Cold runner systems

Sprue bushings
Sprue bushing connects the machine nozzle to the runner system. To ensure clean ejection from the bushing the bushing should have a smooth, tapered internal finish and have been polished in the direction of draw. The use of a positive sprue puller is recommended.  A cold slug well should also be included in the design. This prevents a slug of cold material from entering the feed system and finally the part, which could affect the final properties of the finished part.

The dimensions of the sprue depend primarily on the dimensions of the molded part in particular the wall thickness. As a guideline:

• The sprue must not freeze before any other cross section in order to permit sufficient transmission of holding pressure.
• The sprue must de- easily and reliably.

Sprue design.

Runner geometry
The ideal runner cross section is circular as this ensures favorable melt flow and cooling. However, it takes more effort to build circular runners because one half must be machined in the fixed mold part and the other half in the moving mold part. The higher the surface to volume ratio, the more efficient the runner.

Full trapezoidal channels in one of the two mold halves provide a cheaper alternative (see figure below). The rounded off trapezoidal cross section combines ease of machining in one mold half with a cross section that approaches the desired circular shape. The height of a trapezoidal runner must be at least 80% of the largest width.

Half round runners are not recommended because of their low volume to surface ratio.

Cross sectional properties for various runner profiles.

Runner dimensions
The diameter of a runner highly depends on its length in addition to the part volume, part flow length, machine capacity, and gate size. Generally they must never be smaller than the largest wall thickness of the product and usually lie within the range 3 mm to 15 mm. Recommended runner dimensions are provided in the table below. The selection of a cold runner diameter should be based on standard machine tool cutter sizes.

Maximum runner length for specific diameters.

While large runners facilitate the flow of material at relatively low pressure requirements, they require a longer cooling time, more material consumption and scrap, and more clamping force. Designing the smallest adequate runner system will maximize efficiency in both raw material use and energy consumption in molding. The runner size reduction is constrained by the molding machine's injection pressure capability.

Initially runner diameters can be calculated with the following formula below. Further fine-tuning can then be performed with the use of flow analysis software where effects such as shear heating and skin layer formation can be taken into consideration.

Runner layout
There are 3 basic layout systems used for multi-cavity systems. These can be catorigized as follows:

• Standard (herringbone) runner system
• "H" bridge (branching) runner system
• Radial (star) runner system
  

Unbalanced runner systems lead to unequal filling, post-filling and cooling of individual cavities that may cause failures like:

• Incomplete filling
• Differences in product properties
• Shrinkage differences/warpage
• Sink marks
• Flash
• Poor mold release
• Inconsistency
  
  

Example of unbalanced feed systems.

Although the herringbone is naturally unbalanced, it can accommodate more cavities than its naturally balanced counterparts, with minimum runner volume and less tooling cost. With computer aided flow simulation it is possible to adjust primary and secondary runner dimensions to obtain equal filling patterns. Keep in mind that non-standard runner diameters will increase manufacturing and maintenance costs.

Adjusting runner dimensions to achieve equal filling may not be sufficient in critical parts to prevent potential failures. Special attention is required for:

• Very small components
• Parts with thin sections
• Parts that permit no sink marks
• Parts with a primary runner length much larger than secondary runner length.
  

It is preferred to design naturally balanced runners as shown in the figure below.

Naturally balanced feed systems.

The "H" (branching) and radial (star) systems are considered to be naturally balanced. The naturally balanced runner provides equal distance and runner size from the sprue to all the cavities, so that each cavity fills under the same conditions.

When high quality and tight tolerances are required the cavities must be uniform. Family moulds are not considered suitable. Nevertheless, it might be necessary for economical reasons to mold different parts in one mold. The cavity with the largest component should be placed nearest to the sprue.

The maximum number of cavities in a mold depends on the total cavity volume including runners in relation to the maximum barrel capacity and clamping force of the injection molding machine.

Branched runners
Each time a runner is branched, the diameter of the branch runners should be smaller than the main runner, because less material flows through the branches and it is economically desirable to use minimum material in the runners.

Where N is the number of branches, the relationship between the main runner diameter (dmain) and the branch runner diameter (dbranch) is    

dmain= dbranch x N1/3

At all runner intersections there should be a cold slug well. The cold slug well helps the flow of material through the runner system by stopping colder, higher viscosity material moving at the forefront of the molten mass entering into the cavity. The length of the well is usually equal to or greater than the runner diameter and this is achieved by extending the length of the primary runner at the intersection with the secondary runner (see figure below).

Related subjects:
Runnerless molds / Hot runners

Recommended design of cold slug well or overflow.