How a Used Hurco From a 2022 CNC Sale Taught Me About Runner Types and Laser vs. TIG Welding

At 7:45 on a Tuesday in March 2022, the semi finally showed up in our parking lot. I stood there with coffee in one hand and a Bill of Lading in the other. Inside that crate: a used Hurco VMX42i that we bought at a CNC sale a few weeks earlier. I took a photo of the Hurco logo on the control panel before the riggers even leveled the machine.

I was excited. I was also an idiot.

Not about the machine—that part turned out to be one of the best purchasing decisions we've made. The problem started after the machine was running. We landed an injection mold job that should have been straightforward, and I nearly turned a $40,000 order into scrap because I didn't understand the types of runner in injection molding, or the difference between laser and TIG welding for mold repair.

Let me back up. I'm the production manager at a custom CNC machining shop in Winnipeg. I've been handling machining orders for eight years. In that time, I've made fourteen significant mistakes that I've documented, totaling roughly $42,000 in wasted budget. This one was the closest I've come to losing a customer.

The job that exposed the gap

Our shop doesn't normally build complete molds. We do CNC machining in Winnipeg—precision milling, turning, small batch work. But in late 2021, a local agricultural technology company asked if we could make a sensor housing. They needed 10,000 parts, not 50. What they needed was a mold.

I said yes before I understood what that meant.

The mold was a two-cavity design. We'd machine the cavity blocks from P20 steel on the Hurco, then polish, assemble, and sample. The concept looked simple. The risk was in the details.

Runner types: the first thing I should have learned

Two weeks into the project, the customer's engineer asked, “What runner system are you planning?” I remember pausing. The honest answer was: I hadn't planned one.

The runner is the channel that carries molten plastic from the sprue to the cavity. It's not optional. And if you choose the wrong system, you'll either fill parts inconsistently, create excessive waste, or get stuck with a mold that can't run efficiently.

There are three runner types you'll most commonly see in injection molding:

• Cold runner: The plastic runs through an unheated channel and is ejected with the part. It's simple and affordable, which is why it's common in low-volume molds. The catch: you create runner waste, and recycled regrind might not be acceptable for glass-filled engineering plastics.
• Hot runner: A heated manifold keeps the plastic molten right up to the gate. This eliminates runner waste, shortens cycle time, and can improve part quality. The downside: significantly higher cost, more complex controls, and a tougher maintenance path.
• Insulated runner: A hybrid that relies on a frozen skin of plastic around a molten core. It's rarely used with heat-sensitive or highly filled materials.

I chose a cold runner because the mold was low to medium volume and the tooling budget was tight. That logic was fine. The problem was the geometry I picked inside that cold runner.

Cold runners also come in cross-section profile options. Full-round is best for flow, but you need a groove in both halves of the mold. Trapezoidal is the practical majority—one groove, machined with an end mill, decent flow. Half-round is the lazy version. It has a bigger surface-to-volume ratio, which means faster cooling at the outer edges and a higher pressure drop.

I chose half-round because it looked fine on my screen. That's the most expensive sentence I've ever said.

Laser vs. TIG welding: same goal, very different heat

Around the same time, we found a small nick in one cavity—right on a sealing edge. Maybe two millimeters long. We needed a repair, not a new insert. The toolmaker who helps us handed me the question: laser or TIG?

I almost said TIG because it was cheaper and faster to schedule.

Here's the thing: TIG welding works fine for structural repairs. The problem is heat. A TIG torch puts a lot of energy into the surrounding steel. On a finished mold cavity, that can draw the temper, create stress, and distort the edge you're trying to save.

Laser welding deposits the same filler metal but uses a focused, low-energy beam. The heat-affected zone is much smaller, which makes it the right choice for precision repair on critical surfaces. You pay more, and you may wait longer. But you don't burn the part.

We had nine business days to first articles. The laser welding shop could get us in two days later than the TIG guy. The upside was a professional repair. The risk was missing the deadline. I kept asking myself: is saving two days worth potentially ruining a $40,000 mold?

The answer was no. I still almost said yes. My toolmaker stopped me. “TIG on that edge will be a dent again by Friday,” he said. “Wait for the laser.”

He was right. The laser welder did the repair in about twenty minutes, and the HAZ was so small I could barely feel it with a fingernail. That decision ended up being the only thing that went right that week.

The first trial: short shots and a cold silence

First trial day came. We mounted the mold, set the barrel temperature, ran the injection cycle. Instead of nice sensor housings, we got short shots—incomplete parts with rounded edges.

I stood there watching the robot drop a half-formed housing into the bin. The Hurco had cut every cavity dimension correctly. The mold assembly was pristine. The failure was in what I couldn't see: the flow path.

Our customer's engineer drove across Winnipeg and asked to see the runner layout. He looked at my CAD file, then at me.

“Half-round runner, 1 mm edge gate, 30% glass-filled nylon,” he said. “You're trying to drink a milkshake through a coffee stirrer.”

Gates also have types. Pin gates, edge gates, submarine gates, film gates, fan gates. Each has a role. For glass-filled nylon, the gate needs to be thick enough to avoid high shear and premature freeze-off. Our 1 mm gate was too small. The plastic was hitting what amounted to a bottleneck before it reached the cavity—so it cooled, pressure spiked, and the part didn't fill.

The fix was not glamorous: recut the cold runner from half-round to trapezoidal, open the gate, add a second vent on the parting line. Four days later, the second trial ran clean parts. First articles passed dimensional inspection. The order survived.

The lessons I keep on a checklist now

Since that job, I've added a pre-cut review for every mold project. The checklist doesn't replace experience, but it catches the gaps before the steel hits the spindle.

• Runner type: cold, hot, or insulated—decide deliberately, not by habit.
• Runner profile: full-round or trapezoidal for engineering plastics; never half-round if you need stable flow.
• Gate size: check it against the material data sheet, especially with glass-filled resins.
• Weld repair: if it's a finished surface, use laser welding. TIG stays in the repair block stage.
• Ask the customer engineer to review the flow layout before machining. They know the material better than you do.

In the 18 months since, we've caught 47 potential errors using some version of that checklist. The used Hurco from the 2022 CNC sale has been reliable, and a positioning checkout following ISO 230-2 confirmed it was within the original spec. It wasn't the machine that failed on that mold project. It was my assumptions.

Now, before every mold job, I open CAD and look at the runner first. If the logic doesn't make sense, I stop. That's not a skill I learned in training—it's a reminder I earned by nearly having to explain a $42,000 mistake.

And if you're looking for CNC machining in Winnipeg, don't assume a shop knows injection molding just because it knows milling. Ask about their checklist. Seriously. It might save your mold.

Every time I see the Hurco logo on the control panel, I'm reminded of that Tuesday morning in March 2022. The machine didn't make my mistakes. I did. The logo is just a logo—but the lessons are mine now.