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Rubber Injection Molding
AN OVERVIEW OF COMMON MOLDING
PROCESSES FOR HIGH-PRECISION
GASKETS AND SEALS
TABLE OF CONTENTS
Introduction...................................................................... 1
Molding Processes......................................................... 1
What is Injection Molding?............................................. 2
Determining Which Molding Process to Use................ 6
INTRODUCTION
Many of the objects we see and use every day, from household goods
to automotive parts and medical devices, are the result of injection molding
manufacturing. Molding is the process in which a manufacturer uses a fixed
frame, also known as a mold or tool or die, to shape liquid or pliable raw material
into a finished product. It is most often used in mass-production processes to create
part quantities in the thousands or millions.
Most people only associate injection molding manufacturing with plastic. After all,
plastic bottles are one of the most common products manufactured by the billions
each year. However, injection molding is not limited to plastic.
For the purposes of this white paper, we will focus on rubber injection molding,
specifically the use of synthetic elastomers such as EPDM, NBR, ACM, and FKM.
We will discuss the three most common molding methods for rubber and take a
closer look at injection molding.
MOLDING PROCESSES
The three most common processes for rubber molding are compression, transfer,
and injection molding.
From a high level, each method follows a similar workflow: rubber is heated, either
before or during the process, and forced into a mold. It then conforms to a single
mold cavity shape, producing only one part per “heat”, or molding cycle, or multiple
cavities, producing more than one part per heat.
The rubber is also cured or vulcanized during this process. Rubber in its
natural state is not an elastic material, but through the curing process,
the polymer chains that compose the rubber crosslink and
no longer move independently. Vulcanization makes it
possible for rubber to deform under stress and then return
to its original shape when the stress is relaxed.
We will define how the workflow for each molding method
differs and explain the advantages and limitations of each
process.
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COMPRESSION MOLDING
Compression molding is accomplished in a compression press consisting of a top
and bottom platen that are forced together via a hydraulic cylinder or ram. In this
process, an operator places a rubber preform in an open, heated mold, most often
made up of a top and bottom plate containing the part geometry. The mold is closed
in the press and the press compresses the mold and forces the rubber to conform to
the shape of the mold cavities. Heat and pressure are maintained until the rubber has
vulcanized to create theOPEN COMPRESSION MOLD
final product.
MOLD CAVITY
TOP PLATE
LAND
ID-OD RUBBER PREFORM
BOTTOM PLATE
MOLD CAVITY OVERFLOW GROOVE
Figure 1: Open Compression Mold
The advantage of compression molding is its ability to mold relatively simple parts
with comparatively low mold and equipment costs and fast setup times. Compression
molding is one of the lowest-cost molding methods with regard to tooling and
equipment. However, compression molding is primarily limited to compact parts with
generally simple geometries. Also, production costs can climb higher due to slower
cycle times and the need to utilize an
CLOSED excess of raw material
COMPRESSION MOLDto assure the part is
complete.
FLASH
TOP PLATE
BOTTOM PLATE
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PART OVERFLOW GROOVE
Figure 2: Closed Compression Mold
RUBBER INJECTION MOLDING
TRANSFER MOLDING
Transfer molding is a hybrid process between compression molding and injection
molding where pre-heated rubber is forced into a heated mold, which, unlike
compression molding, is closed. As the compression press closes, it forces the
material from a chamber, known as the pot, though sprue or feed holes in the top of
the mold into the mold cavity. Once filled, the rubber cures inside the mold cavity.
Transfer molding can be performed utilizing a “hot” pot, where the excess material in
the pot is completely cured in each cycle. It can also be performed utilizing a “cold”
pot where the rubber is warmed to promote flow, but not heated to the point where it
cures in the pot. The rubber waste in a hot pot process is much greater than in a cold
pot process.
OPEN TRANSFER MOLD
RAW MATERIAL RAM
CLOSED TRANSFER MOLD
SPRUE TRANSFER POT FLASH PAD TRANSFER POT
RAM
MOLDED RUBBER PART
MOLD CAVITY TEAR TRIM BEADS
Figure 3: Open Transfer Mold Figure 4: Closed Compression Mold
Examples of transfer molding applications are vibration isolator grommets for engine
fastening systems and electrical connector seals.
Compared to compression molding, transfer molding offers many benefits, such
as shorter production cycle times, the ability to mold more complex parts, less
material waste, and better heat-to-heat product uniformity. Transfer molding can
also lend itself to tools with a higher number of cavities since it does not need to
place individual preforms into each cavity, thereby creating a much faster cycle time.
However, the tooling costs for transfer molding are higher than those for compression
molding due to the more complex mold and equipment. So, while transfer molding
may deliver shorter production cycle times than compression molding, it may be
slower than injection molding.
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INJECTION MOLDING
Injection molding is a manufacturing process for producing parts by injecting pre-
heated material into a mold using an injection unit consisting of a screw and piston.
Injection presses offer significantly more control over all process parameters,
including the material’s temperatures at various points in the process and the speed
and volume of the material introduced into the mold. It is a more advanced iteration
of transfer molding, utilizing special equipment, and offers unique benefits, especially
OPEN INJECTION MOLD
forthose looking to produce a high volume of complex precision parts.
UNCURED STOCK
INJECTION NOZZLE
RUNNER
SPRUE
SPRUE BUSHING
FILLED INJECTION MOLD
TEAR TRIM BEADS HOLD CAVITY
Figure 5: Open Injection Mold
CURED STOCK IN RUNNER
MOLDED RUBBER PART
Figure 6: Filled Injection Mold
WHAT IS INJECTION MOLDING?
Injection molding is one of the most popular processes used to achieve
high quality and cost-effective parts. As stated previously, parts are
produced by injecting material into a mold. Specifically:
Step 1: A rotating extruder screw automatically feeds a ribbon of raw
material into a heated barrel where the rubber is pre-heated to the
optimum temperature. With silicone rubber, a secondary ram known
as a “stuffer” introduces a bulk quantity of material into the injection
unit.
Step 2: A hydraulic piston injects the pre-heated and masticated
material into a hot mold cavity.
Step 3: Now, inside the mold cavity, the material cures quickly
due to the heat produced while quickly entering the mold cavity,
combined with the mold temperature and pressure.
BENEFITS OF USING INJECTION MOLDING
The popularity of injection molding is due, in part, to several
unique advantages, which include:
HIGH PRECISION
Injection molding can achieve more complex and tighter
tolerance geometries than other molding methods. This is
because an injection press forces heated, and therefore less
viscous, raw material into a mold under high pressure.
Material handling also plays an integral part in improving
product precision. Instead of an operator handling the
material, the charging of the injection barrel by the rotating
screw is automated, and therefore, the volume of the
injected material is exact. Material feed automation also
contributes to more consistent cycle times with reduced
shot-to-shot size variation.
REPEATABILITY
When performed by experts, the injection molding
process is highly controlled and repeatable. Once a part
is successfully produced, the following parts will be
nearly identical to the original. And because injection 5
molding is a more automated process, there is no
manual handling of materials or molds, removing many
of the process variables caused by press operators.
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Moreover, the faster cycle times for injection molding means that the molding tool
spends less time open. Keeping the tool closed creates a more stable heat profile
within the tool and results in better cycle-to-cycle consistency.
MINIMAL MATERIAL WASTE
Some forms of injection molding can further reduce material waste by employing a
“cold runner” system. In basic injection molding, the rubber is introduced through
a central injection point in the middle of the tool and fed through a gallery of hot
runners to various points of the part geometry. Depending upon the complexity and
size of the part, this can lead to a significant amount of waste rubber in these cured
runners.
One way to mitigate this waste is by using a “cold runner” system where a cooled
manifold splits the central flow of material into multiple streams kept at a temperature
below the cure point. These multiple streams can then feed the raw material into
several points in the part cavity, significantly reducing or, in some cases eliminating
the waste of cured sprues. To further advance a cold runner system, valve gates
systems can be employed to introduce the raw material directly into the part cavity
and then shut off the rubber flow through the use of a pintle valve.
These methods reduce material costs but also increase the complexity and cost of
the tooling. So a valve-gated cold runner system is primarily considered for high-
volume programs where the material cost savings justify the substantial tooling
investment.
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RUBBER INJECTION MOLDING
INJECTION MOLDING
STRIPPER BOLT
NOZZLE BUSHING INJECTION CULL & RUNNER
TOP PLATE
SPRUE OPENING BAR SLOT
DOWEL PIN & BUSHING
BOTTOM PLATE CAVATIES/PARTS FLASH & TEAR TRIM GATE
Figure 7: Injection Molding Equipment
INJECTION MOLDING EQUIPMENT
Of significant importance in the effectiveness of the injection molding process are the
capabilities of the injection press itself. A well-designed injection press should have
some key features that promote satisfactory injection molding such as:
ENERGY EFFICIENCY
With their collection of pumps, heaters, motors, and control systems, large injection
presses consume significant amounts of electricity. So manufacturing with quality
machines that run efficiently can help mitigate energy costs.
CAPACITY EFFICIENCY
Injection presses can also take up large amounts of space in a manufacturing plant,
especially horizontal presses. When choosing an injection press, vertical presses
should be considered to help reduce the physical footprint of the machine and leave
more space on the manufacturing floor.
PRESS MAINTENANCE
Injection presses require maintenance by expert engineers to ensure continued
quality output. To reduce maintenance costs across a line of multiple presses, a
molder should consider sourcing all its machines from a single press manufacturer.
This practice allows the rubber molder to stock fewer spare parts than molders who
maintain equipment from multiple press makers.
FULL-SIZE RAM
The main hydraulic piston or ram applies the force to close and apply the full
clamping pressure to the mold. A press that utilizes a full-sized hydraulic ram will
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more evenly distribute the clamp tonnage to the platen/mold surfaces mitigating
any bending that would lead to tool wear and flashing. A full-size ram will help to
maximize the life of the mold, ensure satisfactory parts over the long term, and
protect customers’ tooling investment.
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FIFO
A “First In, First Out” or FIFO system for injecting raw material is another desirable
attribute for injection molding equipment. The ability for an injection unit to take up
raw material into the injection barrel and then inject that material into the mold in
a FIFO order ensures the press injects only fresh material for each heat. A FIFO
system prevents the potential introduction of old compounds that can lead to part
defects and production delays.
WHEN TO USE INJECTION MOLDING
As mentioned previously, deciding which molding process is ideal for achieving
quality rubber parts is complex. However, there are some generalizations for when
injection molding is the best choice.
HIGH-VOLUME PRODUCTION
Injection molding is primarily ideal for producing part quantities in the hundreds of
thousands or millions. While the setup costs and lead times are substantial, injection
molding becomes a largely automated process with short cycle times once the
upfront work is complete.
COMPLEX PRODUCTS
Injection molding can achieve a higher degree of product complexity compared to
compression and transfer molding. This is because injection machines feed the
rubber into the tool in a precise, automated manner. In contrast, compression and
transfer molding rely on the manual placement of rubber into the tool.
DETERMINING WHICH MOLDING PROCESS TO USE
There are numerous factors to consider when deciding on which of these three
molding processes to use: part size and design, material selection, product quantity,
setup cost, and more. However, there is no decision matrix or if-then algorithm that
can identify which process is ideal. Ultimately, this decision should be made only by
experienced manufacturers who have the substantial technical skill and capabilities
required to mold high-precision parts.
At Morgan Polymer Seals, we understand that choosing the proper molding process
is both a science and an art. Our company founder, Kevin Morgan, is an engineer
with more than 40 years of molding experience, and we employ a team of technical
8 experts and design engineers who understand the benefits and limitations of each
process. Since 1997, we have manufactured quality rubber products for OEMs like
Ford, and GM, delivered across the globe from our headquarters in Baja California,
Mexico.
CONTACT MORGAN POLYMER SEALS
+1 (619) 498-9221
+52 (664) 625-5835
info@morganpolymerseals.com
