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:
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.
Mechanical properties
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. 

          •  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  
   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.).
   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.
   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