EPDM Molded Product Demolding Cracks Revealed: Comprehensive Analysis from Materials to Process

EPDM (Ethylene Propylene Diene Monomer) is a commonly used elastomer material known for its excellent weather resistance, ozone resistance, heat resistance, and aging resistance. It is widely used in industries like automotive, construction, and electronics. However, despite its superior properties, cracks often occur in EPDM molded products during demolding, which can lead to reduced product quality or scrap. This article provides a detailed analysis of the causes of demolding cracks from material, process, mold design, and operating conditions to help address this issue.

1. Material Factors

The formulation and raw material quality of EPDM rubber significantly influence the occurrence of demolding cracks. Issues may stem from the material's characteristics and processing performance.

1. Plasticity and Elasticity of Rubber
   EPDM rubber is highly elastic, with considerable tensile strength and elongation at break. However, the plasticity and elasticity of the material during the molding process may affect its demolding performance. If the proportion of plasticizers in the rubber formulation is incorrect, poor material flow can cause stress concentration during demolding, leading to cracks.

2. Inappropriate Vulcanization System
   The vulcanization system plays a critical role in the quality of molded products. An unsuitable vulcanization system can lead to either under-vulcanization or over-vulcanization, affecting the mechanical properties of the product. Over-vulcanization makes the material brittle, increasing the likelihood of cracks during demolding, while under-vulcanization results in poor elasticity and strength, causing cracking issues.

3. Filler Selection and Distribution
   Fillers like carbon black or silica are often added to EPDM formulations to enhance physical properties. Uneven filler distribution or improper selection can cause demolding cracks due to internal stress concentration. Poor compatibility between fillers and the matrix can exacerbate this problem.

4. Additive Use
   Demolding agents and plasticizers also affect the demolding performance of EPDM products. Incorrect use or insufficient application of demolding agents can increase friction between the mold and the product, leading to demolding difficulties and cracks. Excessive plasticizers can soften the product surface, reducing structural strength and making it prone to cracking under stress.

2. Process Factors

The molding process parameters directly affect the final product quality, and improper settings can lead to demolding cracks.

1. Improper Control of Vulcanization Temperature and Time
   Vulcanization temperature and time are key parameters. Excessive temperature or prolonged time can lead to over-vulcanization, making the rubber brittle and prone to cracking. Conversely, insufficient temperature or time leads to under-vulcanization, causing weak strength and elasticity, which can also result in cracking during demolding.

2. Uneven Mold Temperature
   Uniform mold temperature is essential for product quality. Uneven temperature distribution in the mold may result in localized under- or over-vulcanization. Particularly, if cooling is uneven, temperature differences between the surface and interior can lead to internal stress, causing cracks during demolding.

3. Improper Pressure
   Pressure directly affects the density and uniformity of the product. Insufficient pressure may lead to air bubbles or uneven density, causing weak areas prone to cracking during demolding. Excessive pressure can cause excessive material compression, increasing the risk of tearing during demolding.

4. Incorrect Demolding Timing
   Demolding at the wrong time also impacts product quality. Demolding too early can result in cracks as the product is not fully vulcanized and lacks strength and elasticity. Demolding too late can lead to material aging or over-vulcanization, making the product brittle and prone to cracking.

3. Mold Design Factors

Reasonable mold design is critical to molding performance and demolding efficiency. Improper mold design can contribute to demolding cracks.

1. Insufficient Mold Surface Finish
   A rough mold surface increases friction between the mold and rubber, requiring more force during demolding, increasing the likelihood of cracks. Sufficient polishing is required to reduce friction and lower the risk of demolding cracks.

2. Improper Demolding Angle
   An improper demolding angle (the angle at which the product separates from the mold) can increase resistance during demolding, leading to localized stress concentration and cracks. A properly designed demolding angle reduces friction, ensuring smooth separation without cracking.

3. Poor Mold Venting System
   Mold venting is critical for avoiding bubbles and voids in the product. A poorly designed venting system can trap air, resulting in internal defects that become stress points during demolding, leading to cracks.

4. Inappropriate Mold Opening and Closing Design
   An unreasonable mold opening and closing mechanism can cause excessive tension or compression on the product, increasing the risk of cracks during demolding. A well-designed opening and closing mechanism ensures even force distribution, reducing the likelihood of cracks.

4. Operating Conditions

Operator handling also plays a crucial role in the occurrence of demolding cracks.

1. Operator Skill Level
   The skill level of the operator can affect product quality. Improper application of force during demolding can result in localized stress concentration, leading to cracks. Proper training can help operators achieve even force application during demolding.

2. Choice of Demolding Tools
   Using inappropriate or rough tools can cause scratches or tears, leading to cracks. Selecting smooth and suitable demolding tools is essential to prevent mechanical damage.

3. Use of Demolding Agents
   Improper or insufficient use of demolding agents increases friction between the product and mold, requiring more force during demolding and increasing the likelihood of cracks. Correct use of demolding agents can reduce mechanical damage during demolding.

5. Solutions and Improvements

To prevent cracks in EPDM molded products during demolding, the following measures can be taken:

1. Optimize Rubber Formulation
   Adjust the formulation by selecting the right vulcanizing agents, fillers, and additives to ensure good material flow and strength, reducing stress concentration during demolding.

2. Control Vulcanization Parameters
   Strictly control the vulcanization temperature and time to prevent over- or under-vulcanization. Ensure even mold temperature for uniform vulcanization.

3. Improve Mold Design
   Enhance the mold surface finish, optimize the demolding angle and venting system, and ensure reasonable mold structures to reduce friction and stress concentration.

4. Strengthen Operation Management
   Enhance operator skills, use appropriate demolding tools, and ensure proper use of demolding agents to reduce mechanical damage during demolding.

Demolding cracks in EPDM molded products are caused by multiple factors, including materials, processes, mold design, and operating conditions. By optimizing formulations, improving mold design, controlling process parameters, and enhancing operational management, the issue can be effectively minimized, ensuring stable product quality.