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May 1, 2007

10 Min Read
How to spec the best hot runner (Web-exclusive expanded content)





Husky’s Martin Baumann says the more part specification information you can give to your runnerless system supplier, the better your system will be.

Are you sure the hot runner system you’re about to buy is the right one for the job? Communication with your supplier, early on, is key to job security.

Upfront input from you can have a major impact on the materials cost savings, cycle time reductions, uptime, and overall performance of your new hot runner systems. Your preferred supplier probably has the resources to recommend a runnerless system capable of handling that killer app you’re ready to run, even if it involves limited gate cooling and access, corrosive resins and additives, and a high-temperature resin, or two, or more—if you’re on to a multimolding job.

More often than not, even more mundane applications may require special mechanical requirements in a hot runner system, especially when it comes to available shut height and other factors in your molding system that could influence your supplier’s recommendations for the type of manifold, nozzle size, and gating style you need. Catalog shopping doesn’t cut it, more often than not.

“Everyone is looking for extra part quality. The more upfront information that you provide to your hot runner supplier gives you that extra quality and saves you money down the road,” says Martin Baumann, hot runner marketing manager at Husky IMS in Milton, VT.

He was one of the speakers at a well-attended Husky hot runner symposium IMM sat in on at the National Plastics Museum in Leominster, MA, one of many convened around the United States and Canada this year. The symposium was chaired by Scott Kroeger, GM of Husky’s Boston Technical Center (Westford, MA). In addressing the impact of customer information on the performance of a hot runner system, Baumann’s message was clear: “Don’t rush to drop in a runnerless system until after all the necessary upfront work is done.”









Product specification

An extensive bank of technical resources to assist you in making your hot runner selections is available from suppliers like Husky. The first resource to help get you started that Baumann described in Leominster is Husky’s Product Specification Form (PSF). Available online, the PSF asks you to fill in information on the resin, the part, the processing requirements, and the mechanical and electrical properties.

Husky also asks for digital or hardcopy drawings of the part, and detailed part information—including even a part sample, if you have one available. Using this information, a manifold layout is determined.

“The key question to ask yourself is, ‘What do you need for the part?’” Baumann says. “Suppliers, like Husky, want you to give them part specification information. In this way, your supplier can optimize your runnerless system to the requirements of the part.”

Husky then runs the PSF information through its Melt Channel Sizing (MCS) software to determine how such factors as injection time and resin viscosity affect pressure drop, residence time, and shear ratios.

Melt channel sizing

MCS is a predictor of plastic flow through the channels. Period. It’s used to spot possible system design limitations, such as too much pressure drop through the hot runner, or too long of a residence time. MCS neither accounts for the compressibility of the plastic, nor for part geometry. It’s just about sizing the melt channel.

The data input required for MCS includes resin type, part mass, process temperatures, wall thickness, fill time, machine pressure, and color change. The software uses this information to determine and optimize such factors as shear rate, channel size, residence time, and pressure drop after determining the manifold layout. It outputs the proper melt channel sizes for each leg of the melt channel and computes the proposed system’s performance.

“Take shear rate, for instance. Is the minimum sufficient for proper color changes? Or, is the minimum sufficient for handling a heat-sensitive resin?” Baumann asks. “What’s key is giving your supplier precise information. And, this kind of information is used. It’s not just requested to make your life more difficult.”

He adds this reminder: “Melt channel sizing is not a part flow simulation. MCS is a prediction of how plastic flows through the melt channels.”The next step is determining the right hot runner manifold material and thickness. Did you know different manifold materials have different thermal conductivities? You did? Go to the head of the class.

Manifold material selection

Husky’s Manifold Material Selection (MMS, which also stands for Manifold Material Screening) bases its selection on hot runner pressure drop, melt channel layout (including angles and manifold thickness), fill time, and melt channel sizing. All of these factors reportedly affect the layout and design of the heaters.

“We can predict and simulate how manifold materials and heater layouts react to the temperatures required in specific applications,” says Baumann. “Also, we can use this information to determine manifold thickness, which has a direct effect on shut height determinations.”

Although he stressed the differences between Husky’s MCS and flow analysis software, Baumann didn’t mean to imply that Husky doesn’t use flow analysis in engineering its runnerless systems.

At competitive pricing, it provides Moldflow Part Advisor (MPA) services for basic jobs, such as part filling pressure analysis and verification of supplied injection times for help in nozzle selection. It also can provide more advanced Moldflow Plastics Insight (MPI) services. Both MPA and MPI are from Moldflow Corp. (Framingham, MA).

Husky also performs standardized resin testing techniques at its Boston Tech Center and at its hot runner plant in Vermont to help determine such factors as gate quality, processing conditions over a range of temperatures, and possible cycle interruptions.

“We’ve got to put more science into this process up front to find better ways to reduce time to market and to make better parts,” Baumann says. “To do that we need feedback. Molders can’t say, ‘I’m running production now—don’t bother me,’ and expect to get the best hot runner system they need.”

Computerized troubleshooting

OK, so you supplied all the necessary upfront information to design and build the best hot runner system for that new job and your new system’s running just fine. How do you keep it up and running? Husky’s answer is an interactive CD-ROM it calls the Hot Runner Toolbox. In January it began to field early copies of Release 2.0.

It provides preliminary steps for troubleshooting common hot runner issues, including a Top 10 list of things you can do before calling your supplier. There’s information on troubleshooting guidelines, which includes details for troubleshooting such issues as crown flash, stringing, and hot tip vestige.You’ll also find recommended service procedures, with online links to all the solutions specified in the troubleshooting guidelines, and links to all the calculation and estimation tools referenced in the guidelines that can be used when you’re searching for a specific tool.

A process information sheet allows you to record resin, mold, machine, and process information and an inspection sheet lets you record hot runner or mold measurements. All of this data can be saved to your hard drive.

Topping off the services provided is a pictorial glossary of the most commonly used hot runner components, a global listing of contact information for service and spares, and access to a PDF copy of the hot runner installation maintenance manual.

Web-exclusive: How Mastip specs the best

According to David Shirley, North American GM of of hot runner supplier Mastip Technology Ltd. (Richfield, WI), “Mastip's goal is to avoid the potential pitfalls with any hot runner application, pitfills such as filling issues, which can lead to unacceptably high injection pressures and temperature settings, flow lines, and burn marks, which in turn cause unacceptable parts and drive up reject rates.”

He continues, “We use a design review to determine the ideal hot runner design for specific applications. It's critical in our goal to eliminate the potential for common molding problems-items two and three are particularly important among these steps. The ratio of flow length to wall section is an issue that we stress, especially when the application involves engineering-grade materials. These review steps have been complied into a flow chart to narrow down the final product selection and design.

“1. Application (material, part weight, and geometry) and mold design data have to be supplied to us initially to select the proper nozzle style, whether it is a valve gate or an open thermal gate, and if we are gating directly to the part or into a cold runner.

2. Part data on the flow-length-to-wall-section ratio, part weight, and material must be analyzed to select the proper nozzle size and series. This information is important in determining the proper size flow channel diameters in the systems, and determines if additional gates are needed to fill the part adequately. Ignoring this guideline typically leads to difficult-filling parts, unacceptable weldlines, and molded-in stress, which can result in part warpage and dimensional issues. Reviews for material residence issues and mold cooling are also part of this step.

3. Material specifications including shear sensitivity, filler, and colorant are necessary to select the proper tip design, whether we use a multiflow hole, a single offset hole, or an open-center hole.

4. From the material specs we also know the processing temperature and whether or not there are corrosive or abrasive fillers, which allows us to select the correct tip material to optimize thermal conductivity and resistance to wear and/or corrosion.

5. Cosmetic gate requirements are used to determine the appropriate gate diameter and geometry, as well as tip type. Gate design and size must be carefully matched to the desired gate appearance, part design, and material being molded. Mold cooling also plays a part in this segment of the selection process.

6. Finally, Mastip runs a fill analysis to further confirm the proper selection and design has been made.”

Mastip Technology Ltd.
(262) 628-8725
www.mastip.com

Click here to download a form from Mastip Technology that prospective customers can use to help select the best hot runner for a given application.

Web-exclusive: Seiki Spear's 13 questions

According to Henry Mora, sales and service manager, you should answer the following 13 generic questions if you want to get the best hot runner bang for your buck from Seiki Valve & Spear System/Mitsui Plastics Inc. (Glendale Heights, IL). Mora's worked for other hot runner suppliers before joining Seiki Spear, and he says these basic questions are pretty generic, regardless of which supplier you prefer:

1.What is the resin?
a) What types of fillers are involved?
b) What types of additives?
i) Are they corrosive?
c)What is its V0 rating?
d)Are color changes involved?
i) What colors?
ii) How often are they changed?

2. What is the part?
a)What is the part weight or volume?
b)What is the wall thickness?
i) At the gate?
ii) At the thinnest section?
c)What is the wall-thickness-to-flow-length ratio? (e.g., if it's a .090-inch wall, and the distance from the gate to the farthest point is 5.30 inches, the resultant ratio-5.30/0.090-is 58:1)
d) What is critical about this part?
i) Flatness?
ii) Difficult-to-fill features?
iii) Clarity?
iv) Cosmetics?

3.Why are you looking for a hot runner?
a)Material scrap reduction?
b)More precise process control?
c)Other?

4.Do you have problems with your existing hot runner? If so, what type of problems?

5.What type of gate do you prefer?
a)Valve gate?
i) Will your application require contouring of valve pins?
b)Pin gate?
i) Is there a gate vestige limitation?
ii) Is a gate well acceptable?

6.What kind of control will you be using?

7.What type of injection press will you be using?
a)Press size?
b)Does it have multiple barrels?

8.What type of mold will incoporate the hot runner system?
a)Mold size?
b)Cavitation?
i) Gate pitch?
c) Shut height limitations?
d)Quick mold change?
e)Any features that could interfere with the hot runner?
i) Core pins?
ii) Waterlines?
iii) Inserts?

9.Which market is being served by this hot runner application?

10.What is the cycle time required?

11.What is the mold delivery schedule?

12.What is the standard hot runner system and control that you use in-house?

13.What hot runner system price range are you in?

Seiki Valve & Spear System, distributed by Mitsui Plastics Inc.
(630) 924-8800
www.seiki-hot.com

Contact information

Husky Injection Molding Systems, Milton Campus
(802) 859-8000 | www.husky.ca

Moldflow Corp.
(508) 358-5848 | www.moldflow.com

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