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The Advantages of Aluminium Moulds
Following are some of the main advantages which can be obtained by using Aluminium alloys for moulds. Some are well-known, other perhaps less:
 The importance of machinability: equal to lightness
Extremely high thermal exchange capability: rapid cooling of the injected thermoplastic product
Liberty when designing the thicknesses of the thermoplastic moulded part
Elimination of many electroerosion operations
Actual utilisation of HSC and VHSC systems with Al alloys
Elimination (or drastic reduction) of fitting time
Durability and good maintenance of polished surfaces
Considerable weight reduction: easy-to-handle Al moulds and simplification of investments in infrastructures
Re-design flexibility
The importance of machinability: equal to lightness
The machinability of some Al alloy families is excellent for various reasons, such as reduced shearing stress, self-cooling, and the shape of the metal dust granule shavings.
The alloys which are better suited to be machined are the harder ones featuring an optimised grain structure (II Generation 7xxx alloys).
We can say that under certain conditions, which moreover are still not employed to their fullest, Al alloys react to tool cutting like graphite-based materials or the resins used for models.
The possibility of working at a higher cutting speed than usual (e.g. thousands of meters/min) i.e. from 5 to 10 times with respect to steel, maintaining the good quality of the cutters and of the tools, involves:
- In general, from appreciable to excellent reductions in machine tool time
- A new type of milling machine management (long periods of time during which the machine is not attended to)
- The possibility of carrying out various types of milling work which cannot be done on steel (long and deep slots for the ribbing of plastic, precise draft corners without having to resort to fitting, etc.)
- The feasibility of 'direct cycles': only one placing followed by rough-turning, pre-finishing and finishing, without fitting.
Extremely high thermal exchange capability: rapid cooling of the injected thermoplastic product
Al alloys feature a thermal conductivity up to 7/8 time greater than common alloys for traditional moulds.
Steel is a very poor heat conductor and the constant addition of increasing levels of chrome worsens the already critical characteristics of thermal conductivity. The ordinary experience of exposing a common chrome-steel fork to fire till its tips become red hot without the person feeling the least bit of pain or being scorched, is within everyone's reach.
So, thermal conductivity, which has always been considered important in thermoplastic material injection processes, has acquired special significance in the past few years due to both the increasing content of chrome in mould steel and the development of rapid presses.
Cooling time in the press constitutes 70 ÷ 75% of the injection moulding cycle time: any improvement of the conductivity coefficient of the alloy used for the moulds thus turns into a reduction of the cycle time, if the injection press is intrinsically powerful and - above all - fast.
The opposite occurs if the mould is insulating - made of steel: even with faster presses the open / close rhythm is strongly conditioned by the cooling time of a product moulded in an insulating metal cavity.
In conclusion, by adopting the Al mould it is possible to cut the cycle time in half if the moulding press (the injection machine) is fast enough and features the sufficient installed power - be it hydraulic or electrical.
Currently - aware of the advantages that a high cooling mould can offer - productive systems like "1 single press with Al mould" instead of "2 traditional mould presses" are being studied.
Liberty when designing the thicknesses of the thermoplastic moulded part
The very scarce cooling capacity and the difficult elimination of fusion heat by steel tools cause considerable limitations in the design of the thermoplastic part. These are:
- the difficulties in making injected walls thicker than 3 m/m (the difficulty or the practical impossibility for thicknesses > 5 mm)
- the impossibility of constructing solid elements (full section, which is typical of chair technology)
- the enormous difficulties of moulding parts with considerable thickness variations - e.g.
5mm —> 12,5 m/m —> 5 mm
- the occurrence of deformations caused by the great cooling differences: maintaining uneven temperatures due to the insulating effect of steel
- the need to carefully position the water circuit cooling channels in the critical points
- the ease of superficial rapid cooling to obtain polished, even and compact surfaces over the "core" of the plastic and featuring "different" porousness, or made of other resins or fluids.
It is precisely thanks to the thrust towards maximum technological evolution related to the particular performance of Al alloys that special processes have been developed (proprietary).
Elimination of many electroerosion operations (except when working on local, restricted areas featuring a very limited range / corners etc.)
This particular - and very strategic - feature of the Al mould is one of those that seems to more naturally come under the category 'Technological Hurricane'. This definition was used 18 months ago by the magazine "Utensili ed Attrezzature" (November 1999).
The almost total elimination of EDM - Electro Discharge Machining - and, consequently, the reduction of this technology to small local applications has changed and is changing both the way of conceiving the mould and investing in tool shops.
The impact of the "EDM elimination" hurricane can be seen from many points of view. Let's try to describe some of them.
"There is no slower process - when constructing a mould - than the various phases of both electrode working and the subsequent use of actual electroerosion" - say some of the best Al die-sinkers of northern Italy.
Consequently, having Al mould technologies available (suitable blocks of Al alloys, super-milling machines, etc.) means reducing the use of EDM to a minimum.
A second point is the incidence of the investments in electroerosion machines in old fashioned tool shops. In the past, due to the slowness of the processes, it was necessary to multiply the EDM units: once the investments were made, it was not difficult to conclude (unfortunately) that 50÷60% of the capital invested in tools was invested in a slow process: EDM.
A typical aspect of the slowness connected to the use of EDM on steel is the need to polish all the narrow section slots in order to form the ribbing of the thermoplastic part.
This is the result of the very poor surface left by local microburns - on all surfaces - during the EDM process.
A part from the negative characteristics of EDM technologies on steel, which can be remedied by milling the Al alloys, the worst aspect is the one concerning investment turnover.
A traditional steel-mould tool shop with 'long amortisation' investments, 60% of which are machines / slow processes typical for steel, such as EDM machine batteries, constitutes a block that is difficult to update. The actual updating, in the sense of gradually resorting to 2nd generation mould alloys, is very difficult once everything has been done (when the investments have already been made): traditional investments will condition the set-up of the tool shop up to the end. To obtain the best conditions in the new business - fast moulds, high rhythm presses - it will be necessary to conceive a tool shop featuring mostly different equipment.
This is why many large groups have chosen to create a completely separate Al Tooling department, from scratch.
Actual utilisation of HSC and VHSC systems with Al alloys
Even though the cutting speeds for light alloys are much higher (especially with super-milling machines), the life of the tools improves by a factor of at least 10 and up to 50 times in comparison to steel.
In many cases it is possible to use, with considerably lower costs, tools made of super rapid steel (especially with high Co contents).
The possibility of working on steel at practically impossible cutting speeds is leading to two conclusions: - that the high speed (continuous) is actually reached, and utilised continuously and on a vast scale only when milling light alloys; - that the Very High Speed Cutting speeds are totally excluded for steel.
In the case of steel, especially during rough machining, it is more correct to talk about HPM - High Productivity Machining - than about high speed milling. The two systems have been differentiated only recently (see the diagrams in the box dedicated to the super-milling machines).
Elimination (or drastic reduction) of fitting time
Here, for example, we speak of finishing of surface draft corners, especially in areas which can be reached only with tools featuring a high length/ Ø ratio.
The more the tool works in critical conditions, tending to bend on especially deep steel surfaces, the more cutting errors (generation of the actual surface by the tool) there will be with respect to the theoretic profile of the mathematics - see the continuous line in the drawing. The error will have to be corrected with a fitting cycle.
Durability and good maintenance of polished surfaces
Aluminium does not rust! This typical statement, which is part of everyday life for those who normally work with light alloys, became very important when it was realised that steel moulds rust quite rapidly, especially in certain climatic conditions.
As has already been said, the solution for alloys normally used for traditional moulds (steel) lies in the increase in the chrome content of the tests specific for mould blocks.
Unfortunately this type of solution (in part already adopted) clashes with the already scarce level of thermal conductivity of mould steel, which was originally 'without' or 'low chrome'. By increasing the chrome content, the already scarce thermal conductivity will further decrease. The conflict will further increase with the rapid conduction of the presses.
Considerable weight reduction: easy-to-handle Al moulds and simplification of investments in infrastructures
The positive effect of the light mould on the wear of injection machines, on their dynamics (acceleration and braking), and on the powers in play is by now very well known.
Different - and for certain aspects new - is the case regarding the study of industrial sheds for the new shops, where the placement of the mould weight with the maximum level weight equalling 10 tons (2nd generation Al moulds) instead of 30 tons (steel) has permitted the lightening not only of the runways but also of columns and their foundations.
Re-design flexibility
The improvement of process time and costs, the simplification of the devices required for certain thermoplastic profiles, together with all the phenomena typical of the Al mould mentioned previously, allow us to obtain even very complex profiles and components with moulds featuring minimum technological difficulty and, in the end, minimum risk and cost. The speed with which you obtain the new design depending on the final phase greatly favours the preparation of new catalogues. An example, which for certain aspects is sensational, is the case of the polypropylene "composter" in the figure.
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