Canadian Plastics

Taking the heat off hot runner selection

A well-designed hot-runner system can save material, energy, time and improve part quality. It also typically adds 10 to 20 percent to the cost of the mold. For many projects that can be hot-runner mo...

September 1, 2000   By Michael LeGault, editor



A well-designed hot-runner system can save material, energy, time and improve part quality. It also typically adds 10 to 20 percent to the cost of the mold. For many projects that can be hot-runner molded, the higher up-front cost is often the biggest barrier to the purchase of a hot runner system.

Ken Kurtz, product manager, hot runner systems, D-M-E Company, estimates that about 30 percent of the molds built in North America have hot runners. He says molders and moldmakers need to think about the payback they will receive on the investment when deciding whether or not to purchase a hot runner system.

“There is a total cost scenario for both cold-runner molding and hot-runner molding,” says Kurtz. “The payback on some hot-runner purchases can be quite quick. It depends on the project.”

Hot runner cost justification

Nanda Kramer, applications engineering manager, Dynisco HotRunners, recommends using a worksheet to forecast one’s return on investment for a hot-runner system purchase. A worksheet developed by Dynisco helps customers figure their ROI for a hot runner by taking into account factors such as material costs, estimated scrap rates, molding and machine conditions, electricity rates, cycle time savings and costs to run auxiliary equipment.

In one specific example, the worksheet is used to calculate the cost savings resulting from converting a 16-cavity cold runner mold for an acetal gear to a hot-runner tool with 16 drops and 4 control zones at an estimated cost $39,000, which includes the cost of the temperature control unit. The gear is molded from a material costing $1.50/lb. Each gear weighs two grams and the production run is 650,000 parts per year, with an estimated lifetime for the mold of five years. The calculation shows that the most significant cost savings from hot-runner molding the gear are due to material savings on scrap, a reduction in cycle time (from 35 to 27 sec.) and a reduction in labor costs to run grinding equipment. Total estimated annual savings for switching to a hot-runner tool is $47,777, with an ROI of 9.8 months. Dynisco’s web site (www.dynisco.com) currently allows customers to select hot-runner systems, and will soon be equipped with software that allows users to design their own.

The actual amount of cycle time reduction achieved by conversion to hot-runner molding depends on the number of cavities and the cross-sectional area of the cold runner, says Kurtz. “Parts with large cold runner cross sections take a long time to cool. By converting to hot-runner molding you can significantly reduce cycle time for these types of parts, because you can gate directly into the cavity and eliminate the need for cold runners.” He also notes that the elimination of cold runners reduces mold fill time, which in turn is another factor in shortening cycle times. D-M-E’s technical service department will help customers evaluate the benefits and payback on a hot-runner system for a specific molding application. D-M-E’s website (www.dmeco.com) allows customers to select and order hot runner systems and other equipment.

Selecting a hot runner system

Once a decision has been made to use hot-runner molding, successful implementation hinges on selecting the right hot runner system for the application.

According to Martin Baumann, marketing manager, hot runners, Husky Injection Molding Systems Inc., there are a number of criteria to consider when specifying a hot-runner system. These criteria include the type of heater system, balanced material delivery to the drop, proper channel sizing, gating method, and material type and throughput.

A processor should use externally-heated hot runners for materials that are heat sensitive, such as polycarbonate. The most efficient way to ensure material is delivered equally to each drop is to have a balanced manifold with equal flow lengths and channel sizes from the point of injection to each cavity.

Gating method and channel size depend in great part on the type of material being processed, Baumann reports. For heat-sensitive materials, the channels should be on the smaller side to minimize residence time of the materials in the hot runner system; for shear-sensitive materials the channels should be larger to reduce shear. Baumann suggest using a mold flow software program to optimize channel size.

Husky has launched a dedicated hot runner website (www.hotrunners.com) which includes a web-enabled version of the Husky Hot Runner Design Center, featuring design guidelines and downloadable CAD files.

With off-shore competitive cost pressures increasing, buying a hot-runner system that is both high performance and economical is becoming more important to customers, says Louis Herbert, sales and marketing manager, Hasco Canada. A molder or moldmaker can save one-half to one-third the cost of a new hot-runner system by buying off-the-shelf components and doing the machining, wiring and assembly themselves, says Hebert. Hasco supplies over 50 standard manifolds and kits containing components and directions for assembly of a complete hot-runner system. The company also provides in-plant technical support.

“We are in a position to help the customer,” says Hebert. “Some shops may want to machine the plates and let us do the assembly. We can work with the customer’s requirements.”

Hebert stresses that the first consideration in the selection of any hot-runner system is the application for which it is to be used. Part geometry, material selection and processing conditions may require customized components, as well as technical expertise and time beyond the capabilities of a shop to comfortably manage. In this case Hasco can provide a complete hot runner system designed to the customer’s specifications.

Mold-Masters has launched its Internet-based system, Merlin, that customers can use to design, order and track their hot-runner system. According to sales director Scott Molnar, Merlin allows a person to configure and specify an entire hot runner system on-line, interactively. Context-sensitive help is provided throughout the design process. Additionally, Molnar says, Merlin is the only hot-runner design site that lets the user view and download CAD drawings over the internet. The VRML viewer does not require any CAD system on the user’s end.

The design process begins by keying in the material and required shot weight. Merlin then logically steps a person through the process of selecting the system’s components. A person can download 2-D drawings of the individual components they have selected. The program lets you create “what-if” scenarios for future comparison and allows you to review and change your choices at any time. When the design is complete and you are ready to order; Merlin provides pricing, lead-time and other information.


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