The feed system,
The injection of material from the machine into our cavity must first flow though some type of feed system. For those that are new to molding or the process think of the plumbing system of your home or apartment. We have our source of water from the city that connect to the meter, which after the meter feeds our home, the water than can be gotten by turning on the tap in the kitchen or the bathroom or an out side faucet, along with what we supply to toilets, showers and other sinks or appliances within our unit. The point is we have a piping system that supplies us with water by turning on a faucet (gate). Further we hope to have the same flow at each of those points along with adequate pressure.
Within our mold we have a similar distribution, in that we flow through a sprue, runner and sub-runners to get to the gate. Key here is that we usually fill all at the same time. With our feed system we can have various types, cold runner, insulated runners, and hot runners. This issue I will try and describe the cold runner type.
Layout design of feed:
A cold runner is a runner (feed) that is ejected from the mold each shot and than reground or thrown away. This type of system is often cut into the mold base, and we may have various configurations of the cross section. The key is that we use the runner system (feed) to supply material at the same flow rate into each cavity of our tool.
The overall design of the runner system will depend on cavitations, type of mold, and whether the design is naturally balanced, pressure balanced, or not.
- The Or Not is when we have a tool and keep adding the cavities without regard to how we flow material into our cavities, similar to I have room in the mold for another cavity lets add it. We just need full parts. Creates the most problems I have run into in trying to process good parts.
- Naturally balanced means that the feed of material to each cavity is the same as it relates to diameter and distance of the feed (runner), from the sprue to the cavity and is in a path that is similar in turns and angles.
- Pressure balanced means that someone has done a pressure drop and modified the diameter of the runners to feed various cavities hopefully at the same rate.
Please note that none of the above mention types of designs takes into effect the shear and stress we induce into our flow of material which well be another topic onto itself.
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The lid and box would be presured balanced in the above example, by changing diameter of runner.
Cross section design
The full round runner cross section is the best, yet when we cut it into the mold we have to cut it into both half's of the tool thus it may or may not be your first choice.
Trapezoid runner is a good compromise but the modified trapezoid is even better. With this runner we can cut it into one half of our mold so no matching and or mismatching occurring cutting down on the flow, and or surface area of the OD of our runner.
The ˝ round runner and flat runner are ones to avoid. Why, because the pressure drop through these can be tremendous and the surface area of the OD of the runner compared to the cross -sectional area of flow is too high of a ratio. What you need to think about here is that plastic fountain flows, or better said flows through the center of its cross sectional area. Further when we come into contact with the steel of the tool we cool or form a skin on the plastic, thus our channel of flow is now smaller than when we started, and the skin continues to get thicker as time goes on limiting our flow channel and causing us to use pressure to push the material through this ever smaller diameter.
If you notice from the drawing above the we have a circle in each cross sectional view, this is the material flow, (yes it may vary but not much) so this is what we look at. Further the circumference of the various shapes as compared to circle within is what is key.
KEY:
1- Supplying material to each cavity within our mold at the same flow rate.
2- Having a balanced flow system.
3- Remember that the runner is a volume flow. When opening up a runner please do it based on volume / cross sectional area.
a. Example
i. 3 mm diameter runner = 7.068 square mm
ii. 2 mm diameter runner = 3.14 square mm
iii. 1 mm diameter runner = 0.785 square mm
b. Too many times people just open up the runner without considering the area and end up with wasting material, and cycle time (waiting for runner to cool).
4- WE can always make the runner diameter bigger
a. Start small
b. Use past experience,
c. Watch relationship of runner diameter to wall thickness.
5- Vent the runner
a. Eliminate air from the runner first before it enters the cavity
b. Runner vents should be deeper and or shorter on land length so they can exhaust this air prior to putting that air into the cavity and fighting the burn and or other issues.
6- Use the available tools, mold flow, hand calculations, prior tools and most important your experience or someone else's
