Porous Metallic Tooling May Improve the Productivity and Quality of Injection
Molded Plastic Parts
By David J. Novotnak, Senior Materials Engineer
Carpenter Powder Products
Division of Carpenter Technology Corp.
Bridgeville, PA
June 2006
While significant advances have been made in the design and capabilities of
plastic molding machines during the last quarter century, little has changed in
the materials and processes used to manufacture injection molding tools.
Two major issues that prevail with current tooling include (1) the venting of
gases generated during the molding process, and (2) thermal management of the
molding process itself.
Traditional methods of dealing with the venting problem have had limited effect.
Trapped gases still cause quality problems such as burning, produced by
compressed gases that impede the flow of molten resin in a cavity pocket, and
short shots, caused by low injection pressure and /or gases trapped in pocket
areas of the tool.
In terms of thermal management, manufacturers still struggle with uniformly
removing heat from all areas of the die cavity during the process cycle. Current
tooling and cooling methods make thermal management difficult because the amount
of heat eliminated depends on thermal conductivity of the mold material, the
coefficient of heat transfer of the cooling medium, the temperature difference
between the material and the cooling medium, and the area of the
heat-transmitting surface.
Designing a tool for effective cooling remains difficult because of the complex
thermal management issues involved. Since the plastic injection molded part must
be cooled sufficiently to hold its shape and dimensions before ejection, the
production cycle is detrimentally affected, and the capacity of the injection
molding machine reduced if cooling is not well controlled.
Porous Metallic Tooling
The universal problems of venting can be minimized or solved and productivity
increased significantly through the use of porous metallic molds.
Carpenter Powder Products
(CPP) has demonstrated the feasibility of manufacturing porous tooling with
controlled porosity levels from stainless steel powder. Tooling or molds can be
made with interconnected, uniformly dispersed porosity levels, including
predetermined pore size and distribution.
Powder metallurgy preforms of the specified material and the subsequent finished
machine tooling are produced by combining powder metallurgy technology with a
new and unique rapid consolidation metal powder (RCMP) process. The combined
technologies produce a porous tooling material which can which can minimize or
eliminate the venting problem in plastic injection molding. In addition, this
material, when combined with an advanced cooling concept, has allowed
manufacturers to compress cooling cycles by as much as 50%.
Plastic injection molders have found that porous tooling manufactured by these
new process technologies along with improved tool design, have permitted them
also to increase productivity, improve quality and reduce costs significantly.
Powder metallurgy preforms with controlled porosity can be manufactured from
monolithic alloys or bimetallic alloys (porous and solid combinations) when, for
instance, a superior surface finish of the molded part is important. Pores can
be controlled from 3 microns and higher within a tool, and finished tooling can
range from 80% to 95% dense.
Although porous, this tooling has more than enough strength needed to withstand
the loads encountered in plastics injection molding – typically ranging up to
140 MPa (20 ksi). The tensile strength of Type 420 stainless steel tooling at
about 90% density with an average pore size of 25 microns, for example, has been
measured at 760 MPa (110 ksi).
Unique Powder Process
The RCMP process uses gas atomized powder of any metallic alloy including tool
steels, stainless steels and cobalt/nickel-base alloys. Screened powder is
enclosed in a shaped container, degassed, sealed and consolidated by a unique
quasi-isostatic hot forging process. The contained powder is heated to an
appropriate temperature and forged at pressures up to 828 MPa (120 ksi). This
rapid consolidation process is illustrated in Figure 1.
Figure 1 – Schematic of rapid consolidation metal powder process for
manufacturing porous tools.
– Schematic of rapid consolidation metal powder process for manufacturing
porous tools.
Porous tools can be tailor made with high strength and hardness and/or corrosion
resistance, as required. The pores are connected to one another and to the
surfaces of the tooling to allow gas flow throughout the tool. RCMP
consolidation parameters determine the level of porosity as well as pore shape,
size and distribution. Average powder particle size plays a significant role in
controlling the average pore size (see Figure 2).
Figure 2 – Micrographs showing effect of average particle size changes on porous
Type 420 stainless steel after rapid consolidation metal powder process.
– Micrographs showing effect of average particle size changes on porous Type
420 stainless steel after rapid consolidation metal powder process.
The hydraulic press employed with the RCMP process is capable of producing
bimetallic/composite tools as well as a wide range of near-net shapes. Porous
material made by the process can be used as inserts or as whole tools. In
service, tooling of all sizes and cross section has demonstrated uniform
behavior.
RCMP porous tools can significantly minimize or eliminate trapped gas that can
occur in inadequately vented areas during plastic molding. Traditional methods
of venting trapped gas such as parting line vents, vent plugs and pins are not
always enough to free the large volumes of gases that some resins can generate.
Both the porosity level and tool size can be controlled during RCMP
consolidation to provide required venting capacity.
By eliminating trapped gases, porous tooling also can minimize or eliminate flow
and knit lines, cancel shrink, improve fill through a vacuum assist, enhance
surface appearance, and facilitate ejection. An illustration of the venting
application is shown in Figure 3.
Figure 3 – Schematic showing venting of gas through porous tool
– Schematic showing venting of gas through porous tool
More Effective Cooling
In conventional injection molding, it is difficult to control the amount of heat
lost through conduction and convection because of the variables (previously
mentioned) that affect this process. As a result, designing effective tooling
becomes a real challenge. Cycle time is increased because the heat must be
removed by conduction. Water cooling in small cores is problematical, and good
temperature control is tough to achieve.
These thermal management limitations can be overcome through the use of RCMP
porous tools in conjunction with a newly patented gas cooling technology known
as Toolvac®*. With this approach, liquid CO2 is forced through the
pores of the porous tool, keeping it at a controlled temperature, thus providing
efficient and even cooling.
After the liquid CO2 is introduced into the porous mold, a pressure
drop causes it to vaporize. Due to the relationship between temperature and
pressure, the energy content for CO2 in the gaseous phase is much
higher than in the liquid phase. Consequently, thermal energy is absorbed from
the surrounding porous mold system.
The extent of cooling can be managed by controlling the temperature and rate of
the incoming liquefied gas. Since the gas is distributed by the porous tool, an
even temperature can be achieved over the entire mold cavity surface. In
contrast, uniform temperature cannot be maintained in the mold cavity with solid
tools cooled by water channels. A schematic of the porous tooling approach can
be seen in Figure 4.
Figure 4 – Schematic of CO2 cooling approach with a bimetal solid (A)
and porous die configuration (B) and an all-porous die configuration.
– Schematic of CO cooling approach with a bimetal solid (A) and porous die
configuration (B) and an all-porous die configuration.
Porous tools, when combined with the liquid CO2 cooling concept, can
provide a number of specific benefits for plastic injection molders. Faster heat
dissipation and constant wall temperature, along with venting, can reduce cycle
times and enhance product quality. In addition, ejection marks can be avoided
and "the vacuum effect," or sticking, eliminated.
Parts molded with porous tooling generally have a matte finish, the quality of
which can be predetermined by pore size. If surface finish is an issue, the
porous tooling can be produced as a bi-metallic composite with a thin/thick
solid tool surface bonded to the porous structure. When polished, this bonded
surface gives an excellent finish. The composite tool thus can provide the
desired degree of cooling together with a high quality finish on the molded
part.
Overall, with the combined porous tooling technology and the liquid CO2
cooling, the plastic molder can increase productivity, improve product quality
and gain valuable flexibility in designing complex parts. A simulation of this
benefit is illustrated in Figure 5.
Figure 5 – Productivity enhancement achieved with porous tooling compared with
conventional tooling.
– Productivity enhancement achieved with porous tooling compared with
conventional tooling.
*Toolvac is a registered trademark of AGA Aktiebolag Corp.
* * *
For further information about porous metallic tooling or other Carpenter Powder
Products, access the company’s
technical database or call
1-800-527-6900 .
Carpenter Powder Products is a Carpenter business unit that manufactures, sells
and services gas-atomized nickel-, cobalt- and iron-based alloy powder products
including PM and conventional tool steels.
Carpenter Technology Corporation, based in Wyomissing, PA, is a leading
manufacturer and distributor of specialty metals including stainless steels and
titanium, and various engineered products.
