User Story

Bunch Engineering : Moldflow

Prairie du Sac, WI-based Bunch Engineering is a product and mold analysis consulting firm that uses Moldflow software in product and mold design and process analysis. Owner Michael Bunch performs filling/packing, warp, and cooling simulation to help his clients obtain optimum product and process designs. In business for 17 years, Bunch says he's been successful in business because of his combination of 39 years in the plastic molding field and using Moldflow analysis tools that complement his expertise. "A consultant deals with a very wide range of problems. It's difficult to describe them all but the simple ones 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," says Bunch.

He says a list of more complex problems that he's addressed over the years include co-injection optimization of core resin percentage, co-injection core resin placement, gas injection channel and gate design, fiber orientation changes in a part to improve weak areas, warp relating to resin shrinkage, cooling system design, warp relating to fiber orientation, and overmolding warp predication.

Bunch recently worked on a client's product that was being molded in China. The client 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 was successful with 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 Moldflow tools are most helpful since there are many approaches we can take to solve problems. 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," says Bunch.

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 US markets. This process has the advantage of using these materials internally in the part so that properties and cosmetics of the skin of the part don't suffer from their use.

Bunch explained the 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. He 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. The use of this core material improves the cold temperature impact strength of the part," Bunch adds.

Bunch says, "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. With MPI 5.0, 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."

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?

Bunch adds, "In this case, we're also seeing an improvement in properties in the part. The use of this core material improves the cold temperature impact strength of the part." There are several difficulties associated with co-injection molding including locating optimum gate locations and determining where the core breakout will occur. Bunch adds, "We consider many additional factors such as how will the pressure and clamp tonnage be affected by two different polymers. Our preparatory assessments include determining if valve gate timing 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.

Bunch notes, "Using Moldflow, I can add a flow leader to reduce injection pressure and clamp tonnage. Molding the part initially required high injection pressure and tonnage - 26,000 psi and 3,500 tons, respectively. Parts flashed along the gate edge due to high pressure. The part was initially analyzed without co-injection and a .020 thick flow leader was added to decrease this pressure and tonnage to 18,000 psi and 2,500 tons. However, since this done without co-injection analysis, the core percentage and core distribution suffered. Making the part more moldable also made it more expensive. The part was re-analyzed with co-injection software to change flow leader design to both improve pressure and preserve core percentage."

Advantages to Performing Upfront Simulations
Bunch says, "Our world economy and competitive 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. 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 Go to for more information about Moldflow Corporations, its products and services. To contact the author, go to

Author: Laura Carrabine

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Page last modified on December 10, 2004
Copyright 2004 by John Stark