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.






 
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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. 
Mold Design- critical factors #1
Factor #1—Polymer melt residence time

Mold design goes hand in hand with part design to determine the manufacturability of a product. One factor that is critical to manufacturability—as well as to preserving the performance characteristics of the material being used—is polymer melt residence time.
 
In this blog, the first of a five-part series discussing critical factors of mold design, we answer these questions about melt residence time: 
      •  What is melt residence time?
      •  How does it affect the integrity of a polymer?
      •  How does this relate to Eastman Tritan copolyester?
      •  What is the residence time recommendation for Tritan?

Melt residence time—know when enough is enough (and why) 
The polymer melt residence time is defined as the total time the resin is in the molten state required for injection molding—from the time it is melted in the injection unit’s barrel through to the mold’s runner system and main cavity.
 
Unreasonable melt residence time can reduce the molecular weight of polymers and affect their desirable performance characteristics. 
NOTE: Be sure to consider the size of the injection unit when calculating total residence time. When possible, pair the mold and injection molding machine to prevent excess residence time being used up in the machine’s barrel before the polymer reaches the mold. While this is best practice, it is not always practical for custom molders who produce parts out of multiple molds. 

How residence time affects molecular weight 
While the resin remains heated in the molten state, there is a tendency for the molecular weight to decrease. The loss of molecular weight is not visible in the molded part and is only detected by sophisticated testing. Finished parts may look fine aesthetically only to have their loss of integrity revealed in reduced performance and fitness for use.
 
For example, customers select Eastman Tritan copolyester for the advantages it provides in chemical resistance and impact strength. Both properties can be compromised by reduced molecular weight. Therefore, it is important to retain as much of the original molecular weight of Tritan as possible.

Recommended melt residence time
Reasonable melt residence time for Tritan is 5 minutes or less. Good mold design, combined with part design and efficient processes, can help achieve this goal—and help ensure performance from your molded parts.   
 
Brand owners invest a lot of time and energy selecting the material with the right combination of performance attributes. Proper melt residence time is one way for tool designers and molders to ensure the desired performance will be delivered by the finished molded part. 
 
Questions about melt residence time? The Tritan experts and the Eastman Design Services group will help you find the answers. 



 
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Selecting Medical Grade Polymers
Making the grade—selecting the right medical grade polymer

Eastman understands that the stakes are high when selecting the material for your next fluid management component, blood contact device, or electronic medical device housing. That’s why we want to help make this complicated process simpler when you consider Eastman Tritan copolyester, beginning with two criteria:
•  Making the grade. Selecting medical grade polymers helps ensure your device complies with quality and biocompatibility standards—and that you have reliable support for regulatory approvals.   
•  Matching the application. Each medical grade of Tritan offers a different combination of desirable strengths that help ensure performance in specific medical applications.  

Matching a medical grade of Eastman Tritan copolyester to your needs

Different medical applications have different performance priorities. For example, clear applications, such as fluid management and IV components and dialyzer housings, place a high value on:
•  Clarity (for unobstructed viewing of fluid levels and foreign substances)
•  Durability and toughness (to withstand the applied stress of handling, connecting, and fabricating)
•  Chemical resistance (compatibility with cleaning and bonding agents as well as harsh drugs and their carriers)
•  Color stability after sterilization
•  No black specks
•  Biocompatibility (compliance with FDA/ISO 10993 and USP Class VI biological evaluation)
•  Heat resistance—Some devices require a higher level of heat resistance to deliver their intended performance or for accelerated aging validation.
•  Specific rigidity requirements to comply with ISO 80369 (required in some connectors)
 
On the other hand, opaque medical housings demand a durable combination of chemical resistance and impact strength to resist cracking, breaking, and premature device failure during rigorous daily use and disinfection. They also may require color stability and even true matching of a brand’s color palette.
 
Starting with your specific needs, Eastman provides a range of medical grades of Tritan as well as technical expertise and support to help determine your best material option. Eastman is a reliable partner in problem solving to help bring your product to market as seamlessly as possible.
 
Here is an overview of each of our medical grade polymers and a table with more details of the properties of each polymer. 
 
Eastman Tritan copolyester MX711 is our most commonly preferred medical grade. It sets the standard for most products that require: 
•  Impact strength and toughness
•  Excellent clarity
•  Chemical resistance
•  Good color stability upon sterilization by gamma irradiation or EtO

MX711 is also made without bisphenol A (BPA), BPS, or ortho-phthalates and offers improved processability compared with traditional copolyesters.
 
Eastman Tritan copolyester MX731 offers many of the same advantages as MX711 but with 40% to 50% lower viscosity. Its exceptional flowability is ideally suited for high-cavitation molds or designs with long plastic flow lengths.
 
Eastman Tritan copolyester MX811 offers similar properties to MX711 but adds greater value for products requiring higher Tg or higher heat deflection temperatures (see Table 1) for downstream product-validation testing.
 
For opaque electronic medical device housings, Eastman Tritan copolyester MXF121 offers good impact strength and significantly better chemical resistance than polycarbonate (PC) or PC alloys.  

Table 1. Eastman Tritan copolyesters—property overview
Physical properties MX711 (clear) MX731 (clear) MX811 (clear) MXF121 (opaque)
Suggested application IV components Medical devices Medical devices and housings Electronic medical device housings
Specific gravity (ASTM D792) 1.18 1.18 1.17 1.19
Izod impact strength, notched @ 23°C (73°F), J/m (ft-lbf/in) 980 (18.4) 860 (16.1) 650 (12.2) 416 (7.5)
Flexural modulus, MPa (105 psi) 1550 (2.25) 1575 (2.28) 1585 (2.28) 1748 (2.53)
Elongation @ break (%) 210 210 140 133
Tensile stress @ break, MPa (psi) 53 (7700) 52 (7500) 53 (7700) 47 (6780)
Tensile stress @ yield, MPa (psi) 43 (6200) 43 (6200) 44 (6400) 43 (6200)
Heat deflection temp @ 0.455 MPa (66 psi), °C (°F) 99 (210) 94 (201) 109 (228) 94 (201)
Heat deflection temp @ 1.82 MPa (264 psi), °C (°F) 85 (185) 81 (178) 92 (198) 83 (181)
Specific gravity 1.18 1.18 1.17 1.19
Thermal glass transition temp, Tg, °C (°F) 110 (230) 110 (230) 120 (248) 106 (223)
Clarity—haze % <1 <1 <1
Clarity— transmittance %  
90
 
91
 
92
 
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NOTE: MX710, MX730, and MX810 offer properties similar to MX711, MX731, and MX811, respectively,
but without a mold release. Learn more

 TMI Tip

 
Troubleshooting focus: Sinks and voids
Sink marks appear as surface depressions (seen as dimples or grooves); voids are less obvious and may appear as bubbles in clear parts. Both compromise aesthetics and can reduce the confidence a customer has in a part—but the impact is much greater on applications such as fluid management, where leakage from a Luer or stopcock can result in loss of sterility and contamination.

In opaque medical device housing applications where aesthetic is critical, the matte finish and pastel colors that are often used make sinks and voids obvious.   

Sinks and voids are caused by localized shrinkage of the resin at thick sections during the following steps:   
  1. When excessively heated material expands to fill the mold cavity, it results in excess space between the plastic molecules.
  2. The skin of the material in the mold solidifies (freezes) first.
  3. As the remaining resin core cools and shrinks, it pulls the solidified skin with it away from the main mold wall.
  4. If the skin is sufficiently stiff, core shrinkage may not cause surface deformation but a void can form within the core of the resin as it shrinks.
Here are some of the most common causes of sinks and voids and corrective actions.

Possible cause 1—Insufficient packing
The proper amount of pressure held for the proper amount of time helps ensure consistency throughout the mold while the molten resin cools and solidifies.
Corrective actions:
•  Increase packing time and/or pressure.
•  Check gate, sprue, and tip dimensions to make sure their size is adequate.
•  Check part dimensions (packing thick sections through thin walls).
 
Possible cause 2—Excess wall thickness 
Corrective action: Reduce thickness if possible.
 
Possible cause 3—Hot spots in mold 
Corrective action: Improve cooling.
 
Possible cause 4—Injection speed too fast
Corrective action: Reduce speed to allow more uniform fill and pack.  
 
Possible cause 5—Melt temperature too low
Corrective action: Check and adjust temperature upward if needed. 
 
You can see how Eastman uses mold-filling simulation to predict the fill pattern of a proposed part design by opening the “Reasonable fill pattern” and/or “Eliminating areas of excessive shrink” in the Medical Part Design section of TritanMoldIt.com.

If you have additional questions about sink marks and voids in your parts, talk with your Eastman technical service representative—and ask how to receive a free copy of our Injection Molding Troubleshooting Guide

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