Jason Nelson, New Product Development Tech Center Manager, ProMed Molded Products, Inc

1. ProMed Molded Products
Designing for Success
(Silicone Molded Components)
Designing a silicone component can be a challenge when
trying to balance design for manufacturability and the
optimum design for end use.

2. 3 Key Rules for Success
1. Know your companies capabilities and know the capabilities of the materials you
work with.
2. Always understand the specifications and requirements of any new project before
starting development work. (Build in the voice of the customer)
3. Always align the specifications and requirements with your capabilities before
starting development work.

3. Material Selection
(LSR) Liquid Silicone Rubber
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Consistency of honey
Better flow for complex part geometry or thin walls
Faster cycle times
Usually no secondary post cure needed
(HCR) High Consistency Rubber
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Consistency of clay
Difficult flow for complex geometry or thin walls
Slower cycle times
Usually requires secondary post cure

4. LSR Material Mixing
Mixing and formulating LSR material is typically done using a planetary mixer or through a
pumping system and static mixer.

5. HCR Material Milling
Milling and formulating HCR material is typically done using a Two Roll Mill

6. Colorants
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Silicone Colorants are offered in an array colors including white, black red, orange,
yellow green, and blue.
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Colorants can be custom formulated into silicone at low or high percentages based on
desired color.

7. Material Selection
Manufacturability Tips and Aids
• The most common hardness range for silicone is from 20 to 80 Shore A durometer.
For manufacturability purposes we recommend using between 30 to 70 durometer
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Fully automated molding, higher durometers work best
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Complex part geometry or thin walls, LSR material works best
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Measurement accuracy and repeatability, higher durometers and colored parts work best

8. Materials
Shrink Rate, General Rule
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Liquid Silicone Rubber (LSR) 2.5% to 4.0%
High Consistency Rubber (HCR) 1.5% to 3.0%
Variables that play a role and can affect shrink
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Material, Lot to lot variation
Material, Durometer
Material, Additives and Colorants
Material flow
Process
Gate, Vent size

9. Dimension and Tolerance
ProMed’s standard dimensional policy, on any given dimension, is 2.5% or ± .003”, whichever is
greater. To ensure project success, it’s best to consider using the maximum possible tolerance
for each specific component.
Note: the R.M.A Handbook calls out a tolerance for “A1” high precision Silicone components to
be a minimum of +/-.004” and states that depending on material selection, part geometry and
molding method this may not be attainable.

10. Dimension and Tolerance
Manufacturability Tips and Aids
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Prototype when possible
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When working with a tight tolerance, build the mold steel safe and plan a mold
modification into the project timeline
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If a dimensional feature can be made a reference, then its best to make it a reference
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Indicate critical or inspection dimensions on the part drawing

11. Visual Inspection Criteria
The criticality of visual inspection criteria can have an effect on how parts are manufactured
and cost.
Typical inspection callouts:
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Inspect at 18” inches with the unaided eye having 20/20 vision under normal work space
lighting.
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Inspect at 10X magnification

12. Visual Inspection Criteria
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Material (Raw Silicone) directly from the supplier can have up to .010” in foreign material.
This will end up fully embedded which would be detected under magnification.
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Additional material filtering can be added to the process, but is not always 100% effective on
very small particulate such as fiber.

13. Visual Inspection Criteria
Recommendations to Aid in Manufacturability When Possible
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Notes for reject criteria should always be specific and measureable, for example, embedded
foreign material can be a maximum of .020” x .020” in any direction.
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A note saying nothing is allowed, for example “no nicks, cuts or voids allowed” is difficult to
quantify and should be avoided.
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Inspect at 18” inches with the unaided eye having 20/20 vision under normal work space
lighting.
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Adding a colorant can make a part easier to inspect and improve accuracy of measurement.

14. Flash Extension
The amount of flash extension allowed on a part can make a difference in cost, when
secondary operations are needed
Using the below part for example
With a .004” extension = $1.15
With a .005” extension = $.71
With a .008” extension = $.56
With a .010” extension = $.53

15. Flash Extension
There can be more than one flash extension specification on a part
Example: Maximum flash extension not to exceed
(zone #1 and #3 .010” inch)
(zone #2 .005” inch)

16. Tooling
Mold construction is critical, LSR material will flow into a gap of .0001” during the
injection and hold phase.
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Building a tight mold becomes more difficult when shutting off around core
pins or inserts and when uneven parting lines are present. In these cases tear
flash or dump rings can be used.
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On a tubular shaped part, dump rings can be added to part ends and plucked
away leaving any flash left behind below tolerance.

17. Tooling
Tear flash
Dump Rings

18. Tooling
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Surface finish of mold cavities will have an effect on the appearance of the molded part.
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A polished cavity will produce a clear appearance where an EDM cavity surface will produce
more of a matte appearance.
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For manufacturability purposes it’s good to note that silicone parts are easier to remove from
an EDM finish than a polished finish.
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ProMed’s recommended cavity surface finish callout is 32 Micro-inch Max

19. Tooling
Manufacturability Tips and Aids
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Its good to note all sharp corners depicted on drawing may exhibit up to a .005” maximum radius
condition.
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Use hardened 420SS whenever possible
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Add plating to mold surface
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When possible, gating should be located on a thicker and non-critical area of the part

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At ProMed, we like to work closely with our customers
on the front end of a project offering our knowledge
and expertise in silicone processing and best design for
manufacturability.
Your upfront homework will have more of an impact
on success than anything you can do in development.