Overmolding for soft-touch designs
New self-bonding LSR technology created to optimize Eastman Tritan copolyesters

The medical industry has a great and growing demand for innovative soft-hard designs in devices, housings, and other equipment. A recent advance in liquid silicone rubber (LSR) technology makes it easier to satisfy this demand with medical grades of Eastman Tritan copolyester. 

The advantages of Tritan are well-known to readers of this blog. Medical grades of Tritan offer a unique combination of properties including:
      •  Outstanding resistance to medical disinfectants and solvents
      •  Excellent impact strength and durability
      •  Made without BPA and halogens
      •  Excellent clarity and color retention after sterilization by ethylene oxide (EtO),
         e-beam, and gamma irradiation

Transparent and opaque formulations of medical grade Tritan also feature a lower Tg and require a lower processing temperature than some engineering polymers—a potential challenge to achieving strong in-mold adhesion with elastomers. 

Specially designed to optimize Tritan
Momentive Performance Materials, a leader in LSR solutions, recently introduced a self-bonding LSR technology specifically for one-step overmolding of silicone to Tritan medical grades. This is the first commercially available process of its kind and is tested against USP Class VI and/or ISO 10993 biocompatibility standards.
 
The Momentive Silopren* LSR 47x9 series provides strong in-mold adhesion with Tritan without the need for primers. It also cures rapidly at relatively low temperatures—hitting the sweet spot for achieving functional performance and efficient processing with Tritan medical grades.  
 
The combination is ideal for adding soft-touch designs for applications such as:
     •  Respiratory devices
     •  Sealing elements
     •  Gaskets for joints in housings and hardware
     •  Buttons and switches on electronic housings and hardware
     •  Vibration reduction
     •  Membranes and lenses for electronic device housings
 
The combination also demonstrates excellent adhesion strength, as summarized in this table. 

Adhesion performance of medical grades of Tritan with Silopren* LSR 4739 liquid silicone rubber
Engineering thermoplastic Peeling force (N/mm) 24h/RT Peeling force (N/mm) 4h/100°C
Tritan MX711 copolyester 6.7 6.8
Tritan MX731 copolyester 6.4 7.1
Tritan MX811 copolyester 6.6 7.2
Tritan MXF121 copolyester 6.0 6.5

For more information about how to optimize overmolding with Tritan medical grades, contact an Eastman Technical Service Representative. 


 
Mold design — critical factor #3

Fill pressure and fill pattern 

Fill pressure and Fill patternTo design greater manufacturability into molds—and parts—it’s critical to achieve the most effective fill pressure, to anticipate the fill pattern, and to predict volumetric shrinkage. Eastman Design Services uses mold-filling simulation to evaluate the “moldability” of the part design and engineering resin combination. 

Fill pressure and fill pattern go hand in hand
Just as mold design is inextricably linked to part design, reasonable fill pressure and reasonable fill pattern should be evaluated together. 





Reasonable fill pressure—excessive fill pressure can create several problems:
  •   High clamp tonnage requirements
  •   Reduced life of mold components—due to high stress loading
  •   High ejection force requirements, which can part deformation or breakage
  •   Temptation to raise melt temperature to compensate for high fill pressure. This can break down the molecular weight of the
      polymer and sacrifice some of the mechanical properties of Tritan. (See Mold design #2—Thermal control of cavity surfaces.)
 
When creating mold-filling simulations for Tritan, we start at 15,000 psi or less, before runner systems are added. This provides a form of “insurance” that allows for the additional pressure drop added to the overall system pressure will not exceed the molding machine’s capability to push the plastic.
 
Reasonable fill pattern—using mold-filling simulations can help determine the right fill pattern up front, and reduce costly modifications after tooling construction. Our simulations can identify potential problems such as flow front hesitation, air traps, and volumetric shrinkage—all of which can result in cosmetic defects and undesired stresses on the part. 
 






 
​These two images above show the identical part gated on opposite ends. Both are approximately 4x3 inches with a thickness of 100 mils. Both have an arm off the side that is about ½ as thick (50 mils).
   
        •  In the example on the left, the melt starts to move down the thick section, but when it hits the thinner section, it hesitates
           and freezes. After filling the rest of the part, it tries to backfill the arm, but cannot fill it efficiently, resulting in a short shot.
        •  In the example on the right, it fills the thicker section and flows right into the thinner section. 

 
To demonstrate an air trap, we show a plate-type application with a thicker outer rim and requires an aesthetic, unblemished surface across the face. You see the melt advancing faster around the rim, basically cutting off your ability to vent the face. With a highly transparent resin like Tritan, this will result in appearance defects.
 
We discuss possible solutions to these problems as well as the effect of volumetric shrinkage in a thick-walled part in our free webinar, Improving moldability through part and mold design
 
The pressure is on you to resolve problems early in the process.
It can be very costly to make changes after the mold is constructed. Eastman can use mold filling simulation to help identify fill pattern pitfalls and establish reasonable fill pressure early in the process—for fewer headaches when the pressure is on.
 
 
To see fill patterns animated, contact an Eastman Customer Service Representative. For more information on reasonable fill pressure and fill pattern see the dedicated pages at TritanMoldIt.com


 
Blog categories:
Polymer selection for durables applications
Selecting the best grade for food contact and non-food contact applications

Brand owners and design engineers continue to find new ways to use Eastman Tritan copolyesters in a wide range of food contact and non-food contact applications. 

Its unique combination of clarity, durability, hydrolytic stability, and heat and chemical resistance makes Tritan ideal for many applications, including:
Food contact applications
•  Commercial housewares
•  Sports bottles
•  Small appliances
•  Infant care
•  Water filtration

Non-food contact applications
•  Appliances
•  Leisure and safety
•  Ophthalmics
•  Oral care
•  Tools
•  In-mold decorations

Starting with your specific needs, Eastman can provide technical expertise and support to help determine the best grade of Tritan for your application.
 
Table 1 compares the properties of the three most popular grades of Tritan for durable goods. All three grades exhibit excellent clarity, durability, hydrolytic stability, and good heat and chemical resistance. In addition, Tritan TX1001 and Tritan TX2001 have a mold release derived from vegetable-based sources. TX2001 is excellent for thick parts. Tritan TX1501HF provides viscosity reductions of 40%–50% relative to TX1001 and is well suited for long flow length, in-mold decoration, and in-mold labeling. 

To learn more about Tritan grades for durables, check out the Durables section at TritanMoldIt.com or download Take your housewares from concept to countertops

Table 1. Eastman Tritan copolyesters—property overview
Physical properties TX1001 TX1501HF TX2001
Suggested application Appliances
Consumer goods Durable goods
Housewares
Small appliances
Appliances
Consumer goods Durable goods
Housewares
In-mold decoration
In-mold labeling
Appliances
Consumer goods Durable goods
Housewares
Small appliances
Specific gravity (ASTM D792) 1.18 1.18 1.17
Izod impact strength, notched @ 23°C (73°F), J/m (ft-lbf/in)
ASTM D256)
980 (18.4) 860 (16.1) 650 (12.2)
Flexural modulus, MPa (105 psi)
ASTM D790)
1,550 (2.25) 1,575 (2.28) 1,585 (2.28)
Elongation @ break (%) (ASTM D638) 210 210 140
Tensile stress @ break, MPa (psi)
(ASTM D638)
53 (7,700) 52 (7,500) 53 (7,700)
Tensile stress @ yield, MPa (psi)
(ASTM D638)
43 (6200) 43 (6200) 44 (6400)
Heat deflection temp. @ 0.455 MPa (66 psi), °C (°F)
(ASTM D648)
99 (210) 94 (201) 109 (228)
Heat deflection temp. @ 1.82 MPa (264 psi), °C (°F)
(ASTM D648)
85 (185) 81 (178) 92 (198)
Specific gravity 1.18 1.18 1.17
Thermal glass transition temp, Tg, °C (°F) 110 (230) 110 (230) 120 (248)
Clarity—haze % <1 <1 <1
Clarity—transmittance %  
90
 
91
 
92

Join us March 18–21 at IHHS in Chicago—we’re at booth S4875.


 
Mold design—critical factor #2
Thermal control of cavity surfaces

Thermal control is critical for injection molding Eastman Tritan copolyester. Keeping temperatures cooled below the heat deflection temperature (HDT) of Tritan helps ensure successful demolding with no dragging or sticking of parts. Preparing a cooling strategy early in the tool design process can pay big dividends in cycle time and processability. 

One key to keeping it cool—just add water.     
It’s not that simple. But providing ample and well-positioned cooling water channels is critical to controlling the temperature of the cavity surface and the resin in the mold. Here’s why controlling resin temperature is important: as the resin approaches HDT, it becomes sticky (the coefficient of friction increases) and parts cannot be ejected efficiently. 

NOTE: Excess heat over time also reduces molecular weight and degrades a polymer’s properties, as addressed in Mold design—critical factor #1

To produce the best low-stress parts with Tritan, steel surface temperatures should be kept between 130° and 150°F. For optimal temperature control when molding Tritan, cooling water circuits should be positioned near the cavity steel surface, following the contours of the molded part in a process known as “conformal cooling.”
 
To remove heat more efficiently, baffles and bubblers may be used to generate turbulent water flow within the core steel.
 
In areas of the cavity where the part geometry makes it difficult to place cooling lines near the molten plastic, Eastman often recommends an alloy with a higher heat transfer coefficient. An alloy such as steel/copper can extract excess heat more quickly and help improve good part quality and appearance. 

Being a good gatekeeper—with special attention to the gate area  
Injection molding gates typically have the highest heat load in an injection mold. Therefore, the area around the gate requires the greatest capacity to extract heat quickly. Some considerations to keep in mind:
•  Run the cooling circuit as close to the gate as possible.
•  A water-jacketed gate often works well.
•  For a hot-gated system, we recommend a valve gate that uses a mechanical shutoff rather than a thermal shutoff.  
•  Consider using an independent water supply for the gate area. This lets you control gate water temperature independent from cavity cooling circuit—and can help improve processability and gate aesthetics.

Keep the tool cool beyond the cavity surface.
In addition to the cavity surface, be aware of all high-temperature spots that can lead to sticking—even in small areas of the mold. Designers should make sure there are ample cooling channels to address areas such as:
       •  Core pins
       •  Thin steel areas
       •  Areas near sprue and hot runner channels
       •  Insulation around hot runners

Involve Eastman early—and keep us in the loop. 
The Eastman Design Services group can be a great asset during the tool build. We can review your plans and make sure your mold will give you the best results from Tritan.
 
Questions about temperature control and cooling techniques? The Tritan Experts and the Eastman Design Services group are ready to help you find the answers.






 
Blog categories:
Why specify medical grade polymers
Two words: confidence and compliance
Image of Tritan Medical Device

It is critical for designers of innovative medical devices and packaging to have access to high-performance medical grade materials.
 
To ensure patient safety and long-lasting reliability, manufacturers depend on medical grade material suppliers like Eastman—not only to provide advanced, high quality raw materials but also to have capabilities and systems that help comply with medical protocols and regulations. 

Manufacturers who specify medical grades of clear or opaque Eastman Tritan copolyester* know that Eastman will provide a high level of support throughout the regulatory journey to commercialization of a new product. Eastman’s world of experience helps ensure confidence and compliance in the areas of:
 
•  Biocompatibility—selected tests from the FDA-Modified ISO-10993, Part 1 “Biological Evaluation of Medical Devices”
•  Sterilization—test data for EtO, gamma, and e-beam sterilization methods
•  Quality systems—including current Good Management Practices and traceability processes
•  Dedicated regulatory support—global regulatory support to ensure compliance
•  Regulatory statements and Product Regulatory Information Sheets (PRIS)
•  FDA Drug Master Files
•  Quality systems and cGMP—complies with the requirements of ISO 9001 for the design, development, manufacture, and supply of polymers 
•  Application development and technical service—including but not limited to:
–   Educational webinars and customized lunch and learn sessions
–   Design recommendations
–   Material FFU criteria and specific performance testing
–   Competitive materials analysis or ID
–   Physical property testing
–   Aging and sterilization studies
•  Other supporting servicescontact your Eastman customer service representative
 
Why specify Eastman for your medical grade polymers? 
 Eastman has proven to be a reliable global supplier of raw materials—even proprietary materials like Eastman Tritan copolyester, which has no competition in the marketplace. Eastman has demonstrated leadership not only in innovation and business continuity but also regulatory support and processes that help customers take products to market with greater efficiency and confidence. 


*In addition to several medical grades of Eastman Tritan copolyester, Eastman offers medical grades of Eastar copolyester, Eastman Eastalite copolyester, Ecdel elastomers, and Tenite cellulosics. 

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