The gate is a key component for the filling of the part from a tooling point of view. The gate is not used as a balancing tool. This is usually determined by the runner. The gate geometry should not be adjusted until you have generated data. The data includes but is not limited to: gate freeze time, pressure drop, and shear. The gate should be identical in multicavity tools. The gate is composed of three things: land, height, and width. This provides necessary information for overall effect on the processing window.
Land
The land is the distance from where the runner stops to where the part starts. This length should be as short as possible, and in most cases is only 0.040” to 0.060” in length. The pressure drop across the gate will become greater as the land length dimension is increased. If the gate is properly sized but the land area is too long, there will be a pressure drop. Take a typical 3 lb. polycarbonate part that was not completely filled. How large should we open the gate? The original land length was 0.125” in length; you will only need to reduce the land length to approximately 0.055”. A simple measurement with a caliper is easiest. You can also do short shots to find pressure drops across the gate. If the land length of the gate is altered, the gate freeze time will also change due to the cooling rate and flow characteristics through the gate.
Height
This is the thickness’ of the gate and may be used to determine gate freeze time. It is usually recommended to start at 40% to 60% of the wall thickness for the height of the gate. This ensures that the gate will freeze off prior to the part, thus letting the screw recover while the part continues to cool.
Width
The width is how wide the gate is at the part. A starting point would be at 40% to 60% of the wall thickness. If you determine that you need more volume of material into the part, you can increase the width without increasing gate freeze time.
When both the height and width are the same size, they are equal in determining gate freeze time. Whichever dimension is effected that dimension will determine gate freeze time. Also note that there are many formulas to determine the actual size of the gate. Practical knowledge and prior experience of the tool maker, molder will also help determine gate geometry.
Height/Width
When looking at the height and width of the gate, we need to determine the gate freeze time. If the gate is freezing off too fast, it will usually cause inadequate packing. Short shots, and sinks will be obvious. If it is not freezing off soon enough, there will be problems of size repeatability and quality problems. When actual gate freeze time is determined, multiple data points are generated. This will be used to determine to add more pack time or pressure based on that data. The physical measuring of the gates will help with, relative viscosity analysis, pressure drop and balance of the fill. If the height or width of the gate is increased, greater volumes of flow of material will allow shear rate by increasing the material viscosity. Additionally, the time for the gate to freeze will be increased. The real key is to base the modification of the gates on data. Viscosity curves, pressure drops of flowing materials through a gate will help. If you are not on the flat portion of the viscosity curve and the gate is opened the material viscosity can increase enough to create more problems. If the pressure drop through the gate in not significant in relation to the runner and part then changing the gate geometry may not help at all in filling the part.
In conclusion, this information may seem basic but is very important. Think of the gate as it actually is; an opening to let the appropriate material into a part. Too big will result in uneven fill while too little will result in short shots and sinks.
Dallas Cada is a highly trained plastics engineer with over 20 years of sales support experience. Owner of a plastic consulting business (DDC Consulting), his experience includes technical service, application development, market engineering, injection molding, design, tooling, material suggestions and problem solving for plastic manufacturing companies. For more information with troubleshooting plastic problems or helping with new plastic applications, contact Dallas Cada by e-mail at dallascada@charter.net. Contact Dallas by phone (507) 458-5785 or
(507) 452-1584 or www.ddcconsulting4@webnode.com.
