Published on February 22, 2014
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.
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.
Material Selection (LSR) Liquid Silicone Rubber • • • • 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 • • • • Consistency of clay Difficult flow for complex geometry or thin walls Slower cycle times Usually requires secondary post cure
LSR Material Mixing Mixing and formulating LSR material is typically done using a planetary mixer or through a pumping system and static mixer.
HCR Material Milling Milling and formulating HCR material is typically done using a Two Roll Mill
Colorants • Silicone Colorants are offered in an array colors including white, black red, orange, yellow green, and blue. • Colorants can be custom formulated into silicone at low or high percentages based on desired color.
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 • Fully automated molding, higher durometers work best • Complex part geometry or thin walls, LSR material works best • Measurement accuracy and repeatability, higher durometers and colored parts work best
Materials Shrink Rate, General Rule • • 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 • • • • • • Material, Lot to lot variation Material, Durometer Material, Additives and Colorants Material flow Process Gate, Vent size
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.
Dimension and Tolerance Manufacturability Tips and Aids • Prototype when possible • When working with a tight tolerance, build the mold steel safe and plan a mold modification into the project timeline • If a dimensional feature can be made a reference, then its best to make it a reference • Indicate critical or inspection dimensions on the part drawing
Visual Inspection Criteria The criticality of visual inspection criteria can have an effect on how parts are manufactured and cost. Typical inspection callouts: • Inspect at 18” inches with the unaided eye having 20/20 vision under normal work space lighting. • Inspect at 10X magnification
Visual Inspection Criteria • 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. • Additional material filtering can be added to the process, but is not always 100% effective on very small particulate such as fiber.
Visual Inspection Criteria Recommendations to Aid in Manufacturability When Possible • 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. • A note saying nothing is allowed, for example “no nicks, cuts or voids allowed” is difficult to quantify and should be avoided. • Inspect at 18” inches with the unaided eye having 20/20 vision under normal work space lighting. • Adding a colorant can make a part easier to inspect and improve accuracy of measurement.
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
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)
Tooling Mold construction is critical, LSR material will flow into a gap of .0001” during the injection and hold phase. • 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. • On a tubular shaped part, dump rings can be added to part ends and plucked away leaving any flash left behind below tolerance.
Tooling Tear flash Dump Rings
Tooling • Surface finish of mold cavities will have an effect on the appearance of the molded part. • A polished cavity will produce a clear appearance where an EDM cavity surface will produce more of a matte appearance. • For manufacturability purposes it’s good to note that silicone parts are easier to remove from an EDM finish than a polished finish. • ProMed’s recommended cavity surface finish callout is 32 Micro-inch Max
Tooling Manufacturability Tips and Aids • Its good to note all sharp corners depicted on drawing may exhibit up to a .005” maximum radius condition. • Use hardened 420SS whenever possible • Add plating to mold surface • When possible, gating should be located on a thicker and non-critical area of the part
• • 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.
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