Introduction
Gypsum molds have become indispensable tools in ceramic slip casting due to their low cost, excellent fluidity, and unique setting and water absorption properties. Traditional methods prepare molds by adjusting the ratio of high-strength gypsum to highly absorbent gypsum, but the conflicting properties of these two types make it difficult to achieve both high strength and low water absorption simultaneously. Modern ceramic production demands molds that simultaneously exhibit high strength, high water absorption, excellent wear resistance, and corrosion resistance to enhance production efficiency and green body quality. While mechanical properties can be improved by adding reinforcing agents, this often leads to reduced water absorption performance.
To achieve high-performance gypsum molds, beyond selecting optimal gypsum raw materials, precise control of casting process parameters is essential. These include the paste-to-water ratio, mixing technique, vacuum degassing, and water temperature. Optimizing these aspects is crucial for overcoming existing technical limitations and advancing the ceramic industry toward high-quality development.
First: Water-to-Gypsum Ratio
| The performance of gypsum molds hinges on precisely regulating the water-to-gypsum ratio. This parameter directly determines the setting speed of the slurry and the final properties of the mold: increasing the ratio effectively enhances mechanical strength and extends service life, but reduces water absorption. Traditional processes typically employ a (1.25–1.28):1 ratio to balance strength and water absorption. |  |
With the adoption of high-strength gypsum materials and the widespread use of combined casting one-step forming techniques, the current production range for paste-water ratios has been optimized to (1.3–1.5):1. This adjustment significantly enhances mold strength and durability while ensuring adequate water absorption. Each porcelain region should scientifically determine the plaster-to-water ratio based on the specific characteristics of the gypsum type used to achieve the optimal balance between strength and water absorption.
Second: Mixing Process
The mixing process of gypsum slurry is a core factor affecting mold quality. Thorough mixing ensures uniform blending of gypsum and water, optimizing the distribution of internal pores in the model, thereby enhancing mold strength and slurry absorption performance. However, excessive mixing time accelerates slurry setting, which is detrimental to casting operations. Traditionally, mixing time is controlled at 1–2 minutes. Nowadays, using retarding agents or slow-setting gypsum allows extending mixing time to 3–5 minutes, further improving mold surface hardness and overall quality.
| Vacuum degassing technology is recommended during mixing to effectively remove air bubbles from the slurry. This prevents mold defects caused by air pockets, which could lead to difficult mold finishing or premature scrap, significantly extending mold service life. The mixing sequence is also critical: Gypsum powder should be evenly sprinkled into the measured water volume. After thorough wetting, mixing should commence while controlling the rotation speed to prevent vortex formation and air entrainment. The entire slurry preparation must be completed before initial setting, typically within 4 minutes, to ensure optimal flowability and casting results. |  |
Third: Temperature Control
| During plaster mold casting, water temperature is a key parameter affecting slurry setting speed and final mold quality. Higher water temperatures significantly accelerate plaster slurry setting—for instance, 20°C water reduces initial setting time by over one-third compared to 8°C water—while also influencing mold strength and expansion rate. |  |
Additionally, while tap water is commonly used in production, impurities within it may remain inside the mold, reacting with the gypsum and potentially reducing its service life. To enhance overall mold performance, it is recommended to prioritize the use of higher-purity water when process conditions permit, and to precisely control water temperature. This optimizes the operational workflow and improves the quality of the finished product.
Fourth: Demolding Process
Post-production processes for gypsum molds—including thickness design, demolding timing, and drying control—are critical to final quality.
Mold Thickness: Traditional molds often exceed 65mm thickness due to insufficient gypsum strength, resulting in bulkiness, high costs, and slow drying. By using high-strength gypsum and optimizing porosity, single-layer mold thickness can be reduced to 40-50mm, and double-layer molds to 25-35mm. This approach saves 1/3 to 1/2 of gypsum usage, significantly reducing weight and costs.
Demolding Control: Demolding timing should be scientifically determined based on gypsum's final setting time, not solely on experience. Premature removal damages the internally unhardened structure, compromising strength; delayed removal risks damaging the master mold and causing removal difficulties due to the exothermic expansion of hardening gypsum. For release agents, potassium soap solution outperforms traditional vegetable oils or chemical agents because it provides uniform coating, enhances surface hardness, and does not affect water absorption.
Drying and Curing: Drying temperature must be strictly controlled below 55°C to prevent hydrated gypsum from dehydrating and powdering. It is recommended to first accelerate drying to a semi-dry state at 70°C, then transfer to an environment below 50°C for slow drying until complete. During drying, molds will shrink. They must be placed flat, supported firmly, and clamps tightened to prevent deformation, avoid impacts, and protect from rain exposure.
Modern gypsum mold manufacturing has shifted from reliance on experience to precise, scientific control over materials, formulations, and processes. Only through systematic optimization of these critical parameters can high-performance, long-lasting molds be produced, meeting the stringent demands of the contemporary ceramic industry for efficient, low-cost, and high-quality production.