How to Calculate the Right TCU Capacity for Injection Moulding

Start With the Process, Not the Product Catalogue

Most TCU purchases start in the wrong place. Someone opens a supplier's catalogue, looks at the model range, and picks one that seems about right — usually with a safety margin added on top.

The better approach is to start with your process and work backwards to the specification. What temperature do you need? How much heat goes into the mould? How much needs to come out? What flow rate keeps the temperature uniform? Once you know those numbers, selecting the right unit is straightforward.

Step 1: Define Your Temperature Requirement

This determines whether you need a water-based or oil-based unit.

Water TCUs typically operate up to 90°C (pressurised water units up to 160°C or even 200°C). Oil TCUs handle temperatures from 150°C up to 350°C.

For most injection moulding applications processing standard thermoplastics (PP, PE, ABS, PA), a water TCU operating at 40–90°C is sufficient. Engineering plastics like PEEK, PEI, or PPS often require mould temperatures above 120°C, which means either a pressurised water unit or an oil unit.

Step 2: Calculate Heating Capacity

Heating capacity (in kW) determines how fast the TCU can bring your mould to operating temperature and how well it maintains setpoint during production.

The formula for heat-up: Q = (m × c × ΔT) / t

Where Q is required heating power in kW, m is mass of the mould in kg, c is specific heat of steel (approximately 0.50 kJ/kg·°C), ΔT is the temperature difference (target temp minus ambient), and t is the desired heat-up time in seconds.

Example: A 200 kg mould heated from 20°C to 80°C in 30 minutes (1,800 seconds): Q = (200 × 0.50 × 60) / 1,800 = 3.3 kW

This is the minimum for heat-up. During production, the mould continuously loses heat to the environment and to the cooling water. A practical rule: add 30–50% to the calculated heat-up requirement. In this example, a 6 kW heater would be appropriate.

Step 3: Calculate Cooling Capacity

In injection moulding, cooling capacity is often more critical than heating. The TCU must remove the heat that the molten plastic transfers to the mould during each cycle.

The formula: Q_cool = (m_shot × c_plastic × ΔT_plastic) / t_cycle

Where m_shot is shot weight in kg, c_plastic is specific heat of the polymer in kJ/kg·°C, ΔT_plastic is the temperature drop of the plastic (melt temp minus ejection temp), and t_cycle is cycle time in seconds.

Example: Moulding PP with a 150 g shot, 230°C melt temperature, ejecting at 80°C, 25-second cycle: Q_cool = (0.15 × 1.93 × 150) / 25 = 1.7 kW per cavity. For a 4-cavity mould: 6.9 kW of cooling capacity required.

Step 4: Determine Flow Rate and Pressure

Flow rate ensures even temperature distribution across all cooling channels. Too little flow creates hot spots and uneven part quality.

A practical starting point: turbulent flow through the cooling channels is essential for efficient heat transfer. This typically means a minimum flow velocity of about 1.5–2 m/s.

For a typical cooling channel with 8 mm internal diameter at 2 m/s: approximately 6 L/min per channel. For a mould with 6 parallel cooling circuits: 36 L/min total flow required.

Important: Always check the pump curve at the required pressure — not just the maximum flow rate. A pump rated at 80 L/min maximum may only deliver 40 L/min at 4 bar back-pressure.

Step 5: Put It All Together

For the example above, the specification would be: temperature range up to 90°C (water), heating capacity 6 kW, cooling capacity minimum 7 kW at your actual ΔT, and flow rate 36 L/min at approximately 3 bar.

This is a clear, process-based specification. Compare it to "we had a 12 kW unit before, order the same" — which may be double the required heating capacity, with no consideration of cooling or flow.

Skip the Manual Calculation

The maths above is not difficult, but it requires knowing your process parameters and having the time to work through them. That is why we built an online tool that does this in about two minutes.

The Boe-Therm TCU Configurator asks for your application type, material, mould weight, operating temperature, and basic process parameters. It calculates the required heating, cooling, and flow — and recommends a specific configuration.

Try it at boe-therm.com/configurator.