Walter Jungwirth, head of the packaging business unit at injection moulding machinery producer Engel Austria, and Mathias Hermle, CEO of inspection systems supplier IMD in Switzerland, compared injection and compression moulded closures in a paper delivered at the VDI injection moulding conference in Baden-Baden in February.

A core question raised was how far energy reduction may "help injection moulding to a renaissance" compared with compression moulded closures, which currently dominate the market.

Compression moulding was described in the Engel VDI paper as a simpler process but with inflexible product changeover, especially for parts heavier than 2g. More complex process logistics means the likelihood of reject rates is high, it was suggested.

Sacmi says compression moulding has high machinery but low mould costs, and the reverse applies to injection moulding. But the company says overall machinery and mould costs are lower for compression moulding.

However, compact 24- to 128-cavity hot runner injection moulding tools enable injection moulding to "stand up" to compression moulding by achieving cycle times below 3s, said the Engel paper.

But Sacmi has reported falling compression moulding cycle times. These cycle times dropped from 5.5s using Sacmi's CCM001 system in 1995 to 2.4s with its introduction of the CCM48S system in 2006, and this was cut again to 2.2s by K2010. Meanwhile cycle time fell further to 2.0s with the new CCM48SB machine shown at Interpack 2011 in May.

The new 100-420 tonne clamping force Engel all-electric drive e-Cap machine can produce 28,800-115,200 caps/h (480-1,920 caps/min) on 24 to 128-cavity moulds. At K2010, a 420 tonne e-Cap machine achieved 2.7s cycle time in a 96-cavity Schšttli mould for output of 130,000 caps/h (2,166 caps/min), while a Sacmi CCM80S line reached output of 2,000 caps/min.

Sacmi says shorter cycle time - due to lower melt temperature and therefore faster cooling - means lower energy consumption, while the absence of an injection point in compression moulding means low part stress. There is also practically no wear over the lifetime of compression moulds with their vertical and axial movement, it adds.

In the Engel VDI paper, Jungwirth showed Sacmi 2009 data giving specific energy consumption of 0.87 kWh/kg for a Sacmi CCM48S compression moulding machine, alongside Sacmi-sourced data for a 400-tonne hydraulic injection moulding machine with 1.3 kWh/kg.

Engel-sourced data showed 0.98-1.1 kWh/kg for a 400-tonne hydraulic machine with "state-of-the-art hot runner system and cooling" and 0.77 kWh/kg for the 420-tonne Engel e-Cap machine.

Allowing for differing compression and injection moulded cap weights (see table), Jungwirth calculated 2.26Wh/cap for the hydraulic machine, 1.74Wh/cap for the compression machine and 0.77Wh/cap for the all-electric e-Cap 420 machine.

But the target has meanwhile moved, as Sacmi quoted lower specific energy consumption of 0.6 kWh/kg in early 2011 for both its CCM48S and CCM80S machines. Sacmi's CCM48SB machine shown at Interpack 2011 has even lower specific energy consumption of 0.4 kWh/kg.