What are the special features of PU with mold design?

The design of PU with molds has many special features, and the following is a detailed analysis:

1、 Consideration of material properties

1. Flowability of PU material

-PU (polyurethane) has certain fluidity characteristics during the molding process, which requires the design of the mold's flow channel to be reasonable. The shape, size, and layout of the flow channel need to be carefully planned to ensure that PU material can be evenly and smoothly filled into various parts of the mold. For example, using a suitable gradient flow channel can reduce pressure loss and local retention during material flow, making the material distribution in various parts of the PU strip more uniform during the forming process, and avoiding defects such as local material shortage or excessive thickness.

-At the same time, it is also necessary to consider the surface smoothness of the flow channel. A smooth flow channel surface can reduce the resistance of material flow, improve fluidity, reduce the heat generated by friction during the flow process, and thus reduce the risk of thermal degradation of the material.

2. The impact of shrinkage rate

-PU materials will undergo a certain degree of shrinkage after molding, and this factor must be fully considered in mold design. Generally speaking, corresponding compensation should be made in the mold size design based on the specific shrinkage rate data of PU material. For example, if the shrinkage rate of a certain PU material is known to be 1.5%, then when designing the mold, the corresponding size should be appropriately enlarged to ensure that the size of the formed PU strip meets the design requirements.

-In addition, the shrinkage rate is also affected by molding process parameters such as temperature, pressure, cooling rate, etc. Therefore, in the mold design process, it is necessary to conduct multiple experiments and adjustments based on actual production process conditions to determine the mold size compensation scheme.

2、 Unique design of die structure

1. Selection of parting surface

-The parting surface is one of the key factors in mold structure design. For PU strip molds, the selection of parting surfaces needs to consider multiple aspects. Firstly, it is necessary to ensure that the PU strip can be smoothly removed from the mold, and the position and shape of the parting surface should be convenient for demolding operations to avoid difficulties in demolding or damage to the PU strip.

-Secondly, the design of the parting surface should also consider the difficulty and cost of mold processing. Try to choose a simple and easy to process parting surface structure to reduce the difficulty and cost of mold manufacturing. For example, for PU strips with complex shapes, a design with multiple parting surfaces can be adopted, but attention should be paid to the precision of the fit between each parting surface to ensure the sealing and molding quality of the mold.

-In addition, the selection of the parting surface should also consider the appearance quality requirements of the PU strip. To avoid adverse effects on the appearance of PU tape caused by traces on the parting surface, special parting surface treatment techniques such as fine grinding and polishing can be used if necessary.

2. Design of cooling system

-Due to the heat released by PU materials during the molding process, the design of the cooling system is crucial for PU strip molds. The cooling system needs to be able to quickly and evenly remove the heat inside the mold to control the molding temperature and cooling speed of the PU strip.

-In the layout of the cooling system, a surround or embedded cooling water channel design is usually adopted. The surrounding cooling water channel can be evenly distributed around the periphery of the mold, providing overall cooling for the mold; The embedded cooling water channel can be flexibly arranged at key parts of the mold according to the shape and size of the PU strip, improving cooling efficiency.

-At the same time, it is necessary to choose the cooling medium and cooling parameters reasonably. Generally speaking, commonly used cooling media include water and oil. Water has a better cooling effect, but for some special PU materials, oil may need to be used as the cooling medium to avoid adverse reactions between the material and water. Cooling parameters (such as flow rate and temperature of the cooling medium) need to be adjusted and optimized according to specific production processes and material characteristics.

3. Setting of exhaust system

-During the process of injecting PU material into the mold, the air inside the mold will be expelled. If the gas inside the mold cannot be discharged in a timely manner, defects such as pores and bubbles will form in the PU strip, affecting product quality. Therefore, a reasonable exhaust system must be set up in mold design.

-The exhaust system usually includes exhaust slots, exhaust holes, etc. Exhaust slots are generally set at the parting surface, the end of the flow channel, and other locations, while exhaust holes can be distributed at key parts of the mold as needed. The size and quantity of exhaust slots and exhaust holes need to be reasonably determined based on factors such as the fluidity of PU material, the structure of the mold, and the molding process.

-In addition, some auxiliary exhaust measures can be adopted, such as selecting a mold release agent with certain exhaust function when applying it on the surface of the mold, or pre vacuuming the mold before material injection to reduce the gas content inside the mold.

3、 Requirements for accuracy and surface quality

1. Guarantee of dimensional accuracy

-PU belts are usually used in applications that require high dimensional accuracy, such as transmission devices, conveyor equipment, etc. Therefore, the dimensional accuracy of the mold must be strictly guaranteed. In the process of mold design, high-precision processing equipment and measuring tools are required to accurately process and inspect various components of the mold.

-At the same time, it is also necessary to consider the wear and deformation of the mold during use. Wear resistant materials or surface treatment techniques can be used to improve the wear resistance and deformation resistance of the mold, extend its service life, and ensure stable dimensional accuracy of the PU strip during long-term production.

-In addition, the assembly accuracy of the mold also has a significant impact on the dimensional accuracy of the PU strip. During the mold assembly process, it is necessary to strictly follow the assembly process requirements to ensure that the fitting accuracy between each component meets the design requirements.

2. Control of surface quality

-The surface quality of PU tape directly affects its appearance and performance. The surface quality of the mold is one of the key factors determining the surface quality of the PU strip. In mold design, it is necessary to finely treat the surface of the mold to achieve a certain level of smoothness.

-For some PU strips that require a very smooth surface, mirror processing technology can be used to treat the mold surface. At the same time, attention should be paid to the hardness and corrosion resistance of the mold surface to prevent scratches, corrosion, and other issues that may affect the surface quality of the PU strip during production.

-In addition, special textures or patterns can be set on the surface of the mold to meet the appearance design requirements of different PU strips. But when designing these textures or patterns, full consideration should be given to their impact on the PU tape forming process and performance, ensuring that product quality issues are not caused by the design of textures or patterns.