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Investigative Tools - Computer Simulated Performance
Predictive computer analysis such as mold
fill, mold cool and part stress analysis can help significantly by
identifying potential problems before the mold design is started. It
is important that these techniques are used prior to the lock in of
final part design and mold construction.
These tools provide detailed information about the part design with
respect to its ability to be molded efficiently. A mold fill analysis
will show the various stages at which a part fills, providing
information of where venting is needed to prevent potential problems
with material burn areas. The analysis can be run several times using
different gate locations, making it possible to determine the best
gate location before the mold construction process is started.
These tools can save considerable
time in the project scale up cycle if they are used at the proper time
and are used as tools to guide the final part design process. These
services are available through engineering service companies, mold
makers or through the resin suppliers as a part of their overall
service.
Single Cavity Prototype Tools
Single cavity tools should be used as a part of the project
scale up process. This step is important because it validates the
decisions relating to part and mold design and sets the stage for the
construction of higher cavitation molds to be made with minimal risk.
Parts run form single cavity tooling
improve the entire scale up process in many ways.
i. Increase the cycle time which effects the product cost
ii. Creates particulate matter through the grinding and recycling
process
iii. Creates the need to recycle and regrind resin back into the
process
iv. Uses more energy to process the resin used to form the runner
v. Variations in the virgin resin/regrind mixture causing change in:
Part dimensions
Color
Melt flow of blend
Process conditions
The selection of a hot runner system, the
part design and the mold design that goes with it must all be panned
together to achieve maximum benefits. The Tooling Engineer must work
closely with the hot runner supplier to select the type of system that
meets the requirements for the type of part and the material under
consideration.
The system selected should be designed into the single cavity
prototype mold to insure it meets all the needs of the project. This
stop cannot be emphasized enough. There is a significant amount of
technology involved in the hot runner system. The single cavity
injection mold will prove out most of the decisions that will be made
concerning the hot system design and the tool design that surrounds
it.
Some important areas to consider in the
mold design process are as follows:
Selection of specific hot runner design
with special consideration for:
a. Gate vestiqe requirements
b. Part design
c. Gate location
d. Material specified
e. Mold cavitation
Provision of adequate water cooling
designed close to the gate area
Distance between cavities
The use of a hot runner system will
increase the cost of the production molds, but the money spent up
front will be returned many times through the quality of the parts
made and the savings gained through energy savings and cycle times.
Production Mold Design and
Construction
All the planning and work that has occurred up to this point must now
be incorporated into the product mold design and construction process.
The tooling engineer now understands how the parts ran ton the
prototype mold. The part design, the hot runner design, the cool stack
up mold cooling and the mold action sequence have all been evaluated.
This work must now be translated into production molds.
The size of the production mold now is an important step to consider.
The part design along with its tolerance requirements are two critical
factors that influence mold size. The part design may require the use
of secondary side action. Secondary mold actions will require moving
steel segments that will significantly increase the spacing between
components.
Tolerance requirements are an important consideration in the
determination of mold cavitations. It is well understood that as one
increases the number of cavities in a mold, the ability to control
tolerances becomes more difficult. The graph below shows a study
performed on the relationship
between the number of cavities/mold and the resulting tolerance ranges
for a particular part.
The control of steel component tolerances
and the alignment of these components are also important. If a
particular dimension requires a tolerance of .004" and the steel
dimension varies by .001", 25% of the tolerance range is given away
before plastic is even injected into the mold. If the particular
dimension is carried across a mold parting line, the concern of part
tolerance is further magnified. The alignment of A side to B side can
vary by as much as .005" (or more) through the normal wear of guide
pins and bushings. A .005" misalignment in the parting line would
cause a dimensional change of .010". The result is a total loss of
tolerance control, caused by the steel, before plastic is injected
into the mold.
It becomes clear that the control of steel tolerances and the
alignment of components is critical in the production mold process.
Component dimensions must be carefully reviewed with mold designer and
tool shop during this phase of the process. It may be necessary to
hold a steel tolerance by as much as .0002" in areas where part
tolerance range for a part tolerance of .004".
The use of tapered interlocks between the components is important to
control parting line alignment in many medical applications. Tapered
interlocks will override the problems of plate alignment associated
with leader pin and bushing wear.
When selecting steels for the mold plates and components, it is
preferable to use a stainless steel such as type 420 where there is
contact with cooling water. The purpose being to reduce problems with
rusting wherever possible. The selection of steel for cores, cavities
and strip bushings requires careful planning that goes beyond the
basic concerns for corrosion. In areas if high wear, where there are
parting lines or slide shut offs, special tool steels that offer
toughness and/or shock resistance must be considered. Some examples of
steel with these qualities may be A-6 or S-7.
The heat treatment process must be carefully selected to establish a
finish hardness that is correct for the application in the tool. When
two steel surfaces come in contact in an area expected wear, the two
steels should have a distinct difference in hardness. This will
prevent the incidence of damage from occurring to both components and
will promote a good bearing surface between the two steels.
The use of mold coatings is an important area to be reviewed during
the selection of mold steels. Mold coatings should be used on
components that cannot be stainless steel and come in contact with
cooling water. Typical coatings for this purpose may be chrome, nickel
or armoloy. Mold coatings should also be considered in areas to reduce
the wear between components or in areas where release qualities,
between the plastic and steel, are desired. There are many types that
could be considered for release purposes. The following list covers
some of the most commonly used coatings.
Summary
The process for constructing injection molds for medical components
has become more complex as the medical industry places higher demands
on plastic products. Medical companies are expecting molders to
provide parts that meet all their requirements of dimensional
stability, low cost, fast turnaround and particulate free (clean).
The Tooling Engineer must coordinate
communications between many groups of people in order to meet the
demands listed. It is clear the task required detailed planning and
cooperation form all the resources involved. The Tooling Engineer
should have experience with tool design, mold making, product design
and injection mold processing. In addition, the person should be
trained in good project management techniques. Management and
coordination of the steps outlined in the flow chart (attachment #1)
will keep the project proceeding in the right direction and will
increase the chances of a successful completion and a happy customer. |
