CNC Processing Plastic Mold Pins

• 1
  What these pins do inside a mold
• 2
  How CNC processing plastic mold pins works
• 3
  Which material and hardness hold up
• 4
  Fits, clearances and the tolerance budget
• 5
  When CNC pin making is the wrong route
• 6
  Matching pin type to application

A plastic mold pin is a cylindrical steel or carbide element that either pushes a cooled part out of the cavity or holds a hole open while resin flows around it. The pin looks like a dowel. Its job is not. It has to slide thousands of times a day without galling, and it has to hold a diameter that the molded part depends on.

Three families cover most of the work. Ejector pins push the part off the core. Core pins form through-holes, bosses and small bores that would be too deep to mill in the tool. Guide and leader pins keep the two mold halves aligned so the cavity and core do not rub each other shut.

The common thread is that each pin runs in a bore cut to a clearance measured in microns, and it has to keep that fit after millions of strokes. Surface finish, straightness and hardness decide whether that happens. Diameter alone does not.

Pin sizes in production tools usually run from Ø1 mm to Ø25 mm, with lengths up to about 300 mm. Above that range you are usually looking at a sleeve, a core insert or a machined post rather than a pin, and the process changes with it.

Most pins start as bar stock and are turned on a CNC lathe. The body is roughed, the head or shoulder is formed, and the working diameter is left 0.2–0.4 mm oversize for grinding. A mill-turn center can cut the head and the shank in one setup if the pin has a flange or an undercut.

Turning alone will not hold the tolerance these parts need. Precision pins are finished by cylindrical or centerless grinding, which controls diameter and roundness independently of the lathe. That is where ±0.005 mm and a straightness of a few microns per 100 mm come from.

Heat treatment sits between the two operations. Through-hardened tool steel is quenched, then ground after tempering so the finished size is not lost to distortion. Nitrided pins are ground first and nitrided last, because the case adds a few microns and cannot be cut afterwards.

For pins under Ø3 mm, or pins with a stepped profile, a simultaneous 5-axis machine with a Ø400 mm rotary table can turn and relieve the shank in one chucking. Fewer setups means fewer chances to lose concentricity between the head and the working end.

Finish is chosen by function, not by looks. A sliding ejector pin usually runs at Ra 0.2–0.8 μm to hold a film of lubricant and resist galling. A core pin that only shapes plastic can stay at Ra 0.8–1.6 μm.

Mold pins live in three material groups. Through-hardened tool steel covers most ejector and core pins. Case-hardened grades take nitriding well and keep a tough core under a hard skin. Stainless grades appear where the resin is corrosive, such as PVC or a halogenated flame retardant.

Hardness is the real decision. A pin that is too soft will wear oval against a hardened bore. One that is too hard will chip at the head where the ejector plate strikes it. Most production pins settle in the 58–62 HRC range after heat treatment, with the head often left slightly softer.

For abrasive filled resins such as glass-filled PA or PBT, standard tool steel wears at the working end within a few hundred thousand cycles. Carbide or a nitrided, coated pin lasts longer. The trade-off is cost and the risk of a brittle pin breaking inside a hot tool.

Stainless 420 and 440C are common for medical and food-contact molds because they resist corrosion and tolerate frequent cleaning. They machine well before hardening, but they move more during heat treatment. Build that into the stock allowance.

If the pin runs in a dry, high-cycle application, a hard chrome or electroless nickel layer reduces friction. Our shop applies both as part of the finishing sequence, and we control the plating thickness so the final diameter stays inside tolerance.

A pin is only as good as the hole it runs in. The bore is usually reamed or wire-EDM cut to a tolerance that is tighter than the pin itself. If both are made to ±0.005 mm, the worst-case clearance can be 0.01 mm before any thermal growth is counted.

Mold temperature moves the numbers. A steel pin at 200 °C grows more than the surrounding plate in some designs and less in others, depending on the plate material. For pins longer than 150 mm, check the growth at running temperature rather than at room temperature.

Venting is part of the fit. If air cannot escape past the pin, the pin acts like a piston and either slows the ejection stroke or blows a slug of trapped gas into the part. A small flat ground on the pin shank is often enough.

Concentricity between the head and the working diameter matters more than most drawings suggest. A head that is off-center loads the ejector plate unevenly and bends the pin on every stroke. That is a fatigue failure waiting to happen.

On a Ø6 mm pin, a 0.01 mm error in diameter is about 0.17 percent of the size. On a Ø1.5 mm pin, the same error is 0.67 percent. Small pins demand proportionally tighter process control, which is why they cost more per piece.

CNC turning and grinding win on short runs, odd sizes and pins with features that a catalog cannot supply. They lose when the geometry is a plain cylinder and the quantity is high. A standard Ø5 × 150 mm ejector pin from a catalog costs less than the setup time to make one.

Very long, very slender pins are another weak spot. A Ø2 mm pin at 300 mm long will deflect under cutting force no matter how light the pass. Centerless grinding helps, but the blank still has to be turned first. Below Ø1 mm, consider wire EDM or a purchased blank that is ground to size.

Pins with a complex head, a thread or a cross-hole are worth machining, because the alternative is a multi-part assembly with its own failure modes. A single machined pin removes joints, and joints are where fatigue cracks start.

A mold pin is also not the right answer when the hole could be formed by the core itself. If the feature is a through-hole in a low-volume part, a machined core post is simpler and eliminates a wear pair.

Finally, surface treatment cannot fix a bad fit. If the bore is out of round, plating the pin only moves the problem. Fix the bore, then decide on the coating.