After 39 years’ experience in molding and design consulting, Michael Bunch has amassed quite a repertoire of expertise. One of the areas in which he is seeing a significant increase in activity is coinjection design. His experience has shown him that a thorough understanding of the process, together with CAE tools, is critical to successful design and production.
Bunch Engineering uses software modules from the Moldflow MPI 5.0 suite, including Fill/Pack, Cool, Warp/Shrink, Gas, Coinjection, 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,” Bunch says. “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.”
Lowdown on Coinjection
Bunch spoke to participants at SPI’s 2004 Structural Plastics conference and design competition about the importance of using coinjection software. He explained that coinjection molding is used widely in European markets to facilitate recycling of regrind and reprocessed materials; he also says the technology is becoming better used in U.S. 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,” he adds.
Other benefits of coinjection include cost reduction, improved environmental impact thanks to increased use of recycled materials, and the choice of different materials for core vs. skin. He notes that UV-resistant skin materials work well with high-impact core materials. “In this case, we see an improvement in properties in the part. The use of this material combination improves the cold temperature impact strength,” he says.
Using coinjection 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, and weight of skin and core polymer.
On with Analysis
Coinjection software provides the same results as a normal analysis, including such factors as flow front, flow front temperature, and frozen layer thickness. Bunch notes that several questions should be asked prior to launching the analysis of a coinjected part:
How much core polymer is being considered?
Where will the core polymer penetrate?
How thick will the skin polymer be?
Will family molds produce the same core polymer distribution as single-cavity molds?
There are several challenges associated with coinjection molding, including gate location and determining where core breakout will occur. “We consider many additional factors such as how the pressure and clamp tonnage will be affected by two different polymers. 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.”
Without using coinjection software, Bunch can add a flow leader to reduce injection pressure and clamp tonnage. In the case of a garden tractor hood, molding the part initially required high injection pressure and tonnage—26,000 psi and 3500 tons, respectively. Parts flashed along the gate edge due to high pressure.
The part was initially analyzed without coinjection and a .020-inch-thick flow leader was added to decrease this pressure and tonnage to 18,000 psi and 2500 tons. However, without coinjection analysis, the core percentage and core distribution suffered. In essence, making the part more moldable also made it more expensive. The part was then analyzed with coinjection software to change flow leader design to improve pressure and preserve core percentage.
Core percentage improvements can be traced to the ability to analyze the hot runner system, according to Bunch. “A coinjection 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 coinjection analysis software, 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.”
A word from the wise on using CAE
On competing in today’s environment, Bunch says, “Our world economy and competition from other countries make 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 turnaround were used for special cases, but now they have become the norm. The only way to compete in the world market is to use not only your own special knowledge and experience, but also the best technology available.”
Troubleshooting from a distance
Bunch recently worked on a product 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.”
Getting to know Bunch Engineering
Wisconsin-based Bunch Engineering is a product and mold analysis consulting firm that uses CAE software (Moldflow’s MPI 5.0) in product and mold design and process analysis. Bunch performs filling/packing, warp, and cooling simulation to help his clients obtain optimum product and process designs. “A consultant deals with a very wide range of problems. It’s difficult to describe them all, but the simple ones might include identifying weldlines, tonnage or pressure limitations, and gas traps; assessing moldability; adding flow leaders; determining processing windows; balancing runners; and reducing high shear rates, stresses, and thermoset resin scrap,” he says. A list of more complex problems that he’s addressed over the years includes coinjection optimization of core resin percentage, coinjection 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 prediction.
Contact information
Bunch Engineering, Prairie du Sac, WI
Michael Bunch; (608) 643-4355
michael@bunchengineering.com
www.bunchengineering.com
Moldflow Corp., Wayland, MA
(508) 358-5848; www.moldflow.com(end)
