Sprue Bushing Buying Checklist: 7 Mistakes Every Buyer Keeps Making

Most buyers treat a sprue bushing as a commodity: pick a size, compare three prices, place the order. Then the mold goes in the press and the problems start — leaking, splay, stuck sprues, cold slugs, a carriage that blows back and coats your heater bands in plastic. If you are about to order sprue bushings, or you have bought them before and gotten burned, this checklist is for you.

I have spent years manufacturing mold components for distributors, mold makers and OEM customers, and I have watched the same seven mistakes repeat across purchase orders. Each one is avoidable — usually with a few extra minutes on the RFQ (request for quotation). Here is what buyers get wrong, and what to ask for instead.

1. Match the nozzle to the bushing before anything else

This is the single most common cause of trouble. Field studies by injection-molding engineering specialists have tested brand-new sprue bushings against brand-new nozzle tips and found them mismatched most of the time — as high as nine out of ten in one widely cited survey — even when both parts are nominally the same size.

What does “mismatched” mean in practice?

• The spherical radius (SR) of the nozzle tip and the concave radius of the bushing seat must belong to the same family — either the ½-inch (12.7 mm) family or the ¾-inch (19.05 mm) family. A ½-inch nozzle will never seal in a ¾-inch bushing.
• The nozzle radius should be about 1.5 mm smaller than the bushing seat radius, so the seal forms on the inner ring instead of the outside edge.
• The nozzle orifice should be about 0.5–1 mm smaller than the bushing entry hole, so the solidified sprue pulls free cleanly instead of dragging an undercut.

Get this wrong and you get leaks, venturi suck-in that looks like splay or bubbles, drooling, stringing, and cold slugs. The fix starts on the drawing: put the machine’s nozzle SR and orifice size on your RFQ, and ask the supplier to confirm the bushing seat will match before you order.

2. Put every dimension on the PO, not just “one of those brass bits”

If you only send a length and a material, you are asking for a surprise. A sprue bushing is defined by more than length. The dimensions buyers need to specify on the purchase order are:

• Overall length (L) — measured from the head to the tip, not including the machine nozzle.
• Shank (stem) length (A) — from the underside of the head to the end of the shank.
• Head diameter and head thickness — must sit flush in the clamp plate and clear the locating ring.
• Seat radius (SR) — 0.50″ or 0.75″ spherical, or a flat-face design for certain machines.
• Orifice (O) diameter — the entry hole on the nozzle seat; also called the gate or melt passage diameter.
• Internal taper — the classic standard is 0.5 inch per foot (about 1.19° per side), which helps the sprue release.
• Body (shank) diameter — must fit the bore in the mold base. A tight fit of no more than 0.0004 inch clearance across the mating diameters is widely recommended, because some resins will flash past a looser fit.

Here is the awkward reality on taper and orifice availability: standard bushings come in a limited set of orifice sizes, so your mold may genuinely need a custom bore. If so, say it up front. A supplier that reams a standard bushing to fit is a very different service from one that quotes a true custom.

3. Pick the standard and the seat type — and say which one you mean

Sprue bushings are built to dimensional standards, and the names get mixed up constantly. The relevant ones are ISO 10072:2004 (Tools for moulding — Sprue bushes — Dimensions), JIS B 5112 and DIN design dimensions. ISO 10072 defines types A and B; JIS B 5112 expands the family to types A–F; DIN design dimensions follow the ISO 10072-based German adoption. When you specify one on a PO, make sure your supplier knows you mean a dimensional standard, not a vague “standard part.”

There are also two seat philosophies for cold sprue bushings: a spherical seat that cups the machine nozzle, and a flat seat for flat-nozzle machines. Get the wrong one and the nozzle cannot seal. Ask your supplier which standard their stock is made to, and whether they can switch between spherical and flat without a tooling fee.

4. Match material and hardness to your resin — not to the cheapest quote

This is where low-price sourcing usually goes wrong. A bushing that lasts a million cycles in unfilled ABS can erode in a week in glass-filled nylon. The general rule of thumb: corrosive resins (PVC, acetal) call for stainless steel; abrasive resins (glass- or talc-filled) call for hot-work steel like SKD61/H13, sometimes with carbide-lined bore inserts; very high-temperature resins like PEEK or LCP call for grades rated for continuous operation above 150°C. If cycle time or cooling matters, a copper-alloy bushing can dissipate heat several times faster than carbon steel — at a cost in hardness.

Your resin type	Typical bushing material	Typical hardness
Unfilled ABS, PS, PP, PE	S45C or pre-hardened steel	HRC 40–48
Engineering plastics up to ~30% glass fill	SKD61 / H13 (ESR preferred for long runs)	HRC 48–52
PVC, POM, corrosive grades	Stainless (e.g. S136)	HRC 48–52
High glass content or abrasive fill	DC53, carbide-lined, or coated	HRC 58+
PEEK, LCP, ultra-high temperature	Nitriding grades (e.g. 41CrAlMo7)	HRC 46+

Name the material and the hardness on the RFQ, and ask for the heat-treatment record. If a supplier gives you a price that is dramatically lower than the market, ask what grade they actually used. Swapping a specified grade for a cheaper one is a known risk in imported bushings, and you cannot see it with your eyes.

5. Write the finish and tolerance expectations into the contract

The inside bore of a cold sprue bushing is polished so the sprue releases cleanly — and the polish direction matters. Polishing “in draw” (in the direction the sprue is pulled) prevents micro-scratches that act as undercuts and make sprues stick. Circular polishing can create exactly the scratches that cause sticking. If you mold soft-touch materials, a vapor-honed or lightly blasted surface can actually release better than a mirror finish.

Put numbers on the contract. Specify: surface finish (for example Ra 0.4 µm in the bore), concentricity of the bore to the OD, and the fit tolerance to your mold-base bore. If the bushing will be pressed into an existing mold, give the supplier the hole size so they can confirm the interference fit rather than assuming a standard clearance.

6. Verify the supplier can prove tolerance — a sample is not a batch

Here is the part most importers learn the hard way. A glossy website, a cheap price and one beautiful sample tell you nothing about the next 10,000 parts. A perfect sample only proves a supplier can make one good part, not thousands.

Before you pay, ask for evidence — and make it part of the PO:

• Material certificates (MTR) matching the grade and heat number of your batch.
• Dimension reports with measured values against the tolerance limits, ideally CMM-based or on a clearly labeled gauge.
• Hardness readings from the actual batch, not a copy of the datasheet.
• Batch traceability — can they trace a box back to the raw material lot and production date?
• A pre-shipment inspection by you or an independent third party covering quantity, specifications, workmanship, packaging and documentation.

A disciplined supplier can walk you through this in one email. A vague one will route you back to sales. That is your signal.

7. Agree the commercial terms before you send a cent

Last, the money and logistics. This part is less glamorous and costs the most when skipped:

• MOQ — ask what the real minimum is, and whether mixing sizes counts toward it. Consolidating SKUs can cut unit cost dramatically.
• Lead time — standard stock items versus customs are completely different timelines; get both in writing.
• Customs are non-returnable — if it is made to your drawing, it is yours. Check the geometry before production, not after.
• Packaging and rust protection — parts should be cleaned, oil-protected and packaged for long sea freight, since rust and corrosion from poor shipping practices are among the most common overseas claims on machined steel parts.
• Payment milestones — tie payments to visible milestones (deposit, first article approval, inspection) instead of paying a large balance up front.

The 60-second summary

Buying sprue bushings does not have to be a gamble. Nail the nozzle match, send the full set of dimensions, name the standard and the seat type, pick material and hardness for your resin, write finish and tolerance into the contract, demand verifiable quality evidence — and then talk terms.

If you want to skip the trial-and-error, we manufacture cold sprue bushings to ISO 10072, JIS B 5112 and DIN dimensions with up to 0.002 mm precision, plus custom hot (heated) sprue bushings to your drawings — available under your brand since 2006. Send us your drawing or your machine nozzle details and we will confirm the match before you order. Request a quote here.