Molded thread
- No extra parts, and the port is sealed in one molded wall
- Tapered pipe threads push the plastic outward as they tighten, which is where overtightening cracks start
- Straight threads with an O-ring or flat seal load the plastic far less
Molded part guide: filter housings
Injection Molded Filter Housings
A filter housing is a pressure vessel that someone unscrews by hand every few months. The thread, the seal, and the plastic around them have to survive both the pressure and the person.
The job
Most filter housings fail at the thread
The bowl rarely bursts in the middle. Cracks start where the housing is joined, sealed, or connected.
A typical housing has two molded parts: a head with the inlet and outlet ports, and a bowl that threads onto it and holds the cartridge. An O-ring between them makes the seal. Every time the cartridge is changed, the bowl is unscrewed and tightened again, often with a wrench, and often too hard.
A case published by the Society of Plastics Engineers describes molded filter bodies that cracked just below their internal pipe thread in the field. The parts had been molded properly and passed pre-production testing. The cause was a weld line sitting in the thread, combined with installers tightening the fitting harder than the test technician had. The test was right about the part and wrong about the user.
The lesson carries over to any filter housing: keep weld lines out of threads and sealing faces, give the thread a positive stop so it cannot be overtightened, and test the way the product will really be installed.
Resin
Clear or opaque, and what each choice gives up
General behavior. The fluid, temperature, and any drinking water listing decide the grade.
| Resin | Where it fits | Trade off |
|---|---|---|
| Polypropylene | Opaque bowls and heads for water and many chemicals | Low cost and chemical resistant, but you cannot see the cartridge |
| Glass-filled polypropylene or nylon | Heads and bowls for higher or pulsing pressure | Stiffer and more creep resistant, and glass makes weld lines weaker |
| SAN or copolyester | Clear bowls so the user can see when the cartridge is dirty | Clear resins are more sensitive to chemicals and notches, so thread roots need care |
| Polycarbonate | Clear, tough bowls for air and industrial fluids | Some drinking water and food programs restrict it, so check your specification first |
| Polysulfone | Transparent amber bowls for hot water | Handles heat and hydrolysis well, at a higher resin cost |
Threads and seals
How the thread gets made shapes the tool
A molded thread is an undercut, and the way it releases from the mold drives tool cost and cycle.
External threads on a bowl can often be formed by splitting the cavity across the thread, which leaves a fine parting line along it. Internal threads on the head need a core that unscrews or collapses, or, for a shallow rounded thread in a flexible resin, a stripping action that pushes the part off. Unscrewing tools cost more but give a clean thread with no parting line.
The O-ring groove should sit where the tool forms it in one piece, away from the parting line. Flash or a step across a sealing face is a leak path. Axial seals, where the O-ring is squeezed between the bowl rim and the head, are more forgiving of thread wear than seals that depend on the thread pulling the parts tight.
A round bowl gated at the center of its bottom fills evenly with no weld line in the wall. A drain port or a hole in the bottom changes that, so its weld line has to be steered to a spot that does not carry pressure.
Threads
Molded plastic thread or a metal insert in the head
The bowl thread is almost always molded. The choice comes up on the head's pipe connections.
Metal insert
- The fitting bears on metal, so installer torque is much less of a risk
- The insert needs knurling or grooves to resist torque and pull out
- Adds a loading step to every cycle, or a post-mold insertion step
Drawing
What to send with a filter housing
Send both halves, even if you only need one of them molded.
Head and bowl CAD, plus the cartridge that goes inside
Working pressure, peak pressure, and the fluid temperature
Whether the bowl must be clear, and why
Port connection type, and whether it is a tapered or straight thread
The O-ring size and compound you plan to use
Any drinking water or food contact listing the product needs
Keep reading
Related guides
Threads, weld lines, and resins for pressurized housings.
Molded in threads
Thread forms and how they release from the mold.
Molded in threadsWeld lines
Where they form and how to move them.
Weld linesMolded threads vs inserts
Choosing how the head takes a fitting.
Molded threads vs insertsSAN
A clear, stiff option for bowls.
SANPolysulfone
Transparent and suited to hot water.
PolysulfonePolypropylene
The opaque workhorse for water filtration.
PolypropyleneFlash
Why a step on a sealing face leaks.
FlashPump housings
The other pressurized housing in the system.
Pump housingsMolded parts hub
Every part-type guide, grouped by family.
Molded parts hubFAQ
Questions about molded filter housings
Can you match an existing standard cartridge size?
Yes, if you send the cartridge or its drawing. The bowl length, the seal surfaces, and any center post are designed around the real cartridge.
Why did our clear bowls craze after a few months?
Clear resins are prone to stress cracking when a chemical meets a stressed area. Common triggers are cleaning agents, thread lubricants, or O-ring grease that the resin does not tolerate, acting on the thread root or the gate. Check every chemical that touches the bowl.
Do you burst test the housings?
Pressure and burst testing is normally run by your team on molded samples. DFM feedback before the tool is built shows where weld lines are expected, so the test can target the right spots.
Is there a minimum order quantity?
No. There is no MOQ, and you own the tool.
Next step
Quote the head and bowl together
Upload both parts with the cartridge and pressure details. DFM feedback flags weld lines at threads and ports, and how each thread releases from the tool.