Active Elements Tools Made by Selective Laser Sintering
Active Elements Tools Made by Selective Laser Sintering
Using the classical production route of the active elements for
injection molding can be too expensive and the time to market could be
too long. Interest in the development of injection molds by rapid
prototyping technology has been very keen. Making injection molds by
subtractive CNC milling and spark erosion methods is slow and
expensive. Skilled craftspeople are in short supply, plastic products
complexity is increasing and product development times are becoming
shorter and shorter. This means that an increasing number of more
precise tools have to be created. There is therefore a great deal to
gain from a process that provides great time and labor savings
The long-term prospect is for direct additive fabrication of
injection molds with the same level of precision and durability as CNC
methods. While great strides have been made in that direction and
important time and labor savings are realized today by RP methods, the
technology is still immature. This means that the benefits realized are
not universal and must be evaluated for each case. RP injection mold
fabrication methods should be considered for projects in which the
reduction of time to market is important, such as for prototype and
short, medium-volume production runs and for parts that may be very
difficult to machine because of their complex geometry.
SLS Technology and case Study
Rapid tooling by selective laser sintering (SLS) of the steel metal
powder is a new modern technology now being used at the Technical
University of ClujNapoca. SLS could be a good solution for rapid
tooling for injection molding in low-volume production of the complex
plastic parts.
As always, when a new technology appears, there are still many
experiments to be done to find good answers to many questions regarding
the details and implementation of these new emerging technologies.
Case Study. The authors performed a complete case study of rapid
tooling for injection molding, in cooperation with a company, which
could be one of the beneficiaries of implementing these new
technologies. Starting from the part's design, a complex team from SC
Plastor SA and Technical University of Cluj-Napoca worked to design the
mold assembly necessary for injection molding of the lid shown in
Figure 1.
The part (shown in Figure 1) selected for this case study is a lid
component of a grass cutting machine that has been designed at SC
Plastor SA (Oradea, Romania) using the ProEngineer 2001. This lid is a
component of the button for adjustment of the cutting height for the
grass. The part has many fine details and complex surfaces. Some of the
components of the mold assembly are standard modular components, which
contain guiding rods, support plates for the active elements (punch and
die), cooling system, extracting elements and more. All of these
modular components were designed and manufactured at SC Plastor SA.
The active elements (punch and die) presented in Figure 2 were
designed and manufactured at the Technical University of Cluj-Napoca,
within the National Rapid Prototyping Centre, using the Sinterstation
2000 SLS machine.
These active elements have a decisive role in getting a good final
part (the lid), as they are in full direct contact with the molten
plastic during the injection molding process. The 3-D modeling of these
active elements took into account not only the dimensions and features
of the final part, but also some important manufacturing aspects, such
as during the selective laser sintering process and during the
injection molding process.
SLS Rapid Tooling Technology. Figure 3 briefly presents the main
successive steps that we have carried out at TUCN to make the active
elements by rapid prototyping.
The authors tried to estimate when and how the shrinkage occurs,
within different stages of the SLS rapid tooling technology, as shown
in Figure 3. The long-term objective is to calculate the amount of
different deformations along the x, y and z-axes, to introduce the
necessary software compensations to the 3-D virtual model before the
STL file is saved and sent to the RP machine. If that could be done,
there would be no need for the machining operations to do the finishing
of the active elements made by RP.
Unfortunately, the optimal SLS parameters have not been found. If
optimal parameters were discovered, we would be able to use SLS tools
directly in the injection molding process, without having to machine
the active elements.
There are two main stages of the SLS process when the 3-D
deformations occur. First is while building the SLS model on the
Sinterstation 2000 machine. At this stage, it is difficult to even
measure the SLS part because it is in a so-called "green stage," when
it is very fragile and difficult to handle without damaging it.
Noncontact measurement methods need to be used to measure the actual
shrinkage of the green stage part. The second shrinkage takes place
during the post-processing into the oven of the SLS tools.
Thermal Shrinkage Modeling and Finite Element Analysis
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