Bunch Engineering uses Moldflow design and analysis tools to solve difficult problems

Michael Bunch, owner of Prairie du Sac, WI-based Bunch Engineering, performs filling/packing, warp and cooling simulation in his product and mold analysis consulting business to help his clients obtain optimum product and process designs. Bunch attributes his ability to help clients solve difficult problems to two things: extensive knowledge of plastics acquired by working in the plastics industry for 39 years and the use of Moldflow analysis tools.

"A consultant deals with a range of problems," says Bunch. "Simple problems might include identifying weld lines, tonnage or pressure limitations, gas traps, moldability assessment, addition of flow leaders, processing windows, runner balancing, reducing high shear rates, reducing stresses, and reducing thermoset resin scrap."

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Recently, he says, he worked on a product being molded in China with a client who was experiencing significant warp problems. "The molder was unable, in spite of many attempts at gating changes, to correct the warp and produce consistent parts," Bunch explains. "There were severe time constraints – the problems had to be solved and new tooling completed in just four weeks. Through analysis, we made several design and gating recommendations and the first production run achieved acceptable and consistent parts. That project and many others proved that upfront CAE efforts preserved the schedule and allowed the client to deal with problems at a great distance."

Bunch Engineering uses Moldflow Plastic Insight modules including Fill/Pack, Cool, Warp/Shrink, Gas, Co-Injection, Thermoset, Overmolding, and Fiber. "In dealing with design and molding problems, it's important to understand which tools are most helpful since there are many approaches we can take to solve problems," says Bunch. "In some cases, clients have their own opinions about problems. But, sometimes they have no idea what's causing the problem. Sometimes I don't understand the cause, either. But with information from an analysis, we can start to piece together a solution. I started consulting with analysis tools because I thought they gave qualitative and quantitative answers, rather than intuitive or speculative answers to problems."

Getting the most from co-injection molding

Recently, Bunch spoke to participants at the 2004 Structural Plastics Show and explained how to take the mystery out of co-injection molding using Moldflow software. He said that co-injection molding is used widely in the European markets to enable recycling of regrind and reprocessed materials. And it is becoming better utilized in the U.S. markets because the co-injection molding process has the advantage of using materials internally in the part so that properties and cosmetics of the skin of the part don't suffer from their use.

Additional benefits of co-injection molding include cost reduction, environmental benefits of using recycled materials, the use of regrind on the inside while preserving the properties of virgin materials on the skin, and choosing different properties for core versus skin. Bunch noted that ultraviolet-resistant skin is preferable to high-impact core skin. "In this case, we're also seeing an improvement in properties in the part," says Bunch. "The use of this core material improves the cold temperature impact strength of the part.

"A co-injection hot runner system is a dual system that delivers melt from two barrels, through two separate flow channels in the manifold, to a valve pin that controls delivery of either skin or polymer to the cold runner," he says. "Using the proper analysis tools, we can choose two injection points so the whole hot runner system can be analyzed. This is particularly important since resin flow rates through each flow channel can affect core percentage."

However, there are difficulties associated with engineering and/or predicting co-injection molding. Bunch notes that several questions should be asked prior to launching the process such as:

How much core polymer is in consideration?

Where will the core polymer penetrate?

How thick will the skin polymer be?

Will the family molds produce the same core polymer distribution as single cavity molds?

Certainly, difficulties associated with co-injection molding including locating optimum gate locations and determining where the core breakout will occur. "We consider many additional factors such as how will the pressure and clamp tonnage be affected by two different polymers," Bunch notes. "Our preparatory assessments include determining if valve gate timing is necessary or possible, if family molds are possible, and if we can maximize core polymers. But in addition to this, plots and graphs relating to the skin and core are provided."

Using Moldflow co-injection molding software specific to two polymers, Bunch can determine the thickness fraction of a core material, distribution of skin and core polymers through thickness, percent volume of the skin and core polymer, as well as the weight of skin and core polymer.

Besides helping users get the most benefits from co-injection by working through the associated difficulties, analysis software helps his clients keep up with the rapidly expanding world market. "Our world economy and competition from other countries makes it imperative that clients use the latest and best technology for product and process design. Engineers and designers from other countries are using these technologies and it's a mistake to think that we know and understand the injection molding process better than they do," says Bunch. "Early in my engineering career, rapid design and development and fast tooling turn-around were used for special cases. But now they have become the norm. The only way to compete in this world market is to use not only your own special knowledge and experience, but also the best technology available."

For more information about Bunch Engineering, visit www.bunchengineering.com. Go to www.moldflow.com for more information about Moldflow Corporations, its products and services.

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