DESIGNING THREADED HOLES IN PLASTIC PRODUCTS:

The right threading matters. We have the following points to consider before making a decision.

1. Select the right material for the job
   1. Choose materials compatible with your application's mechanical and chemical requirements. If you're having trouble figuring out which is best, consult with our  experienced engineers to find the right resin or material to get the job done right.
2. Choose the right thread size & pitch
   1. This crucial step works to ensure a leak-free, tight, and secure connection to the other parts of your assembly. Understanding the application's pressure and flow requirements is crucial.
3. Know the threat standards in your industry
   1. Familiarize yourself with industry standards to ensure compatibility with fittings and accessories. Questions? Feel free to book a consultation with our staff for some guidance. 

In the materials world, we commonly refer to two types of plastics: stress-sensitive and not stress-sensitive (or stress-insensitive). Stress-sensitive plastics will be more likely to experience cracking and breaking under load. Majority of cases are with threaded fasteners where others have accidently over-torqued. Stress-insensitive 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.

*These plastics, while non-stress sensitive, are amorphous

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 with the ability to insert NPT threaded inserts thermally. 

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 non-stress sensitive plastics 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.

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. However, do not use Helicoils in stress-sensitive plastics. Threads will begin to propagate stress-cracking over time and result in the component’s premature end. 

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.

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.