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1. Steps to Getting a Rotomolded Part Manufactured
If you
are new to the rotational molding process and are looking to get a hollow
plastic part manufactured, this page will provide basic guidelines on how
to proceed.
The first thing that we recommend is to locate a rotational molder (see
section 2). We can refer you to a rotational molder qualified to produce
the product that you wish to make. A good rotational molder will also walk
you through the following steps.
Product Design
Blue-Reed can assist you with all stages of product development and
design. If your plastic product is in the concept stage or requires
extensive modifications to fit this process, we will work closely with
your rotational molder to ensure that your product is launched
successfully. We provide a 3d solid model from which a mold can be made.
If your
product is already designed, Blue-Reed provides a service whereby we
review your product design and provide recommendations to improve
moldability, reduce part and mold cost and reduce any long-term
maintenance issues.
Rotational Mold Makers
Blue-Reed has relationships with all the leading moldmakers -
worldwide. After reviewing your design, budget and geographical
location, we'll provide a list of recommended mould makers who can provide
the quality you're seeking within your budget.
The
Aluminum Cast Mold Making Process
Model/Pattern Production
Once the part design has been finalized, production of your mold can
begin from the solid model data that our design studio provides. From this
data, the mold maker typically produces a wooden model, a replica of the
finished part, only slightly large to allow for aluminum and plastic
shrink. In instance where cost is a driving factor, the toolmaker may
bypass the model and utilize a negative foundry pattern.
Casting
After the model has been produced, customers have the opportunity to
view the model/part design before it is cast in aluminum. From the
approved model or foundry pattern, foundry tooling and sands are produced
into which the mold maker casts their aluminum. The casting is then
polished and/or textured, parting lines fit, framed, inspected and shipped
to the molder.
Molding 1st Articles for Approval
Upon receipt of the mold, the molder typically runs several parts to
break the mold in. After achieving the desired cure and specifications for
the part, the molder will run several parts for customer approval. On
parts that have critical tolerances or require urethane foaming for
insulation or structure, fixtures may need to be manufactured.
2. Locating and Selecting a Molder
At
Blue-Reed, we foster relationships with rotational molders throughout
North America and possess an intimate knowledge of their equipment,
capabilities and competencies. We would be more than happy to assist you
in locating a molder that fits both your geographical and part complexity
requirements. We even have relationships with molders as far away
as China.
3. Locating and Selecting a Roto Mold Maker
Again,
Blue-Reed has relationships with all the leading moldmakers - worldwide.
After reviewing your design, budget and geographical location, we'll
provide a list of recommended mould makers who can provide the quality
you're seeking within your budget.
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4. About the Rotational Molding Process
If you
are new to this process and would like a brief non-scientific overview,
here it is.
Overview
Rotational molding is a plastics process to produce hollow plastic parts.
Since the molds are relatively inexpensive compared to injection or
blow-molding tooling, this process lends itself to low volume production.
It is ideal for part quantities of 2,000 or under per year or the
production of very large hollow plastic parts. Another reason designers
choose rotational molding is due to highly complex geometric shapes, deep
draws and zero draft that can be incorporated into each part. No other
hollow parts process has the design freedoms and potential of rotational
molding.
The Process
The process utilizes plastic ground into powder form typically 35 mesh.
The powder is placed in a hollow metal shell (the mold) which is most
often mounted onto a steel grid (spider) on a machine arm. The machine arm
rotates the grid both horizontally and vertically (biaxially) in very slow
motion.
The rotating grid with the mold attached is then moved into an oven which
can range in temperature from 600°F to 850°F. The mold begins to heat and
as the internal surface of the mold reaches approximately 350°F the
plastic powder begins to melt and stick to the surface of the mold. Since
the mold is rotating slowly biaxially, the power uniformly coats the
entire inside of the mold.
After all the powder is melted, the grid with mold attached is removed
from the oven and moved to a cooling station where is cooled with high
velocity air and/or water mist. Once the part has cooled to a temperature
that will allow it to hold the required shape and tolerances, the mold is
open and hollow part removed.
Cycle Time
There are many variables in the rotational molding process that can effect
cycle time. Some of the shortest cycles for liquid PVC are around 4
minutes. For 1-1/2” thick walled 20,000 gallon tanks, the cycle could be
as long as 60 minutes. It is best to consult your molder for this
information.
Materials
Predominantly rotationally molded parts are produced from polyethylene;
LLDPE, LDPE, MDPE, HDPE and XLPE. Other materials that can be rotationally
molded include acrylic, polypropylene, nylon, PVC, pur and TPU.
Development work is currently being undertaken to produce grades of ABS
that can be rotationally molded, at this time, none are commercially
available.
5. Product Design Tips to Reducing Initial Mold Cost &
Future Maintenance
As
mentioned in the second paragraph of this page, we strongly recommend
working closely with both your molder, plastic mold maker and ourselves to
ensure that your product design is suitable for the rotomolding process
and optimized to reduce mold, part and long-term maintenance costs.
Since processing parameters and capabilities vary molder to molder, so do
design constraints. Some molders may say absolutely no small radi while
others have minimal problems molding these, so please check with your
molder as to their tolerances and specific requirements.
Following are a few general design guidelines to keep in mind. If we are
doing the design for you, we work closely with your molder and mold maker
to ensure that each of these criteria meet the requirements of your chosen
suppliers.
Minimize The Number of Mold Pieces – Flatten The Parting Line
The first recommendation, probably obvious to most design engineers is to
visualize the parting line. How does the mold open and close. The lowest
cost, least maintenance mold has just two pieces and a flat parting line.
As you increase the number of mold pieces, inserts, pull pins and core
pins or increase the irregularity of the parting line, particularly
vertical drops, you increase your rotomold cost, molding operator labor
and potential for a high mold maintenance bill down the road.
Account For Shrinkage
Since nearly 90% of today’s rotationally molded products are produced from
polyethylene, it is important to note that while you gain great properties
such as impact strength, those properties also lend themselves to varying
shrinkage. Molders can typically hold several critical dimensions, but not
all dimensions. As the part cools in the mold, there are areas or
restricted shrinkage, such as around the parting line, that make it
impossible to control the part shrinkage. Therefore, when there are mating
parts, you must allow for varying shrinkage.
Avoid Flat Surfaces.
The one thing that polyethylene does not do is stay very flat particularly
in rotationally molded products. When cycling, the powder tends to puddle
in corners and radi causing these areas to have increased wall thickness.
This is great for rigidity and part structure but that build up of
material around a radius can also play havoc with large flat surfaces
stretching, pulling and distorting them. We therefore recommend you avoid
large flat areas. Design with domes, curves, contours, ribs, grids, waffle
patterns anything but flat surfaces.
Avoid Sharps.
As the powder tumbles in the mold, it will bridge across a sharp or small
radius causing pin holeing in the finished product. This is an area that
you definitely need to check with your molder on. There are additives,
surface enhancers and PE filling compounds that can be used to create
sharps even solid areas in a part, but these all add to the part
processing cost and slow cycle times. Wherever possible, design with large
generous radi.
Avoid Designing Part Walls To Close Together.
If you are producing a double wall part, be conscious of how close the
part walls are to each other and make sure that there is enough room in
the cavity to hold the required material. When walls are too close
material will bridge, kissoff, create voids and distort outer surfaces.
This is an area to discuss with your molder but a rule of thumb to follow
is leave a gap of three times your part wall thickness between the walls.
Part Properties.
If you are basing your required part properties upon a material spec.
sheet, be aware that there are many factors inherent in the rotational
molding process that can impact part properties. Easily identifiable
variables are dry blending of colors vs. compounded resins, achieving
optimum cure, cooling methods and of course additives such as fire
retardants and UV stabilizers. This is an area that can vary from molder
to molder and must be discussed upfront with your molder.
For
additional information, please feel free to contact me - Greg Stout
e-mail:
info@blue-reed.com -
phone/fax: 330.688.1324
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