Rapid Prototyping with Medical Grade Silicone: An Overview of Material Options and Processing Methodologies

Matt Bont, Albright Technologies

Why Silicone?• Good compression set• High temperature resistance• High elongation• Softness• Good bio-compatibility

Materials General Overview

• Long-term implantable grade• Short-term implantable grade < 29 days• Skin contact only• Liquid silicone rubber materials are

typically two parts formulations, commonly mixed 50/50, part A and part B

• Colorant dispersions are typically added at a rate of 2%

Silicone Applications

Wide range of device applications including strain reliefs, gaskets, seals, diaphragms, thermal/electrical insulators, cardiac catheters, valves, drug delivery devices, punctal plugs, ocular implants, dental devices, orthodontic devices, laparoscopic lenses, medical device handles, and equipment keypads

Silicone Prototyping

• Feasibility testing for development and early stage marketing– Is silicone the right material for the function?– Can this geometry even be made? And demolded?– Which material should be used?– Is the combination of geometry and materials

appropriate? – Is the geometry and wall thickness appropriate?

Silicone Prototyping

• Design validation/scale up– Validate design

• Confirm tolerances and specifications for final design reviews

• Production capability• Feedback for testing performance

– Clinical trials– Produce marketable components to use during scale

up and early production

Materials and Prototyping: RTV• RTV – Room Temperature Vulcanization• Advantages

– Typically low temperature cure in minutes to hours– Low viscosity, pourable– Often available in small kits

• Disadvantages– Many grades have poor properties– Not well suited for injection molding or scaling up

• Applications– Coating/encapsulating of heat sensitive components

• Electronics

– Pourable molding and casting• 3D molds

Materials and Prototyping: LSR• LSR – Liquid Silicone Rubber is Injection and Compression

molding friendly• Advantages

– At elevated temperatures cycle is in seconds– Viscosity is relatively low – Properties are competitive

• Disadvantages– Generally not pourable

• Applications– Compression and Injection molding– Most device applications

Materials and Prototyping: HCR• HCR - High Consistency Rubber• Advantages

– High tear strength– High modulus– Good compression set

• Disadvantages– Highest viscosity– Low elongation

• Applications– Valves, strain reliefs, gaskets, and seals– Some high performance applications– Legacy designs

Silicone Prototyping Methods

• 3D printed casting• Compression molding• Transfer molding• Injection molding

3D Casting• Casting tooling is often

Synthetic ABS or Polyetherimide

• Advantages– Inexpensive– Quick trials of complex

geometry

3D Casting (Continued)• Disadvantages

– Material may not represent end material

• Limited to RTV and some LSR materials

– Limited dimensional accuracy

– Excessive flash– Surface quality is limited by

the printer resolution– Short tool life (1-20 parts)– Long cycle time (>20 min)

Compression Molding• Aluminum or steel tooling• Material is overfilled and excess

is displaced as the geometry is created

Compression Molding (Continued)

• Advantages– Dimensionally accurate– Use of final material selection possible– Rapid material changes and cleaning– Simpler tool construction that can be

easily modified– Complex parts are possible– Inexpensive to setup

Compression Molding (Continued)

• Disadvantages– Cycle time is typically in minutes

depending on complexity of mold and material cure characteristics

– Operator intensive– Limited number of cycles

• Tens to thousands – May not represent production process

for validations– Flash may be harder to control than

Injection molding

Magnified edge

Transfer Molding• Slug is compressed

by mold action and transferred into cavity

• Shot size control by weight or fluid dispenser

• Most practical for HCR or thicker LSRs

Transfer Molding (Continued)• Aluminum or steel tooling• Advantages

– Favorable for HCR processing– Dimensionally correct– Simple material changes

• Disadvantages– Requires additional setup– Limited process controls– Operator intensive

Injection Molding• Aluminum vs. Steel – Production quantity, strength,

durability, and cost.• Primarily LSR

– Less commonly HCR.• Injection molding often most represents final process

– Feasibility• Gating• Parting line

– Validation and scale-up• Cold deck• Part removal

– Robot– Sweeper– Compressed air

Injection Molding (Continued)

• Advantages– Long tool life, especially in harder

materials• Thousands to millions

– Higher production rate, in seconds– High repeatability– Test parting line, injection location,

and handling processes– Develop manufacturing processes

and procedures for operators– Build quantities of parts to transition

into production line startup

Injection Molding (Continued)

• Disadvantages– Greater initial tooling complexity and cost– High processing temperature required– Limit of substrates due to processing conditions– More time to setup – die setting and processing– Part removal requires operator or automation– Time consuming material changes

• Purge vs. clean barrel, screw, static mixer and pumping unit

Tooling

• 3D printed plastic tooling• Aluminum• Steel

Aluminum Finish: No

Cutter Marks

Aluminum Finish:

Sand Blast

Finish: 3D

Printed Tool

Molding Substrates• Bonding

– Primers– Surface treatments – corona and plasma– Self-adhesive and adhesive promoters– Temperature– Inhibition

• Metal– Aluminum– Steel– Titanium

Molding Substrates (Continued)• Plastic

– Limit Heat Deflection Temperature or Vicat Softening Temperature

– Inhibition– 3D printed inserts

• Temperature restrictions• Cure inhibition

– Materials• Bonding – ABS, Nylon, Peek, Polysulfone, and

polycarbonates • Poor or no bonding to – Polypropylene and

polyethylene

• Silicone– Post cure vs. bonding– Chemistry alignment

Questions