Hot Runner Selection

By: Trevor Pruden D-M-E, Mechanical Engineering 

Considerations when thinking of incorporating a
hot runner system 

Market trends are continuing to drive toward the use of hot runner systems. Like many technologies, hot runner systems were created out of a need to do things better, faster and at a lower cost. Cold runner systems result in a wasted runner every time the mold opens. When molding small parts, a high-cavitation cold runner tool produces significant material waste, cutting deep into the bottom line for the molder.  

The expected material cost savings is often the primary driver for switching to a hot runner system. Lead time reduction is another factor. Molders and moldmakers are becoming increasingly sophisticated in the technology and processes they use, generating more interest in hot runners.  

While hot runner systems have significant benefits, it is always important to look at each individual application, and consider part size, aesthetic requirements, production quantities, material, additives and more to ensure the proper and most cost-effective molding method.   

Hot and cold runner choices and what to expect from each 

Cold runners

The benefit of a cold runner is its ease of use. It also can be beneficial if dealing with certain aesthetic requirements. Cold runners can reduce jetting in parts such as clear acrylic or polycarbonate used for the transmission of light. Jetting can produce ribbon-like effects in a part that may become visible.  

In addition to material waste, cold runners’ downsides include added steps to the process through automation and/or labor. Cold runners must be removed from the parts while in the mold requiring a robot or person physically separating the part from the runner. Depending on the size of the runner, scrap material can amount to hundreds of thousands of dollars, especially if using expensive material such as polycarbonates. With molders’ profit margins squeezed to the max, hot runners are often a money-saving solution.  

For higher cavitation systems, cold runners may be undesirable. In these cases, the sprue mass can exceed the combined mass of the molded parts. When this happens, the cooling time for the sprue exceeds the cooling time for the molded parts, leaving molders at the mercy of sprue cooling as a cycle time driver – an undesirable processing condition to be in. Hot runners address this issue and can effectively increase the total number of part cavities since the sprue is kept molten throughout the process.

Cold Runners:

·  are ideal for family molds where gates remain attached

·  can have fast delivery times

·  are good when molding symmetrical parts

·  can be low in cost to produce

·  can be more sensitive to running semi-crystalline materials

·  have relatively low maintenance

·  are flexible with gate locations

·  work well with edge gate requirements  

·  have a limit on the maximum number of cavities

·  are well understood and easy to use 
 

Hot Runners

Eliminating the need for ejecting a large secondary cold runner is important when simplifying mold action as well as reducing part waste. The key remains keeping the melt molten throughout the entire runner system so that it doesn’t solidify until it hits the cool walls of the part cavity.  

Internally heated

Internally heated hot runner systems were the first wave of hot runners to the market, emerging in steps.  

The first solution developed for cold runner problems was to heat the sprue bushing, producing the most benefits for the production of small parts. The next method developed was a distributor tube system. With this technology, major runner legs are heated, and the molten resin moves from there to the unheated vertical legs, called drops, prior to passing through gates into the part cavity. Around the same time as the development of the distributor tube system, another group of molders started placing heated probes into each drop leg, fed by an unheated insulated runner. Since insulated runners were hard to keep running “on cycle” due to freeze-off at the gates, the key was to keep the resin molten while passing through the gates. These three features were later combined to create an “internally heated” hot runner system. 

The internally heated runner system may be more complex than a cold runner system but it allows increased processing flexibility for more difficult resins and reduces resin waste. Due to the annular geometry of the flow paths, internally heated hot runners work well with resins that are not overly sensitive to shear. Amorphous materials also lend well to this method, with the exception of materials that are residence time-sensitive, such as polycarbonate. 

To this day, some molders remain stalwart users of internally heated hot runner systems, or even cold runners, due to the higher cost and experience needed with an externally heated hot runner system.   

Externally heated

The externally heated hot runner started as a steel block, machined with an internal passageway for molten resin. Heaters affixed to the outside  perimeter of the steel block heat the manifold from the outside in, enabling an even heat inside the block. Melt is then distributed throughout the heated manifold block, fed into a heated nozzle, then sent into the final gate well or “bubble,” just prior to passing through to the part cavities. 

The benefits of a hot runner system are clear. When designed properly, the system will achieve maximum mold-processing capability while effectively eliminating resin waste per injection cycle, a “win-win” solution. At D-M-E Company, we recommend hot runners whenever possible because of the huge material savings alone, not to mention the other efficiencies they bring to a molder’s process.  

Some tips: It’s difficult to produce a perfectly flat thermal profile from the entrance of a hot runner system to the tip of each nozzle assembly. Some hot runner systems get close, but there are times resins require a drop in temperature right at the gate to prevent drooling or stringing. Also, flow rate and injection speed have great effects on resin processing, as well as on color changes. Hot runner system designs that work perfectly with one resin, but not another, are not uncommon. A small tweak to the thermal profile will usually correct these problems. No matter what hot runner solution a designer chooses, the intended resin and molded part application should be considered.   

Hot Runners:

·  lead to significant material cost savings

·  support lean manufacturing

·  serve high-volume needs

·  are good for molding with multiple, identical cavities

·  create lower labor and automation costs

·  increase production speeds

·  increase process efficiencies

·  lower material recycling costs

·  are still in the early stages of full understanding for
    some molders

·  are ideal for high-cavitation molds 

Each runner system, ranging from unheated “cold” runners to externally heated “hot” runners, has its unique benefits and drawbacks. Hot runners require a greater initial investment, but also produce greater efficiencies. The process and speed improvements can pay for a system in just a few months, making it an attractive option for molders.  

A subsidiary of Milacron, Inc., D-M-E Company is an essential mold technologies resource to customers worldwide, offering the plastics industry a broad range of products including hot runner systems, control systems, mold bases and mold making and molding equipment and supplies. D-M-E also manufactures and sells standard tooling for the die-casting industry. Visit www.dme.net for more information.

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