SPRING 1998
Volume 4 • Number 2
ROTATIONAL
MOLDING: All Systems "Go" for Growth and Innovation
Rotational
molding is one of the processes available that converts plastics
from its raw bulk form into a molded component of useful shape
(a finished good). It is a process useful for producing hollow,
seamless products, such as liquid holding tanks, containers, housings,
boat hulls, mannequins, outdoor play equipment, and toys, and
has found its way into many different markets and industries.
New applications continue to be developed every day, and many
new designs are displacing components that have been fabricated
previously in steel, concrete, wood, or other complex plastic
assemblies.
In
rotational molding, the motion of the mold distributes material
under low-force levels of gravity and centrifugal acceleration,
as the plastic tumbles in a bi-axial rotation and conforms to
the geometric shape of the heated mold tooling. This process creates
moldings of low stress, as opposed to other plastic processes
such as injection molding, where the plastic is driven under very
high pressures through thin walls, or in blow molding, where the
plastic is stretched under air pressure. One of the biggest advantages
to the process is the ability to change wall thickness without
changes to the tool itself. This allows additional flexibility
to the manufacturing company for a low cost.
The
traditional mindset for rotational molding includes the fabrication
of parts that are hollow and tank or vessel-like in appearance.
For example, rotational molded tanks are replacing designs that
were historically made from sheet metal or steel. The plastic
version offers the advantage of being lighter weight because of
the high strength-to-weight ratio characteristic of plastic. There
is also the benefit of reduced obsolescence, as polymers are often
resistant to corrosion and perform well in aggressive environments
that are inappropriate for metals.
Smaller
parts are also excellent candidates for rotational molding. Some
may include doll parts, bowling pills, and balls for sporting
goods. Rotational molding may also serve as the first step in
a series of processes for a part. For extremely rigid applications,
urethane foam is injected as a secondary process to the hollow
interior to add stiffness, such as in medical backboards.
Because
the plastic powder in rotational molding will show no affinity
for unheated regions of a mold, and thereby leaving it "un-coated",
a product does not have to be a fully enclosed volume, such as
a cylinder or box, to be considered for rotational molding. Shrouds,
covers, canopies and large panels can be manufactured by innovative
use of shielding and insulation. Other applications have recently
been developed to reduce costs in a complex assembly. For example,
several labor intensive injection or extruded components may be
needed to complete a finished good, but only one rotationally
molded component could be molded to complete a final assembly.
Since
rotational molding is a low-pressure process, mold construction
is simple and less expensive than other processes. The mold construction
is often cast aluminum, of approximately 1/4 inch in thickness.
Fabricated sheet metal can also be used for tooling, especially
for parts that are straightforward in shape, where molds can be
generated from a series of components that are welded together.
Electroformed tools are used for detailed or seamless part construction.
Rotational
molding allows manufacturers to introduce and produce quantities
of new products without creating the financial burden of a large
up-front capital investment in molds as seen in other processes.
If the product is successful, additional sets of tooling can be
made in response to escalating product demand. Injection molding,
for example, would require a much larger capital investment in
tooling before the first production part is generated.
There
are a variety of materials that can be used for rotational molding.
About 80% of the applications use polyethylene (LLDPE to HDPE).
Other resins include polycarbonate, polyurethane, PVC, polypropylene,
and nylon. There are some great advances in the nylon family and
high heat requirements are forcing rotational molders to use this
material. For instance, aircraft ducting is currently utilizing
this type of application.
Rotational
molding worldwide is growing at a rate of eight to 10% per year.
The Association of Rotational molders lists 348 molding members
worldwide, with the highest concentration of rotational molders
found within a 100-mile radius of Akron, Ohio. From a global perspective,
the number of machines installed for this process is growing most
rapidly in the Far East and South America, and domestic processors
are also enjoying rapid growth.
With
the recent introduction of process control systems such as the
Rotolog II, rotational molding is becoming more of a science and
less of an art. Through the use of these systems, processors can
build a processing window that can document processing parameters
to be programmed into the machine. This ensures quality parts
with each order produced by the processor.
Recognition
for the rotational molding industry is on the increase. The Association
of Rotational Molders has been in existence for a long time and
is currently experiencing a rapid growth curve. Recently, the
Society of Plastics Engineers recognized rotational molding as
a potential separate plastic process inside of their organization.
The rotational molding committee for this organization is in the
process of becoming fully recognized by the year 2000.
(This
article was written by Marty Dropik, PTDC senior project engineer,
and Greg Cronkhite, owner and president of Sterling Technologies,
a rotational molder located at 1801 Peninsula Drive, Erie, PA.
For more information regarding the rotational molding process,
contact the PTDC at (814) 898-6145 or Cronkhite at (814) 836-8700,
or visit Sterling's web site at
http://moose.erie.net/~sterling/.)
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Extrusion
or Injection Molding
Penn
State Erie is looking at the development of a new plastics processing
method called continuous injection molding technology (CIM TECH).
This unique process has been patented and partially developed
by a local manufacturer who is interested in turning the project
over to the Plastics Center for further development.
The
innovative process consists of a mold wheel which contains the
shape of the parts being molded. A metal band is held against
the mold wheel and plastic is continuously extruded, as the wheel
rotates past a plastic melt channel contained in a "shoe" that
holds the strap against the wheel. The plastic cools during the
time between injection and when it is pulled off the wheel. The
resulting plastic product can be continuously placed on reels
or separated as it comes off the wheel.
Two options are currently available to place discrete, injection
molded parts on reels to accommodate subsequent automated assembly
processes. The first is to place the parts on a carrier in a process
that is secondary to the injection molding process. The second
is to semi-continuously mold discrete sections of the parts, with
the carrier attached, in a standard injection molding machine.
The discrete sections are then indexed forward after each molding
cycle, creating a continuous strip of discrete parts.
Currently,
continuous plastic profiles are made by placing a die with the
required shape on the end of an extruder. However, when secondary
operations are required to punch holes or form special shapes
within the profiles, or in cases where very tight dimensional
tolerances are required, this new process may be an alternative.
CIM TECH also has the advantage of simultaneously molding and
attaching parts which can reduce assembly time and overall cycle
time. That time savings could potentially make this technology
an attractive alternative to standard injection molding for certain
parts.
If
you have applications that would be a possible candidate for this
new process, please call Brad Johnson, lecturer in engineering,
at 814-898-6148 or e-mail him at bgjl@psu.edu.
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A
New Application for an Old Process
Traditionally, blow molding was a process mainly used to produce
hollow forms, such as bottles, containers, tanks, and toys. Today,
the blow molding industry is on the verge of breaking into markets
that were once dominated by injection molding. Automotive and
consumer products markets are the most likely candidates to begin
using the blow molding process. With new techniques that are currently
being developed and refined, parts can virtually drop out of the
blow molding machine without need for further assembly. For example,
applications that require a front, back, and insulation layer
can easily be molded, as in-mold filling is one feature possible
using the blow molding process.
In
the future, interior trim and dashboards of some automobiles will
be blow molded, using new advances in smooth-wall blow molding
and area-selective multiple material layering techniques. To illustrate,
an elastomer can be placed on outside regions where it will be
visible, while a less expensive, but compatible, material can
be used for the hidden areas and inner layers of the parison.
While
the automotive industry will probably reap the benefits of blow
molding first, it is not the only market to take advantage of
new advances. Molding doors for refrigerators, as well as cabinet
and drawer fronts using blow molding technology is also a possibility
for the future. In fact, as designers begin to see the cost and
time saving benefits of converting an injection molded assembly
to a single blow molded part, even more markets will consider
switching to the blow molding process.
Here
at Penn State Erie, there is a consortium that can help your company
with the research needed to get started or advance with blow molding.
The Blow Molding Consortium mission has three elements: education,
applied research, and technology transfer. Equipment and projects
are provided through the consortium, which allow undergraduate
students to get hands-on experience operating production-type
equipment. It is this relationship that allows students to do
applied research in areas that are of concern to the consortium
members. Penn State faculty and staff are also available for specific
company research projects. Technology transfer occurs through
the research reports to the members, graduating students, and
professional presentations.
Entering
its forth year, the consortium consists of companies that represent
a cross section of the blow molding industry. Members include
custom and captive molders, resin suppliers, equipment manufacturers,
and research and software groups.
The
consortium holds two meetings each year. The fall meeting is normally
held in conjunction with the Penn State Erie Engineering Career
Fair, and hosts guest speakers on topics for the enrichment of
all members. The spring meeting features guest speakers, updates,
and the presentation of research from throughout the year. Both
meetings address research concerns of members and entertain requests
for specific research topics.
Your
company can benefit from its membership in numerous ways. Member
companies are entitled to four blow molding simulations using
a choice of available software packages. During fall and spring
meetings, members gather to share interests and concerns. Research
results and software and computer updates will also be reviewed.
This gathering is a valuable source of technical assistance among
co-members. Members also have access to the Penn State Erie blow
molding equipment, as well as active participation in the direction
of research projects and research teams. Members are the first
to be notified of future technical conferences and technical training
seminars in blow molding, and will be granted discounted rates
at these meetings.
Currently,
the consortium is exploring several areas of interest to blow
molders, one of which is evaluating blow molding simulation software.
The consortium has acquired licenses for the three top software
packages, and is in the process of evaluating the effectiveness
of each. Preliminary studies have been performed, and exhaustive
case studies with each software package are goals for the immediate
future. Another area of research is extrusion blow molding pinch-off
studies using design of experiments. This is an area where there
is little technical information, and is mainly done with rules
of thumb or experience. Research is being conducted to develop
several equations to quantify the pinch-off strength. The last
area of focus is developing a new state-of-the-art prototype extrusion
blow molding machine. The University has received an unfinished
prototype machine from a company, and within a year the machine
should be running. Looking toward the future, the consortium will
be investigating rapid prototype blow mold tooling. This could
provide a company with prototype tooling virtually over night.
With
the exciting new advances in technology, the blow molding industry
is sure to be an expanding market. The Blow Molding Consortium,
located on the campus of Penn State Erie, can be your source to
explore the new technology and information using the lab facilities
and faculty, staff, and student resources. For more information
on the Blow Molding Consortium, please contact Lucy Lenhardt,
research assistant, at (814) 898-6146, or Jon Meckley, senior
project engineer, at (814) 898-6147.
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Memo
To Manufacturers: Concurrent Product and Process Design
Nothing shakes up our businesses so much as failure — failure
to get the order, failure to meet scheduling deadlines or budgets,
failure to get a product to market and gain the competitive advantage.
So much has been written and shared about the importance of developing
a strategic approach to product development, involving all functions
in the process, and accelerating the product development process
from idea to launch. Included, obviously, is the all-important
front-end work associated with idea development, investigation,
market analysis, and business planning. Notwithstanding, (and
since we are a technology-focused center) my remarks focus on
the technical.
Many project management techniques and commercial technologies
help us avoid failure by supporting efficiency and effectiveness
in design and engineering analysis. However, many of our region's
leading manufacturers are also avoiding failure by recognizing
the importance of process design as a critical component in the
overall product development cycle.
Process
design should take place simultaneously with product engineering
design and materials selection. This is, in part, because grades
and resultant properties of materials are generally process dependent.
With the advent of both new and improved processing technologies
and various new and integrated processing methods — the design,
materials, and the process are clearly interdependent. Coupling
this interdependency with today's need to reduce the overall product
development cycle often drives us to improved methods.
Many
of today's computer-aided (CAE) technologies greatly assist in
process design and rapid iteration and evaluation of alternative
processes. The effective use of predictive simulation modelers
(MoldFlow®, C-Mold®), design for manufacturability
and assembly (DFMA), factory flow/capacity planning simulation
programs, and other engineering software tools are helping our
customers minimize lead-times and product development costs, while
at the same time maximizing product quality and reliability. Use
of these tools and technologies allow for concurrent engineering
of both the product and the process.
We
know that the right tools in the right hands will minimize risk
and ensure success in most endeavors. Although we learn from our
mistakes, we prefer success to failure — and we know you do as
well. Let us help you succeed. The PTDC has the right tools and
the right hands to help you — in both product and process design.
Please call us at (814) 898-6145, as we can be of service.
-
David Thomas-Greaves, Director
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