Cold Runner Design – Attention to Details, Part I

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We previously talked about Flow Group ID’s and how they relate to the pressure drop equation (Figure 1) to help identify root cause of mold filling and part quality variations. Using Flow Groups help separate out the root causes into either steel or viscosity variations.

Figure 1

Figure 1


The term “steel variation” is a catch-all phrase used to describe any source of variation not related to viscosity variations. Below are a few common sources of steel variations:

• Gate land
• Venting
• Gate diameter
• Cold slugs
• Machining of the runner
• Hot runner temperature distribution
• Wall thickness (core/cavity dimensions)
• Hot runner manufacturing

This tech tip will focus on one particular source of steel variation that shows up in both two-plate and three-plate molds, and is actually designed into the mold! We are talking about “puller pins (sucker pins), sprue picker stems, and vacuum pads”. These features aid in removing the runner system from the mold, and/or the gate from the part. Figure 2 is an illustration of a gate puller pin used in a three-plate mold design.

Figure 2

Figure 2


The placement and overall design of these features will impact the mold performance, flow balance, process window, and overall part quality. Our recommendations when designing these features into your molds are as follows:

1. Design puller pins such that they do not restrict the main flow path
2. Where appropriate, add the particular feature to both sides of the sprue and at each side of the preceding intersection

Figure 3 below shows a short shot of a runner from small 32-cavity, three-plate mold. As indicated by the red lines/arrows, one half of the mold is filling before the other half. Can you figure out why (no, the root cause is not gravity in this case)? How would you fix it? What other design improvements can be made?

The general perception is “it’s only the runner, so who cares?” But as with anything, the devil is in the details. And those details are often overlooked and misunderstood. A runner design that has been engineered for plastic flow needs to consider these details and a long list of others.

As you begin to understand plastic flow and the variables in the pressure drop equation you will begin to understand how small differences can have a big impact on the molding process and part quality. After all, the plastic that makes up the parts has to travel through the runner system. And what the plastic experiences along that trip will influence the final destination…the molded part!

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