1. Molding temperature: When the formulation is fixed, temperature is the key factor determining whether vulcanization proceeds properly. If the temperature is too low, the rubber compound will not react; if it is too high, the product becomes brittle and deformed. Typically, below 170°C, the vulcanization rate is slow, requiring longer curing times. Above 180°C, vulcanization occurs very rapidly, yielding a high output, but the mold often becomes fouled, the surface develops a glossy sheen, the product may warp, dimensions shrink, wall thickness exceeds specifications, and flash becomes brittle.
2. Curing Time: The duration of the curing process directly affects product quality. If the curing time is too short, the product will be under‑cured, prone to significant deformation, easily turn white, and lack durability. Conversely, excessive curing can lead to over‑curing, reducing tensile strength and hardness, resulting in brittleness or a sticky surface. The relationship between temperature and time must be carefully calibrated based on the product’s thickness and the type of rubber used. Thick‑walled parts require low‑temperature, extended‑time curing. If, after demolding, insufficient curing is detected—causing the part to stick to the mold and become difficult to remove—re‑mold and re‑cure the part; this usually allows the material to release cleanly. One of the most common issues in molding is employee‑initiated machine adjustments that produce scrap.
3. Mold pressure: Refers to the set pressure of the molding vulcanizing press, which is controlled via a pressure gauge and computer. Sufficient pressure ensures that, during mold closing, the rubber compound is fully compacted into the cavity and prevents the formation of bubbles caused by trace moisture in the compound during vulcanization; combined with venting, it helps control bubble defects in the finished product. For compounds with high hardness and poor flowability, a higher pressure should be selected; however, excessively high pressure can easily damage the mold’s cutting edges. The appropriate pressure also depends on the mold’s dimensions: for smaller molds, the pressure should be reduced accordingly to prevent deformation of both the mold and the press platens. When adjusting the vulcanizing press pressure, never exceed the machine’s maximum rated pressure, as doing so may result in equipment damage. Leaving empty molds unpressurized (except for injection molds) is an effective measure to protect the mold’s cutting edges and extend its service life.
4. Shape of the rubber compound: This refers to the pre‑formed shape of the rubber, which may be a long strip, small square blocks, or a circular ring. The thickness, length, and width of the rubber compound must closely match those of the final product. The closer the rubber’s shape is to the product’s geometry, the shorter the flow time and distance within the mold, resulting in thinner flash, reduced rubber consumption, and a higher yield of conforming parts.
5. Rubber Compound Weight: The weight of the rubber compound must exceed that of the finished part; otherwise, underfilling will occur. However, using more compound than necessary results in thicker flash and larger dimensions. The compound’s weight is directly related to its shape. Achieving optimal production with minimal material and a well‑designed mold—while avoiding underfilling—is a clear indicator of the molding operator’s skill level. Even if the compound weight is sufficient, an excess length beyond the mold can still lead to underfilling. Similarly, when both the compound’s shape and weight are appropriate but it is positioned off‑center, one side may be thin while the other is thick; in severe cases, this can result in underfill on one side and excessive flash on the other. A properly shaped and appropriately weighted compound, if cut unevenly in thickness or width, or if additional material is applied inconsistently, can also cause underfilling. When underfilling occurs, avoid immediately increasing the compound feed; first, remove the complete mold—including the flash—and verify the weight. Next, inspect the part for uniform thickness, and only then decide whether to add more material. The technical expertise and experience of the molding supervisor are critical!
6. Operation Speed: Refers to the speed at which rubber is placed in the mold and products are removed during vulcanization. Vulcanization is a chemical reaction; once the rubber compound is positioned on the heated mold, it begins to cure. For multi-cavity molds, the shorter the time required to place each piece of rubber—from the first to the last—the better. After placing the rubber, the mold should be closed immediately; otherwise, issues such as mold fouling, scorching of the compound, thick flash, and deformation may occur. During product removal, pneumatic assistance can easily blow the parts out, but it also lowers the mold temperature. Inexperienced operators are most likely to encounter a situation where the mold temperature drops progressively, resulting in undercured parts that stick to the mold—this is a common problem for beginners. When producing smooth-surfaced rubber sheets, the speeds of rubber placement, mold closing, and mold opening have the most pronounced impact on surface quality. This is one of the key reasons why certain products with stringent appearance requirements are processed using the overhead‑mold technique.