What Is Cold Runner?
I get asked about cold runners versus hot runners all the time, especially from guys getting into insert molding. So here's the deal.
A cold runner is just the channel that feeds plastic from your barrel into the cavity. Nothing fancy. The plastic in that channel freezes every shot, you eject it with your part, and you either throw it away or regrind it. That's it.
The name comes from the fact that there's no heater keeping it molten. Hot runners have heaters in the manifold. Cold runners don't. The steel sits at mold temperature, the plastic hits it, it freezes.
Why cold runners still matter
Plenty of shops have moved to hot runners over the years, but cold runner tooling isn't going anywhere. I've seen brand new programs in 2024 spec cold runners, and there's good reasons for that.
The obvious one is money. A cold runner two-plate mold for a small insert molding job might run you $25K. Put a hot runner in that same tool and you're looking at $40K, maybe more depending on who's building it. That's real money, especially if you're running five or six different part numbers and each one needs its own tool.
The other thing nobody talks about enough is material changeover. We run a lot of connector work. PBT, nylon 6/6, some LCP. Different colors, different glass loadings. With a cold runner I can switch materials in maybe twenty minutes. Purge it out, run some shots, good to go. Try that with a hot runner and you're babysitting the thing for an hour making sure every drop is clear.
The insert molding angle
Insert molding adds wrinkles that make cold runners more attractive in some cases.
Your insert sits in the cavity before the shot. Plastic flows around it. The gate location matters a lot here because wherever that plastic front meets itself on the backside of the insert, you get a weld line. Weld lines are weak spots. On a connector pin or a threaded brass insert, that weld line location can make or break the part.
The insert molding angle
Insert molding adds wrinkles that make cold runners more attractive in some cases.
Your insert sits in the cavity before the shot. Plastic flows around it. The gate location matters a lot here because wherever that plastic front meets itself on the backside of the insert, you get a weld line. Weld lines are weak spots. On a connector pin or a threaded brass insert, that weld line location can make or break the part.
With a cold runner setup, relocating a gate is cheap. Mill out the old gate, cut a new one somewhere else. Maybe a few hundred bucks and a day of machine time. On a hot runner, moving a drop means serious surgery on the manifold. I've seen quotes come back at $8K just to relocate one gate.
We had a job last year, automotive sensor housing with a steel sleeve insert. First shots looked fine but the parts were failing pull tests. Weld line was landing right where the sleeve met the housing wall. Moved the gate 15 degrees around the parting line, problem solved. If that had been a hot runner tool we'd have been in trouble.
Runner sizing
I'm not going to give you a formula because every material runs different and your flow length matters more than anything else. But as a starting point, most insert molding work in the 5 to 20 gram shot range does fine with 3/16" diameter runners. Bigger parts or longer flow paths, go up to 1/4". Glass filled stuff needs more room because the fibers don't like to turn corners.
The shape matters too. Full round runners flow the best but they cost more to cut because you need matching halves in both mold plates. A lot of shops use trapezoidal runners instead, cut into one side only. You lose some flow efficiency but the tooling is simpler.
One thing I see guys mess up is the sprue. They'll size the runners right but leave the sprue too small. That sprue is the bottleneck of your whole system. The orifice at the nozzle end should be at least as big as your main runner, bigger if you can get away with it.
The regrind question
Here's where cold runners get expensive in ways that don't show up on the tool quote.
Every shot makes scrap. Your runner might weigh 12 grams and your part weighs 4 grams. That's a 3:1 runner to part ratio, which isn't unusual for multi-cavity insert molding tools. On a hot runner that 12 grams stays in the manifold. On a cold runner it hits the floor every cycle.
Most shops grind it up and feed it back in. Twenty percent regrind mixed with virgin is pretty standard. Some customers won't allow any regrind at all, which means you're either selling the scrap for pennies or throwing it away.
Glass filled materials are the worst. Every time you regrind, the glass fibers get shorter. Run that same material through three or four times and your mechanical properties are shot. I've tested parts where the regrind-heavy shots failed impact testing that virgin material passed easily.
Nylon picks up moisture too. If your regrind bin sits open overnight, that material needs four hours in a dryer before you can run it again. Cold runners with nylon mean you're drying regrind constantly.
When I'd pick cold runner
Low volume work
Anything under half a million parts a year, the math usually favors cold runners. The tooling savings more than offset the regrind costs.
Development programs
When you're still figuring out where the gate should go or what material you're actually going to run production in, cold runners give you flexibility.
Multi-material tools
If you're running three different resins through the same cavities for different customers, cold runner wins every time.
Tight budgets
Sometimes the money just isn't there for hot runner tooling. Cold runner gets you making parts.
When I wouldn't
High volume automotive or medical
If you're running 24/7 and cycle time matters, that extra second or two ejecting the runner adds up. Hot runners also give you better process control when you need Cpk numbers for PPAP.
Expensive engineered resins
PEEK at $50 a pound, you don't want to be grinding and reprocessing that. Go hot runner.
Zero-regrind specifications
Some aerospace and medical specs prohibit regrind entirely. At that point your cold runner scrap is pure waste.
Cold runners are simple. They work.
For a lot of insert molding jobs they're still the right answer, even if hot runners get all the attention in the trade magazines. Know what your program needs and pick the tool that fits.