Moulding experts look to pulsed heating

For some in the plastics injection moulding industry, attempting to maintain a constant temperature regime within what is essentially a cyclical production process means accepting compromised results. A number of engineering teams are working right now across Europe on technologies that use cyclical thermal regulation to improve the quality of demanding plastics mouldings.

The latest to demonstrate its capabilities is an engineering team at the Kunststoff Institut L?denscheid, which demonstrated its Indumold induction mould heating technique in production of a black ABS structural foam bottle opener at the Fakuma show last autumn.

Running on a Wittmann Battenfeld 110 tonne HM machine, the process was applied to one surface of the mould to yield a high gloss finish in place of the usual porous and swirled structural foam surface. The high gloss surface was achieved by keeping the mould surface close to the polymer melting point during filling to delay the onset of freezing, followed by rapid cooling of the filled mould cavity.

In the Indumold process, this is achieved through the use of induction heating coils integrated into the mould together with two separate conformal cooling circuits, which are positioned close to the mould cavity surface to achieve very fast response. During the filling stage, a high temperature water circuit maintained at 60C provides background control, while integrated induction heaters capable of lifting the temperature of the mould surface to 330C within six seconds provide almost instant additional heat input. Once the mould is filled, a second water cooling circuit maintained at 20C cools the part.

KIMW project partner Wittman Battenfeld says the high gloss foamed mouldings achieved using the Indumold technology provide higher rigidity and lower weight than compact alternatives and also eliminate visible knit lines. The traditional structural foam benefits of freedom from sink marks and warping in thick sections are maintained. Cycle time is "slightly longer" than conventional structured foam technology, the company points out.

The Indumold technology comes out of a long standing project carried out at KIMW that initially looked at induction heating to speed up and enhance thermosetting plastic moulding. This has since been applied to thermoplastics moulding and some 20 partner companies are now involved in the project.

While most companies are reluctant to acknowledge the use of the technology for competitive reasons, KIMW manager for moulded part surface technology Jorg Gunther says that Siemens in Bochum has a small number of Indumold tools producing cordless phone parts. And a number of automotive OEMs are also said to be benefiting from the process in serial production.

Induction heating coils can be either integrated into the mould, as is the case with the Indumold demonstration, or can be introduced into the open mould after part removal to heat the mould surface. The latter technique is used by Roctool in its Cage induction heating system, the latest injection moulding version of which was demonstrated at the PEP plastics technology centre in Oyonnax in France in October last year.

The Roctool Cage process has also shown that dynamic heating and cooling processes - some have coined the name "varioform" - deliver extremely good surface reproduction, whether the surface is polished or textured.

Roctool founder and CEO Alexandre Guichard says that there are equipment differences between the Roctool Cage system demonstrated in Oyannax and KIMW's Indumold induction heating system. However, he says Roctool has a number of patents covering induction heating of injection moulds. "We are waiting to see how it [Indumold] develops before discussing the IP [intellectual property] situation," he says.

Austrian machinery maker Engel has also been looking at cyclical mould heating technologies. The company's process analysis manager Josef Giessauf says that dynamic temperature control of the mould cavity during the injection moulding cycle to achieve a high tool temperature during the injection part of the cycle and a low temperature during the cooling phase through the use of either separate or common heating and cooling channels, is certainly not new.

"It has been known for around 20 years that knit lines can be avoided if the tool surface temperature is kept close to the glass transition or crystalline melt temperatures," he says.

In a presentation given at last year's VDI injection moulding conference in Baden-Baden, Giessauf outlined a number of alternative electrical "variotherm" processes.

The use of low cost electrical heating cartridges has not been very successful due to difficulties of control and short lifetime, he says. More success has been achieved using flat electrically conductive sintered ceramic elements located in the mould close to the cavity surface. These elements can achieve heat densities of up to 150 W/cm2 and surface heating rates of up to 20C/s. However, they can only be applied to relatively flat parts.

While the induction heating technologies used by Roctool and KIMW also provide heating rates of up to a 20C/s, Giessauf says they are limited with regard to generator performance, which restricts potential in large part applications. Obtaining even temperature distribution with curved 3D parts is also difficult.

Giessauf says short wave infra-red (IR) radiation provides an alternative option for variotherm processes. The company claims to have overcome the challenge of effectively heating highly polished tools using IR by applying a special IR-absorbing coating to the cavity. Further improvements can be made by employing a spring-mounted cavity insert in the tool. This lifts away from the cooled mould base as the tool opens to provide an insulating air gap.

Using commercially available 40 W/cm2 IR heaters, Giessauf says that a 13C/s heating rate can be obtained on such a coated tool compared to 6C/s for an uncoated high gloss surfaced tool. This heating rate is broadly similar to the that achieved using steam heating in a close contour (conformal) mould, he says, yet cooling is faster due to the reduced heated mass. Investment cost can also be lower, as the IR method does not require pulse cooling or emptying of cooling channels.

However, Giessauf says there is a disadvantage over systems that integrate the source of heat within the mould as the IR heating cycle cannot start until the mould opens. Engel's solution to this has been to trial a double insert approach, where the heat is applied at the rear of the insert using one or more IR radiators mounted above the moving platen.

The inserts are indexed to the heating stations using an arrangement similar to multi-component moulding. This eliminates the need for robotic handling of the radiator, as well as the need for a heat absorbing coating on the mould surface to be heated.

In Asia, steam heating and cooled water is used to cycle mould temperatures for production of large high gloss parts such as TV frames, and this is becoming more popular in Europe. UK-based Gas Injection Worldwide markets a steam heated technology. It estimates there are around 400 steam heating installations in operation, principally in China, South America and Eastern Europe, producing mouldings for TVs.

GIWW claims that its Rapid Temperature Cycling (RTC) process provides improved surface finish, reduces filling pressures and eliminates weld lines. It claims to have improved ease of use of the steam technique through development, together with Oxford Moulding Technology and the universities of Oxford and Swansea, of a new controller which regulates introduction of steam, compressed air and cooling water to enable RTC to used with a single set of media channels.

Giessauf believes considerable progress has been made in the past few years in the detail and implementation of variotherm technologies. Aside from the introduction of alternative electrical heating techniques, this has included improved ability to manufacture close contour 3D cooling channels. This has helped reduce cycle times and energy consumption.

Meanwhile, he says users are realising that the available technologies deliver more than just improved surface finish. As a consequence, Gie§auf believes variotherm moulding will be more widely utilised in the future but he says it is difficult now to predict which of the various methods may win out.