different chemical types, including harsh corrosives. They have added strength and durability when the component needs that feature.
However, we must note that steel inserts often take more time to assemble with heat staking, which can drive up their initial cost. We urge designers to keep this in mind if considering these for manifolds.
Brass inserts usually cost less initially. If a component does not need staunch chemical resistance or added strength, brass inserts can offer a fair compromise. However, if a designer is looking for something RoHS compliant, they may need to seek another option as some brass inserts contain lead.
Concerning Heat Affected Zones in Manifold-Insert Construction
As staked inserts melt the plastic around the insert, there is a zone where any features cannot be placed. Insert must be a full diameter away from an edge to avoid sidewall bulge.
Press-in inserts (PIIs) offer similar advantages as the others, but these specialized inserts require mechanical force to place. Due to this, these 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). Moreover, these come in limited choices in thread length and offer limited pull-out strength. If the assembly needs repeated disassembly, we recommend an alternative more suited to the product's purpose.
With these, pipe threading is possible. They consist of same materials as their other counterparts: brass and stainless steel in common thread sizes.
On valves & press-in inserts
For small valves, the mounting holes are often close together. Inserts cannot be used due to the heat affected zone. Machined threads for small valves are acceptable as long as the tightening torques are managed properly.
While Helicoils are common in metal machining to improve thread strength, they are not used in many applications for plastics due to several limitations. Cost factors as one of those top limitations. It often comes out as one of the most expensive options because (depending on size) these can add an extra $2-$4 per hole in each manifold. Those costs add up quickly.
Why so expensive? Helicoils have marked themselves as time-consuming and difficult to insert — particularly for smaller holes. Moreover, there is an inherently higher risk of damage when installing in a threaded hole, which limits even which plastic materials would take Helicoils. We recommend that if using Helicoils to avoid any stress-sensitive plastics as they add more mechanical stress to the manifold.
All that said, Helicoils do have some advantages. If the product needs taking apart and putting together on a regular basis, these offer that feature with security. We recommend them for thermoplastics and thermosets as they can take the mechanical stress involved when installing as well as future assembly/disassembly. These also work well in small places near other features. Pipe threads are also possible here.
Regarding tanged vs. tangless helicoils, we have seen that designers tend to prefer tanged over tangless as the installation tools are less expensive and more robust. If tang removal is required, designers then often choose tangless.