Plastic thermoforming is a key manufacturing method for plastic products, widely used in packaging, automotive, electronics, construction, and other sectors. It offers advantages such as high production efficiency, low cost, and the ability to produce complex shapes, meeting diverse industry demands. However, various quality issues frequently arise during the thermoforming process, adversely affecting product performance, appearance, and market competitiveness. Consequently, in-depth research into these common problems and their solutions is crucial for enhancing product quality and reducing production costs.
Causes and Solutions for Product Deformation in Plastic Thermoforming
(A) Cause Analysis
Uneven Heating: Malfunctions in the heating system, such as damaged heating elements or faulty temperature control, or incorrect heating time/temperature settings can lead to inconsistent softening of the plastic material. For instance, localized overheating or prolonged heating causes excessive softening in some areas while others remain inadequately softened, resulting in deformation due to uneven stress distribution during forming.
Non-uniform Cooling: Improper cooling speed, cooling medium, or cooling method causes uneven shrinkage. For example, inconsistent cooling medium temperature leads to differential cooling rates across the product, causing uneven shrinkage and subsequent deformation.
Mold Design or Installation Issues: Poor mold structure, such as an improperly designed cavity, results in uneven stress distribution during forming. Tilting or insecure mold mounting subjects the product to unintended lateral forces, causing deformation.
Plastic Material Properties: Certain plastics exhibit high shrinkage rates or poor thermal stability, making them prone to deformation due to shrinkage and heat distortion during forming. Materials like polypropylene (PP), with relatively high shrinkage rates, are susceptible to deformation if specific countermeasures are not implemented.
(B) Solutions
Optimize Heating Process: Precisely adjust heating temperature and time based on the plastic material's properties and product requirements. Implement zonal heating to provide differential heating control for various sections, ensuring uniform softening. For complex shapes, apply targeted heating to critical areas.
Improve Cooling System: Design cooling channels rationally to ensure the cooling medium flows uniformly across all product sections. Regulate the flow rate and temperature of the cooling medium to achieve consistent cooling rates. Utilize systems like recirculating water cooling with flow control valves and temperature controllers to ensure balanced cooling.
Inspect and Adjust Mold: Regularly inspect the mold structure to ensure the cavity, runners, etc., are undamaged and undistorted. Adjust mold mounting position and securing methods. Employ precision alignment devices to ensure the mold is installed level and firmly fixed, preventing deformation caused by mold issues.
Select Suitable Plastic Material: Choose plastic materials with low shrinkage rates and good thermal stability based on product requirements and deformation control objectives. For high dimensional accuracy parts, consider materials like polycarbonate (PC).
Main Causes and Countermeasures for Surface Defects (such as Orange Peel, Scratches, and Uneven Gloss) in Thermoformed Plastic Products
(1) Orange Peel Phenomenon
Causes: The uneven flow of plastic melt can lead to orange-peel-like textures on the surface of products; large surface roughness of the mold will make the product surface replicate the unevenness of the mold surface; too fast cooling speed will cause the plastic surface to solidify rapidly while the internal melt continues to shrink, thus producing orange peel phenomenon.
Countermeasures: Optimize the plastic formulation and add appropriate plasticizers or lubricants to improve the fluidity of the plastic melt. Improve the surface quality of the mold by polishing, plating treatment and other methods to reduce the surface roughness of the mold. Adjust the cooling speed, appropriately reduce the temperature or flow rate of the cooling medium to make the plastic cooling process more stable.
(2) Scratch Problems
Causes: Impurities on the mold surface will scratch the product surface during the forming process; when the product is demolded, if the demolding process is improper, such as too fast demolding speed or improper use of demolding agent, scratches will also be produced; when the products are handled, mutual collision will also leave scratches on the surface.
Countermeasures: Regularly clean the mold, use special mold cleaning agents and tools to ensure that there are no impurities on the mold surface. Optimize the demolding process, use demolding agents reasonably, adjust the demolding speed to make the products demold smoothly without damaging the surface. Strengthen the management of product handling and storage, adopt appropriate packaging and handling tools to avoid mutual collision of products.
(3) Uneven Gloss
Causes: Unstable heating temperature will make the melting degree of the plastic surface inconsistent, resulting in uneven gloss; uneven mold surface temperature will make the cooling speed of different parts of the product different, affecting the surface gloss; the difference in the gloss of the plastic material itself will also cause uneven gloss of the product.
Countermeasures: Adopt a stable heating system with high-precision temperature controllers to ensure stable heating temperature. Control the mold surface temperature, install temperature control devices to monitor and adjust the mold surface temperature in real time. Select plastic materials with uniform gloss, or add an appropriate amount of gloss agents to the plastic formulation to improve the overall gloss of the products.
How to Solve the Quality Problems of Uneven Thickness or Local Thinness/Thickness in Plastic Thermoforming Process
(1) Analysis of Influencing Factors
Mold Design: Inconsistent depth of the mold cavity will cause thickness differences in different parts of the product; unreasonable runner design will lead to uneven distribution of plastic melt during filling, thereby affecting the product thickness.
Flowability of Plastic Materials: Properties such as the melt index and viscosity of plastic materials will affect their flow performance in the mold. If the material has poor flowability, local thinness or thickness is prone to occur when filling complex cavities.
Thermoforming Process Parameters: Improper setting of process parameters such as heating temperature, pressure, and forming time will change the melting state and filling performance of plastics, leading to uneven product thickness. For example, too low heating temperature or insufficient pressure will cause insufficient plastic filling and local thinness.
(2) Solutions
Optimize Mold Design: Improve the mold cavity structure to ensure consistent cavity depth and a flat surface. Reasonably design the runner, adopt a balanced runner layout, and enable the plastic melt to uniformly fill all parts of the mold cavity.
Select Appropriate Plastic Materials: According to the shape and thickness requirements of the product, select plastic materials with good flowability and stable melt index. For example, polystyrene (PS) and other materials with high flowability can be selected for thin-walled products.
Adjust Thermoforming Process Parameters: Determine the optimal heating temperature, pressure, and forming time through trials. During the forming process, conduct real-time monitoring and adjustment of these parameters to ensure that plastics fill the mold in a suitable state and the product thickness meets the requirements.
Common Influencing Factors and Adjustment Methods for Dimensional Deviation of Thermoformed Products Beyond Tolerance Range
(1) Common Influencing Factors
Mold Wear and Tear: After long-term use, the cavity dimensions of the mold will change due to wear, leading to dimensional deviations in products. For example, wear on the edges of the mold cavity can cause product dimensions to increase.
Plastic Material Shrinkage: The shrinkage characteristics of plastic materials vary under different temperature and pressure conditions, and changes in shrinkage rate will affect product dimensions. If the shrinkage rate is inaccurately predicted and corresponding compensation is not made in mold design and the forming process, product dimensional deviations will occur.
Precision of Thermoforming Equipment: The positioning accuracy and motion precision of thermoforming equipment will affect the forming dimensions of products. For instance, inaccurate positioning of the equipment can cause the product to shift within the mold, leading to dimensional deviations.
Environmental Factors: Changes in ambient temperature, humidity, etc., will affect the properties of plastic materials and product dimensions. A rise in ambient temperature can soften plastic materials, causing dimensional changes; changes in humidity may affect the moisture content of plastic materials, thereby influencing product dimensions.
(2) Adjustment Methods
Mold Repair or Replacement: For severely worn molds, professional repair techniques should be used to restore the dimensional accuracy of the cavity. If the mold cannot be repaired, it should be replaced with a new one in a timely manner.
Prediction and Compensation of Plastic Shrinkage: Determine the shrinkage rate of plastic materials under different conditions through testing, and make corresponding compensation in mold design and the forming process. For example, adjust cavity dimensions according to the shrinkage rate during mold design; control shrinkage by adjusting process parameters during the forming process.
Improvement of Equipment Precision: Carry out regular maintenance and calibration of thermoforming equipment, inspect the positioning devices and moving parts of the equipment, and ensure that its positioning accuracy and motion precision meet requirements. High-precision sensors and control systems can be adopted to monitor and adjust the operating status of the equipment in real time.
Control of Environmental Factors: Conduct thermoforming operations in a production environment with constant temperature and humidity. Install air conditioners, dehumidifiers, and other equipment to control ambient temperature and humidity within appropriate ranges, reducing the impact of environmental factors on product dimensions.
Mechanisms and Improvement Measures for Internal Defects such as Weld Lines, Bubbles, and Burn Marks during Plastic Thermoforming
(1) Weld Lines
Mechanism: When plastic melt flows in the mold, two or more streams of melt meet. Due to factors such as temperature and pressure, if they fail to fully fuse, weld lines are formed. Weld lines can reduce the strength and appearance quality of the product.
Improvement Measures: Increase the mold temperature to enable better fusion of the plastic melt when they meet; raise the injection pressure or speed to promote the flow and fusion of the melt; optimize the design of the mold runner to minimize the splitting and confluence of the melt during flow, thereby reducing the probability of weld lines.
(2) Bubbles
Mechanism: Excessively high moisture and volatile content in the plastic material can vaporize and form bubbles when heated during the forming process; poor venting in the forming process can cause gas to fail to escape in time, becoming trapped inside the product to form bubbles.
Improvement Measures: Dry the plastic material to reduce the moisture and volatile content in the material; optimize the mold venting structure by adding venting slots or holes to allow gas to escape smoothly; adjust the forming process parameters, such as lowering the heating temperature and shortening the heating time, to reduce gas generation.
(3) Burn Marks
Mechanism: Excessively high heating temperature, too long heating time, or locally high mold temperature can cause the plastic material to decompose and produce burn marks. Burn marks seriously affect the appearance and performance of the product.
Improvement Measures: Lower the heating temperature and set it reasonably according to the characteristics of the plastic material; shorten the heating time to avoid the plastic material staying at high temperature for too long; reasonably design the mold cooling system to ensure that the local mold temperature is not too high, and cooling water channels or air cooling can be used for cooling.
Common quality problems in the plastic thermoforming process include product deformation, surface defects, uneven thickness, dimensional deviations, and internal defects. The causes of these problems involve multiple aspects such as heating processes, cooling systems, mold design, plastic material properties, equipment precision, and environmental factors. By optimizing heating processes, improving cooling systems, inspecting and adjusting molds, selecting appropriate plastic materials, repairing or replacing molds, predicting and compensating for plastic shrinkage, improving equipment precision, controlling environmental factors, and taking corresponding improvement measures, these problems can be effectively solved, and the quality of plastic thermoformed products can be improved.
In actual production, regular equipment maintenance, optimization of process parameters, selection of suitable plastic materials, and strengthening of production management are crucial. Enterprises should establish a sound quality control system, strengthen the monitoring and management of the production process, and continuously improve product quality to meet market demands and enhance the competitiveness of the enterprise. In the future, with the continuous development of plastic thermoforming process technology, quality control will place greater emphasis on intelligence, automation, and refinement. By adopting advanced sensors, control systems, and data analysis technologies, real-time monitoring and precise control of the production process can be achieved, further improving product quality and production efficiency.
