Straight vs. Tapered Runner Lock Pins: Which Shaft Type Do You Need?

Shaft Geometry Comparison

How Each Shaft Works

Straight Shaft

A straight shaft runner lock pin is a cylindrical pin with a uniform body diameter along its length. It is installed by pressing it into a reamed bore in the mold plate (H7/h6 interference fit) or by sliding it into a clearance bore and securing it with a set screw.

The press fit holds the pin in position during the thousands of mold opening and closing cycles. Over time (100,000+ cycles), the press fit can loosen slightly due to micro-wear of the bore surface, which may require bore re-reaming and pin replacement.

Tapered Shaft

A tapered shaft pin has a slight conical taper along its body. When inserted into a matching tapered bore, the wedge geometry creates a self-locking grip that increases with seating force. This taper provides two advantages:

• Self-centering — The taper acts as a centering guide during installation, achieving ±0.005 mm concentricity without special tooling
• Increasing retention — Unlike a press fit that loosens with wear, a tapered fit can be re-tightened by tapping the pin further into the bore

According to mechanical engineering handbooks referenced by ISO 2339 standard for taper pins, taper pin connections provide superior axial and radial positioning compared to cylindrical interference fits.

Selection Decision Guide

Choose based on your primary requirement:

• Easy maintenance, frequent replacement → straight shaft (push out and replace in minutes)
• Precision runner alignment (±0.005 mm) → tapered shaft (self-centering taper)
• High lateral force (large runner cross-section, stiff resins) → tapered shaft (wedge resists lateral loads)
• Standard three-plate mold, moderate volumes → straight shaft (industry default)
• High-volume automotive/medical production → tapered shaft (better long-term stability)

Note that shaft type is independent of tip type. Both straight and tapered shafts are available with standard, hard lock, edge, and cone tips. Choose the shaft based on installation and retention requirements; choose the tip based on runner grip requirements. See the runner lock pin tip comparison guide for tip selection guidance.

Plate Bore Preparation

Bore preparation differs between the two shaft types:

For tapered shaft pins, always use the reamer recommended by the pin manufacturer. A generic taper reamer may not match the pin's exact taper angle, resulting in poor contact and reduced retention force. According to JIS B 5111 mold component specifications, taper pin bores must achieve full contact over at least 80% of the taper surface for proper function.