EVA Tool Bag Thermoforming Technology

EVA Tool Bag Thermoforming Technology

In the production of EVA tool bags, thermoforming technology is the core engine that determines product quality and performance. This process, which precisely shapes EVA material through heating and pressure, not only gives the tool bags a regular structure and a snug-fitting lining, but also directly impacts the product’s durability, protective properties, and aesthetics. For companies engaged in EVA tool bag production and those interested in industry technology, a thorough understanding of the key technical aspects of thermoforming is crucial to enhancing product competitiveness. This article will comprehensively analyze the technical essence of EVA tool bag thermoforming from five perspectives: process fundamentals, core steps, parameter control, common problems, and development trends.

I. Thermoforming Technology Basics: Understanding the “Shaping Logic” of EVA Material

Before discussing the specific process, it is necessary to first understand the characteristics of EVA material and the basic principles of thermoforming. This is a prerequisite for understanding the key technical aspects.

(I) Key Properties of EVA Materials and Compatibility with Hot Pressing
EVA (ethylene-vinyl acetate copolymer) is a thermoplastic material with excellent elasticity, toughness, and processability. Its properties are determined by the vinyl acetate (VA) content and melt flow rate (MFR), two parameters that directly impact hot press molding results:
VA Content: A higher VA content increases the EVA’s flexibility and adhesiveness, but also reduces its hardness and heat resistance. For tool bag linings, EVA with a VA content of 15%-30% is typically selected to ensure structural stability after molding while providing good cushioning.
MFR Value: The MFR value reflects the material’s melt flowability. A higher value indicates the material’s ease of filling the mold cavity after heating. In hot press molding, EVA with a medium MFR value (2-8g/10min) is easier to control, avoiding flashing caused by excessively fast flow or incomplete molding caused by excessively slow flow. Furthermore, the degree of crosslinking in EVA material is crucial to its performance after hot pressing. Crosslinked EVA exhibits improved compression set resistance and aging resistance. Adding a crosslinking agent (such as dicumyl peroxide) during the hot pressing process significantly increases the tool kit’s lifespan. (II) The Core Principles of Hot Press Forming
Hot press forming essentially utilizes the thermoplastic properties of EVA material, achieving a material transformation through three stages: heating and melting, pressurizing and shaping, and cooling and solidification.

Heating and melting: The EVA material is heated to its melting temperature (typically 120°C-160°C) in the hot press mold, intensifying molecular chain motion and transforming the material from a solid state to a fluid, viscous state.

Pressing and shaping: Under pressure (typically 5-20 MPa), the molten EVA material is extruded and filled into the mold cavities, achieving a precise fit to the mold cavity.

Cooling and solidification: The mold’s cooling system lowers the temperature, slowing the molecular chain motion and realigning the EVA material, ultimately solidifying it into a tool kit component that meets the requirements.

During this process, the synergistic effects of heating temperature, pressurizing pressure, and cooling rate directly determine the dimensional accuracy, surface quality, and mechanical properties of the molded part.

II. Core Process Steps in Hot Press Forming: Full Process Control from Preparation to Forming

Hot press forming is a systematic process. From preliminary preparation to forming operations, every detail of each step can affect the quality of the final product. The following will analyze the key technical aspects of the process.

(I) Preliminary Preparation: Precise Matching of Mold and Material
Mold Design and Manufacturing: The mold is the “blueprint” for hot press forming, and its accuracy directly determines the dimensional error of the product. Given the characteristics of EVA tooling, mold design requires three key considerations:

Cavity structure: Based on the concave and convex structure of the tooling lining, the mold draft angle (typically 1°-3°) should be appropriately designed to prevent product sticking after molding. For complex cavities, venting slots (0.1-0.2mm width, 0.05-0.1mm depth) should be provided to vent gases generated during heating and prevent surface bubbles and material shortages.

Temperature uniformity: The mold should utilize zoned heating to ensure that the temperature difference between different areas of the cavity does not exceed ±5°C to avoid inconsistent material melting caused by localized temperature variations.

Cooling channels: Cooling channels should be located close to the cavity surface, with spacing of 20-30mm to ensure uniform cooling and minimize product shrinkage and deformation. EVA Material Pretreatment: To improve molding quality, EVA material requires pretreatment before use:
Drying: If the moisture content of the EVA material exceeds 0.1%, it must be dried in a drying oven at 80°C-90°C for 2-4 hours to prevent moisture vaporization and bubbles from forming during heating.
Cutting Pretreatment: Based on the mold cavity volume, cut the EVA sheet into blanks that are slightly larger than the cavity by 10%-15%. This ensures that the cavity is fully filled during pressurization and prevents overflow and waste caused by oversized blanks. (II) Molding Operation: Dynamic Balancing of Three Key Parameters
Heating, pressurizing, and cooling are the core stages of hot press molding. Their parameter settings must be dynamically adjusted based on the EVA material’s properties and product requirements. The following are key points for parameter setting:
Heating Temperature: Understanding the “Melting Critical Point”
If the heating temperature is too low, the EVA material will not fully melt, resulting in poor fluidity and an inability to fill the mold cavity. This can easily lead to problems such as material shortages and unclear textures. If the temperature is too high, the material will decompose, discolor, and produce an odor, while also reducing the product’s mechanical properties. Parameter Setting: Adjust based on the VA content. The higher the VA content, the lower the melt temperature can be (e.g., for EVA with a 30% VA content, the melt temperature can be set between 120°C and 140°C; for EVA with a 15% VA content, the melt temperature should be set between 140°C and 160°C).

Practical Tips: Use a “step heating” method, first raising the temperature to the lower limit of the melt temperature, holding for 1-2 minutes, and then raising it to the target temperature to avoid internal stress in the material caused by sudden heating.

Pressure: Achieve “precise filling” and “avoid damage”

Pressure is the driving force that pushes the molten EVA into the mold cavity. However, excessive pressure can easily cause mold deformation and excessive flash, increasing subsequent trimming work. Excessive pressure can prevent adequate cavity filling, affecting the integrity of the product. Parameter Setting Basis: Adjust the pressure based on the complexity of the product structure. For simple flat structures, the pressure can be set to 5-10 MPa, while for complex concave-convex structures, the pressure should be increased to 15-20 MPa. Also, consider the thickness of the blank: for every 1 mm increase in thickness, the pressure can be increased by 1-2 MPa.

Pressing Timing: Apply pressure only after the EVA material is completely melted. Typically, hold the temperature for 30-60 seconds after reaching the target temperature before applying pressure to avoid “cold pressing” and other molding defects.

Cooling Rate: Control “shrinkage deformation.”

Excessively fast cooling results in a large temperature difference between the inside and outside of the EVA material, which can easily generate internal stress and lead to warping and cracking. Excessively slow cooling reduces production efficiency and may cause “post-shrinkage.”​
Parameter Setting Basis: Typically, a “rapid cooling – slow holding” method is used, with an initial cooling rate of 5-10°C/min. Once the temperature drops below 80°C, the cooling rate is reduced to 2-3°C/min until the temperature drops below 40°C before the mold can be opened.
Cooling Medium Selection: Air cooling is suitable for small molds, while water circulation cooling is required for large molds to ensure uniform cooling.

(III) Post-Processing: Improving Product Refinement

The EVA tool kit components after molding are not ready for immediate use and require post-processing to optimize quality:

Trimming: Use a utility knife or a dedicated trimming machine to remove burrs from the edges of the molded part. Trimming should be done 0.5mm from the edge of the part to avoid damaging the product itself.

Surface Treatment: If surface smoothness is required, lightly sand the surface with sandpaper. If wear resistance is required, a special EVA protective agent can be sprayed on.

Inspection and Screening: Dimensional accuracy is measured with a caliper (error must be within ±0.2mm). Visual inspection is performed to check for defects such as bubbles, missing material, and cracks. Unqualified products require rework.

III. Common Problems and Solutions: Overcoming the Technical Difficulties of Hot Press Molding

Even if process guidelines are strictly adhered to, various problems can still occur in actual production. The following summarizes the causes and solutions for four common problems:

(I) Surface Bubbles

Causes: 1. Excessive material moisture content, causing water to vaporize during heating; 2. Clogged or improperly designed mold vents, preventing gas from escaping; 3. Excessive heating temperature, causing material decomposition and gas generation.

Solutions: 1. Enhance material drying to ensure a moisture content below 0.1%; 2. Clean the vents, or increase their number and depth; 3. Reduce the heating temperature by 5-10°C and shorten the holding time.

(II) Material Shortage/Incomplete Filling

Causes: 1. Heating temperature too low, resulting in insufficient material fluidity; 2. Insufficient pressurization, preventing the material from filling the cavity; 3. Insufficient blank size, resulting in insufficient material. Solution: 1. Raise the heating temperature to the upper limit of the melt temperature; 2. Increase the pressurization pressure by 3-5 MPa and extend the pressurization time; 3. Increase the blank size to ensure that the blank volume is 110%-120% of the cavity volume.
(III) Product Warpage
Causes: 1. Uneven mold temperature, resulting in varying material cooling rates; 2. Excessively rapid cooling, generating internal stress; 3. Insufficient draft angle, resulting in uneven force during demolding.
Solution: 1. Overhaul the mold heating system to ensure uniform temperature across the cavity; 2. Reduce the cooling rate, especially by extending the holding time in the 80°C-100°C range; 3. Adjust the mold draft angle to ensure it is at least 1°.
(IV) Mold Sticking
Causes: 1. The mold surface is not release treated or the release agent is ineffective; 2. Excessively high heating temperature, causing material to adhere to the mold surface; 3. Excessively small draft angle, resulting in high demolding resistance. Solution: 1. Spray a specialized silicone-based release agent on the mold cavity surface, reapplying every 5-10 molds; 2. Reduce the heating temperature by 5-8°C; 3. Increase the draft angle to 2°-3° and polish the mold cavity.

IV. Hot Pressing Technology Development Trends: Towards a New Era of “Efficiency and Precision”

With the increasing demand for quality, efficiency, and environmental protection in the EVA tooling market, hot press forming technology is developing in the following directions:

(I) Intelligent Parameter Control
Traditional hot press forming relies on manual parameter adjustment. In the future, the combination of “intelligent temperature control systems + pressure sensors” will gradually become more common. By monitoring mold temperature and pressure changes in real time and combining AI algorithms to automatically adjust process parameters, this system achieves “one-click molding,” improving product consistency while reducing labor costs. (2) Mold Technology Upgrade

Using 3D printing technology to create mold cavities enables rapid prototyping of complex structures, shortening mold development cycles. Simultaneously, we are developing molds with “self-cleaning vent grooves” to reduce vent groove blockage and improve production continuity.

(3) Environmentally Friendly Process Innovation

To address the potential VOC emissions associated with traditional hot press molding, we will gradually promote the “low-temperature hot press + solvent-free crosslinker” process. This reduces energy consumption and pollutant emissions, complying with environmental regulations.

(4) Composite Molding Technology

Combining hot press molding with foaming and laminating technologies, we produce a composite tool bag lining composed of “EVA foam + rigid EVA support.” This provides excellent cushioning and enhanced structural stability, meeting the performance requirements of high-end tool bags.

Conclusion
The hot press molding technology for EVA tool bags may appear simple, but it is actually a comprehensive reflection of material properties, mold design, parameter control, and practical experience. From the early preparation of molds and materials, to the dynamic balance of heating, pressurization and cooling during the molding process, to the later refined processing, every link needs to be strictly controlled.