Mold design—critical factor #5
Tips for a clean release 

Tips for clean releaseThe coefficient of friction of copolyesters such as Eastman Tritan™ copolyester is at its highest near the heat deflection temperature (HDT) of the material-and competitive materials generally freeze at a higher temperature than Tritan. Keep this in mind if you're repurposing a mold that was originally designed for the cooling temperature of another plastic. Parts molded with Tritan must be adequately cooled to withstand ejection forces during demolding.

Three mold design and maintenance factors can also help ensure a clean and efficient release: 

Draft—Part design features with minimal draft, such as long cores and deep ribs, often result in high ejection forces. The Eastman Design Services team recommends that all wall​ surfaces have a minimum draft angle of 1° in the direction of draw. 

Polish—Polishing mold cavity features in the direction of draw will reduce the coefficient of friction and result in lower ejection forces required. 

Mold steel coatings—Several mold steel coatings have been successfully used to reduce the coefficient of friction and required ejection force. Ask Eastman Design Services for a description of these coatings. 

NOTE: A textured surface may affect all three factors and should be considered when you're designing the mold. Working with Eastman Design Services early in the process can help reduce frustration and cost later.

TMI Tip




 
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设计适用消毒剂的医疗设备
Are the devices you design ready for aggressive medical disinfectants? 

The designer is the point where innovative creativity crashes head-on into the real-world needs of durable functionality in a health care environment.
 
Unfortunately, many device housings and hardware designed just a few years ago were made with materials that lack the right combination of impact strength and chemical resistance. They crack, craze, and ultimately fail when challenged by the increased use of aggressive medical disinfectants-combined with the applied stress of greater portability.
 
Connecting the dots between design flexibility, chemical resistance, and impact strength is critical for selecting the best material for reliable clear and opaque medical devices. That’s why Eastman created a 60-minute webinar to provide new information about: 

Patient safety and HAI prevention
      •  The cost of device failure
      •  The relationship between chemical resistance and impact strength
      •  A practical new 4-step testing protocol for comparing suitability of different material 

                                                

Use this valuable information to identify material liabilities earlier in the development process. It can help you make design adjustments before you bring a new device to market-and be more confident of its reliability and safety in the future.






 
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Mold design — critical factor #4
Gate style, location, and size  

Gate style location and sizeThe gate is where it all comes together in injection molding. Mold design and part design. The molten resin and the solid molded product. Aesthetics and mechanical performance.
 
Skillful decisions about gate style, location, and size early in the mold and part design process can pay big dividends when it is time to start molding parts. And the benefits pay off in greater part performance and reliability.

Overriding considerations—aesthetics and mechanical properties
Regardless of gating type, location, or size, these two factors will drive most decisions made regarding gating:
                       
美观性
The gate on an injection molded part leaves a "witness," or a vestige, where the part is separated from the runner system. This is considered an appearance defect and is typically hidden in an area of the part where it will not be obvious.
 
机械性能
In addition to causing cosmetic defects, the gate can affect mechanical properties of the resin. As the flow enters the gate at high pressures and temperatures, the gate can be a source of molded-in stress. Since it has the longest heat history (it is the last to cool), resin in the gate areas is far more susceptible to inferior mechanical properties compared to the molded resin out in the cavity. Therefore, the part surface in the gate area is far more likely to include defects that can behave as stress concentrations during tensile loading or drop testing.

Style of gate  
Customers who mold parts with Eastman Tritan copolyesters for medical and durable goods applications generally use one of these gate styles. We provide a brief description of each style here and will dive deeper into the pros and cons of each style for molding Tritan in a future blog. If you’d like to receive an email when this blog posts, contact us
     MANUALLY TRIMMED (requiring an operator to separate parts from runners during a secondary operation)
           •  Direct (sprue) gate—Commonly used for single-cavity molds where the sprue feeds directly into the cavity, filling it rapidly
              and symmetrically with minimum pressure drop
           •  Edge gate (standard)—Located at the parting line of the mold, generally filling the part from the side, top, or bottom
           •  Edge gate (tab)—Typically employed for flat and thick parts to reduce the shear stress in the cavity. Shear stress can be
              confined to the auxiliary tab, which is trimmed off after molding.
           •  Edge gate (fan)—A wide edge gate with variable thickness, permitting rapid filling of large parts or fragile mold sections
              through a large entry area
           •  Flash gate—Similar to the ring gate (below) but for straight edges. It consists of a straight runner and a gate across the
entire length or width. Frequently used with acrylics and flat designs where warpage must be minimized.

    AUTOMATICALLY TRIMMED (including features to facilitate breaking or shearing the gate as the molding tool is opened to eject
    the part)
           •  Hot runner (hot probe) gating—Used to deliver hot material through heated runners and electrically heated sprues.
              Material is delivered directly into the cavity, so there is no runner to trim. Valve gates are the preferred gating style for
              hot runner systems when molding Tritan to avoid sticking and improve part aesthetics and performance.
          •  Ring gate—Often used for cylindrical or round parts that have an open inside diameter. A ring gate is used when
             concentricity is important and a weld line is objectionable.
          •  Tunnel (submarine) gate—Often used in two-plate mold construction, this style features an angled, tapered tunnel
              machined from the end of the runner to the cavity.  

Location of gate  
Since the gate is a focal point for both inherent and applied stress, putting one in the wrong place can produce parts with inferior functionality and reliability. Generally, the gate should be placed so it will not affect the processing efficiency or structural integrity of the finished part.
    •  It should not be placed where internal molded-in stresses might relieve themselves over time.
    •  It should not be placed in areas that will experience high tensile loading in its end application. 

    GENERAL CONSIDERATIONS 
          •  Minimize flow length—Flow lengths can be minimized by locating the gate near the center of the mold. This reduces the
             pressure required to fill the cavity, optimizes wall thickness necessary for easy molding, and reduces part cost.
          •  Consider weld line (knit line) location—Although Eastman Tritan copolyester has relatively low-visibility weld lines, gate
location can determine where weld lines will form. This should be considered early in tool design.
          •  Minimize gate blush—Tritan is known for low occurrence of small gate blush, but gate design and location can be major
factors in preventing blush. Low-shear gates are essential, and edge gates can be used with a small transition distance for
             aesthetically demanding parts.   
          •  Apply good gate geometry—Gate geometry is very important to part appearance near the gate. Sharp corners or abrupt
features in the gate or runner may need to be radiused, and gate thickness may need to be adjusted. (We recommend
             gate thickness be no smaller than 1.65 mm [0.065 in.].) 

Size of gate  
   TOO SMALL
   When molding clear parts where appearance is critical, an undersized gate can result in excess shear heating and blushing
   on the surface. For example, Eastman recommends avoiding hot runner valve gates that are smaller than 1.02 mm (0.040 in.).
 
   TOO LARGE
   On the other hand, if the gate is too large in diameter, it makes the parts difficult to degate and often results in a mark or
   vestige that is unacceptable.
 
   JUST RIGHT
Eastman prefers to work with customers early in the process to find the "Goldilocks" solution for your specific needs. If you are
   molding Tritan in tooling designed for another material, we can help you determine how to change gate size to account for the
difference in viscosity. (HINT: A rule of thumb for polyester-based materials like Tritan is that the gate should be 50%-80%
   of the wall thickness of the part.)
 
For more information about gate design recommendations, contact an Eastman customer service representative or download a copy of the Eastman Tritan copolyester Processing guide










 
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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:
• 锁模力要求过高
  •   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


 
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