Causes of Sprue Sticking in Injection Molding and How to Solve It

Sprue sticking is a common molding issue in injection mold production. It occurs when the sprue or runner does not release smoothly from the mold during opening, which can affect cycle stability, part removal, and production efficiency. In actual production, sprue sticking may be caused by mold structure, insufficient cooling, poor nozzle matching, excessive holding pressure, or insufficient draft angle.

To solve this problem effectively, both the mold design and the molding conditions should be reviewed together. In many cases, sprue sticking is not caused by a single factor but by the combined effect of runner design, temperature control, shrinkage behavior, and demolding resistance.

Main Causes of Sprue Sticking

1. Runner Size Is Too Large

   If the sprue or runner is too large, the plastic material may grip the sprue area too tightly during cooling, making release more difficult. In this case, the mold design should be modified appropriately.

2. Insufficient Runner Cooling

   If the sprue or runner does not cool sufficiently before mold opening, the material may remain too soft and stick in the sprue area. Extending the cooling time or reducing the temperature of the nozzle area can help improve release.

3. Insufficient Draft Angle

   If the draft angle in the sprue or runner area is too small, demolding resistance increases. The mold should be modified to provide a more suitable draft angle for smooth release.

4. Poor Matching Between Nozzle and Sprue Bushing

   If the concave radius of the sprue bushing does not match the injection machine nozzle correctly, material flow and release behavior may be affected. Proper adjustment and matching of the nozzle and sprue bushing are necessary.

5. Rough Inner Surface of the Sprue

   If the inner surface of the sprue is not polished properly, or if there is no suitable demolding chamfer, the sprue may stick during mold opening. Polishing and repair of the sprue surface can help reduce resistance.

6. Damage to the Sprue Opening

   If the outer opening of the sprue is damaged, worn, or deformed, the molded sprue may not release correctly. The damaged area should be repaired to restore proper release conditions.

7. No Effective Sprue Pulling Mechanism

   If the mold does not include a suitable sprue puller or sprue locking structure, the sprue may fail to separate reliably from the fixed side during mold opening. A proper sprue puller design should be added where necessary.

How Holding Pressure and Mold Temperature Affect Sprue Release

When the demolding resistance is caused by a sprue that is too tight because of shrinkage behavior, increasing the holding pressure and mold temperature may help suppress excessive molding shrinkage and improve the release condition in certain cases.

On the other hand, if the part or sprue is overpacked and becomes too tight because of excessive filling, it may be necessary to lower the holding pressure and mold temperature so that molding shrinkage can increase appropriately and reduce the gripping effect. In practice, this adjustment must be evaluated carefully according to the actual mold structure and resin behavior.

Structural Improvements to Reduce Sprue and Demolding Resistance

Improving draft angle is one of the most effective structural methods for reducing demolding resistance. The draft angle around the cavity, hub, ribs, and other release-sensitive areas should be reviewed and increased where necessary. This is especially important in areas where the plastic part tends to lock mechanically into the mold.

If the product or sprue area still requires a stronger ejection force, ejector pins should be added or their location should be optimized in order to improve the release force distribution. In some cases, stronger or more direct ejection support is necessary to prevent sticking or unstable demolding.

How to Improve Sprue Release in Actual Production

To improve sprue release, manufacturers should evaluate runner design, cooling performance, draft angle, nozzle matching, holding pressure, mold temperature, and the effectiveness of the sprue puller structure. If process adjustment alone is not enough, the mold structure may need to be repaired or optimized to achieve stable demolding.

Because sprue sticking directly affects cycle time and production consistency, solving the issue early is important for stable mass production and lower maintenance cost.

Injection Mold Optimization Support from FITMOLD

FITMOLD supports custom injection mold development and plastic part production for projects that require better demolding stability, practical mold optimization, and improved manufacturing efficiency. For sprue sticking or runner release problems, we help evaluate mold structure, cooling layout, process settings, and ejection design to improve tooling performance and reduce production risk.

If you are looking for a manufacturing partner for custom injection molds and plastic part production, FITMOLD can support your project from design review to tooling and production.

Contact us: sales@fitmold.com