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Purchasing Basics: Hot runner systems

May 23, 2001

6 Min Read
Purchasing Basics: Hot runner systems

Hot runner technology will probably never take over the moldmaking business altogether, but, increasingly, molders are willing to pay the premium up front to have the benefits during the molding process. And there are definite benefits: 

  • Elimination of material waste from the sprue and runner system. 

  • A faster cycle time, largely related to the cooling cycle; the runner is often larger in diameter than the thickest part of the part wall, and thus is the slowest part of the shot to cool. 

  • Elimination of secondary operations like degating the sprue or grinding the scrap.

For many molders, these benefits outweigh the one-time additional cost of the mold components and the often somewhat touchier molding process. Especially because of the larger investment, it's important that the purchase of hot runner components and controls be carefully planned. Here are some guidelines to help you request a quote for hot runner equipment. 

The Part 
The best place to start is with the basics. Provide a description of the part to be molded. What size is it (length, width, height, wall thickness)? What does it weigh? What's the material, including filler type and percent, if applicable? Is it a heat-sensitive material that might require a lower-voltage system? 

What kind of colorants or other additives are involved that might be affected by temperature? Will color changes be important in this mold? Does the compound contain a flame retardant? Valve gates, for example, don't perform well when a flame retardant is present. A fully dimensioned part drawing should be provided. 

Related to the part itself is information on the cosmetic requirements the part will have to meet. Are there limitations where the gate vestige can be? Are you looking for minimum gate vestige or no gate vestige at all? (See the bottom of the page for a chart on gate design.) Will you want to gate on the part or on the runner? Must stringing be eliminated altogether? 

The Mold 
What are the general specifications of the mold? How many cavities? How many feed points? A general assembly drawing of the mold should provide the following details: 

  • Overall mold dimensions. 

  • Mold plate dimensions, including thickness. 

  • Part feed centers. 

  • Centers of all bolts and dowels. 

  • Mold heights. 

  • Any special requirements in the feed gate area. 

  • "Top of the mold" indicated. 

  • Dimensions of the spacers required on either side of the manifold that prevent the machine clamp force from being supplied to the bushing and manifold. 

  • Reference dimension from feed point to rear of bushing plate. 

  • Will the cavity layout in a multicavity mold fit a standard H, X, or inline I pattern?

The Machine 
To provide the hot runner system that most closely meets your needs, your vendor would like, if possible, to know about the machine in which you are planning to run this mold. If known, include the following: 

  • Make of molding machine. 

  • Hydraulic, toggle, or hydromechanical clamp. 

  • Clamp force. 

  • Space between tiebars. 

  • Machine platen dimension. 

  • Maximum mold height (on toggle clamps). 

  • Maximum daylight. 

  • Location ring diameter. 

  • Injection pressure. 

  • Injection speed.

The Application 
In addition to understanding the part itself and its cosmetic requirements, there are other parts of the process it is useful for your hot runner vendor to understand. He needs to know which of these factors are important. Is fast cycle time a preeminent concern? Will there be frequent color changes? Is the cycle likely to be interrupted frequently? Certain types of gates are better under those circumstances than others. 

Is the mold to be used for foam molding, gas-assist molding, or backmolding? Are there knitline issues that might be addressed by repositioning the flow lines? Will volume balancing among cavities be a problem? 

One of the issues to be addressed in specifying a hot runner mold is how maintenance can be performed. Do you want to be able to do maintenance while the mold is mounted? If so, you'll need to investigate modular designs in which heaters, thermocouples, power connections, and tips are replaceable. 

With our thanks . . .
. . . to hot runner suppliers Günther Hot Runner Systems, Incoe, and Plasthing Hot Runner Systems, for sharing material used in this article. 

Gate selection guide 

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