How the cold runner system can work wonders!

What is a Runner System?

Runner system is defined by molten plastic flow path from the sprue of the injection molding press to the cavity gate. The runner system incorporated in the mold without insulation is called a cold runner system. The hot runner system consists of heated and insulated flow path from sprue of injection molding press to the gate or cold runner.

Either hot or cold runner system is an integral part of the injection mold/tool. Hot or cold runner system is selected during the tool design. The selection of type of runner system depends on several factors including cost, time and maintenance. The selection of runner system can be very cost effective. The runner system change is very difficult and costly.

Cold runner system is the conventional and most commonly used type of runner system. The cold runner system is created by cutting a channel in the mold/tool. The cold runner system reduces the cost of mold/tool. The runner system design is based on the type of mold and number of cavities. Following are the commonly used cross sections for cold runner

Types of Runner Systems

Round runner

Most commonly used and most effective cross section for cold runner system. Material flow through the circular cross section minimizes the material loss and pressure loss in the cold runner system the round cross section is equally divided in the core and the cavity blocks. Any miss match or imperfections in the round cross section results in flash and pressure loss during the injection cycle. Round runner provides the greatest proportion of polymer in molten state. Also most easy to come out of the mold/tool during ejection.

Trapezoidal runner

Trapezoidal runner is cheaper to construct than the round runner. Trapezoidal runner is constructed by cutting channel in the core size of the mold/tool. The cavity side of mold/tool does not need any matching features, this reduces the risk of pressure loss and flash at the runner. Molten polymer material flows in circular shape even if the cross section of the runner is trapezoidal. As the molten polymer flows in the circular shape encompassed by the trapezoidal cross section of the runner. Compared to circular cross section material wastage is higher.

Modified trapezoidal runner

This is a mix between the circular and trapezoidal cross sections. The cross section is on the core side of the mold/tool, hence cheaper to construct and easy to eject. Half of the cross section is semicircular and half is trapezoidal. Reduces material waste than the trapezoidal runner.

Semicircular runner

The cross section is on the core side of the mold/tool, hence cheaper to construct and easy to eject. Semicircular runner does not provide the greatest proportion of polymer in molten state. Compared to circular cross section reduced chance of flash at parting line, but material waste in the runner is higher.

Effective runner size

Advantages

  • Cheaper to produce and maintain
  • Accommodate a large range of thermoplastics
  • No impact on color change

Disadvantages

  • Cycle times are slower
  • Plastic waste from runners
  • Difficult to balance for more than 4 cavities
  • Flow across the cold runner system

Diagram

Cold runner systems are usually in the order of 4, as the split of molten polymer flow can be better predicted, controlled and use the tool space efficiently. The molten polymer flow is circular and cooler as it progresses towards the gates. Ideally length from sprue to the gate should be each gate should be same. As the number of cavities increases above 4 the molten polymer becomes unbalanced reaching to the cavities. The molten flow as it splits from sprue to secondary runner the high temp molten polymer flows is off center. Some cavities get higher flow and pressure which causes imbalance while the filling the cavities. The cause of imbalance can be explained as below

Runner tree

Runner Tree Diagram

Cold runner systems are usually in the order of 4, as the split of molten polymer flow can be better predicted, controlled and use the tool space efficiently. The molten polymer flow is circular and cooler as it progresses towards the gates. Ideally length from sprue to the gate should be each gate should be same. As the number of cavities increases above 4 the molten polymer becomes unbalanced reaching to the cavities. The molten flow as it splits from sprue to secondary runner the high temp molten polymer flows is off center. Some cavities get higher flow and pressure which causes imbalance while the filling the cavities. The cause of imbalance can be explained as below

SPRUE cross section thermal gradient

The molten polymer flows through the runner the high temperature polymer stays in the center of the cross section. The temperature of molten of polymer reduces and becomes solid as the polymer comes in contact with the steel. The core of the molten polymer core tries to stay in the center till the runner flow path is straight.

(Diagram)

 

 

Primary runner cross section thermal gradient

As the flow is split from sprue to primary runner; the high temp in the center of sprue is also split but the high temp of the molten polymer is moved off center.

(Diagram)

 

 

Secondary runner cross section thermal gradient

(Diagram)

 

 

 

 

 

The main characteristic of cold runner system the molten plastic cools as the flow splits from primary to multiple flow paths. As the flow has high temperature at the center of the sprue is split in to multiple flow paths. The core high temperature is shifted off center. The high temperature is associated with high viscosity. The plastic flow from sprue to the gate is restricted as each turn reduces the flow velocity and pressure. The cold runner contributes to significant portion of pressure drop in the required total injection pressure. Each time the molten polymer flow changes direction or split the molten flow pressure and temperature reduces.

Symptoms of incorrect cold runner system

  1. Unbalanced cavities
  2. High requirement of injection pressure

Possible solutions

A. Gate resizing: The off center high temperature due to split in the flow to multiple divisions. The off center high temp can contribute to cavity imbalance. Adjusting gate size can be an option to balance the melt flow between the cavities. The gate size can be increased for cavities with least material. But this method also increases the total injection molding pressure.

B. Melt flipping: The off center high temperature can be resolved by method called as melt flipper. The melt flipper is a patented technology used to reduce off center high temperature. The melt flow in the cold runner system gets multiple turns in the straight path. This method increases the length of the cold runner. The increase in the length of cold runner increase in the total injection pressure and loss of material

(Diagram)

C. Constant cross sectional are: by maintaining cross sectional are of the primary to the total cross sectional area of secondary runner total injection pressure losses can be reduced in the runner system.

Conclusion

In plastic injection molding part, tool design and process settings are unique. Multiple solutions can be offered during the mold/tool validation and trouble shooting. Multiple possible solutions are available for each issue.

The cold runner system design is critical as most of the concerns related to cavity balance, pressure loss and loss of material. If designed correctly the cold runner system can be very effective. The cold runner system is almost maintenance free.

About the author:

Mr. Tushar Naik (RJG trained Master Molder)

Tushar Naik is a mechanical engineer with international experience in plastic injection molding. He has more than 17 years of experience in research and product development. A mechanical engineer by education and trained by RJG, he has worked with organizations and individuals from USA, CHINA, MEXICO, TAIWAN, KOREA, JAPAN, and most of EUROPE, in diverse industries like medical, surgical, automotive, consumer audio, professional audio, consumer appliances and consumer products. He has successfully worked on 100+ products and several patented technologies.

He started working as an intern in a custom injection molding company while doing research of injection molding process. He has worked with scientific molding technology since 2004. He shares his knowledge of scientific molding to save time and money for the plastic injection molding professionals.

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