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In the mold manufacturing industry, there is a critical step often referred to as the "final quality inspection" of a mold — this is known as die spotting (also called mold tryout or fitting).

What is Die Spotting?

Die spotting, also known as mold fitting or tryout, is the process of assembling the core and cavity halves of a mold to check whether their parting surfaces mate perfectly. If the fit is poor, molten plastic will escape through the gaps during injection, causing undesirable flash and significantly increasing the rejection rate of molded parts.

Core Method: The Red Lead (Prussian Blue) Test

The most fundamental inspection method in die spotting is the red lead (or Prussian blue) test. The operator applies a thin, even layer of red lead paste onto the parting surface of the cavity side (or core side). The mold halves are then closed under pressure. After opening, the transferred marks on the opposing parting surface are examined. Uniform and continuous blue/red marks indicate good contact. An area with no marks reveals excessive clearance, while a dark or shiny spot indicates excessive interference that requires stoning or grinding.

The video linked below shows our factory's mold technicians performing manual mold flipping on the molds.（https://youtube.com/shorts/4b17oLrfqhk）

This process is repeated iteratively until the marking pattern is uniform and consistent in width — typically required to be between 3 to 5 millimeters.

Manual Die Spotting vs. Die Spotting Press

Traditional manual die spotting relies on skilled craftsmen using copper bars and hammers to tap the mold, relying on feel and experience to assess contact. While this method offers high flexibility and the experienced judgement of a master toolmaker remains irreplaceable, it has significant drawbacks. The impact force is uneven and can damage the mold, the labor intensity is high, and it demands exceptional skill — a qualified spotter often requires several years of apprenticeship.

Die spotting presses use dedicated hydraulic equipment to apply uniform, controlled pressure across the entire mold. This method delivers superior consistency, can be operated by a single person, causes zero impact damage to the mold, and is far more efficient than manual methods. However, the capital investment is substantial, with a large spotting press potentially costing over one million yuan.

The Role of Die Spotting in the Industry

Die spotting is the final gateway for a mold to transition from individual components to a qualified production tool. No matter how good the design or how precise the machining, if the spotting is inadequate, the mold cannot produce acceptable parts.

Furthermore, die spotting is the process that best demonstrates the technical skill of a mold maker. A qualified spotter must possess solid theoretical knowledge, extensive hands-on experience, and rigorous analytical judgment. As automation advances, die spotting is evolving from purely manual to semi-automatic operations. Consequently, the demand for talent is shifting from pure manual dexterity toward a combination of process judgment and equipment operation.

Conclusion

Die spotting may appear to be a traditional craft, but it directly determines the quality and service life of a mold. Whether performed manually or with a press, its core objective remains unchanged: to achieve a perfect match between the two halves of the mold, ensuring that every injection-molded part meets quality standards. As the industry saying goes: "Spot the mold well, and it will last for years."