Key issues of material control and process optimization in thick sheet vacuum thermoforming

How does the thick sheet vacuum thermoforming machine solve the forming defects caused by uneven heating of thick sheet materials?

In the process of forming thick sheet materials, uneven heating is an important factor leading to forming defects, including but not limited to surface unevenness, internal stress concentration, dimensional deviation, etc., which seriously affect product quality and production efficiency. To solve this problem, comprehensive measures need to be taken from multiple dimensions. ​

Heating uniformity can be improved by optimizing the heating equipment. Use heating elements with higher precision and uniformity, such as specially designed infrared heating tubes or heating plates, to ensure more uniform heat distribution. At the same time, adjust the layout of the heating equipment, and reasonably arrange the position and spacing of the heating elements according to the shape and size of the material to avoid heating blind spots. ​

It is crucial to introduce intelligent control systems. Temperature sensors are used to monitor the surface and internal temperature of materials in real time, and the heating power is dynamically adjusted through a feedback mechanism. For example, when a certain area is detected to have a lower temperature, the system automatically increases the power of the heating element in that area to achieve precise temperature control. In addition, simulation technology can be combined to simulate the heating process before production, predict possible uneven heating problems, and optimize the heating plan in advance.

The skills and experience of the operators should not be ignored. Operators should be trained regularly to master the correct heating process parameters and operating methods, and be able to flexibly adjust the heating process according to different material properties and product requirements, thereby effectively reducing molding defects caused by uneven heating. ​

l Heating plate zone temperature control strategy

Heating plate zone temperature control is an effective means to solve the problem of uneven heating of thick sheet materials. By dividing the heating plate into multiple independent control areas, the temperature of different areas can be accurately adjusted to meet the heating needs of complex shapes and different materials. ​

When zoning the heating plate, the shape, size and molding requirements of the material must be fully considered. For thick sheets of irregularly shaped materials, the areas can be divided according to their contours and key parts to ensure that the key areas can obtain the appropriate temperature. For example, for materials that are thinner at the edges and thicker in the middle, the edge area and the middle area can be controlled separately to make the edge area slightly lower in temperature to avoid overheating. ​

The choice of temperature control strategy is also crucial. Common temperature control methods include PID control, fuzzy control, etc. PID control has the characteristics of high control accuracy and good stability, and is suitable for occasions with high requirements for temperature control; fuzzy control can better adapt to complex nonlinear systems and has strong robustness to uncertain factors. In practical applications, you can choose the appropriate temperature control method according to the specific situation, or combine multiple temperature control methods to achieve the best temperature control effect. ​

In addition, the heating plate zone temperature control system needs to be regularly maintained and calibrated to ensure the accuracy of temperature measurement in each area and the reliability of temperature control. A reasonable zone temperature control strategy can effectively improve the uniformity of heating of thick sheet materials and lay a good foundation for subsequent molding processes. ​

l Synergistic optimization of infrared radiation and convection heating

Infrared radiation heating and convection heating are two commonly used methods for heating thick sheet materials, each with its own advantages and disadvantages. Infrared radiation heating has the characteristics of fast heating speed and high efficiency, but it is easy to cause a large temperature difference between the surface and the inside of the material; convection heating can make the material heat more evenly, but the heating speed is relatively slow. Therefore, the coordinated optimization of the two can give full play to their respective advantages and improve the heating quality. ​

In the collaborative optimization process, it is necessary to determine the reasonable ratio of the two heating methods. According to the characteristics of the material and product requirements, through experiments and data analysis, find the optimal power distribution ratio of infrared radiation heating and convection heating. For example, for materials with poor thermal conductivity, the proportion of infrared radiation heating can be appropriately increased to increase the heating speed; for products with high requirements for temperature uniformity, the proportion of convection heating can be increased. ​

To optimize the working sequence of the two heating methods, you can first use infrared radiation heating to quickly increase the surface temperature of the material, and then switch to convection heating to gradually even out the temperature inside the material. You can also use the two heating methods alternately according to the heating process of the material to achieve a steady rise and uniform distribution of temperature. ​

The structure of the heating equipment also needs to be optimized to ensure that infrared radiation and convection heating can work together effectively. For example, the shape of the heating chamber and the ventilation system should be reasonably designed to allow hot air to flow better on the surface of the material, enhancing the convection heating effect while avoiding affecting the transmission of infrared radiation. Through the coordinated optimization of infrared radiation and convection heating, the efficiency and quality of heating thick sheet materials can be improved and the occurrence of molding defects can be reduced.

l Real-time monitoring method of material surface temperature

The material surface temperature is a key parameter in the thick sheet forming process. Real-time and accurate monitoring of the material surface temperature is of great significance for controlling the heating process and ensuring the forming quality. At present, the commonly used methods for real-time monitoring of material surface temperature are mainly divided into two categories: contact and non-contact. ​

Contact temperature monitoring methods mainly include thermocouples and thermal resistors. Thermocouples have the advantages of fast response speed and high measurement accuracy, and can directly measure the temperature of the material surface. However, they need to be in close contact with the material surface during the measurement process, which may cause certain damage to the material surface, and are not suitable for high temperature, high-speed movement or difficult-to-contact material surface measurement. Thermal resistors have the characteristics of good stability and wide measurement range, but their response speed is relatively slow. ​

The most commonly used non-contact temperature monitoring method is infrared temperature measurement technology. Infrared temperature measurement measures temperature by detecting infrared radiation emitted from the surface of an object. It has the advantages of non-contact, fast response speed, and wide measurement range. It can achieve fast and accurate temperature measurement without affecting the surface state of the material. In addition, infrared thermal imagers can be used to obtain temperature distribution images on the surface of the material, intuitively observe temperature changes, and promptly discover abnormal temperature areas. ​

In order to improve the accuracy and reliability of temperature monitoring, multiple monitoring methods can be used in combination. For example, thermocouples can be used in conjunction with infrared thermometers to measure local precise temperatures and infrared thermometers to monitor overall temperature distribution, thus achieving comprehensive and real-time monitoring of the material surface temperature. At the same time, the temperature monitoring system needs to be calibrated and maintained regularly to ensure the accuracy of the measurement data.

How to avoid local thinning and rupture under high stretching ratio?

In the process of thick sheet forming, when the material needs to be formed with a high stretch ratio, local thinning or even cracking is prone to occur, which not only affects product quality but may also lead to production interruptions. To avoid such problems, it is necessary to start from multiple aspects such as material selection, process parameter optimization and mold design.

In terms of material selection, materials with good tensile properties and ductility should be preferred. The mechanical properties of different materials are different. Choosing the right material can improve the molding ability of the material under high stretch ratio. For example, some polymer materials with added plasticizers or special additives have significantly improved tensile properties and are more suitable for high stretch ratio molding.

Optimization of process parameters is the key. In the stretching process, it is crucial to reasonably control the stretching speed, stretching temperature and stretching force. If the stretching speed is too fast, it is easy to cause local deformation of the material and there is no time to adjust, resulting in thinning and rupture; if the stretching temperature is too low, the plasticity of the material will be reduced and the risk of rupture will increase. Therefore, it is necessary to determine the best combination of stretching process parameters through experiments and simulation analysis. At the same time, the segmented stretching method is adopted to gradually increase the stretching ratio to avoid excessive one-time stretching, so that the material has enough time for stress relaxation and deformation adjustment.

Mold design also plays an important role in avoiding local thinning and cracking. Reasonable design of the mold's transition radius, surface roughness and demoulding slope can reduce the friction and stress concentration of the material during the stretching process. In addition, setting up a suitable support structure or auxiliary molding device on the mold, such as support blocks, stretching ribs, etc., can effectively constrain and guide the material to prevent local instability of the material under high stretching ratios. ​

l Pre-inflation (pre-stretching) pressure and speed matching

Pre-inflation (pre-stretching) is an important process in the thick sheet forming process. The reasonable matching of pre-inflation pressure and speed directly affects the material forming quality and product performance. Improper pressure and speed matching may lead to problems such as uneven material stretching and large thickness deviation. ​

When determining the pre-inflation pressure and speed, the material characteristics must be considered first. Different materials have different sensitivities to pressure and speed. For example, for harder materials, a larger pre-inflation pressure and a slower speed are required to ensure that the material can be fully deformed; while for softer materials, the pressure can be appropriately reduced and the speed increased.

Secondly, it is necessary to adjust it according to the shape and size of the product. For products with complex shapes and large depths, the pre-inflation pressure needs to be set differently according to different parts to ensure that the material can evenly cover the mold cavity. At the same time, the pre-stretching speed also needs to be coordinated with the pressure. When the pressure is high, the speed should not be too fast to avoid material rupture; when the pressure is low, the speed can be appropriately increased to improve production efficiency. ​

In addition, the matching of pre-blowing pressure and speed can be optimized through experiments and simulations. During the experiment, the material forming conditions under different pressure and speed combinations are recorded, and various indicators such as thickness distribution and surface quality are analyzed to find the best matching parameters. By simulating the pre-blowing process with simulation software, the deformation process of the material can be observed intuitively, possible problems can be predicted, and a reference can be provided for actual production. By reasonably matching the pre-blowing pressure and speed, the quality and efficiency of thick sheet forming can be improved and the scrap rate can be reduced. ​

l Relationship between mold contour design and material flow

Mold contour design is a key factor affecting material flow during thick sheet forming. Reasonable mold contour design can guide the material to flow evenly, avoid local accumulation, thinning and other problems, and ensure the molding quality of the product.

The shape and size of the mold contour directly determine the flow path and deformation mode of the material. For molds with complex shapes, it is necessary to reduce the resistance to material flow through reasonable transition fillets, draft angles, ribs and other structural designs so that the material can smoothly fill the mold cavity. For example, setting a larger transition fillet at the corner of the mold can avoid stress concentration during the flow of the material and prevent cracking; a reasonable draft angle helps the material to smoothly leave the mold during demolding, and is also conducive to the flow of the material during the molding process. ​

The roughness of the mold surface will also affect the material flow. A surface that is too rough will increase the friction between the material and the mold, hindering the material flow; while a surface that is too smooth may cause the material to slip on the mold surface and fail to flow along the expected path. Therefore, it is necessary to select the appropriate mold surface roughness based on the material characteristics and molding requirements. ​

In addition, the temperature distribution of the mold is also closely related to the material flow. Reasonable control of the temperature of different parts of the mold can adjust the viscosity and fluidity of the material. For example, appropriately increasing the mold temperature in the parts where the material is difficult to fill can reduce the material viscosity and promote material flow; lowering the mold temperature in the parts prone to deformation can increase the material stiffness and control material deformation. By optimizing the mold contour design and fully considering the characteristics and requirements of material flow, the quality and efficiency of thick sheet molding can be improved. ​

l The influence of lubricant and anti-stick coating selection

In the process of thick sheet molding, the selection of lubricants and anti-stick coatings has an important impact on molding quality and production efficiency. They can reduce the friction between the material and the mold, prevent the material from adhering to the mold surface, and reduce the occurrence of molding defects. ​

The main function of lubricants is to form a lubricating film on the surface of the material and the mold to reduce the friction coefficient. Different types of lubricants have different performance characteristics and should be selected according to the characteristics of the material and the requirements of the molding process. For example, for high-temperature molding processes, high-temperature resistant lubricants such as molybdenum disulfide lubricants are required; for products with high surface quality requirements, water-based lubricants without residue can be used. At the same time, the application method and amount of lubricants also need to be strictly controlled. Too much or too little lubricant may affect the molding effect. ​

Anti-stick coating forms a special coating on the surface of the mold to prevent the material from sticking to the mold. Common anti-stick coatings include polytetrafluoroethylene (PTFE) coatings and silicone rubber coatings. These coatings have excellent non-stick and wear resistance, which can effectively prevent the material from sticking to the mold and increase the service life of the mold. When selecting an anti-stick coating, the adhesion, corrosion resistance and compatibility of the coating with the mold material should be considered. In addition, the thickness and uniformity of the anti-stick coating will also affect its anti-stick effect, and it is necessary to ensure that the coating is evenly coated on the mold surface.

Reasonable selection of lubricants and anti-stick coatings, as well as proper use and maintenance, can significantly improve the friction and sticking problems during thick sheet forming, improve product surface quality and production efficiency, and reduce production costs.

How to optimize vacuum and air pressure systems when molding complex geometries?

In the process of forming thick sheets with complex geometries, the optimization of the vacuum and air pressure system is crucial to ensure that the material can accurately fill the mold cavity and obtain good molding quality. By reasonably adjusting the vacuum and air pressure parameters, the deformation and flow of the material can be effectively controlled. ​

First, the layout of the vacuum and air pressure pipelines should be reasonably designed according to the shape and size of the product. Ensure that the vacuum and air pressure can act evenly on the surface of the material to avoid insufficient or excessive local pressure. For parts with complex shapes, the number of vacuum holes or air pressure nozzles can be increased to improve the pressure transmission efficiency. ​

Secondly, optimize the timing control of vacuum and air pressure. In the early stage of molding, appropriately increase the vacuum degree so that the material can quickly fit the mold surface and capture the detailed shape of the mold; during the molding process, dynamically adjust the size of vacuum and air pressure according to the deformation of the material to ensure that the material can evenly fill the mold cavity. For example, in areas where the material is difficult to fill, increase air pressure assistance to promote material flow; in areas prone to wrinkles or deformation, appropriately increase the vacuum degree to make the material close to the mold surface. ​

In addition, it is necessary to select and maintain the equipment of the vacuum and air pressure system. Select a vacuum pump and air pressure source with sufficient suction capacity and air pressure output capacity to ensure that it can meet the requirements of the molding process. Regularly check and clean the vacuum and air pressure pipelines to prevent blockage and leakage to ensure the stability and reliability of the system. By optimizing the vacuum and air pressure system, the success rate and quality of complex geometric thick sheet molding can be improved. ​

l Multi-stage vacuum timing control

Multi-stage vacuum timing control is an important means to improve the quality of thick sheet molding. By setting different vacuum degrees and vacuuming times at different stages, the deformation and bonding process of the material can be better controlled to avoid defects such as bubbles and wrinkles. ​

In the early stage of molding, a higher vacuum degree and a shorter exhaust time are used to allow the material to quickly fit the mold surface and expel most of the air between the material and the mold. The purpose of this stage is to allow the material to capture the general shape of the mold as quickly as possible, laying the foundation for the subsequent molding process. ​

As the molding process progresses and enters the intermediate stage, the vacuum degree is appropriately reduced and the pumping time is extended. At this point, the material has initially fitted the mold, and a lower vacuum degree can provide a certain buffer space for the material during the deformation process, avoiding excessive stretching or rupture of the material due to excessive vacuum; a longer pumping time helps to further expel the residual air between the material and the mold, improving the fitting accuracy. ​

In the final stage of molding, the vacuum degree is adjusted again and fine-tuned according to the specific requirements of the product. For some products with high surface quality requirements, the vacuum degree can be appropriately increased to make the material fit the mold surface more closely and eliminate tiny bubbles and unevenness; for some materials that are prone to deformation, a lower vacuum degree can be maintained to prevent excessive deformation of the material before demolding. ​

By rationally designing the multi-stage vacuuming sequence, the material forming process can be accurately controlled according to the material characteristics and product requirements, thus improving the quality and stability of thick sheet forming. ​

l Parameter setting of air pressure assisted forming (APF)

Air pressure assisted forming (APF) is an effective thick sheet forming technology, and its parameter setting directly affects the forming effect. The main parameters of APF include air pressure, air pressure application time, pressure holding time, etc. Reasonable setting of these parameters is the key to ensuring product quality. ​

The setting of air pressure needs to comprehensively consider factors such as the characteristics of the material, the shape and size of the product. For harder materials or products with complex shapes and greater depth, a higher air pressure is required to push the material to fill the mold cavity; for softer materials or products with simple shapes, the air pressure can be appropriately reduced. Generally speaking, the air pressure should be within an appropriate range. Too high air pressure may cause material rupture or mold damage, while too low air pressure will not allow the material to be fully formed. ​

The time of applying air pressure is also crucial. Applying air pressure too early may cause the material to be stressed without sufficient preheating or deformation, resulting in molding defects; applying air pressure too late may miss the best molding time for the material. Therefore, it is necessary to accurately determine the time point of applying air pressure according to the heating state of the material and the molding process requirements. ​

The setting of holding time is related to the curing and shaping process of the material. Sufficient holding time can allow the material to fully fill the mold cavity under the action of air pressure and maintain a stable shape to avoid deformation after demolding. However, too long holding time will extend the production cycle and reduce production efficiency. In actual production, the best holding time can be found through experiments and data analysis. ​

In addition, parameters such as the rate of increase and decrease of air pressure need to be considered. Smooth air pressure changes can reduce stress fluctuations in the material during the molding process and improve molding quality. By reasonably setting various parameters of air pressure-assisted molding, the advantages of APF technology can be fully utilized to produce high-quality thick-sheet molding products. ​

l Layout and efficiency analysis of mold exhaust slots

The reasonable layout of the mold exhaust groove is crucial for the exhaust of gas during the thick sheet molding process, which directly affects the molding quality and production efficiency of the product. A good exhaust groove layout can effectively avoid the generation of defects such as bubbles and pores, so that the material can smoothly fill the mold cavity. ​

When designing the layout of the mold venting groove, we must first analyze the material flow path and gas gathering area in the mold. Usually, gas is easily gathered at the corners of the mold, the parting surface, and the last part of the material filling. Venting grooves should be set in these areas. The shape and size of the venting groove also need to be carefully designed. Common venting groove shapes include rectangle and trapezoid. The depth of the venting groove should not be too large, otherwise it will easily cause material overflow; the width should be reasonably selected according to the fluidity of the material and the size of the mold to ensure that the gas can be discharged smoothly. ​

The efficiency analysis of the exhaust groove is an important means to evaluate the rationality of its design. The gas flow during the molding process can be simulated through simulation analysis software, the gas discharge in the mold can be observed, and the layout of the exhaust groove can be evaluated to see whether it is reasonable. In actual production, the effect of the exhaust groove can also be tested through mold trials. According to the defects such as bubbles and pores that appear during the mold trial, the exhaust groove can be adjusted and optimized. ​

In addition, the mold exhaust grooves need to be cleaned and maintained regularly to prevent them from being blocked by impurities and affecting the exhaust effect. By rationally arranging the mold exhaust grooves and conducting effective efficiency analysis and maintenance, the quality and production efficiency of thick sheet molding can be improved and the scrap rate can be reduced.

How to improve the dimensional stability and cooling efficiency of thick sheets after forming?

In the field of thick sheet forming, dimensional stability and cooling efficiency after forming are key indicators for measuring product quality and production efficiency. As the core equipment, the performance and process parameter optimization of the thick sheet vacuum thermoforming machine play a decisive role in achieving these two goals. Dimensional instability will cause the product to fail to meet the precision requirements, while low cooling efficiency will extend the production cycle and increase costs. To improve the performance of the two, it is necessary to comprehensively optimize the cooling process, material properties, and post-processing links based on the thick sheet vacuum thermoforming machine. ​

l Effect of cooling rate on crystallinity and shrinkage

The intelligent temperature control system equipped in the thick sheet vacuum thermoforming machine is the key to regulating the cooling rate. For crystalline polymer materials, the thermoforming machine can achieve a faster cooling rate by quickly switching the cooling medium circuit, inhibiting the orderly arrangement of the molecular chains, reducing the crystallinity, and thus reducing the volume shrinkage caused by crystallization. However, too fast cooling will produce greater thermal stress inside the material, leading to problems such as warping and deformation. Taking polypropylene (PP) as an example, in a thick sheet vacuum thermoforming machine, when the cooling rate is too fast, its crystallinity decreases and the shrinkage rate of the product decreases, but the internal residual stress increases significantly, and warping and deformation may occur during subsequent use.

On the contrary, a slower cooling rate helps the molecular chain to fully crystallize, improve the crystallinity and mechanical properties of the product, but it will prolong the cooling time, and excessive crystallinity will increase the shrinkage rate and affect the dimensional accuracy. The thick sheet vacuum thermoforming machine supports the setting of segmented cooling program. The operator can suppress crystallization through the rapid cooling function of the thermoforming machine at the beginning of molding, and switch to the slow cooling mode to release stress when it is close to room temperature, and use the precise temperature control ability of the thermoforming machine to achieve better molding effect. ​

l Configuration optimization of water cooling / air cooling system

The integrated design of the cooling system of the thick sheet vacuum thermoforming machine provides a basis for the efficient use of water cooling and air cooling. The water cooling system has the advantage of fast cooling speed due to the precise pipeline layout inside the thermoforming machine. When configuring, the mold cooling pipeline of the thermoforming machine adopts a combination of parallel and series to ensure uniform distribution of the coolant. For large thick sheet products, the density of cooling pipelines can be increased at key parts of the thermoforming machine mold (such as corners and thick wall areas). The circulating water pump of the thermoforming machine can accurately adjust the coolant flow rate and cooperate with the temperature control device to control the coolant temperature to avoid thermal stress in the material due to excessive temperature difference. ​

In thick sheet vacuum thermoforming machines, the air cooling system takes advantage of gentle and uniform cooling through an adjustable speed fan. Operators can adjust the wind speed on the thermoforming machine's control panel according to the material properties and the forming stage, which can ensure the cooling effect and reduce energy consumption. The thermoforming machine's unique air outlet design can be reasonably arranged at a certain position and angle so that the airflow can evenly cover the surface of the material and prevent uneven local cooling. Some high-end thick sheet vacuum thermoforming machines also support intelligent switching and composite cooling modes between water cooling and air cooling, giving full play to the advantages of both and achieving efficient cooling. ​

l Post-molding shaping process​

The thick sheet vacuum thermoforming machine is closely connected with the shaping process after demoulding to jointly ensure dimensional stability. The common mechanical shaping method can be achieved through the automatic clamping device equipped with the thermoforming machine. These clamps are linked with the demoulding mechanism of the thermoforming machine to fix the product and limit its deformation. It is suitable for products with simple shapes and large sizes. During operation, the pressure sensor of the thermoforming machine monitors the pressure distribution of the clamp in real time to ensure uniform pressure and avoid damage to the surface of the product. ​

The heat setting process relies on the secondary heating function of the thick sheet vacuum thermoforming machine, which heats the product to a certain temperature and maintains it for a period of time to release the internal stress and rearrange the molecular chains. For some materials that are easy to deform, such as polycarbonate (PC), after the thermoforming machine completes the forming, the heating chamber can be directly used for heat setting. The temperature control accuracy of the thermoforming machine can ensure that the temperature and time of heat setting meet the requirements of the material properties, significantly improving the dimensional stability of the product. In terms of chemical setting, the thick sheet vacuum thermoforming machine can be linked with the subsequent spraying equipment to coat certain plastic surfaces to limit the shrinkage and deformation of the material. The automated process design of the thermoforming machine ensures the efficiency and accuracy of the chemical setting link.