Solid Models Make Molds Faster

Solid modeling technology has enabled this piston manufacturer to design prototype casting molds in a fraction of the time it once took. Here's their entire design-to-manufacturing process.

Article From: 2/1/1996 Modern Machine Shop,

Shrinking time-to-market requirements and increasingly strict emissions standards are having a large impact on automotive parts producers. Suppliers must be more flexible, more responsive and more efficient than ever before. For piston manufacturer Zollner Pistons (Fort Wayne, Indiana), the ability to produce prototypes quickly is key.

The combination of stiffer competition and environmental regulations has substantially increased the number of new molds that must be made and compressed the time available in which to build them. "Emissions (standards) changes are driving the entire industry," says Jeff Castleman, product design manager at Zollner. "Because of this, we have to pay close attention to the changes that engine designers are implementing. We want to produce quick prototypes that can be put in an engine and tested. In today's market, the first organization to produce the product will most likely receive the new business."

Zollner has committed themselves to achieving success in this area. New methods have helped reduce overall concept-to-finished-piston time from 21 weeks to 12 weeks. According to Mr. Castleman, a new strategy for piston design at Zollner is largely responsible for this time reduction, as well as other improvements in the manufacturing process. The new method involves the creation of solid models in CAD to quickly and unambiguously establish new product designs, and then to use those same models to swiftly create the piston mold designs and the NC code to cut them.

More specifically, Zollner uses the same modeling system to design both piston and mold. This way, tolerances and product geometries are always consistent and rely on a simple data transfer within the same CAD/CAM environment. The tooling geometry is then passed along to tooling manufacturers who cut the piston mold components on CNC machines. To shorten the tool build time, all standard tooling components are pre-machined so that the only critical machining added is the specific piston geometry. Once the tooling is complete, the pistons are man-ufactured by conventional means.

According to Mr. Castleman, the overall piston design process has gone through drastic changes as a result of new CAD technology. "The age of designing pistons on a 2D drawing board and with CAD drawings has passed, and a new 3D solid model design has arrived. The solid model of the piston is used as a base for all further processes."

Zollner creates their solid models in the Pro/Engineer CAD system from Parametric Technology Corp. (Waltham, Massachusetts). The reasons this modeler is a good fit for this particular application, says Mr. Castleman, have to do with its feature-based capabilities, parametrics, full associativity and ease of use.

Feature-based means that primary shapes can be called up on the system--rather than having to be constructed from lines, arcs and curves. And because the system is parametric, these models can be developed independently of fixed dimensions, which can be added later. Moreover, the designer can define physical relationships between features in such a way that a change in one feature is also reflected in any other affected features. These capabilities substantially speed the design process.

Full associativity means that changes made to any module of the system will be reflected in all modules. For example, a dimensional change in the solid model will automatically be reflected in the 2D drawings. This capability makes the entire design-to-manufacturing process more manageable, particularly in the prototyping environment where engineering changes are frequent.

Zollner maintains a library of piston and mold parts in their system. This capability allows designers to go to a library menu and scan the data inventory for specific parts. Then they can pull out part assembly files for modification and restore them as new parts or features. This ensures that the experience gained from each new design is stored in an easily accessible area.

"Piston design is an evolutionary process," Mr. Castleman explains. "Often we need to design a new piston that may be similar to one we already have. So, we'll go ahead and modify the geometry on the previous model to meet our new needs. This could involve changing machining dimensions or casting dimensions, for example. Then we add any new features that are needed. We build on the information we already have."

The ability to do quality checks was also important to Zollner. "We try to perform finite element analysis on all new piston designs. Another module lets us make finite mesh models right from our solid models and saves time so we can perform additional quality checks."

3D Piston Design, Step By Step

Piston design starts with a solid model of a cylinder that is the length and diameter of the overall piston casting. This solid model cylinder is then hollowed to form the undercrown and skirt thickness of the piston. Protrusions are added for the ring belt, skirt open end belts and pin boss with its supports. Cuts are made for the skirt tails, pin boss outer reliefs and cast pin bores. All cast piston top features are add-ed along with the radii on all the sharp corners--and the solid model for the piston casting is complete.

The piston casting model then undergoes all the necessary "virtual" machining cuts to produce a finished piston solid model. These include ring grooves, skirt and lands, piston top features like bowls or dishes, and pin bores.

Refinements to the solid model are made using customer requirement data and Zollner's internal design standards. Next, the solid model is used to create the views and dimensions needed for an accurate 2D drawing of the piston. This 2D drawing is used as a reference to inspect pistons in the prototype production phase.

Mold Design_Ensuring Quality

Once a solid model of the piston design is complete, work begins on the mold that will be used to produce the piston. Zollner's designers use the 3D solid model of the piston to transfer geometries to various mold components.

Mold design begins with the selection of the proper pre-designed mold components. The geometries for these components are stored in the CAD system's library of piston and mold parts. Says Mr. Castleman: "We have a core detail mold that consists of five parts (so the piston casting can be easily removed). In addition to having the core detail solid model, we actually have this geometry sitting on the shelf at our mold shop assembled and ready. When we get jobs that are rushed--sometimes as short as 12 weeks--we can fit the piston right onto that core and design the rest of the mold components around it. Because the core makes up the majority of the lead time (an average of three weeks before part geometry can be applied), we have cut three weeks from our prototype time.

"I think the most time saved has come from the actual machining of the mold components, because we can use the solid models. We pass the solid models over to the mold manufacturers, and they create tool path directly from that geometry. Then they pass that tool path to a CNC mill and cut the part."

Other mold components that are considered standard parts are contained in a separate parts library and can be automatically placed within a new design. Because the size and shape of these mold parts never change, 3D models never need to be generated from scratch, and design time is greatly reduced.

The next step in designing the mold involves transferring the piston geometry to the mold. Using a CAD module specifically created for mold design, all piston geometry is transferred to the mold, while maintaining associativity between the piston and its mold--meaning the design of the mold will always conform to the design of the piston exactly. The piston geometry is used to make a reference part that incorporates a defined shrink factor. A reference part, with shrinkage, is made for every mold component that will contain part geometry.

Another capability that saves time is the ability to manipulate features at various phases in the tool design cycle. "We can take the piston design with all of the machining and casting features on it, and suppress the machining features and get back to a casting state," says Mr. Castleman. "Every cut to the solid model is actually its own feature, so you have the ability place one feature ahead of other features. You also have the ability to modify these features or suppress them. For our pistons we build the solid model in a way that we have the casting first. So we have all the casting features and then we machine after that.

"We take the casting and apply a uniform shrink factor to it. Then we locate the piston onto each of the various mold components. We do it part by part, including the core detail that forms the underside of the piston and the ring that forms the bottom of the piston. Once all the parts are located on the core detail, we create our mold."

Once they have designed each individual piece of the mold, designers employ an assembly module to ensure that all of the mold pieces fit together. "This is a very powerful tool for finalizing a mold design before the actual building of the mold," says Mr. Castleman. "Our use of 3D models means there are no errors with our piston part geometry. With a simple software command any interference between components is detected. We simply take all of the pieces of the mold and put them together in one large assembly and do a global interference check."

Time Savings, Quality Improvements

According to Mr. Castleman, the use of 3D models has resulted in some critical improvements in a market that demands quick turnaround and high quality. They've achieved significantly greater accuracy in the initial designs, while still cutting design time by some 30 percent.

The initial dimensional accuracy also provides efficiencies downstream in the manufacturing modeling phase. "We can use the same tool path to cut every die now," says Mr. Castleman. "Once we know our geometric accuracy is there, we can go back to the same program every time. And when the clerical worries are taken care of, we have more time to be creative, to think of different ways of making pistons--in terms of the core geometry, for instance."

Explaining the difference between their current system and the previous 2D system, Mr. Castleman says, "It used to be that for each piston, we would provide a chart of dimensional information that would be used to cut the parts on a manual mill. It usually required three or four days for just one part. Now, this same process with 3D requires about 14 hours to cut the first part, but with multiple sets of a component, each additional one only takes about four hours after the tool path is generated. This is where we start to see the important benefits."

The greater total process accuracy derived from that initial solid model also flows through to the supplier shop floors where Zollner's molds are cut. On a recent project for Navistar, Zollner had three manufacturers each produce ten molds. The same solid model was given to all three, and all three generated tool path differently. However, all three ended up with the same geometry. "We check all the mold geometry when it comes in, and when we checked this, all three were within 0.002 inch, which is very close. These are the kind of results we are coming to expect working from a solid model."

Mr. Castleman's belief in the effectiveness of 3D modeling is highlighted in his predictions for the future. "I believe that engine part suppliers will start using the geometry to interface with other 3D designers. If somebody is making a crankshaft and another supplier is responsible for the connecting rod, for instance, they will be able to work together and know that the two parts will fit together properly.

"I think that the world will start revolving around 3D design to compete more effectively and provide a higher level of quality."

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