AIM Processing Small Plastic Parts Blog

In plastic injection molding, the “gate” is the opening in a mold through which the molten plastic is forced into the cavity, the tooling representation of your part. The type of gate (which I’ve blogged about previously) and its location are critical. In fact, it’s no exaggeration to say that gate location can make or break a part. The type and placement of the gate can affect many aspects of a project including:

• Structural issues including the overall strength of the part
• Ability to hold your desired roundness or flatness
• Operational considerations like what injection pressure is needed
• Aesthetics such as the appearance of sinks, voids, and flow lines

Even so, many mold designers give little thought to where the gate will be positioned. It’s true that they have many other aspects of the mold to consider in their design, but nevertheless, gate location must be high in importance on the list of considerations.

Better Parts through Proper Gate Placement

 Here are some gate location tips to keep in mind as you work with a designer or mold maker.

• Avoid obstructions. Cores, pins, and sidewalls can impede the flow of plastic. Place gates away from severe obstructions but near some features that will resist the flow a little so that you don't face an excessive pressure drop when your part begins to fill.

• Seek strength. The area around a gate is subjected to significant stress during the injection process. Place gates at a thick section of the part to optimize part packing and help prevent sinks and voids.
• Consider function and aesthetics. Again, keeping in mind the stress associated with gates, position them where they won’t affect how the finished part looks or functions.
• Prevent “gate pucker.” If your part will use a plastic that has a high shrink rate, it may pucker near the gate location if there is high molded-in stress at the gate. Keep this in mind in your design.
• Design for degating. Your gate location can affect how easily a part can be degated from the runner (both manually and automatically). Consider that process as you develop your design to help save you manufacturing cost.
• Roundness and flatness.  If roundness or flatness is a critical requirement, tread carefully.  Placing the gate on one side may save tooling money, but is a virtual guarantee that you will struggle against stress imbalances that will warp your part.
• Consider two gates. With some parts, you may have to have two gates to ensure proper filling. Don’t hesitate to add a second gate if appropriate, but be sure to keep proper gate location in mind for both gates and consider where the two flows will meet.  This meeting place can cause cosmetic problems and weakness.
• Minimize flow marks. The longer the flow path in a part, the greater the chance of flow marks. Position your gate to keep that path as short as possible.
• Get a second opinion. If you feel a suggested gate location may not be optimal, be sure to ask the opinion of another plastic injection molding expert. Better to correct the design now than deal with imperfect parts later.

Two Decades of Insight on Mold Designs

In our more than 20 years in business, we’ve seen a wide range of mold designs and the finished parts they produce. Consequently, we bring a great deal of insight to the table when discussing a customer’s plastic injection molding project; specifically the mold design.

Have a new project coming up? We’re happy to share our expertise on gate location and other aspects of the design. Give us a call.