Chiller and TCU Sizing Calculation for Injection Molding

Introduction to Mold Cooling Systems

Proper chiller and TCU sizing is critical for injection molding success. The cooling system removes heat from the mold to solidify the plastic part consistently, directly impacting cycle time, part quality, and dimensional stability. Undersized cooling leads to longer cycles and warpage; oversized systems waste energy and capital.

This comprehensive guide provides the exact engineering formulas to calculate cooling requirements based on material enthalpy, cycle time, and mold design. We'll cover both chiller sizing for general cooling and Temperature Control Unit (TCU) selection for precise mold temperature control, with practical examples and Tederic integration guidelines.

Heat Load Fundamentals

Mold cooling calculations start with understanding the heat that must be removed. During injection molding, heat enters the system through three primary sources:

• Sensible heat from the melt: Heat content of the molten plastic as it enters the mold
• Friction heat: Heat generated by viscous shear during flow
• Exothermic heat: Heat released during crystallization (semi-crystalline plastics)

The dominant factor is typically the sensible heat from the plastic melt. As the material cools from processing temperature to ejection temperature, this heat must be absorbed by the cooling water circulating through the mold.

The Core Heat Load Formula

The fundamental heat load calculation uses the basic thermodynamics equation:

Q = m × Cp × ΔT

Where:

• Q = Heat load (BTU/hr or kW)
• m = Mass flow rate of plastic (lb/hr or kg/hr)
• Cp = Specific heat capacity of the plastic (BTU/lb·°F or kJ/kg·°C)
• ΔT = Temperature change (processing temp - ejection temp)

This formula gives us the theoretical heat that must be removed. In practice, we add factors for system inefficiencies, safety margins, and additional heat sources.

The Complete Engineering Formula

The comprehensive cooling load calculation includes additional factors:

Total Heat Load = (Shot Weight × Cp × ΔT × Cycles/hr) + Friction Heat + Exotherm + System Losses

For most applications, the simplified formula with a 20-30% safety factor covers 80% of sizing needs.

Material Enthalpy & Specific Heat Data

Specific heat capacity (Cp) varies significantly by material. Use this reference table for accurate calculations:

Material	Specific Heat (BTU/lb·°F)	Specific Heat (kJ/kg·°C)	Typical Processing Temp (°F)	Typical Ejection Temp (°F)	ΔT (°F)
Polypropylene (PP)	0.48	2.01	400-450	140-160	240-310
Polyethylene (HDPE)	0.55	2.30	400-500	140-160	240-360
Polycarbonate (PC)	0.30	1.26	550-600	200-220	330-400
ABS	0.35	1.47	450-500	160-180	270-340
Polyamide (Nylon 6)	0.40	1.68	500-550	160-180	320-390
PBT	0.35	1.47	480-520	160-180	300-360
Polystyrene (PS)	0.32	1.34	400-450	140-160	240-310

Note: Specific heat values are averages and can vary by grade and filler content. Consult material data sheets for precise values.

Calculating Shot Weight Per Hour

To calculate the hourly plastic throughput, we need to know the shot weight and cycle time:

Plastic Flow Rate = Shot Weight (lb) × (3600 seconds/hr ÷ Cycle Time)

For example, if you're running 8 oz (0.5 lb) shots with a 25-second cycle:

Plastic Flow Rate = 0.5 lb × (3600 ÷ 25) = 0.5 × 144 = 72 lb/hr

This flow rate represents the mass of plastic that must be cooled each hour.

Multi-Cavity Considerations

For multi-cavity molds, multiply the single cavity shot weight by the number of cavities:

Total Shot Weight = Single Cavity Weight × Number of Cavities

Don't forget to account for runner and sprue weight in family molds.

Converting to Chiller Capacity (Tons)

Once we have the heat load in BTU/hr, we convert to cooling tons:

Cooling Tons = BTU/hr ÷ 12,000

The industry standard is that 1 ton of cooling capacity removes 12,000 BTU/hr (288,000 BTU/day).

kW to Tons Conversion

If working in metric units:

Cooling Tons = kW × 0.284

Or more precisely:

1 Ton = 3.516 kW

Flow Rate & Turbulent Flow Requirements

Proper water flow rate is as important as temperature control. The Reynolds number determines whether flow is laminar (inefficient) or turbulent (effective heat transfer):

Re = (Velocity × Diameter × Density) ÷ Viscosity

For effective cooling, target turbulent flow with Re > 4,000.

Flow Rate Calculation

GPM = (Heat Load (BTU/hr) ÷ (500 × ΔT)) × 1.1

Where:

• 500 = Heat capacity of water (BTU/gallon·°F)
• ΔT = Water temperature rise (typically 2-3°F)
• 1.1 = Safety factor

For optimal heat transfer, limit water temperature rise to 2-3°F across the mold. Higher ΔT indicates insufficient flow.

TCU vs. Chiller: Application Guide

Choose the right cooling system based on your precision requirements:

When to Use a Chiller

• Temperature control within ±2-3°C
• Large heat loads (>5 tons)
• General mold cooling
• Cost-effective for basic applications

When to Use a TCU

• Temperature control within ±0.5°C
• Small to medium heat loads (<5 tons)
• Precise mold temperature control
• Hot oil heating capability
• Variotherm processes

TCUs excel at maintaining stable mold temperatures for dimensional consistency, while chillers provide brute-force cooling capacity.

Step-by-Step Sizing Example

Let's calculate the cooling requirements for a polypropylene container mold.

Process Parameters

• Material: Polypropylene
• Shot weight: 2.5 lb (including runner)
• Cycle time: 35 seconds
• Processing temperature: 425°F
• Ejection temperature: 150°F
• Number of cavities: 4

Step 1: Calculate Hourly Throughput

Total shot weight = 2.5 lb × 4 cavities = 10 lb

Cycles per hour = 3600 ÷ 35 = 102.9 cycles/hr

Hourly plastic flow = 10 lb × 102.9 = 1,029 lb/hr

Step 2: Calculate Temperature Differential

ΔT = 425°F - 150°F = 275°F

Step 3: Calculate Heat Load

Cp (PP) = 0.48 BTU/lb·°F

Q = 1,029 lb/hr × 0.48 BTU/lb·°F × 275°F = 134,916 BTU/hr

Step 4: Add Safety Factors

Total heat load with 25% safety factor = 134,916 × 1.25 = 168,645 BTU/hr

Step 5: Convert to Cooling Tons

Cooling capacity needed = 168,645 ÷ 12,000 = 14.05 tons

Step 6: Calculate Flow Rate

GPM = (168,645 BTU/hr ÷ (500 × 3°F)) × 1.1 = (168,645 ÷ 1,500) × 1.1 = 112.4 × 1.1 = 123.7 GPM

Recommendation: 15-ton chiller with 125 GPM capacity

Tederic Auxiliary Integration

Tederic injection molding machines feature integrated auxiliary interfaces for seamless chiller and TCU connectivity. Key integration points include:

• OPC UA communication for real-time temperature monitoring
• Alarm integration with machine control system
• Automatic startup/shutdown sequences
• Data logging for process optimization

When selecting Tederic auxiliaries, ensure the cooling capacity matches your calculated requirements. The integrated control system allows for precise temperature control and automatic fault detection.

Recommended Tederic Cooling Solutions

• Small applications (1-5 tons): Tederic TCU series with ±0.5°C accuracy
• Medium applications (5-20 tons): Tederic chiller series with variable speed compressors
• Large applications (20+ tons): Tederic central chilling systems with redundant pumps

Summary & Best Practices

Proper chiller and TCU sizing requires careful calculation of material enthalpy, cycle rates, and system requirements. The key formulas are:

• Q = m × Cp × ΔT (heat load)
• Cooling Tons = BTU/hr ÷ 12,000 (capacity)
• GPM = (BTU/hr ÷ (500 × ΔT)) × 1.1 (flow rate)

Always include 20-30% safety margins for process variations and future capacity needs. Consider TCUs for precision applications and chillers for high-capacity general cooling. Tederic's integrated auxiliary systems provide seamless connectivity and monitoring capabilities.

Remember: Cooling system sizing affects cycle time, part quality, and energy efficiency. Proper calculations prevent costly over-sizing or under-performing systems.