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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Hot runner molding systems #4

Focus on gate design valve gate systems 


Valve gate systems are recommended for hot runner systems used to mold Eastman Tritan copolyester. Advantages compared with other hot melt delivery systems include:

        •  The melt channel is externally heated.
        •  Mechanical shutoff allows better gate vestige control.
        •  The gate size is generally large.
        •  The valve pin is retracted during the filling process, resulting in a less
           obstructed flow. The end result is less shear heating and pressure drop
           when processing Tritan. 

Early collaboration of all stakeholders to ensure efficient hot runner design and proper heat management of the mold and the mold cavity will pay lasting dividends when molding Tritan.  



Valve gate considerations for your hot runner mold

Gate size and aesthetics
Special care should be taken to ensure the valve pin seats well to ensure good contact. Even with adequate cooling and good contact, there are limitations with gate size. Gate sizes 3.0 mm (0.125 in.) and below generally result in the best aesthetics. Gates larger than this are often difficult to cool and result in poor gate aesthetics due to sticking.
 
Another factor affecting gate area aesthetics is crystallization. An Eastman technical service representative can help you determine if this will be an issue. 
 
Cooling at the gate
Drooling, sticking, and stringing may occur if the gate does not cool properly. Steel that is directly heated as part of the hot drop should not contact the part directly; it should be insulated from the cooled portion of the mold. An independent cooling circuit in close proximity is often suggested.  
 
(See Part 2 of this series for information about mold heating and cooling and recommended temperature guidelines for Tritan.) 

Another viable solution for temperature control is a water-jacketed insert. These are sometimes custom fabricated but are also available as standard items from some manufacturers. They usually result in a witness line around the gate, which may need to be considered.
 
The mold should be designed so that heat is quickly removed from the gate. This is best accomplished by the gate orifice being an integral part of the cavity steel, rather than the hot runner system being an insert.
 
When the gate is in the cavity, cooling channels can be incorporated to provide the cooling needed for the cavity in the gate area.
 
Plumbing this circuit independent from other cavity cooling channels can be beneficial, as separate water temperature control can be used to optimize molding performance in both the gate area and the mold cavity. 

For more information about hot runners, download the Eastman Tritan copolyester Processing Guide. Or talk with your Eastman technical service representative. 

In Part 5 of our hot runner series, we’ll look at a hot runner success story molding Tritan. 
Blog categories:
Many Medical hardware housing complaints. One new technology solution.

The increased use of aggressive cleaners, medical disinfectants, and disinfectant wipes is taking its toll on traditional plastics. Many handheld and bedside devices are becoming sticky, wearing thin in high-touch areas, or even cracking, crumbling, or shattering after only a few months of service.
 
The problem is that device housings that were designed just a few years ago are often made with materials that lack the right combination of impact strength and chemical resistance for today’s demanding medical environments.
 
In one of our most popular webinars, from November 15, 2016, we discussed the chemistry and stress behind these premature failures as well as how Eastman Tritan™ copolyester for medical housings is helping to prevent costly repairs and replacements. By replacing traditional housing materials with Tritan, you can improve patient safety and customer satisfaction. 

So in case you missed it—our medical device housing webinar is available on-demand.
  • Understand what is actually causing part failures and how to address it
  • Learn how to improve your medical device reliability, reduce repair costs, and extend product life
  • See test results comparing Tritan with traditional housing materials based on their ability to retain impact strength after exposure to common medical disinfectants and stress
WATCH THE WEBINAR NOW.
 
TMI Tip: When designing opaque housings for today’s medical environment, always consider both chemical resistance and impact strength. The combination of these two properties is critical for success.
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