Rubber injection molding is not a new idea. The process has been successfully developed in the United States since the 1940s, albeit on a very limited scale. Today, however, injection molding has become an increasingly important manufacturing process for molded rubber parts, since it is an essentially high-speed operation, which conducts itself through a semi-automatic or fully automatic system. Although injection molding‚ is used for a relatively small proportion of molded artifacts today, by the mid-1970s it will be the method of choice for approximately 25% of all technical molded parts.

1. Rubber Injection Moulding
Luis Antonio Tormento
04/07/2022
Luis.tormento@outlook.com

2. Rubber Injection Moulding
• Synthetic rubber is the most widely applied elastomeric macro-
molecular material in vehicle, aerospace, shoe and tire industries. After
creating molecular stereo structure by vulcanization, synthetic rubber
based products will be strengthened for high elastomeric, better
insulating, perfect sealing, high chemical resistance, high temperature
resistance and all other excellent mechanical performance.

3. Rubber Injection Moulding
• Rubber injection molding is not a new idea. The process has been
successfully developed in the United States since the 1940s, albeit on a
very limited scale. Today, however, injection molding has become an
increasingly important manufacturing process for molded rubber parts,
since it is an essentially high-speed operation, which conducts itself
through a semi-automatic or fully automatic system.
• Although injection molding‚ is used for a relatively small proportion
of molded artifacts today, by the mid-1970s it will be the method of
choice for approximately 25% of all technical molded parts.

4. Rubber Injection Moulding
• The advantages obtained over conventional compression moulding
methods stand out through:
• Reduced molding cycle, therefore increased productivity per cavity.
• Minimum material preparation.
• Minimal or almost no burr, therefore reduced scrap level and lower final
costs.
• More uniform cure on thick pieces.
• Increased dimension accuracy, resulting in greater product uniformity.
• Generally lower scrap rate.
• Reduction of labor in the finished product.

5. Rubber Injection Moulding
• All these factors combined contribute to achieving reduced production
costs and increased productivity, the main reasons for producing parts
on large scales.
• The biggest disadvantage of injection molding machinery compared to
compression or transfer molding equipment is the complexity and high
capital expenditure. Reasonably large amounts of a given rubber
product are needed to justify the expense of tooling.

6. Rubber Injection Moulding
• The increased automation associated with injection molding was
responsible for the proportional reduction of the total costs
attributable to the labor cost from more than 80% to less than 25%.
Higher working temperatures as the putty penetrates into the cavity
and higher cure temperatures facilitate the use of cure cycles that are
5 to 20 times faster than those associated with conventional
compression molding and therefore contribute to. the desired
production increase. On a general average, we can consider that the
cure cycle is 10 times faster with injection molding compared to
compression molding

7. Rubber Injection Moulding
• It is the ultimate goal of the injection process to achieve the shortest
possible cure times for a given molding cycle, where cure
temperatures ranging from 177ºC to 232ºC are common. By reducing
to a minimum the temperature differential between the mold and the
mass being injected into the mold, both require heat transfer during
the cure cycle and the cure cycle time is reduced.

8. Rubber Injection Moulding
• The rubber injection molding process starts with an uncured rubber
ribbon stock that is fed into a rotating screw of the injection unit. A
controlled amount of material is pulled into the injection unit. Here the
material is plasticized to a target elevated temperature. The rubber
material is then injected into the mold cavity through a runner and gate
system where it is held in the mold under high pressure and elevated
temperature to activate the cure system in the rubber compound
(rubber is vulcanized). The cycle time is established to reach an
optimal level of cure. At the end of the cycle, the parts are removed or
ejected from the cavities and the next cycle begins.

9. Rubber Injection Moulding
• Fi
• Figure captured on http://usa.datwyler.com/rubber-injection-molding.html

10. Rubber Injection Moulding
 Rubber Injection Molding Steps:
1.) Material in injection unit ready for injection into cavity.
2.) Material is injected from injection unit through runner system
& gates into cavities.
3.) Parts (material) are cured in mold until the cure process is
completed.
4.) Molded rubber parts are removed from mold and the
process is ready to begin again.

11. Rubber Injection Moulding
• Advantages of Rubber Injection Moulding:
Suits automation with automatic material feeding
Complementary to high precision molding applications
High level of repeatability
Closed mold injection supports molding of complex geometries and
overmoulding
• Disadvantages of Injection Moulding vs. other rubber moulding
methods
Higher start-up / shutdown costs, better suited for high volume applications
Runner systems can lead gross material weights when cold runner systems
or other low waste options are not utilized.
Not all cure systems and elastomers are suitable for injection molding.

12. Rubber Injection Machines
There are three main types of injection machine: the ram type, the in-
line reciprocating screw type, and the out-of-line non-reciprocating
screw type.
Simple ram machines cost less than screw machines and because the
ram can be made to fit very tightly in the cylinder, they can develop
very high injection pressures. However, as the mix receives heat only
by thermal conduction from the barrel, high injection temperatures and
thermal homogeneity are difficult to achieve, and they are not widely
used.

13. Rubber Injection Machines
Simple ram machines cost less than screw machines and because the ram can
be made to fit very tightly in the cylinder, they can develop very high injection
pressures. However, as the mix receives heat only by thermal conduction from
the barrel, high injection temperatures and thermal homogeneity are difficult to
achieve, and they are not widely used.
The screw in the in-line reciprocating screw type acts both as an extruder and a
ram. In this type of machine, the mix is heated and plasticized as it progresses
along a retractable screw. When the necessary shot volume has accumulated in
front of the screw it is injected by a forward ramming action of the screw. With this
system, a more uniformly controlled feeding of the material can be achieved,
together with more rapid heating of the stock from mechanical shearing,
additional distributive mixing from the rotational screw action, a greater degree of
thermal homogeneity, and a temperature 20 to 30 °C higher than the jacket
temperature. However, during the injection stage, when the screw is acting as a
ram, there is inevitably some leakage back past the flights and this limits the
achievable injection pressure.

14. Rubber Injection Machines
The out-of-line non-reciprocating screw machines have
separate screw and injection chambers and combine the
advantages of the above two types. The screw plasticizes the
compound and delivers it through a non-return valve into a
separate injection chamber. Machines of this type can generate
injection pressures of up to 200 MPa and can efficiently mold
high viscosity mixes and effectively fill large volume molds.

15. Rubber Injection Machines
In the standard injection process described above, the compound is
injected into a closed mold; there are two variations on this:
• Injection-compression molding: The mold is partially opened and a
vacuum applied to the cavity area which is sealed by a compressible
silicone gasket. A measured amount of rubber is injected into the partially
opened mold. The mold is then closed and the excess rubber is forced
outward to flow off channels. This process is used for articles such as
precision O-rings, where runner marks are unacceptable.
• Injection-transfer molding: The rubber is injected into a transfer chamber
and then forced from the transfer chamber into the mold. This combination
process uses the plasticization and heat generation advantages of the
injection unit with the controlled flash pad and cavity layout advantages of
the transfer press.

16. Rubber Injection Machines
All of the above systems have been in use for many years. The
equipment manufacturers are constantly improving the design,
operation, and control of their machines, but in general, available
equipment is based on the systems described above. The equipment
manufacturers' concentration has been on data acquisition and
process control systems to enable the processors to implement on-
line statistical process control. This has been in response to the end-
users demands, especially from the automobile industry, for defect-
free products. Another major pressure on suppliers to the automotive
industry is cost. This, in turn, is reflected onto equipment
manufacturers to provide cheaper, more efficient machines to allow
the processors to make parts more cheaply, but with no loss in
quality.

17. Rubber Injection Compounds
Rubber compounds for injection molding differ mainly in the shapes of the vulcanization
curves. Appropriate induction period with constant plasticity and high speed of
vulcanization are required. This is achieved by a suitable combination of vulcanization
accelerators and retarders in the selected vulcanization system. In case of rubber
injection molding, screw plastication units are used. The material is filled either in form of
a belt or granules. Due to the properties of rubber compounds the plastication is carried
out in a cylinder with a significant assistance of dissipated energy. The plasticated
material is often transferred into an injection cylinder, goes through it and is injected into
the cavity of the mold. When filling the mold, the material also flows in the surface layer
because the temperature of the mold is higher than the temperature of the injection
molded material. It is necessary to choose sufficient diameters of the runners and the
cavity of the mold, too. The mold must have impeccable breathing. The ejecting system
must be selected with respect to the high elasticity and low strength of the injection
molded pieces. The injection molding of rubber compounds allows production of thick-
walled products in a reduced time and higher quality of the vulcanized rubber. However, it
requires more complex processing equipment and, unlike the previous technologies, it is
less convenient for piece production.

18. Rubber Injection Compounds
Organic rubber injection starts with more efficient material preparation. The material is mixed,
and then stripped into continuous strips and fed into a screw which charges a barrel as needed
with a pre-defined amount of material. When the mold is closed, the material in the barrel is
injected into the mold cavities and cured. Benefits of injection molding include:
 Complete elimination of operator placement of pre-forms
 The injection screw pre-heats the material before forcing it into the cavities. This decreases the viscosity of
the material, allowing it to flow more easily into the cavities.
 Moderately quicker cycle times than compression and transfer molding
 Liquid Injection Molding (LIM) or (LSR) Injection is the process where a two part liquid silicone compound (A &
B parts) are delivered at a fixed ratio into a static mixer. The LSR mixture blends with a platinum cure system
and is delivered into the injection unit where it is injected through a runner and gate system into the closed
mold until cured. At the end of the cycle the parts are removed or ejected from the cavities and the next cycle
begins. Benefits of LIM molding include: •
 Automated closed-loop systems limit contamination
 Nearly “flash-less” parts
 Optimized cycle times
 LIM is well suited to the unique needs of the medical product industry

19. Rubber Injection Compounds
LIM materials are biocompatible, inert and stable, flexible, have a low compression set
with a wide range of Durometers, and offer superior heat resistance.
High quality components with complex shapes can be repeated in high quantities in a
cost effective manner.

20. Rubber Used in Injection Molding
Not every type of rubber can be used in the injection process, parameters such as
temperature, injection speed, and pressure can affect the vulcanization system, making
its use unfeasible. The most common rubbers are:
EPDM,
 NBR,
NR,
SBR,
FKM,
Silicone,
Fluorsilicone

21. Raw Material used in Injection Molding
Other key points to consider are:
 Acceleration system (process temperature can affect its effectiveness)
Type of filler (Moisture can lead to the formation of bubbles that show defects in the molded
artifact)
Process oil (volatility)
Process Aids

22. Raw Material used in Injection Molding
Other key points to consider are:
 Acceleration system (process temperature can affect its effectiveness)
Type of filler (Moisture can lead to the formation of bubbles that show defects in the molded
artifact)
Process oil (volatility)
Process Aids

23. Typical Problems with Injected Moulded
Rubber
• There are countless quality control issue that can arise from manufacturing injection
molded rubber. Here are five common problems and possible solutions.
• Short shot
• A short shot is the incomplete filling of a mould cavity, resulting in a incomplete injection
moulded rubber part. There are several possible solutions to remedy short shote issues.
• Possible solutions:
• Increase shot size
• Use higher melt flow rate material
• Increase melt or mould temperature
• Increase fill speed, pack pressure or injection time

24. Typical Problems with Injected Moulded
Rubber
• Shinkage
• A common problem when manufacturing rubber products is the fact that moulded rubber parts
always come out of the mould smaller than the cavity that produced then. In other words, they
shrink.
• Possible solutions:
• Maintain adequate cushion
• Delay gate sealing to allow pack out
• Increase the size of the mould cavity
• Increase cavity pressure and hold time

25. Typical Problems with Injected Moulded
Rubber
• Excess flashing
• With injection moulding, the rubber material can leak between the mould´s surfaces along
the parting line. Flashing can affect an injection moulded rubber part´s surface finishing
and sealing capabilities
• Possible solutions:
• Use a larger press
• Increase clamp force
• Decrease peak cavity pressure

26. Typical Problems with Injected Moulded
Rubber
• Warpage
• Warpage is when the intended shape of the moulded rubber part becomes distorted during
the cooling process, which can cause it to fold, twist, bend or bow.
• Possible solutions:
• Increase cycle time
• Decrease injection fill rate
• Minimise hot spots in the mould
• Mould at high temperatures and low pressures

27. Typical Problems with Injected Moulded
Rubber
• Brittleness
• Brittleness can result from shorter molecular chain lengths. This Impairs the physical
integrity of an injection moulded rubber part, resulting in cracks or breakages.
• Possible solutions:
• Increase injection fill rate
• Increase mould temperature and cool time
• Eliminate contamination from other polymers

28. References
1. https://www.timcorubber.com/rubber-resources/rubber-molding-process/
2. https://www.applerubber.com/blog/so-you-want-to-be-a-rubber-injection-molding-
expert/
3. Mixing of Rubber Compound – Andreas Limper – Hanser Publication, 2012
4. Rubber Curing Systems – R.N. Datta – Report Nº 144 – Rapra Review Reporting –
2012
5. Practical Guide to Injection Moulding – Vanessa Goodship _Rapra – Arburg – 2004
6. Product Guide: Injection Molding of XIAMETER®Liquid Silicone Rubber, 2009
7. Practical Guide to Rubber Injection Moulding – John A. Lindsay - Smithers Rapra
Technology Ltd – 2012
8. Troubleshoting guide to rubber injection moulding, Martins-Rubber, UK,