What Is a Pin Gate Extension Bushing and When Do You Need One?

What Extension Bushings Do

A pin gate extension bushing is a cylindrical insert with a through-bore that connects the runner plate bore to the pin gate bushing bore. It is used when the total mold plate stack height exceeds the maximum length of available pin point gate bushings.

Standard pin gate bushings are manufactured in lengths up to approximately 60–100 mm depending on the body diameter. For mold plates thicker than this — common in large automotive and appliance molds — the extension bushing provides the additional reach needed.

The extension bushing itself has no gate tip. It simply provides a smooth, hardened bore for the resin to flow through on its way from the runner to the gate bushing tip.

When You Need an Extension Bushing

Use an extension bushing in these situations:

• Thick mold plates — When the distance from the runner plate to the cavity surface exceeds the gate bushing's maximum available length
• Stripper plate molds — Where an additional plate between the runner plate and cavity plate adds to the stack height
• Insulated runner molds — Where extra plate thickness is needed for thermal management
• Mold modifications — When adding plates to an existing mold increases the stack height beyond original gate bushing reach

Sizing an Extension Bushing

The critical dimensions are:

The length calculation is critical. An extension that is too long will push the gate bushing tip too far into the cavity. An extension that is too short will leave a gap that fills with stagnant resin, causing degradation and black specks. According to mold component sizing standards referenced in ScienceDirect's injection moulding engineering resources, bore step misalignment is one of the top causes of material degradation in multi-component flow assemblies.

Material Options

For most applications, SKD61 is the default choice. It offers the best combination of hardness, toughness, and thermal conductivity. Switch to stainless steel only when processing corrosive resins that generate acidic off-gases — PVC releases hydrochloric acid, and some flame-retardant additives produce corrosive byproducts. According to AZoM's H13 tool steel data sheet, SKD61/H13 achieves optimal hardness and toughness after double tempering at 550–600°C.

Installation Best Practices

• Ream the assembled bore — After installing the extension and gate bushing, ream through the entire bore assembly to ensure perfect alignment. Any step at the joint traps resin and causes degradation.
• Apply anti-seize compound — The extension bushing operates at mold temperature (40–120°C) and can seize in the plate bore over time. Apply a thin layer of high-temperature anti-seize to the OD before installation.
• Check bore surface finish — The bore inner surface should be polished to Ra 0.4 or better to prevent material hang-up. Rough bore surfaces cause streaking and color inconsistency in the molded part.
• Verify concentricity — Use a pin gauge or bore alignment tool to verify the extension-to-gate bushing bore alignment is within 0.01 mm before running production.