The live pages mention SCM-440, SKD-61 (H13) and hardened steel. The exact grade depends on the required hardness, wear resistance, operating temperature and injection pressure. We confirm the actual material, heat treatment and hardness specification based on the product series before manufacture.
SKD-61 (H13) is a hot-work tool steel preferred when the mould processes abrasive resins (e.g. glass-filled), requires high operating temperatures, or needs superior toughness to resist cracking under repeated nozzle impact and high injection pressure.
The live pages state 40 ± 2 HRC, 45 ± 2 HRC and 52 ± 2 HRC across different products. A hardness of ~45–52 HRC is common for H13 bushes to balance wear resistance against toughness. A bush that is too hard may shatter under nozzle impact; a soft bush will deform or wear quickly.
Deformation occurs if the machine nozzle contacts the bush with excessive carriage force, if the nozzle radius is mismatched (e.g. flatter than the seat), if the bush hardness is too low, or if the mould is misaligned. Repeated harsh impact crushes the seat and causes leakage.
Sticking can result from an inadequate internal taper, rough surface finish, scoring, an inlet diameter smaller than the nozzle (creating an undercut hook), insufficient cooling time, or a damaged puller mechanism. Inspect the extracted sprue and the bush bore to identify the cause.
Leakage indicates a poor seal between the machine nozzle and the seating radius. Causes include mismatched radii (e.g. nozzle radius larger than seat radius), worn or deformed seating faces, poor locating-ring alignment, insufficient carriage contact pressure, or polymer residue preventing contact.
Flash at the parting line or runner interface occurs when the overall length of the bush is too long (holding the mould open) or too short (leaving a gap). It can also happen if the bush is not clamped securely by its flange, allowing it to move backward under injection pressure.
Light longitudinal polishing can remove minor residue and improve release. However, aggressive radial polishing or grinding can alter the taper, create undercuts, and remove hardened layers. If the bore is heavily scored or damaged, the bush should be replaced.
The thickest section of molten plastic is usually inside the sprue bush, making it the slowest part to cool. The surrounding mould plate must have adequate cooling channels near the bush to solidify the sprue quickly. If the sprue is ejected while hot and soft, it may stick or break.
If the flange is loose or not clamped securely by the locating ring, the bush can move axially under nozzle pressure and injection pressure. This movement causes flash at the parting line, inconsistent sealing, and potential damage to the mould plates and locating ring.
Most standard sprue bushes are supplied with extra length on the straight section so they can be cut and ground flush with the runner or cavity profile. When grinding, ensure the face is perfectly flush and free from burrs. Do not modify the seating radius or flange dimensions.
A sprue can break if the puller-pin mechanism is weak, the internal bush taper is scored or too small, the inlet bore creates an undercut against the nozzle, or the material is not cooled sufficiently. Inspect the broken sprue and bush bore to determine the sticking point.
A smooth internal finish (e.g. fine reamed or polished longitudinally) prevents the shrinking polymer from gripping the bore. Machining marks that run around the circumference act as micro-undercuts, increasing friction and causing the sprue to stretch, stick or break during extraction.
A standard (cold) sprue bush produces a solid runner branch (sprue) that must be ejected and scrapped. A hot sprue bush contains an internal heater to keep the plastic molten, injecting directly into the cavity or hot runner without producing a cold sprue. Their design, material and control are entirely different.
If a bush is tight due to press-fit, rust, or polymer leakage, remove the locating ring and gently tap the bush out from the parting-line side using a soft brass or aluminium drift. Avoid striking the hardened seating radius or internal bore with steel hammers.
If the sprue bush exit is contoured or machined to match a specific runner shape, cavity profile, or curved parting line, it must not rotate in the mould plate. An anti-rotation flat on the head or a dowel pin ensures the bush remains perfectly aligned with the intended geometry.
Yes. If your mould requires a specific overall length, unique seating radius, non-standard taper, anti-rotation flat or custom exit profile, we can manufacture the sprue bush to an approved dimensioned drawing.
