I. Mass Production Shrinkage Porosity Reaches 40%, Riser Sleeve Enlargement Fails Rectification

The case study focuses on QT600-10 planet carrier casting. The part features a square profile with bearing bosses arranged at four corners and an inner tooth bore at the center. As a critical ductile iron component for transmission systems, it demands strict internal density.

During prototype trial production, only minor shrinkage porosity appeared on the bottom surface of the inner tooth bore, which could be screened manually to meet delivery requirements. After switching to small-batch mass production, defects erupted sharply, pushing the shrinkage porosity reject rate up to 40%. A large number of castings revealed internal cavities after machining and failed flaw detection.

The technical team first adopted conventional industry rectification ideas to increase feeding capacity. They expanded the diameter of original side riser sleeves from 90mm to 105mm, adjusted the gating entry direction, and tested pouring temperatures across multiple ranges. After one and a half months of repeated on-site pouring trials and simulation verification, shrinkage porosity showed no obvious improvement. Solidification flow interruption additionally occurred, so the hot spot could not receive molten iron supply. The project once faced risks of customer claims and order loss.

II. Defect Root Cause: Insufficient Riser Sleeve Capacity Is Not the Issue; Feeding Path Solidifies in Advance

Based on solidification simulation and casting dissection analysis, the technical team confirmed the core failure cause. The problem was not insufficient molten iron stored in riser sleeves, but premature solidification and blockage of the feeding path.

Castings cool following the rule: thin sections solidify first while thick hot spots solidify later. After mold filling completes, thin sections dissipate heat faster and form a solid shell preferentially, directly cutting off the path connecting riser sleeves and hot spots. Even if liquid molten iron remains inside riser sleeves, it cannot be delivered to thick hot spots, rendering the feeding system completely ineffective.

1. Risk evaluation by wall thickness ratio: When the wall thickness ratio is less than 1.5, the casting solidifies nearly synchronously with low shrinkage porosity risk. A ratio of 1.5~3 indicates moderate risk, and unobstructed feeding paths must be guaranteed. If the ratio exceeds 3, thin sections solidify fully and rapidly while thick sections still retain liquid metal, leading to extremely high shrinkage porosity risk. Chills or redesigned riser sleeve layout are mandatory.

2. Staged characteristics of metal shrinkage: Liquid shrinkage accounts for 1.5%~2%. Molten iron maintains good fluidity and feeding is easy to achieve. Solidification shrinkage reaches 3%~4%, the primary source of shrinkage porosity. At this stage, molten iron turns semi-solid with drastically reduced fluidity, making feeding highly difficult. Solid shrinkage stands at approximately 1%. The casting is fully cured and feeding is no longer feasible.

3. Material property difference: QT600-10 ductile iron has higher pearlite content. Its volume expansion from graphite precipitation to compensate shrinkage is weaker than low-grade ductile iron such as QT500-7. This material imposes stricter requirements on feeding path patency and feeding time window, and mature processes for lower-grade ductile iron cannot be directly copied.

III. Abandon Side Riser Sleeve Enlargement; Adopt Central Exothermic Riser Sleeve

The technical team discarded the conventional idea of simply enlarging riser sleeves and redesigned feeding routes. The optimized solution is to fill the central inner tooth bore and apply central exothermic riser sleeves.

1. Riser sleeves are placed directly above hot spots to greatly shorten feeding distance and reduce heat loss during molten iron transportation;

2. Long narrow side feeding paths are eliminated, fundamentally avoiding blockage caused by premature solidification of feeding channels;

3. Exothermic materials continuously release heat to extend the liquid holding time of molten iron inside riser sleeves, ensuring continuous molten iron supply for hot spots throughout solidification.

Simulation results verify that this process can establish stable sequential solidification and completely eliminate shrinkage porosity on the bottom of the inner tooth bore. Subsequent trial production will effectively reduce scrap rate and stabilize internal quality of mass-produced parts.

For a long time, many foundries tend to enlarge riser sleeves once shrinkage porosity occurs. This method increases molten iron consumption yet often fails to eradicate defects and raises production costs. When confronted with shrinkage porosity, enterprises should first assess whether feeding paths will solidify prematurely. Targeting rational sequential solidification, process optimization can be realized by adjusting riser sleeve positions, shortening feeding paths and properly using chills instead of blindly enlarging riser sleeves. This methodology is especially applicable to high-strength ductile iron transmission parts like QT600-10, helping enterprises cut trial material waste, shorten rectification cycles and stabilize mass production quality.