Molding Process Introduction of Unsaturated Polyester Resin in Sanitary Ware Production
1. Introduction
Unsaturated polyester resin has found widespread application in the production of sanitary ware due to its favorable properties. It offers good mechanical strength, chemical resistance, and can be easily processed into various shapes. The molding process of unsaturated polyester resin in sanitary ware production is crucial as it determines the final quality, appearance, and performance of the products. This article will comprehensively introduce the typical molding processes involved.
2. Raw Material Preparation
2.1 Unsaturated Polyester Resin Selection
There are different types of unsaturated polyester resins available, and the choice depends on the specific requirements of the sanitary ware. For example, for products that need high heat resistance, resins with appropriate chemical structures are selected. Resins with low viscosity are often preferred as they can better flow during the molding process, ensuring uniform distribution and good filling of the mold. Some common types include orthophthalic, isophthalic, and terephthalic unsaturated polyester resins. Isophthalic resins, for instance, offer enhanced chemical resistance, making them suitable for applications where the sanitary ware may be exposed to cleaning agents or other chemicals.
2.2 Additives
2.2.1 Promoters
Promoters are added to accelerate the curing process of the unsaturated polyester resin. Cobalt naphthenate is a widely used promoter. The amount of promoter added needs to be carefully controlled. If too much is added, the resin may cure too quickly, resulting in poor mold filling and uneven product quality. On the contrary, insufficient promoter will lead to a long curing time, reducing production efficiency. Generally, the addition amount is in the range of 0.1% - 0.5% of the resin mass, depending on factors such as the type of resin and the curing temperature.
2.2.2 Curing Agents
Curing agents, such as methyl ethyl ketone peroxide (MEKP), initiate the cross - linking reaction of the unsaturated polyester resin. The ratio of curing agent to resin is also critical. Usually, it is around 1% - 3% of the resin mass. An improper ratio can cause incomplete curing, leading to products with low strength and poor durability. For example, in the production of a bathtub, if the curing agent ratio is too low, the bathtub may be prone to deformation under normal use.
2.2.3 Fillers
Fillers are added to improve the properties of the final product and reduce costs. Common fillers in sanitary ware production include calcium carbonate, alumina trihydrate, and glass microspheres. Calcium carbonate can increase the hardness and stiffness of the product. Glass microspheres, on the other hand, can reduce the density of the product, making it lighter. The particle size and surface treatment of the fillers also affect the performance of the composite. Fine - sized fillers can result in a smoother surface finish, while surface - treated fillers can enhance their compatibility with the resin, improving the overall mechanical properties.
2.2.4 Colorants
Colorants are used to give the sanitary ware the desired color. They can be in the form of pigments or dyes. Pigments are more commonly used as they offer better color stability and opacity. The amount of colorant added depends on the intensity of the color required. For example, for a bright - colored washbasin, a relatively higher amount of colorant may be added, while for a more neutral - colored product, a smaller amount is sufficient.
3. Molding Processes
3.1.1 Process Steps
Mold Preparation: The mold is thoroughly cleaned and a release agent is applied evenly to ensure easy demolding later. The release agent can be a wax - based or silicone - based product.
Resin Mixing: The unsaturated polyester resin is mixed with the appropriate amount of promoter, curing agent, fillers, and colorants in a mixing container. The mixture is stirred well to ensure uniform distribution of all components.
Laying Up: A layer of resin mixture is first applied to the mold surface using a brush or roller. Then, a layer of reinforcement material, such as glass fiber mat, is placed on top of the resin layer. The reinforcement material helps to increase the strength of the product. The process of applying resin and laying reinforcement is repeated until the desired thickness is achieved.
Compaction: After laying up the layers, a roller or a squeegee is used to compact the laminate. This helps to remove air bubbles and ensure good adhesion between the layers.
Curing: The mold with the laid - up laminate is left to cure at room temperature or in a controlled - temperature environment. The curing time can range from a few hours to overnight, depending on factors such as the type of resin, the amount of curing agent, and the curing temperature.
3.1.2 Applications and Advantages/Disadvantages
Hand lay - up molding is suitable for small - scale production of sanitary ware with complex shapes, such as custom - made bathroom sinks or artistic bathtub designs. Its advantages include low equipment cost and high flexibility in terms of product design. However, it is a labor - intensive process, and the quality of the product may vary depending on the skill of the operator. The production speed is also relatively slow.
3.2 Spray Molding
3.2.1 Process Steps
Raw Material Preparation: Similar to hand lay - up, the unsaturated polyester resin is mixed with additives. In addition, the resin and curing agent are separately fed into a spray gun system.
Mold Preparation: The mold is cleaned and treated with a release agent as in hand lay - up.
Spraying: The resin and curing agent are sprayed simultaneously onto the mold surface from separate nozzles of the spray gun. At the same time, chopped glass fibers are also sprayed from a central nozzle of the gun. The resin, curing agent, and glass fibers mix in the air and deposit on the mold surface.
Compaction and Curing: After spraying to the desired thickness, a roller is used to compact the sprayed layer to remove air bubbles. The mold is then left to cure, and the curing process is similar to that of hand lay - up.
3.2.2 Applications and Advantages/Disadvantages
Spray molding is often used for the production of large - scale, relatively flat - shaped sanitary ware products, such as shower trays. It offers a higher production speed compared to hand lay - up. The process can also achieve a more uniform distribution of fibers and resin. However, it requires more specialized equipment, and the over - spray of materials may cause some waste.
3.3 Injection Molding
3.3.1 Process Steps
Material Preparation: The unsaturated polyester resin is compounded with additives and reinforcement materials in advance to form a molding compound. The compound may be in the form of pellets or granules.
Mold Installation: A precision - made mold is installed in an injection molding machine. The mold is designed according to the specific shape of the sanitary ware product.
Injection: The molding compound is fed into the injection molding machine's hopper. The machine heats the compound to a suitable temperature to make it flowable. Then, it injects the molten compound into the closed mold cavity under high pressure.
Cooling and Demolding: After injection, the mold is cooled to allow the resin to solidify. Once the product has cooled and hardened sufficiently, the mold is opened, and the finished sanitary ware product is ejected.
3.3.2 Applications and Advantages/Disadvantages
Injection molding is suitable for high - volume production of standardized sanitary ware products, such as toilet seats and small - sized sink bowls. It offers high production efficiency, good dimensional accuracy, and consistent product quality. However, the initial investment in equipment and mold manufacturing is high, and the process is less suitable for products with extremely complex shapes.
4. Post - Molding Processing
4.1 Trimming
After demolding, the sanitary ware product may have excess material, such as flash or sprue. Trimming is carried out using tools like saws, grinders, or trimming machines to remove these excess parts and give the product a clean and finished edge.
4.2 Polishing
Polishing is an important step to improve the surface finish of the sanitary ware. First, rough polishing is done using abrasive papers of relatively low grit, such as 800 - 1000 grit, to remove any surface imperfections. Then, finer polishing is carried out with higher - grit abrasive papers, like 2000 - 3000 grit, followed by buffing with a polishing compound and a polishing wheel. This process makes the surface smooth, shiny, and more resistant to dirt and stains.
4.3 Quality Inspection
A series of quality inspection procedures are carried out on the finished sanitary ware. Visual inspection is done to check for surface defects, such as cracks, bubbles, or uneven color. Mechanical tests, such as hardness testing and impact resistance testing, are also conducted to ensure that the product meets the required performance standards. Only products that pass all the inspection criteria are considered suitable for the market.
5. Conclusion
The molding process of unsaturated polyester resin in sanitary ware production is a multi - step and complex procedure. From careful raw material selection and preparation to the choice of an appropriate molding method and post - molding processing, each step plays a crucial role in determining the quality and performance of the final sanitary ware products. By optimizing these processes, manufacturers can produce high - quality, aesthetically pleasing, and durable sanitary ware items that meet the demands of consumers in the market.
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