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SLM tooling for die casting with conformal cooling channels
Antonio Armillotta*, Raffaello Baraggi
Dipartimento di Meccanica, Politecnico di Milano
Via La Masa 1, 20156 Milano, Italy
Simone Fasoli
Bruschi SpA
Via Mendosio 26, 20081 Abbiategrasso (MI), Italy
* Corresponding author:
e-mail: antonio.armillotta@polimi.it
tel.: +39 02 23998296
fax: +39 02 23998585
Abstract
The paper reports an experimental study of die-casting dies with conformal cooling fabricated by
direct-metal additive techniques. The main objective is to compare the benefits and limitations of the
application to what has been widely discussed in literature in the context of plastics injection molding.
Selective laser melting was used to fabricate an impression block with conformal cooling channels,
designed according to part geometry with the aid of process simulation. The tool was used in the
manufacture of sample batches of zinc alloy castings, after being fitted on an existing die in place of a
machined impression block with conventional straight-line cooling channels. Different combinations
of process parameters were tested to exploit the improved performance of the cooling system. Test
results show that conformal cooling improves the surface finish of castings due to a reduced need of
spray cooling, which is allowed by a higher and more uniform cooling rate. Secondary benefits
include reduction of cycle time and shrinkage porosity.
Keywords
Selective laser melting; rapid tooling; die casting; conformal cooling; spray cooling; surface finish.
Title page
Click here to download Manuscript: titlepage.doc
Click here to view linked References
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SLM tooling for die casting with conformal cooling channels
Abstract
The paper reports an experimental study of die-casting dies with conformal cooling fabricated by
direct-metal additive techniques. The main objective is to compare the benefits and limitations of the
application to what has been widely discussed in literature in the context of plastics injection molding.
Selective laser melting was used to fabricate an impression block with conformal cooling channels,
designed according to part geometry with the aid of process simulation. The tool was used in the
manufacture of sample batches of zinc alloy castings, after being fitted on an existing die in place of a
machined impression block with conventional straight-line cooling channels. Different combinations
of process parameters were tested to exploit the improved performance of the cooling system. Test
results show that conformal cooling improves the surface finish of castings due to a reduced need of
spray cooling, which is allowed by a higher and more uniform cooling rate. Secondary benefits
include reduction of cycle time and shrinkage porosity.
Keywords
Selective laser melting; rapid tooling; die casting; conformal cooling; spray cooling; surface finish.
1. INTRODUCTION
Direct-metal additive manufacturing techniques [1] are increasingly used to build tools for the serial
production of plastic and metal components. Their choice was initially intended as a rapid tooling
option, which could help to reduce time-to-market and increase product competitiveness. It has been
recognized, however, that they enable much bigger improvements on process performance and product
properties.
The most evident example of such opportunities is plastics injection molding, where steel tools for
final production have been fabricated by several additive techniques. The response time of involved
process chains is often comparable to traditional moldmaking practice due to the need of secondary
operations such as CNC machining, EDM, thermal treatments, grinding and polishing. Much more
valued in those process chains is the chance to provide molds with conformal cooling channels as an
alternative of conventional straight-line channels made by deep drilling. During the injection molding
process, conformal channels allow a more effective and uniform heat transfer from the resin, thus
reducing cycle time and shrinkage defects such as warpage and sink marks [2].
The present work aims at verifying if similar benefits can also be achieved for metal die casting,
*Manuscript
Click here to download Manuscript: manuscript (revised).doc
Click here to view linked References
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where applications of additive techniques are less studied. It could be argued that an improved cooling
system is likely to be less profitable for die casting, as the cooling phase takes a lower fraction of cycle
time and material properties are less prone to shrinkage defects. Yet it is interesting to evaluate such
differences and investigate on possible additional benefits not seen in injection molding.
For this purpose, an experimental study has been conducted at Bruschi SpA (Abbiategrasso, Italy),
a supplier of die castings for several manufacturing sectors. A die for a zinc alloy casting was
modified for the use of an additive technique (selective laser melting) and provided with conformal
cooling channels. The die was tested on a die-casting machine, where process parameters were tuned
for the new cooling system and improvements were evaluated in terms of productivity and cost.
Castings were inspected in order to evaluate possible improvements in surface finish and shrinkage
porosity.
The remainder of the paper is organized as follows. Section 2 recalls some literature results about
conformal channels for injection molding and cooling issues for die casting. Section 3 describes the
reference case and its relevance to the objective of the work. The design and fabrication of the die with
conformal cooling channels are described in sections 4 and 5. Results of process tests and inspections
of castings are reported in section 6 and discussed from an application viewpoint in the conclusions of
section 7.
2. BACKGROUND
Studies on conformal cooling channels in injection molds have been recently reviewed in [2]. They
usually refer to the manufacture of parts having deep recesses with nonstandard shapes, for which
conventional straight-line channels cannot be placed close to mold cavity and commercially available
baffles or bubblers cannot be used. Conformal channels are designed with intricate layouts to match
the geometry of mold cavities, and with noncircular and variable sections in order to finely adjust
cooling conditions. In most cases, mold blocks with conformal channels are fabricated in production
tool materials (hot-work or stainless steels) by means of direct-metal additive techniques. As detailed
below, specific design criteria for the cooling system have been developed from experimental tests on
different techniques.
In a pioneering work, the 3D printing technique was used on stainless steel powders with a
polymer binder and secondary operations including debinding, sintering, infiltration with bronze and
CNC/EDM finishing [3]. The main technical issues identified, and later confirmed for other
techniques, included the removal of unbinded powder from cooling channels and the protection of
channels from infiltration. A suitable sizing of channels helped to solve these problems and to
optimize heat transfer with the help of analytic modeling and numerical simulation. Compared to a
conventional cooling system, it was shown that conformal channels reduce mold temperature in both
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average values and variation along the molding cycle, and allow to reach thermal steady-state
conditions in a shorter time. In [4], a thorough analysis of conformal cooling performance led to a
systematic procedure for sizing and layout design considering a comprehensive set of thermal,
structural and manufacturing constraints.
In [5], the selective laser sintering technique was first used on powders of hot-work and stainless
steels with polymer coating. Required secondary operations were debinding, sintering, infiltration and
possible EDM finishing. Powder removal and infiltration issues turned out to be more difficult to solve
and required channels of larger cross section. The industrial deployment of this process chain was later
discussed in [6]. More recently, the process was improved in [7] through the use of a different hot-
work steel, the development of a new way of removing powder from channels (impression blocks split
into pieces to be brazed during the infiltration phase) and the provision of a small allowance for high-
speed CNC milling. Another experience on the same technique is reported in [8]. Variations of the
laser sintering process based on uncoated metal powders (direct metal laser sintering, selective laser
melting) are discussed in some commercial reports [9, 10], which point out the chance of partially or
totally eliminating debinding, post-sintering and infiltration phases. A similar simplification of the
process chain was achieved by using the electron beam melting technique on hot-work steel powders
with CNC finishing [11].
Other process options were attempted for conformally cooled molds. A technique similar to direct
metal deposition was tested in [12]: the process is hybrid since it uses bent copper tubes which are
placed in CNC-milled slots and later cladded by tool steel, with CNC/EDM finishing and polishing as
secondary operations. Lamination techniques from laser or abrasive-waterjet cut steel sheet are
reported in [13] and, for applications on thermoforming molds, in [14-17]. Prototype injection molds
were fabricated by epoxy tooling with bent copper tubes [18] and by spray-metal techniques on
sacrificial cores in soluble material [19]. Bent copper tubes were also used on impression blocks
fabricated in multiple CNC machined pieces [20].
Conformal channels are also regarded as essential for rapid thermal cycling, an innovative thermal
conditioning strategy for injection molds which consists in heating the cavity during filling (to avoid
early freezing of plasticized resin) and cooling it quickly during the rest of the cycle. Recent studies in
that direction include [21-23].
Apart from experimental studies, further attempts were made to improve the design of conformal
channels. The automated generation of their layout from part geometry was studied in [24-27].
Methods for the optimal sizing of channel sections were proposed in [28-32]. Innovative concepts to
improve heat transfer were also proposed and validated by analytic modeling and simulation; they
include unusual shapes of channel sections [33] and hydro-abrasive treatments to increase the
roughness of channel surfaces [34]. Thermal simulations were also reported within case studies in [35-
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38].
Conformal cooling was first applied to metal die casting in [39]. A multiple-impression die was
fabricated from steel laminations and tested in the manufacture of aluminum castings. As part
geometry did not pose special cooling difficulties, conformal channels were only provided to ease
solidification in the shot sleeve and to reduce average die temperature with no special focus on cycle
time reduction. An innovative design of conformal channels, suitable to either additive and subtractive
techniques, was patented in [40]: heat transfer was improved by increasing coolant flow rate through
channel with larger-than-usual cross sections.
Additive techniques were used for die-casting dies with conventional cooling channels. Initial
concerns were related to the higher thermal stresses and to possible loss of accuracy and structural
integrity; such issues were succesfully verified on direct metal laser sintering [41], direct metal
deposition [42] and spray-metal processes for soft tools [43].
Apart from the choice of additive techniques, the design of conventional cooling channels for die-
casting dies has been treated in several papers. The heat transfer between casting and coolant was
modeled and measured in [44]. The effect of boiling in cooling channels was analyzed in [45] to
propose new criteria for the optimization of channel layout. The thermal distortion of the die during
the die casting cycle was simulated in [46] to optimize some design variables related to cooling
channels.
As it will be confirmed in this paper, the design of cooling channels for die casting cannot ignore
the use of die lubricant to remove heat from the impression, usually referred to as spray cooling. As
pointed out in [47], thermal shocks during lubricant application can damage both the impression and
the casting; to keep them at a minimum, the needed amount of lubricant should be reduced by
oversizing the cooling channels. In [48], the heat transfer from die impression to lubricant was
analyzed under different combinations of spray cooling parameters (lubricant pressure and
composition, die temperature). Heat transfer during lubricant spraying was measured by several
techniques as described in [49-50].
In [51], the use of computational fluid dynamics (CFD) simulation is reported along the
development cycle of a die casting process. Although the study did not involve the use of conformal
channels and additive techniques, it has some interest here due to the choice of the reference case: the
casting is similar to the one chosen in the present paper, although in a different design with less
cooling difficulties.
3. CASE STUDY AND OBJECTIVES
This work will refer to the manufacture of a handle cover in zinc alloy Zamak 5 (ASTM AC 41A,
UNS Z35531), whose outside shape and dimensions are shown in Fig. 1. The part has a hollow shape
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