Open a fibre broadband router or an ONT and the plastic housing looks like the simplest part inside. It is not. That shell has to fit together cleanly, handle heat, resist fire, survive years on a wall or pole, and still look good on a customer’s shelf. Getting all of that right, batch after batch, is what precision plastic injection molding is really about.
A quick note on terms: an ONT (optical network terminal) is the box that brings fibre into a home or office, and an OLT (optical line terminal) sits at the service provider’s end. Both rely on well-made plastic housings. This guide covers the choices that matter most, which are material, tolerance and process control, and what to ask your molder before you commit.
Why housings are harder than they look
A housing seems like a simple box, but a handful of features quietly cause most of the trouble:
- Thin walls. They cool unevenly, which can cause warping and a poor fit between the two shells.
- Snap-fits and screw bosses. These small features have a large influence on how well the housing closes and how long it lasts.
- Ventilation slots. They help with heat, but add mold complexity and can affect strength.
- Cosmetic surfaces. Sink marks, flow lines and colour variation are easy to see on a visible cover.
- Wireless performance. Plastic near antennas can influence signal, and some fillers and pigments interact with it more than others, so it is worth raising early.
Each of these is manageable, as long as it is designed for at the start rather than discovered at the end. Our guide to designing parts for plastic injection moulding covers the design rules in more depth.
Choosing the right material
Material choice shapes strength, heat resistance, flame behaviour and surface finish. These four families come up most often for electronic housings:
Material | Why teams choose it | Watch out for |
|---|---|---|
ABS | Easy to mold, good surface finish, economical | Standard grades have limited heat and UV resistance |
PC/ABS blend | Balances impact strength, heat resistance and moldability; widely used for electronic enclosures | Costs more than ABS; material must be dried properly before molding |
Polycarbonate (PC) | Tough and heat resistant; can be clear for windows and light pipes | Sensitive to stress and some chemicals |
PBT (often glass-filled) | Dimensionally stable and stiff | Glass fibres can affect surface finish and increase warpage |
Then there is fire behaviour. UL 94 is the standard series of tests for how plastics burn, and V-0 is one of the highest ratings in the vertical burn test. It is commonly requested for electronic enclosures. One detail catches many teams out: a UL 94 rating applies to a specific material at a specific thickness and colour, so confirm it holds for the wall thickness and colour you plan to use.
Outdoor units add two more questions. UV-stabilized grades help prevent yellowing and brittleness in sunlight, and the housing design needs to meet the ingress protection (IP) level your customer specifies, as defined in IEC 60529. Some customers also require halogen-free flame retardants, which narrows the choice further. Ask about all of these early, and let your customer’s specification, not a generic table, make the final call.
Tolerances that actually matter
Precision does not mean every dimension is tight. It means the right dimensions are controlled. On a housing, those usually are:
- The fit between top and bottom shells
- The gap and flush along the seam
- Hole and standoff positions for the circuit board
- Port and connector openings
- Snap-fit engagement
Plastic shrinks as it cools, and by how much depends on the material, the part’s shape and the process settings. Good molders account for this in the mold design, then fine-tune it during trials. The international standard ISO 20457 covers tolerances and acceptance conditions for moulded plastic parts, and it is a handy reference for agreeing on what is realistic between buyer and molder.
A practical tip: mark the dimensions that are critical to function on your drawing and leave everything else on general tolerances. Tight limits everywhere raise the cost without improving the product. Our post on how tight tolerances affect product performance goes deeper.
How process control keeps parts consistent
Precision is not achieved once. It has to be repeated on every shot, and that comes from a chain of good habits:
- Before tooling: a design review and, where useful, mold-flow simulation to predict filling, weld lines and warpage.
- First trials: the first samples are measured against the drawing and the mold is corrected.
- In production: stable settings for temperature, pressure and cycle time, with monitoring to catch drift early.
- Inspection: dimensional checks, visual standards for cosmetic surfaces, first-article reports and sampling plans.
- Traceability: material and batch records so any issue can be traced back.
The goal is to build quality into the process rather than inspect it in at the end. When you assess a molder, ask to see how they do this, not just whether they say they do. Clixroute is ISO 9001 certified and offers testing applications alongside molding, so durability and functionality checks sit in the same workflow as production.
Questions to ask a molder before you commit
A short list of well-chosen questions tells you more than any brochure. Try these:
- Which exact material grade will you use, and can I see its datasheet and flame-rating documentation?
- Do you run mold-flow simulation before cutting the mold?
- How do you measure and report the critical dimensions on my drawing?
- How do you control material drying and keep batch records?
- What happens if the mold needs correction after the first trials?
- Can you support the flame, ingress and durability testing my customer requires?
Thermal design deserves a mention too. Routers and ONTs generate heat, and the housing is part of how that heat escapes. Vent placement, wall thickness and material all play a role, so involve your molder in that conversation early instead of handing over a finished design.
What export customers expect
Building for Europe or the US raises the bar on documentation and consistency, not just on the parts. One Clixroute customer, which supplies ONT products to European and US buyers, told us they had found it hard to rely on Indian plastics manufacturers and had sourced from neighbouring countries instead. After working with Clixroute on their ONT housings, they no longer depend on outside suppliers for those parts, and they credit reliable turnaround for helping them meet project deadlines.
The lesson is that export customers care about repeatable quality, clear records and on-time delivery as much as they care about price. If you are building for global markets, ask every molder how they document their process and inspection results. You can see the products we support on our ONT, OLT and Wi-Fi router pages.
Planning a new housing? Talk to our engineers through the contact page and share your drawings, material requirements and volumes.
Frequently Asked Questions
What is precision plastic injection molding?
It is injection molding carried out with tight control over the mold, the material, the process settings and the inspection, so parts stay accurate and consistent from the first shot to the last. It is used where parts must fit, seal or perform reliably, as in electronic housings and connectors.
How is it different from standard injection molding?
The molding principle is the same. The difference is the level of control and verification: more careful mold design, tighter process monitoring and more thorough inspection. Our post on precision versus conventional injection molding compares the two in detail.
What tolerances can be achieved?
It depends on the material, part size, geometry and process. Rather than chasing one number, identify the dimensions that are critical to function and discuss what is realistic with your molder. Standards such as ISO 20457 can serve as a shared reference for what to expect.
Which plastic is best for router and ONT housings?
PC/ABS blends are a common choice for electronic housings because they balance impact strength, heat resistance and moldability. ABS suits cost-sensitive indoor products, and polycarbonate suits parts that need extra toughness or transparency. The final choice should follow your customer’s specification and testing.
What does UL 94 V-0 mean?
UL 94 is a series of flammability tests for plastics. V-0 is a classification in the vertical burn test, reflecting how quickly a burning sample stops burning and whether it drips flaming material. Ratings are specific to a material, thickness and colour, so always check them for your actual part.
Do outdoor ONT housings need special materials?
Usually, yes. Sunlight can yellow and embrittle standard plastics, so UV-stabilized grades are common for outdoor units. The housing must also meet the IP rating your customer specifies (see IEC 60529), which depends on both material and design details such as seals and joint shape.
What is mold-flow analysis?
It is a computer simulation of how molten plastic fills the mold. It helps predict problems such as weld lines, air traps and warpage before the mold is built, when fixing them is far cheaper than modifying steel.
Why do plastic parts warp?
Warpage comes from uneven shrinkage, usually caused by varied wall thickness, uneven cooling or material flow patterns, especially with glass-filled resins. Good part design, balanced cooling and stable process control all reduce it.
How do you keep quality consistent across large batches?
Through stable process settings, monitoring for drift, regular inspection against agreed standards, first-article reports and batch traceability. Ask any molder to show how these are done in practice, not only to say they are done.
What should I send a molder to start a housing project?
Share 3D and 2D files with critical dimensions marked, along with material, flame and UV requirements, colour, cosmetic expectations, any IP rating, expected volumes and your timeline. If your end customer has a specification, include that too.
