A practical walkthrough of mold structure and process essentials, covering design, machining, and assembly with real-world considerations for toolmakers.

In mold design, the first thing we lock down is the parting line and the gate location—these decide everything downstream. For a typical two-plate injection mold, we start with the cavity and core layout, then work through the runner system, cooling channels, and ejection mechanism. A common mistake is underestimating shrinkage: for ABS, we use 0.5–0.7% shrinkage, but for PA66 with 30% glass fiber, it jumps to 0.3–0.5% in flow direction and 0.8–1.0% transverse. If you don't account for this in the steel, you'll end up with warpage or short shots. Also, venting depth should be 0.02–0.03 mm for general plastics—any deeper and you get flash; any shallower and you trap gas, causing burn marks. For high-cavitation molds, we always add a balanced runner layout with a cold slug well at the end of each branch, otherwise filling pressure becomes uneven and part weight varies.

On the machining side, electrode design for EDM is where most shops lose time. We rough with graphite (EDM-3 grade) for steel removal, then finish with copper for better surface finish—typically Ra 0.4 µm or better for optical parts. The key is to leave 0.15–0.2 mm stock for EDM after CNC milling, and always use a separate electrode for the finish pass, because wear on the roughing electrode will ruin the texture. For hardened tool steel like H13 (48–52 HRC), we wire-cut the insert pockets first, then EDM the details. Don't forget to add a 0.5° draft angle on all vertical walls—even if the part drawing says straight, you need it for ejection. When it comes to cooling, drilled straight lines are fine for simple cores, but for deep ribs, we use bubblers or heat pipes. A rule of thumb: keep the cooling line within 1.5 times the wall thickness from the cavity surface, and use turbulent flow (Reynolds number above 4000) to get consistent cooling.

Assembly is where the design meets reality. We start with the ejector plate, checking that all return pins are flush and the springs have at least 10 mm preload. Then we mount the cavity and core, using a dial indicator to verify parallelism within 0.02 mm across the parting surface. For interlocking inserts, we use a locating ring with a 0.005 mm interference fit—any looser and you get mismatch lines on the part. After torquing all bolts to spec (usually 120 Nm for M12), we do a trial shot with a low injection pressure first, then gradually increase. Watch for flash at the parting line—if it appears at 80% of full clamp tonnage, check the venting depth or the steel hardness. Also, always run a short-shot test to see the flow pattern; if the weld line is in a structural area, move the gate or add a flow leader. Finally, keep a maintenance log: measure the mold base wear every 50,000 cycles, and re-polish the cavity every 100,000 shots if you're running glass-filled materials. For more mold sourcing details and process tips, visit MoldWorld at www.moldw.com.