Polyethylene Thermoforming: Material Characteristics and Process Practice

Polyethylene Thermoforming: Material Characteristics and Process Practice
Polyethylene (PE), as one of the most widely used plastics in the field of thermoforming, occupies an important position in packaging, industrial components, daily necessities and other fields due to its excellent toughness, chemical resistance and cost advantages. Compared with other thermoformed materials such as ABS and PET, the molecular structure of polyethylene gives it a unique molding behavior - outstanding low-temperature toughness and excellent tensile properties, but relatively limited dimensional stability and high temperature resistance. A deep understanding of the material properties of polyethylene and its compatibility with hot forming processes is the core prerequisite for achieving high-quality product production.
Characteristics of Polyethylene Materials and Thermal Forming Adaptability
The hot forming performance of polyethylene is closely related to its molecular structure. The differences in density, crystallinity, and melt flow rate among different types of PE directly affect the selection of forming process parameters.
Low density polyethylene (LDPE)
Material properties: Density of 0.910-0.925g/cm ³, low crystallinity (50-60%), excellent flexibility and transparency (light transmittance of 70-85%), but low strength (tensile strength of 7-15MPa), limited temperature resistance (operating temperature -60 ℃ to 60 ℃).
Thermoforming adaptability: The heating temperature range is 110-130 ℃, and the stretching rate can reach 400-600%. It is suitable for producing thin-walled and flexible products, such as food packaging bags and toy shells. Its low melting point characteristic results in a short molding cycle (cooling time of 3-5 seconds), but a high shrinkage rate (1.5-3%), requiring compensation to be reserved in mold design.
Typical application: The self standing packaging bag of a certain snack brand is made of 0.1mm thick LDPE thermoformed and sealed through double-sided heat sealing technology, with a light transmittance of 80%, meeting the needs of product display.
High density polyethylene (HDPE)
Material characteristics: Density of 0.941-0.965g/cm ³, high crystallinity (80-90%), significantly better strength than LDPE (tensile strength of 20-30MPa), excellent chemical resistance (able to withstand acid and alkali concentrations of 50%), temperature range for use -40 ℃ to 100 ℃.
Thermoforming adaptability: Heating temperature of 130-150 ℃, stretching rate of 200-300%, suitable for producing products with strong rigidity, such as industrial pallets and turnover boxes. Due to high crystallinity, the cooling rate has a significant impact on dimensional stability, and forced cooling (water temperature 15-20 ℃) is required to control the shrinkage rate (1-2%).
Typical application: The acid-base solution transport tray of a certain chemical enterprise is made of 3mm thick HDPE thermoformed, and the 50kg load-bearing capacity is achieved through reinforced rib design. The corrosion resistance test (soaking for 30 days) shows no cracking or swelling.
Linear Low Density Polyethylene (LLDPE)
Material characteristics: Density of 0.915-0.925g/cm ³, between LDPE and HDPE, combining the flexibility of LDPE with the strength of HDPE (tensile strength 12-25MPa), outstanding puncture resistance, and operating temperature range of -60 ℃ to 80 ℃.
Thermoforming adaptability: Heating temperature of 120-140 ℃, better tensile uniformity than LDPE, suitable for producing products that require complex deformation, such as irregular packaging and corrugated pipes. Its melt strength is high, making it less prone to local thinning during hot forming, and the wall thickness deviation can be controlled within ± 10%.
Typical application: The PE corrugated pipe used for agricultural irrigation adopts LLDPE thermoforming, with a diameter of 50mm and a corrugated depth of 10mm. It has passed the 1.0MPa water pressure test without leakage and has a service life of more than 5 years.
Key points of polyethylene hot forming process
The hot forming process of polyethylene needs to be optimized based on its material characteristics, with a focus on controlling heating temperature, forming pressure, and cooling rate to balance the appearance, strength, and dimensional accuracy of the product.
Heating parameter control
Temperature range: Adjust the heating temperature according to the PE type. LDPE should be controlled at 110-130 ℃, HDPE at 130-150 ℃, and LLDPE at 120-140 ℃. Low temperature can cause insufficient softening of the sheet and incomplete molding; If it is too high, it will cause material degradation (manifested as surface yellowing and decreased mechanical properties), for example, when HDPE is heated above 160 ℃, its tensile strength will decrease by more than 15%.
Heating method: Adopting infrared heating furnace zone temperature control to ensure uniform heating of the sheet. For HDPE sheets with a thickness greater than 3mm, progressive heating (preheating to 80-100 ℃ and then raising to the target temperature) should be used to avoid internal stress caused by excessive temperature difference between the inside and outside.
Insulation time: proportional to the thickness of the sheet, 0.5mm thick LDPE insulation takes 5-8 seconds, and 3mm thick HDPE insulation takes 15-20 seconds to ensure that the material is fully softened but not in a molten state (melting temperature of PE: LDPE 105-115 ℃, HDPE 125-135 ℃).
Molding pressure and method selection
Vacuum forming: suitable for simple structural products, with a vacuum degree controlled between 0.08-0.09MPa. LDPE can be formed by simple vacuum molding due to its high tensile strength; HDPE has strong rigidity, and it is recommended to use an auxiliary air pressure of 0.1-0.2 MPa to ensure the complete filling of deep cavity structures (depth>100mm). Through this method, the corner filling rate of a certain HDPE turnover box has been increased from 85% to 100%.
Pressure molding: For complex structural products (such as parts with protrusions and grooves), 0.3-0.5MPa air pressure molding is used, combined with the vacuum pumping of the mold, to achieve more precise size control. This method is commonly used for forming LLDPE corrugated pipes, and the corrugated size tolerance can be controlled within ± 0.2mm.
Mold gap: The mold gap during polyethylene hot forming should be slightly larger than the thickness of the sheet (increased by 0.1-0.2mm) to avoid uneven wall thickness caused by excessive compression. By adjusting the mold gap, the wall thickness deviation of a certain LDPE toy shell was reduced from ± 0.15mm to ± 0.08mm.
Cooling and demolding control
Cooling rate: Polyethylene has a fast crystallization rate, and insufficient cooling can lead to excessive shrinkage of the product (exceeding 3%), requiring the use of a forced cooling system:
Thin walled products (<1mm): Water cooling (water temperature 20 ℃) for 5-8 seconds to ensure demolding temperature ≤ 40 ℃. Thick walled products (3-5mm): a combination of water and air cooling, with a cooling time of 15-25 seconds, to reduce the center temperature to below 50 ℃. Selection of release agent: Due to the low surface tension of PE, it is easy to adhere to the mold. Therefore, a special silicon-based release agent (concentration 0.5-1%) should be used, but care should be taken to avoid residual effects on subsequent printing or bonding. Food grade release agent (such as polytetrafluoroethylene spray) shall be used for food contact products. Demoulding slope: The demoulding slope of HDPE products should be ≥ 1 °, and LDPE can be appropriately reduced to 0.5 ° due to its good flexibility. However, for deep cavity structures (depth>150mm), it should be ≥ 2 ° to prevent scratching the surface during demoulding.
Design points and structural optimization
The design of polyethylene thermoformed products should fully utilize its flexibility and impact resistance advantages, while avoiding the shortcomings of insufficient dimensional stability.
Wall thickness and strength design
Wall thickness selection: Determine the basic wall thickness according to the load-bearing requirements. LDPE products are usually 0.1-1mm, and HDPE products are 1-5mm. By designing reinforcement bars, the wall thickness can be reduced while maintaining strength. For example, a certain HDPE pallet can reduce the wall thickness from 4mm to 3mm, while adding 2mm high mesh bars. The load-bearing capacity remains unchanged (50kg), and the material consumption is reduced by 25%.
Rounded corner design: All corners must be set with rounded corners. The inner corner radius of LDPE products should be ≥ 0.5mm, HDPE should be ≥ 1mm, and the deep cavity structure should be increased to 3-5mm to avoid stress concentration during molding and cracking during use. By increasing the rounded corners (from R1mm to R3mm), the damage rate of a certain LLDPE container during drop testing (1.5 meters) decreased from 8% to 1%.
Avoid sharp corners: Polyethylene has poor tear resistance, and sharp corners below 90 ° need to be eliminated in the design. A circular arc transition (curvature radius ≥ 2mm) is used. Through this design, the tear rate during installation of a certain agricultural PE pipeline interface is reduced by 90%.
Dimensional stability control
Shrinkage compensation: Reserve shrinkage according to the PE type, with a transverse shrinkage rate of 1.5-3% and a longitudinal shrinkage rate of 1-2% for LDPE; HDPE horizontal 1-2%, vertical 0.5-1.5%. When designing the mold, it is necessary to calculate according to "product size=mold size x (1+shrinkage rate)". For example, if the design size of a certain HDPE turnover box is 500mm and the shrinkage rate is 1.5%, the mold size needs to be set to 507.5mm.
Symmetrical structure: Adopting a symmetrical design to reduce warping, a certain LDPE cover plate reduced the amount of warping from 2mm to 0.5mm by changing the single-sided reinforcing ribs to a symmetrical distribution.
Post processing correction: For products with high dimensional accuracy requirements (such as tolerance ± 0.5mm), cooling with shaping fixtures can be used. Through this method, the dimensional qualification rate of a certain HDPE part has increased from 85% to 98%.
Functional structure design
Sealing structure: For products that require sealing (such as food packaging), a flange sealing surface (width ≥ 5mm) is designed with a surface roughness Ra<1.6 μ m, and a silicone sealing ring can achieve IP65 waterproof rating. A certain LDPE food box has passed this design and no leakage test (underwater 1 meter 30 minutes). Stacking structure: Industrial pallets need to be designed with stacking guidance, with a protrusion height of 2-5mm and a groove tolerance of ± 0.2mm to ensure stacking stability. When the stacking height of a certain HDPE pallet reaches 2 meters, the verticality deviation is less than 1 °. Easy to tear design: Packaging products can be designed with 0.5mm deep easy to tear grooves with a spacing of 5mm, making it easy to manually open. The easy to tear design of a certain LDPE snack bag reduces the opening force from 50N to 20N while maintaining sealing during transportation. Quality Control and Testing Standards The quality control of polyethylene thermoformed products should cover appearance, mechanical properties, and usability, and special testing items should be developed based on their material characteristics. Appearance quality inspection Surface defects: Scratches (depth>0.1mm), bubbles (diameter>1mm), impurities (area>0.5mm ²) that affect use are not allowed. The number of defects per square meter is ≤ 3. A certain food grade LDPE packaging has passed the visual inspection system, and the appearance qualification rate has reached 99.5%.
Color consistency: According to the CIE LAB color difference standard, the Δ E value should be ≤ 2.0, the whiteness (for white products) should be ≥ 70, and the color difference of a certain HDPE turnover box should be controlled within Δ E=1.5, meeting the brand's visual consistency requirements.
Wall thickness uniformity: Ultrasonic thickness gauge is used for detection, and the wall thickness deviation should be ≤ ± 15%, with a focus on corners and deep cavity areas. The wall thickness difference of a certain LLDPE corrugated pipe is controlled within ± 10%.
mechanical properties
Tensile strength and elongation at break: tested according to GB/T 1040 standard, LDPE tensile strength ≥ 7MPa, elongation at break ≥ 300%; The tensile strength of HDPE is ≥ 20MPa, and the elongation at break is ≥ 100%. The test values of a certain industrial HDPE pallet are 25MPa and 150%, far exceeding the standard requirements.
Impact resistance: Adopting the simply supported beam impact test (GB/T 1043), the impact strength of HDPE at -40 ℃ is ≥ 2kJ/m ², and the test value of a certain HDPE container for refrigerated transportation reaches 3.5kJ/m2, meeting the requirements of low-temperature environment.
Environmental Stress Cracking Resistance (ESCR): According to GB/T 1842 standard, when immersed in 50 ℃ Igepal solution, the rupture time of HDPE needs to be ≥ 1000 hours. The test result of a certain chemical HDPE storage tank is 1500 hours, which is suitable for long-term contact with chemical media.
Use performance testing
Temperature resistance test: Conduct temperature cycling test according to the usage environment. For example, food packaging needs to be cycled 50 times between -20 ℃ and 60 ℃. After the test, there should be no cracking or deformation. A certain LDPE cling film passed the test and maintained good sealing.
Chemical resistance test: Products that come into contact with specific media need to undergo immersion testing. For example, HDPE containers soaked in 50% sulfuric acid for 30 days have a weight change rate of ≤ 1% and a size change rate of ≤ 0.5%. The test results of HDPE trays used in a certain laboratory are 0.3% and 0.2%, respectively.

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