Cold runner systems, which bridge the molding machine to a part forming cavity, are a significant factor in determining the success or failure of an injection molded part. Optimally sizing a runner system provides several critical advantages:
- Optimizes filling/packing ability
- Minimizes sink mark/internal void potential
- Manages pressure drop
- Minimizes cycle time
- Reduces gate vestige/allows auto-degating
- Minimizes shear rates
- Reduces material usage
- Improves part quality
- Reduces warpage
The importance of a properly sized runner system is no mystery in the industry, but the steps to achieve it can be! The concept is simple: the gate normally freezes first– but not too quickly, followed by the part, and ideally, the runner freezes last. Preferably, the gate should stay open as long as possible, all the way up to where the part freezes. Note that this is sometimes limited by part geometry. The trickiest part of the process is having a runner that freezes quickly after the part to ensure the shortest cycle time.
Runner diameter plays a significant role: if it’s too small, it can freeze too quickly, creating too small of an effective flow channel to pack the part thoroughly. Plus, a small diameter runner will increase fill pressure. Size the runner too big and the mold just sits there, waiting on the runner to freeze, before ejecting and starting the next cycle. That’ll cost time and money.
Factors to Consider When Sizing a Runner System
There are many factors that must be optimized to achieve a properly sized runner system:
minimizing runner volume (scrap/regrind volume), minimizing fill pressure, and ensuring
adequate packing ability. The inverse relationship between these factors causes challenges in
the sizing process. If a smaller diameter runner is used to minimize scrap, the fill pressure
increases and the packing ability decreases. At Beaumont, we overcome this problem and provide our customers with an optimized runner system by utilizing our plastics engineering knowledge, tools such as Autodesk® Moldflow®, and many years of industry experience.
Sizing a runner can be a long and expensive process when not using the proper tools, like Autodesk® Moldflow®. Once steel is cut and runner related molding problems are found, it’s difficult to reverse the process. Restarting by cutting a new runner plate unnecessarily adds to tool costs and delays mold commissioning. Alternately, welding up the troublesome runner and re-cutting can compromise the structural integrity, functionality, and longevity of a tool. Every iteration of this process costs money in shipping, tooling modification, and TIME! Beaumont Development offers a runner optimization analysis that minimizes the potential need for runner modifications, with an added ready-to-machine runner file optimized for an injection molded part provided.
The Process
First thing’s first: identify the desired gate type, then optimize per nominal wall thickness and gate vestige specifications. Next, a minimum runner diameter is estimated as being 1.5x the wall thickness of the plastic part. This is a common guideline to help assure that the part can be adequately packed out. For the following, we will reference this as the PRD (or “packing runner diameter”). If a graduated runner is to be used, this PRD would be the diameter of the final branch connecting to the gate. Each progressive feed runner diameter, reaching back to the sprue, would be progressively increased.
The following formula is initially used to determine runner diameters in a graduated runner.
These are then verified via Moldflow® simulation. The initial d branch, in the formula below, would
be the minimum diameter runner branch that is connected to the gate.
- dfeed = dbranch x N1/3
- dbranch is the diameter of a downstream branch runner.
- dfeed is the diameter of the runner feeding the branch runner.
- N is the number of branches
The use of the graduated runner is often applied blindly without understanding its purpose or potential negative consequences. When considering filling, packing, and effect cycle time, a constant runner can often be more advantageous.
When deciding between a constant vs graduated diameter, it first must be determined what factor is controlling the size of the runner.
- A properly sized graduated runner will require less pressure to fill than a constant diameter with the same total volume of material.
- If packing controls the size of the runner (i.e. when the part can be safely filled when the entire runner system utilizes the PRD), then the runner should have a constant diameter.
- If fill pressure controls the size of the runner (i.e. when the fill pressure is excessive when the entire runner system utilizes the PRD), then a graduated runner should be considered.
- However, if the largest diameter primary runner section in a graduated runner controls cycle time, a constant diameter runner should again be considered.
Factors to Evaluate
Fill pressure tends to be at the front of the line when evaluating runner size based on the injection molding machine’s capabilities. Typically, the target is 50%-80% of the maximum machine pressure unless otherwise specified.
Packing ability is another critical factor– and is determined by interpreting multiple factors and how they relate to each other. Some of these factors include:
- Evaluation of relative packing ability of the runner system
- Identification of excessive thick or thin regions of the part
- Investigation of how the part geometry/feed system solidifies
Additionally, runner volume is an important consideration to minimize material cost. Excessive cold runner material is considered scrap and can end up being reground and again used for molding, discarded as scrap, or sold at a significant loss. All of these come at a cost. Runner sizing therefore is a balancing act of maintaining adequate volume to minimize fill pressure and maximizing packing ability, while also trying to minimize runner volume in order to minimize the scrap. Designing a runner system that adequately addresses each of these factors is critical to a successful injection molding application.
Although sizing runners is a relatively simple process, each situation is different and consequently needs to be individually considered for customer goals and concerns.
Don’t waste time and money sizing runners the old-fashioned way. Get in touch with Beaumont Development to get a quote on your next tool build! We can also optimize runner systems in existing production tool applications to minimize current molding issues. Interested in getting a balanced fill by adding our patented MeltFlipper® technology to your runner? Simply check the box on the request form.
