Designing With Ductile Iron

The company's previous line of injection molding machines had platens made from AISI 1020 plate steel. These steel plates ranged in size from 5 inches thick by approximately 40 inches wide and 40 inches long for smaller machines to over 20 inches thick by 100 inches wide and 100 inches long for large 3,000-ton machines. Because the plates required a large amount of machining time for cleanup and lost a large amount of material due to machining chips, it was decided that Ductile Iron castings should be used for the platens, and several other parts, in the new design. This decision was made for several reasons; first, the properties of Ductile Iron are similar to the properties of the AISI 1020 steel that had previously been used; second the use of castings would allow the company to consolidate several parts into one; and third, the use of Ductile Iron would substantially reduce machining costs.

Above and Below:  Cincinnati Milacron's VISTA VL1000 injection molding  machine is used for making large, plastic parts.

Most of the parts for which the design team at Cincinnati Milacron was considering Ductile Iron castings were subject to high cycle fatigue. Therefore, the mechanical properties of the Ductile Iron to be used had to be comparable to the properties of the plate steel. It was decided that grade 60-45-15 ferritic Ductile Iron was a suitable match for these critical parts. Table 1 below lists the typical mechanical properties of the AISI 1020 steel plate and the properties of the gide 60-45-15 Ductile Iron that the company specified as a replacement for the majority of the cast parts in the new design, In addition to the typical mechanical properties listed, Ductile Iron has fatigue and fracture toughness properties that allow it to be used in fatigue applications.      back to top

The design team for the castings included people from several different groups, including design engineers, manufacturing engineers, purchasing personnel and representatives from Cast-Fab Technologies, a foundry. Working together from the start of the project, the team designed the parts with a collective eye not only toward surpassing strength and fatigue requirements, but also toward improving both castability and machinablity.

One of the goals for the redesign of these complicated machines was a simplification of the existing design through a consolidation of parts. Toward this end, several parts on the moving platen of the VLl1000 (a hydraulic machine with 1,000 tons of clamping force) were eliminated by casting the stuffing box as an integral part and by casting on several support brackets that otherwise would have been machined and bolted to the platen. Other instances of parts consolidation include the die height and moving platens on the company's Vista toggle line of machines. In this instance the steeples were cast as integral parts of the platens, which not only greatly reduced the number of parts but also reduced the amount of machining required to finish them. (See Photographs below.)

These platens, made of ANSI 1020 steel plate, were used in Cincinnati Milacron's previous line of injection molding machines.

Cincinnati Milacron's newly designed line of injection molding macines uses Ductile Iron castings for several parts, including platens.

Finite Element Analysis (FEA) was used in the design of the critically stressed parts to determine the stress levels in all areas of the casting. FEA was completed before any patterns were made. Once stress levels were determined, the designs were altered to add material in highly stressed areas to reduce stress levels and remove material in areas of low stress. 

Critically stressed parts were designed using finite elements analysis to determine stress levels in all areas of the casting.

This design technique gave the design engineers the ability to design highly reliable parts with stress levels well within the fatigue limit values and to remove any material that does not contribute to the strength of the casting, thereby reducing weight. An example of an FEA is shown at right. Each of the colored areas represents a stress level. FEA can also be used to determine part deflection in various areas of the casting when rigidity is an important factor.     back to top

Foundry that input in the early stages of the redesign project proved to be very important because it averted several potential casting problems. By showing the foundry high stressed areas of the casting, they were able to design the pattern and gating and risering systems to assure sound metal in these areas. They were also able to suggest design changes that improved castability and prevented designed-in casting problems. Machining stock was adjusted to the proper thickness to avoid having casting defects, dross, slag or sand on machined surfaces, highly stressed areas or areas that have hydraulic sealing surfaces. Casting dimensional tolerances were determined to avoid interference between castings at assembly and assure that core tolerances and machining stock could be adjusted so that core holes would clean up without excessive machining.

Manufacturing engineering was also involved at this early stage, designing castings both for ease of machining and fixturing and for reduced tooling costs. The use of Ductile Iron reduced machine costs since it machines more like gray iron than steel. It also permitted an increase in rough machining feeds and speeds, thus reducing machine time as compared to steel. In addition, tool life increased due to the fact that the graphite in Ductile Iron reduces wear. And deburring was also reduced because the size of the burr was reduced or eliminated.

The design advantages described above were only realized thanks to the superior capabilities of Ductile Iron. With mechanical properties similar to steel and castability and machinability similar to gray iron, Ductile Iron enabled Cincinnati Milacron to optimize the design of its new line of injection molding machines, making them more reliable at lower cost.

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