Four tips for successful silicone molding LİNK-> http://medicaldesign.

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Mar 18, 2011 12:44 PM, Written by: Mark Hammond General Manager GW Silicones Royalton, VT
GWSilicones.com GWPlastics.com
Edited by: Leslie Gordon leslie.gordon@penton.com

These silicone prototype inserts fit a representative production mold base, allowing exact production
simulation.

Although liquid silicone rubber (LSR) has been commonly used in Europe for a long time, some North American
OEMs still view it as a “black magic” material because of a lack of internal expertise as compared to
thermoplastics. But the performance advantages that LSR provides in medical devices and disposables are
great. Therefore, leaving more about the material’s proper use and how to select the best molding method and
molder is important. Critical factors for a successful silicone molding program include:

Material selection. Engineering groups can take the path of least resistance and use existing approved
materials already in their database. They really can’t be faulted for doing this because it ensures the selection
of a qualified and validated material, greatly reducing the risk of potential program delays or problems with
biocompatibility.

However, LSR material providers have significantly improved their formulations over the last 10 years, so it
doesn’t make sense to rely only on familiar materials and potentially overlook performance gains, processing
stability, and lower prices. The latest LSRs have improved upon original formulations and have closed the gap
on the mechanical properties of high consistency silicone (HCR) — a material typically compression or transfer
molded. LSRs can provide a better choice than HCRs because its platinum cure system takes significantly
shorter cure times.

Also, LSRs come in sealed drums from material suppliers, reducing the risk of contamination. Materials typically
have more batchto- batch consistency because they are produced on a larger scale at the raw-material
manufacturer and not on a two-roll open mill at a molder or custom mixer. Needless to say, in medical
molding, material consistency is critical, based on the requirement to operate the process within a tightly
validated window.

In addition, LSRs better suit automated production because of the material consistency and the large-drum
delivery systems allow the extended runs without stoppage. HCR can have good tear resistance, compression
set, and abrasion resistance, but an alternative and suitable LSR grade is typically available for most
applications.

The 2 + 2 mold handles a plastic substrate and a silicone overmold.

Another issue with material selection is a tendency for engineering groups to want to use thermoplastic
elastomers instead of silicone. While thermoplastic elastomer (TPE) or thermoplastic vulcanizate (TPV)
materials have improved over the years, silicone is still a better choice when it comes to sealing applications.
TPEs don’t reach liquid silicone rubber’s clarity, biocompatibility, compression set, and temperature resistance.
In addition, the raw-material pricing between LSR and a specialty grade TPE is not that different. It becomes
even less of a factor when working with a silicone molder that uses the latest in flashless and automatic
molding.

Vendor selection. Because of a lack of expertise in silicone at the OEM level, many molding programs are
sourced based on an Internet search, a past relationship, or what company is closest or cheapest. But just
because a company owns a silicone machine does not make it a qualified and experienced silicone molder.
Silicone molding is much different from thermoplastics and can pose many unique problems. For example,
strict silicone tooling requirements mean gating, venting, and demolding parts is always a challenge. So silicone
experience counts a lot in part design, mold design, and the molding process.

Additionally, there is a wide variety of methods for molding silicone. OEMs should ask their suppliers if they use
flash free tooling and automated molding or if they rely on human operators and secondary flash removal. Each
approach has its own merits, but operators and secondary operations add more variables and costs into an
already complicated process.

Prototype and production tooling. There are two kinds of prototype tooling. One type is intended just to give
engineers a soft silicone part to touch and feel. This tooling can be made quickly and relatively inexpensively.
Often, parts are compression or transfer molded with little regard to dimensions or part quality.

Prototypes when the next step is production are a different matter. One of the most important phases of a
silicone project is the prototype stage. Not only does this give the OEM’s product development organization
molded parts to assemble and complete trial builds but this is the opportunity for the molder to get hands on
with the part and process and understand how the component will behave in simulated production.
Prototyping using the intended production in process and tooling is critical to success. Changes such as moving
from a traditional hot runner in prototype to a valve-gated cold runner in production can affect shrink rates and
how the part fills. Changing the gate diameter slightly can affect the shear rate of the material and alter
dimensions. Compression molded or transfer molded prototypes moving to production develop different cavity
pressures, altering dimensions and providing different mechanical testing results. Prototyping and producing
with the same tooling and method is the best bet.

With silicone production tooling, the old expression, “You get what you pay for” is true. Expect to pay premium
prices as compared to thermoplastics for a high-quality silicone tool. There are no SPI standards for tool quality
such as in the thermoplastics industry, so it is hard to know what to expect. When discussing tooling with
suppliers, OEMs should ask to see examples of similar parts they have produced and verify whether parts are
as-molded or need secondary operations.

Also OEMs should find out where the tools are built and the tool maker’s experience level. Processors using
traditional hot runners means added piece price, so it is best to insist they research cold runners to eliminate
runner waste. A high-quality silicone mold at the beginning of a project may seem like a large out-of-pocket
expense, but the investment will pay for itself many times over during the life of the program in piece price and
part consistency.

The use of technology. A lot of silicone molding has come up through the traditional rubber molding industry
and with this have brought many bad habits in waste, tooling quality, and lack of process control. Fortunately,
some silicone processors have made significant improvements. One is in flashless molding. Quality tools leave a
maximum flash extension of only 0.001 to 0.002 in.
The high-quality silicone production mold uses automatic demolding for flashless and automatic operation.

Another improvement is automatic molding. Process consistency and repeated cycle times are critical to the
dimensional stability of molded parts. LSR molds are typically heated to approximately 400º F, so open time
lets mold temperatures drop and results in part variation. Automatic molding means that machines control the
cycle, not human operators.

Also helpful is the use of valve-gated cold runners. They let processors gate directly on parts without waste,
which can speed-up curing. A more recent innovation: valve gates with a servo-controlled valve-gate pin that
controls pin stroke. Processors can thereby tune each individual cavity for volume and ensure a balanced fill on
multicavity molds.

Additionally, self-adhesive silicones are revolutionizing the overmolding of silicone onto plastic substrates.
Traditionally, a primer and adhesive are applied to a substrate offline. This creates a secondary operation and
forces the plastic and silicone operations to be done on separate machines.

In contrast, current materials mix chemical adhesives with the silicone, allowing the molding of the plastic and
an immediate silicone overmold. Multimaterial injection machines can complete a silicone overmold in one
production cell. For these reasons, don’t neglect to think “twoshot molding” during the design phase.

Another advancement comes from servo-electric injectionmolding machines. They provide process
repeatability never before possible and help ensure part consistency. Additionally a closed-loop LSR metering
system can measure and verify the exact percentage of LSR metered into the machine which can guarantee a
50-50 mix of the A and B components.

Design considerations

So, what are important design considerations for LSR parts? Silicone molds must be built to tight tolerances
because LSR flashes at 0.0002 in., which is approximately 10 times tighter than a standard thermoplastics mold.
It is therefore important to prove-out many factors at the prototype phase to ensure a smooth transition into
production including where the last point of fill is, how the part fills based on the gating location, and the
parting line locations selected. These are critical items to address potential gas-trap areas and the resulting
need to use different venting techniques. Also critical is how the silicone part demolds. Because silicone is
elastic, removing the part from the cavity can be difficult. Unlike thermoplastic molds, LSR molds don’t typically
use ejector pins or core pulls because of silicone flashing. Silicone molds usually are comprised of two or three
plates. Molders are always looking for a feature that will give consistency to part location during the mold
opening. For designers, this translates into the need to use an undercut in the part or some other design
feature.

Note that what works for a 40-durometer silicone might not work for a 70- durometer silicone because the
materials have a different modulus of elasticity. The higher the durometer of the material, the greater force it
takes for the part to release from an undercut, making a retention feature more effective. However, when the
retention feature is too large, silicone materials are prone to hot tear, so there is a delicate balance to
maintain. Parts that don’t consistently stay in one location of the mold make automating extremely difficult.
Another consideration is how the silicone part shrinks. Silicone shrink rates are typically 2% to 3%, which is
fairly high relative to thermoplastic materials. Most material suppliers do not publish specific guidelines for
shrink rates of their materials and simulation software is not readily available or proven for silicone. What’s
worse is it’s typical to get a nonlinear shrink of the part, based on geometry. Therefore, determining the correct
shrink can be difficult. That’s why the need to prototype is so important to a molder and the need to work with
an experienced silicone processor is essential.

Choices pose challenge

Medical OEMs have more choices than in the past of molders for silicone projects. Because silicone is still
relatively unknown to many, it is easy to accept the first opinion that sounds like it makes sense.

However, the capabilities and experience of molders are varied. Silicone molders that have been in business for
years may not be using the latest techniques, while new companies might have the technology but not the
needed experience in silicone processing. Medical OEMs should look for companies that have a technical
background

in silicone, use the latest in technology, have a strong presence in the medical industry and can supply both
high-quality thermoplastics and silicones from one source of responsibility. This helps eliminate multiple
suppliers and streamlines the supply chain.

Finding the right mix in a molding partner takes a little research and a willingness to dive into the details but
this can make all the difference in a successful program launch.