How to Calculate Cooling Time in Injection Molding: The Time Magic for Plastic "Cooling Down"

1. Cooling Time: The "Cooling Countdown" in Injection Molding

• Core Principle:\(\text{Cooling Time} \approx \frac{\text{Part Wall Thickness}^2}{\text{Plastic Cooling Rate}}\)Analogy: Wall thickness is like the "size of food chunks," and cooling rate is like the "refrigerator’s cooling power"—larger beef chunks take longer to stew, and thin-walled parts cool faster.

Key Influencing Factors

1. Part Wall Thickness (t): Thicker walls dissipate heat more slowly (e.g., a 4mm-thick part takes 4 times longer to cool than a 2mm-thick part!).

2. Plastic Thermal Conductivity:

   • High thermal conductivity materials (e.g., metal-filled plastics) are like "aluminum pots that conduct heat quickly," requiring shorter cooling times;

   • Low thermal conductivity materials (e.g., PS) are like "ceramic pots that dissipate heat slowly," requiring longer cooling times.

   
   

3. Mold Temperature and Cooling Medium:

   • Low mold water temperature (e.g., 15°C) is like "turning on strong air conditioning," cooling faster;

   • High water temperature (e.g., 50°C) is like "turning on heating," cooling slower.

   
   

2. Calculation Steps: Three Steps to Calculate Plastic’s "Cooling Time"

Step 1: Measure the "Food Chunk Size"—Part Wall Thickness

• Regular Parts: Directly measure the maximum wall thickness (e.g., the thickness of a rectangular part).

• Complex Parts: Use average wall thickness (e.g., for thin-wall + thick-wall structures, calculate by volume equivalence).Example: A PP water cup has a wall thickness of 3mm, classifying it as a medium-thick-walled part.

Step 2: Check the "Refrigerator Cooling Power"—Plastic Cooling Rate

• Lookup Method: Select values based on material type (see table below).

Step 3: Plug into the Formula to Calculate Time

• Formula:\(\text{Cooling Time} = \frac{t^2}{v} = \frac{3^2}{0.7} \approx 12.9 \text{ seconds}\)(Note: v takes the median cooling rate of PP, 0.7 mm²/s)

• Analogy: Similar to moving a 3cm-thick steak from a pan to a refrigerator, taking about 13 seconds to reach a safe eating temperature.

3. Practical Mnemonic: "Measure Thickness, Check Rate, Divide Square, Get Time"

1. Measure wall thickness,   2. Check material cooling rate,   3. Divide thickness squared by rate,   4. Get the cooling time in seconds.   

• Producing a PS phone case (wall thickness 2mm, cooling rate 0.5 mm²/s):\(\text{Cooling Time} = \frac{2^2}{0.5} = 8 \text{ seconds}\)In actual production, set it to 8–10 seconds with a safety margin.

4. Why Control Cooling Time Precisely?—Avoid "Outer Cool, Inner Hot" or "Slow Pace"

1. Too Short a Time: The Deformation Crisis of "Outer Cool, Inner Hot"

• Problem: The part’s surface cools and hardens, but the interior remains uncured, causing "sink marks" or "warpage" during ejection.

• Case Study: A factory reduced the cooling time for PC gears from 40 to 25 seconds, increasing the scrap rate from 5% to 30%, with excessive warpage on gear surfaces.

2. Too Long a Time: The Efficiency Killer of "Procrastination"

• Data: Every 10-second extension in cooling time reduces annual production by approximately 200,000 parts per injection machine (based on a 30-second cycle).

• Economic Impact: At a profit of $0.5 per part, this results in an annual loss of about $100,000!

3. Optimal Balance Principle

• Cooling time should ensure the part’s core temperature is below the "demolding temperature" (typically 10–15°C below the material’s glass transition temperature).

  • Example: ABS demolding temperature ≈ 90°C, so cool to 75–80°C before ejection.

  
  

5. Advanced Techniques: Cooling Optimization for Complex Scenarios

1. Multi-Material Composite Parts: "Layered Cooling" Strategy

• For ABS+TPE two-color parts, cool the rigid ABS layer first (60% of the time), then the soft TPE layer (40% of the time).

• Logic: Similar to freezing ice first and then juice, ensuring sequential curing of inner and outer layers.

2. Mold Waterway Design: "Precision Temperature Control" Black Technology

• Thin-wall areas: Use spiral cooling waterways (e.g., water pipes coiled around the core) to increase cooling efficiency by 30%.

• Thick-wall areas: Insert "cooling rods" (copper alloy material) to accelerate central cooling via high thermal conductivity.

3. Real-Time Monitoring: "Take Temperature" with an Infrared Thermometer

• Measure the part’s surface temperature immediately after demolding. If it exceeds the demolding temperature, extend cooling time by 5–10 seconds.

• Tool: Handheld infrared thermometer (accuracy ±2°C), providing results in 1 second.

6. Ultimate Checklist: Rapidly Verify Cooling Time Rationality

Conclusion