Injection Molding (IM) is a common plastic processing method and is a vast
business in the worldwide plastics industry (Rosato et
al., 2003). Process cycle time is the key factor in IM affecting the
productivity of the process. The process cycle time in injection moulding process
depends greatly on the cooling time of the plastic part, which is facilitated
by the cooling channels in the injection mould. Conventional cooling channels
are normally fabricated with straight drilled holes in the mould, which have
geometric and cooling fluid mobility limitations. The technique of conformal
cooling is being introduced as effective alternatives to conventional cooling
(Saifullah and Masood 2007).
The cross section of conventional cooling channels is circular due to the manufacturing technique of drilling. In conformal channels fabricated with Rapid Prototyping (RP) and Rapid Tooling (RT) technologies, the geometry is again circular. Thermal management of injection mold tools is very much enhanced with the application of conformal cooling channels as compared with conventional method of cooling with straight drilled holes. The concept of conformal cooling channels is shown in Fig. 1 and 2.
|| Conventional and conformal cooling channels
One problem might arise in the cooling channels with circular geometry that the distance between the edges of cooling channel and the edges of cavity in mold cannot be constant due to the geometric constraints. This distance is variable even in the case of conformal channels which conform to the shape of the cavity but have a circular cross section. If the cross section of the cooling channel is so designed that not only it conforms to the cavity but the side of the channel facing the cavity is following the profile of the cavity side. This concept can be called as a Profiled Cooling Channel (PCC). This concept can add further improvements in the field of conformal cooling, enhance the injection molding process and reduce the cooling cycle time.
The use of analysis tools for the simulation of the molding process is a neglected
area amongst many manufactures of plastic products. Mostly, the design and the
choice of different process parameters are based on the experience of mold designers
and other engineers. Studies have shown that costs up to 50% can be cut for
mold modifications and up to 15% for cycle time when using simulation(Saifullah
and Masood, 2007). Thermal analysis is done with different profiles of cooling
channels and cavity geometries to compare the results of circular and profiled
For the fabrication of IM tools, Rapid tooling is a technology for either indirectly
utilizing a rapid prototype as a tooling pattern for the purposes of molding
production materials, or directly producing a tool with a rapid prototyping
system. Manufacturing of aluminium filled epoxy molds are reasonably quicker
in comparison with machined molds. It is a relatively inexpensive and quick
way to create prototype and production tools. If the molds are designed properly,
they can withstand the injection or the compression pressures with the use of
aluminium frames. But the process cycle time is higher due to poor thermal conductivity
of the material (Kovacs and Bercsey, 2005).
MATERIALS AND METHODS
The objective of the current research is to design an experimental mold for
analyzing the concept of Profiled Conformal Cooling Channels for injection mold
tooling. The cooling stage is the most time taking stage (about 70%) (Villalon,
2005) during injection molding process cycle. The cross sectional geometry
of conventional and conformal cooling channels in injection molding is circular.
In the current research, design and modeling of molds will be done with circular
and profiled channels and thermal analysis and comparison was done to simulate
the heat dissipation in injection molds and using different geometries of cooling
channels (Fig. 3).
The design and modeling of mold cavity and cooling channel was done using CAD solid modeling software. The three dimensional CAD model is imported into ANSYS Workbench environment for thermal simulation. Initial and boundary condition and thermal loads are applied to the model.
Following molds were designed and thermally analyzed.
||Mold with Circular Conformal Channels (Fig.
||Mold with Profiled Conformal Channels (Fig.
The diameter of the circular channel in the mold is taken as 10mm with an area of 78.5mm2. The cross sectional area of the profiled cooling channel used in the mold is also same as the cross sectional area of a circular channel of diameter 10 mm. This area for profiled channel is designed as 78.5mm2. The mold dimensions are 125 mm (W), 140 mm (L) and 61 mm (H). The convection value for cooling water is taken as 5000 W/m2K and for air 20 W/m2K.
A steady-state thermal analysis calculates the effects of steady thermal loads
on a system or component. Engineer/analysts often perform a steady-state analysis
before doing a transient thermal analysis, to help establish initial conditions.
A steady-state analysis also can be the last step of a transient thermal analysis,
performed after all transient effects have diminished (Sadegh
et al., 2009).
Transient thermal analysis determines temperatures and other thermal quantities
that vary over time. Engineers commonly use temperatures that a transient thermal
analysis calculates as input to structural analyses for thermal stress evaluations.
Many heat transfer applications- heat treatment problems, nozzles, engine blocks,
piping systems, pressure vessels, etc. involve transient thermal analyses.
|| Mold with circular conformal channels
|| Mold with profiled conformal channel
|| Reaction probe in ansys
A transient thermal analysis follows basically the same procedures as a steady-state
thermal analysis. The main difference is that most applied loads in a transient
analysis are functions of time (Sadegh et al., 2009).
The channel geometry which is dissipating heat more will have the effect of
having more heat flow at the cavity side. The cumulative heat flow rate will
be determined on the cavity side of the mold, done with Ansys simulations. For
determining the results of simulations, Ansys uses different types of probes.
These probes can be structural, thermal and magnetostatic. For the current work,
thermal probes were used in the Ansys simulations. For determining heat flow,
Reaction Probe was applied on the cavity planes (Fig. 6).
The reaction probe calculates the heat flow respective to each time interval
set in the Ansys analysis settings.
RESULTS AND DISCUSSION
A Profiled channel was designed, modeled and simulated to give a comparison between conventional circular channels with profiled channels.
Steady state thermal analysis was performed for molds with different configurations of circular and profiled channel geometries. Heat flow was measured for all mold configurations channels geometries by placing a reaction probe on the cavity side (Fig. 6). The channel geometry which is giving more heat at the cavity end is dissipating more heat and therefore has faster cooling which can result in lesser cooling time.
The results for the two channel types i-e circular and profiled are given in Fig. 7.
|| Heat flow - circular vs. profiled
||Temperature distribution in circular conformal channel
|| Temperature distribution in profiled conformal channel
The Ansys simulated analysis results showed that the profile channel give more
heat flow at the cavity side which results in shorter cooling times for injection
mold process (Fig. 8).
The percentage increase in the heat flow value was about 14.6% for the profiled channel over circular channel (Fig. 9).
The current research has shown that the concept of Profiled Conformal Cooling Channels (PCCC) removes heat faster resulting in more heat flow at the cavity side of the mold. The cooling phase in injection molding process is the most important and crucial part as it directly links with cycle time and part quality. This concept can be utilized in injection molding process for better heat dissipation which can lead to improved cycle times and part quality. A Profiled channel was designed, modeled and simulated to give a comparison between conventional circular channels with profiled channels. The Ansys simulated analysis results showed that the profile channel give more heat flow at the cavity side which results in shorter cooling times for injection mold process.