We have streamlined our rapid silicone prototyping process so that you will have samples shipped in 15 business days and have done this in as little as 3 business days. In order to get started, please send us your part drawing, 3D CAD model, and contact information via our RFQ page. When sending in 3D CAD models, please use Solidworks, Iges, Parasolid, or Step formats. We will review your project and provide a quotation within 2 business days.
We will review your project and provide a quotation within 2 business days.
What can you expect from a silicone prototyping company?
- Engineering support – Material selection, manufacturability, and a review of cost and time reduction opportunities.
- High-quality products manufacturing in a ISO 13485 certified Quality Management System.
- Engineering samples with every tool.
- Shipping of engineering samples in 15 business days after purchase order acceptance.
- Full material selection available and ability to work with most silicone materials including RTV, LSR, LIM, HCR, and custom materials.
- Full-color selection available.
- We can prototype in multiple durometers and colors.
What are the advantages of rapid silicone prototyping?
Prototyping answers questions such as feasibility, form, fit, and function1. It serves to identify materials, tolerances, and behavior. The methods for silicone prototyping typically balances quality, speed, and cost. These fall into a few major categories:
1. Mock up: This is simple representation such as using handcrafted parts such as clay or cut material slabs. Alternatively, 3d Printing of the part may represent ideas. This provides a basis for testing and refinement of individual concepts but leaves functionality and accuracy to be desired.
2. Rough/Working prototype: This typically starts to finalize materials and the general form by building a tool and molding samples. These are often used to develop a process and for testing functionality and design refinement.
3. Functional prototype: These are molded in the right material using tight tolerances in refined tooling to create dimensionally accurate parts. In medical these are often used for design qualifications and early-stage use.
By taking advantage of rapid prototyping, you can quickly conduct several different iterations resulting in real parts for measuring performance and analysis of the trade-offs, implement feature changes and benchmark the resulting performance for intended approaches. This can save time, but it also ensures that you make the correct design decisions. This allows you to:
- Gather more accurate requirements that reduce rework later in the development cycle.
- Technically understand the problem by building a functional prototype so that you can identify and address both the foreseen and unforeseen technical challenges.
- Settle conflicts in differences of opinions by helping your team to understand the product better. Designers, engineers, and specialists develop solid opinions about how a given feature element should be implemented or achieved. Differences of opinions result in conflicts.
Why we use the silicone prototyping process?
Rapid prototyping of parts for new product development helps to assure reduction in time and cost of the project. At Albright, we commonly create silicone prototypes at the working and functional levels but also provide support at mock up level. Quality System supports medical device prototyping, which tends to have greater record and traceability requirements required for populating and maintaining a design file. The silicone fabrication process generally flows from the idea to final products as shown in
Figure 1. Flow diagram of prototyping process.
You have options when it comes to creating silicone parts. Each with advantages and disadvantages. Table 1 provides a basic overview of molding methods used in prototyping and production silicone parts.
Table 1. Silicone Molding Method Comparison
|Prototyping Method||Description – Silicone Materials||Pros||Cons|
|Casting||Pourable materials at room temperature, typically RTV||Quick, communicates concepts||Poor Surface finish|
|Compression Molding||High pressure casting, used for RTV, LSR, LIM, HCR||Better quality compared to casting. Can be dimensionally correct. Surface Finish is achievable||Higher cost and more time than casting|
|Injection||LSR, LIM friendly||Fastest cycle, high repeatability, lower part cost at higher volumes||Longer implementation time, higher cost for tooling|
1. Choosing a Silicone Material
a.i. RTV – These are pourable for casting and can be self-leveling. The common starting point for prototypes. RTV materials generally have favorable thermal processing conditions for handling at lower temperature allowing for encapsulating electronics during early stage development.
a.ii. HCR – These have a high viscosity like clay or gum and generally require high pressure and special handling to mold even during the prototyping stage. Some of these materials have special properties that may be harder to find in other materials.
a.iii. LSR – These silicones have a wide range of processing capabilities including compression and injection molding. These often represent the middle ground for properties for many applications and are often scalable to high volume production processes. They tend to have the viscosity range between honey and cold molasses.
2. 3D Printing and Silicone
a. 3D printed molds can be used for casting silicone and provide a quick method to produce a few very low-quality parts to formulate a concept. These typically have high flash, poor feature definition, and tolerances2.
b. Direct silicone 3D printing has not yet been commercialized.
3. Design for Manufacturability
a. Research and Development versus Fabrication – Understanding the molding process and tooling is important to a successful product. We recommend talking with our engineering and technical account managers for answering those ‘can it be done’ questions. Projects tend to fit into two categories:
a.i. Research and Development, where prototyping methods are used to create a new product and have many of the following topics as open questions. These products may also be working in a blue-sky space that pushes the technology to the edge what has been previously done.
a.ii. Well understood and complete designs, where the design, materials, requirements and processes are established and understood. These have many of the following topics already addressed.
b. Silicone Material selection – processing ease, physical properties, shrink medical products have typically the greatest impact. We mold most silicone materials upon request and stocks a wide range for delivery of those quick turn projects.
b.i. Short and long term silicone implantable materials tend to have limited number of grades compared to more common commercial grades. The biocompatibility of silicones is favorable compared to most materials but grade dependent3-4.
b.ii. Silicone Materials on the high and low end of the durometer spectrum tend to be more challenging to mold. Large undercuts are often processed more easily in a lower durometer materials (with high elongation).
b.iii. Silicone Optical materials have very different processing behavior than typical silicone materials. Some have very poor tear strength and low elongation and are often prone to bubble entrapment compared to others.
c. Silicone part geometry drives cost and delivery time. These include complicating features such as holes, undercuts, long core pulls, etc.
c.i. Undercuts – These features lock the part into the mold or interfere with part removal. In smaller features with softer materials, some of these may be pulled out anyway due to the easy deflection and high elongation of the material while others require more sophisticated mold disassembly to remove the part.
c.ii. Silicone part wall thickness – Most products have a range where the quality is better. Very thin walls such as less than 0.010in (0.25mmtend to rip more easily during handling. Very thick parts such as greater than 0.5in (12.7mm) require longer cycle times due to longer heating and curing and molding quality challenges.
c.iii. Shut offs on silicone parts– Shut-offs are focused on where the mold will close or in over molding, where the tool interacts with the substrate to prevent flashing and control material location and formation.
Shutoffs drive flash placement.
c.iv. Long holes require longer pins in the mold that may deflect especially in more viscous silicones. A 10:1 maximum length to diameter ratio is a good reference point for most applications. Much greater ratios are possible but should be reviewed carefully.
Thru holes – These are holes that go all the way through a part and will typically have flash on the shutoff end.
d. Silicone Flash – Is a thin flap of material at the parting line where the mold splits. Silicone can flash at 0.0001in compared to a TPE flashing at 0.004 at exterior edge and holes. This is because the material is a low viscosity and builds up pressure during heating and curing. This causes pressurized silicone material to flow into even the smallest spaces. Flash may be minimized with favorable designs and better tooling and processing. Post processing of flash such as hand or cryogenics deflashing is possible in many projects for very tight requirements5-6.
e. Dimensional Tolerances of silicone parts are often over-specified in prototyping and these should be carefully considered. RMA A3 tolerances are a good starting point for most projects. There is inherent variability in materials and processes that lead to natural variance in final dimensions. Silicones have a relatively high thermal expansion. The silicone expands during heating and curing causing part shrinkage when ejected and cooled. Shrinkage varies from 1.5-4%7-10 based on geometry, processing, and material and material lot. This may be identified during rapid prototyping and corrections designed into the tooling to form tighter tolerance parts within the limitation of the materials.
f. The surface finish of silicone parts mimics the finish of the mold. The basic finish is typical to remove the cutter marks left from machining the mold.
f.i. Sandblasting for a matte finish or high polish for smooth or optical finish increases costs quickly during prototyping. We offer a wide range of surface finishes in-house for quick delivery.
f.ii. Silicone part release from the mold is eased or hindered by surface finish. Highly polished molds cause parts to stick more while sandblasting eases release of parts from the mold.
g. Medical silicone molding requires traceability and reliability. We have integrated ISO13485 quality requirements directly into all our processes from start through delivery.
g.i. Many molding projects benefit from clean room molding, which reduces the risk of contamination. This is especially important in medical and optical applications.
h. Rapid prototyping of silicone micro parts or micro features on larger part requires specialized handling. The definition of micro varies by industry but often requires a microscope to manufacture in order to fully see features or part detail and handle them. These require tighter tolerances tools to prototype and often have greater cost associated with handling. We have cut and molded features well into the 0.005in (0.125mm) range.
i. Optical Clear Silicones requires two components.
i.i. Clear material. Thin walls tend to be transparent in most materials but in order to have glass clear, an optical grade material is required.
i.ii. The second component is a highly polished tool, because a rough surface creates diffraction of the light preventing transparency in molded products.
j. Silicone Overmolding is the process of molding silicone to a base material or substrate such as metals, plastic and silicone.
The silicone is formed to shape, while in direct contact with the substrate. Liquid silicone is typically bonded to metal, thermoplastics and silicone. Silicone overmolding takes advantage of either mechanical bonding and chemical bonding, or both. A mechanical silicone bond takes place by physically creating undercuts and interlocks for the liquid silicone to “grab” hold of the substrate. Chemical silicone bonding silicone to silicone is the strongest bond is typically stronger and more reliable than mechanical bonding, since it takes place on a molecular level. The resulting chemical bond welds the cured silicone to the substrate material. At the prototyping level is often starts with material compatibility.
j.i. Bond ability to substrate may require testing.
j.ii. The dimensional tolerances of the part to be overmolded is critical to managing flash and tool fit.
4. Silicone Molding – Tooling Choices and Background
a. Number of parts required should be considered because it impacts the tool design, delivery schedule, and cost structure. A few parts for trial can often be done with the simplest tooling while higher quantities may require more complex tooling to reduce part cost and ensure tool life.
a.i. Pro tip – Communicating what you really need to the team molding your products tends to help get to the right number of cavities and the right tool design to meet your needs.
b. Silicone Compression Molding
b.i. Advantages – Typically very fast for develop parts. Can create dimensionally accurate parts in the right material and surface finish. The tooling cost tends to be lower than injection due to the flexibility of being run by hand.
b.ii. Disadvantages – This is a slower process and may have greater variability than injection depending on materials and geometry. Flash may also be more obvious in some geometries. Some projects may be more difficult to scale than others depending on requirements.
c. Silicone Transfer Molding
c.i. Advantages – Can produce production grade parts and is as repeatable as compression molding but can be at a faster rate due to the semi-automation of the molding handling.
c.ii. Disadvantages – Typically takes more time to develop than more basic mold types and has some limitations of speed. Materials still require significant handling.
d. Silicone Injection Molding
d.i. Advantages – Can create production grade parts that are very repeatable. Often has faster processing times resulting in a lower part price for higher quantities. This is typically very scalable.
d.ii. Disadvantages – Typically takes more time to develop and build a tool and process resulting in greater initial cost.
e. Steel vs Aluminum Molds for Silicone Molding
e.i. Steel offers strength and durability at a higher cost of time and money. Tool life and number of parts is generally higher. For prototyping, steel is seldom used due to cost and time for fist parts. Steel also slows change iteration.
e.ii. Aluminum offers speed and lower cost to develop. Many tools may be good for thousands of shots or more depending on geometry, parting line structure, process, and materials.
f. Cores create geometry in the part and with sufficient notice can often be changed within the same cavity to allow iteration through different designs. This offers designers the ability to test and validate different designs quickly and efficiently.
g. Pick apart silicone molding is a process involving an operator removing the part or the part and an insert by hand from the machine. This typically saves cost and time during early stage development and low quantity production.
5. Inspection of Parts
a. Commonly silicone products are over specified. A good starting point for designers is RMA A3 commercial. This provides a reasonable cost effective tolerance that is acceptable for many applications.
b. Inspection often includes visual at a set distance such as 12 inches or microscope at a specified setting.
c. Durometer is commonly referenced but testing is typically not sensitive enough to detect small differences accurately within a batch.
d. Pro tip – Regrind is not a standard in silicone processing due to the thermoset nature.
6. Secondary Operation of Silicone Parts
a. These include assembly or slitting such as commonly done for duck bill valves. prototyping tends to be a learning process at the start. Prototyping of secondary operations often serves to establish standards early on that lead to the better fabrication when expanding into production.
b. Common considerations include holding for jigs, wells for bonding, and features for alignment.
7. Communications with Customer
a. Communication is critical in rapid prototyping since the information is being developed and evolves between all parties.
b. Many rapid prototyping projects may not have fully developed specifications and so technical successes and challenges inform designers.
c. Managing changes in scope
c.i. Many projects have goals that may shift over time as information becomes available and priorities shift. Some changes may be simple to implement while others may require retooling, new materials, additional processes, and more resources to implement.
c.ii. Changes in scope impact pricing and success of completing goals.
1. Wikipedia, Prototype. https://en.wikipedia.org/wiki/Prototype
2. P. Chung, J. Heller, M. Etemadi, P. Ottoson, J. Liu, L. Rand, S. Roy, Rapid and low-cost prototyping of medical devices using 3D printed molds for liquid injection molding, Journal of Visualized Experiments, 2014, (88). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4208739/.
3. G. Polyzois, A. Hensten-Pettersen, A. Kullmann, An assessment of the physical properties and biocompatibility of three silicone elastomers, Journal of Dental Prosthetics, 1994, May, 71(5). https://www.ncbi.nlm.nih.gov/pubmed/8006847.
4. D. Chauvel-Lebret, P. Pellen-Mussi, P. Auroy, M. Bonnaure-Mallet, Evaluation of the in vitro biocompatibility of various elastomers, Biomaterials, 1999, February, 20(3). https://www.ncbi.nlm.nih.gov/pubmed/10030606.
5. Plastics Today, “It may be molding, but it’s Liquid”, September 30, 2004, https://www.plasticstoday.com/content/it-may-be-molding-its-liquid/6182642922797.
6. W. Lynch, Handbook of silicone rubber fabrication, Van Nostrand Reinhold, New York, 1978.
7. Wacker Solid and Liquid Silicone Rubber Material and Processing Guidelines, 6907_EN. http://www.wacker.com/cms/media/publications/downloads/6709_EN.pdf
8. Dow Corning, Rubber Processing Solutions: Liquid injection molding: Processing guide for Silastic Liquid Silicone Rubber (LSR) and Silastic Fluoro Liquid Silicone Rubber (F-LSR), Process. Guide LSR FLSR. (20012). https://www.dowcorning.com/content/publishedlit/45-1014_Processing-guide-LSR-FLSR.pdf
9. Bluestar Silicones, Healthcare Liquid Silicone Rubber – Injection Molding Guide, Healthc. Liq. Silicone Rubber – Inject. Molding Guide. (2011). http://www.silbione.com/wp-content/uploads/2014/02/LSR-Users-Guide.pdf
10. Shin-Etsu Silicone: LIMS Liquid Injection Molding System, Shin-Etsu LIMS Guide 2014. http://www.shinetsusilicones.com/files/literature/Shin-Etsu%20LIMS%20Guide%202014.pdf.
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