From Art to Part in 9O Days

While most manufacturers are heavily dependent on prototyping to decrease time to market, eliminating prototype tooling in blocks and cylinder heads actually yields a complete new engine design in 90 days

Direct Shell Production Casting (DSPC), is a rapid manufacturing process dedicated to the production of functional cast metal parts. DSPC works by producing ceramic casting molds, with integral cores, directly from CAD files, thereby eliminating the need for patterns and core boxes. This enables engineers to exploit multiple design iterations concurrently (including testing engine heads and blocks cast in different alloys), consequently expediting the design phase.

DSPC has essentially taken rapid prototyping to the next stage as it not only allows engineers to see and touch their design, but also to test it functionally in real life situations within a few days of design completion. Once the part has been tested and approved, production tooling (permanent molds, patterns or dies and core boxes) can be generated from the CAD file of the approved part using DSPC technology. Subsequently combining DSPC with conventional casting and CNC maching results in a smooth transition from computer aided design to production.

the challenge
Successfully launching a new automotive engine depends on a fast and efficient Cylinder head development program coupled with a quick and smooth transition to mass production. The engine development is a very lengthy design process. It often requires multiple design iterations to optimize the combustion chamber, ports and water cooling and oil passage geometry for proper lubrication and heat removal. The resultant engine has complex core cavities.

Having to produce patterns and core boxes prior to making a first article part is the main weakness of the traditional metal casting process. This is because each time the customer wishes to incorporate a design change, which affects the casting geometry or requires a different alloy for the part, a new tool is required. Furthermore, tooling production is time consuming and expensive.

Fabricating engine blocks and water-cooled cylinder heads remain two of the slowest, most difficult and costly steps of manufacturing.

The new water-cooled DOHC cylinder head using DSPC represents the highest geometrical complexity, including multiple core cavity structures, in cylinder heads. The goal of this study was to verify the possibility of a paradigm shift Engine housing and heads in engine development, in which tooling design and production are postponed until after dynamometer testing of the engine is completed. This paradigm shift is possible with the use of DSPC, a rapid manufacturing process that makes traditional casting obsolete.

In addition, the DSPC ability to produce the ceramic casting molds with complex integral cores gives the engine designer freedom. New, more effective and efficient engine components can be created by this ability to ignore parting lines, draft angles and core prints considerations while iterating the design. Consequently, the use of DSPC eliminates tooling design and fabrication until after the cast part is functionally tested and approved for production.

Once the product design is completed, and the cast prototype engine has been functionally tested, it becomes essential to create production tooling rapidly and cost-effectively for the mass manufacturing processes. This process minimizes scrap and meets target production costs, which determines the speed and the success of launching the new product. This successful transition will shorten time-to-market and optimize profitability--the ultimate goal of a product launching program. A way to do this is to produce the production tooling last, only once, and right the first time.

This case also studied the possibility of using DSPC to cast net shape production tooling from the same CAD file as the approved cast part. The results of this case study verified the ability of the DSPC process to simulate accurately the casting process, test the gating design, and the direction and rate of solidification, as well as other casting parameters.

DSPC is a patternless casting process, that makes conventional casting techniques obsolete for creating a first article part. Unlike rapid prototyping, DSPC produces a functional metal part without the need for temporary tooling. It uses the customer's CAD file to create the actual ceramic casting molds, enabling the production of any cast part before tooling (prototype or production) is made or even designed.

Eliminating the need for temporary tooling expedites the design and the functional testing phases of cast parts. It enables the customer to perform an unlimited number of design iterations, including testing the design with different alloys, without the cost and time consumption associated with tooling production. Also DSPC's reliance on CAD files provides for stringent configuration control, ensuring that all design changes are properly documented.

For production quantities, production tooling (steel or aluminum patterns and core boxes) is cast as net shape tools from the same CAD file of the approved part. This production tooling is now created only once, guaranteeing a smooth and cost-effective transition from the prototype to production. By combining DSPC and conventional casting practices, long lead times and costs, as well as geometrical inconsistencies between the different tooling are eliminated.

Figure 4: DSPC 300 printing a cylinder head

The DSPC machine is like a 3D printer that uses the designer's CAD to create the actual ceramic casting molds. The DSPC system operator adds a gating system to the designed part's CAD file and adjusts it for alloy shrinkage, converting it into a cavity of a ceramic mold file in CAD space. This is a one-off process, which is performed off-line to the DSPC machine. The computer file of the ceramic mold is used to automatically generate the actual ceramic mold in layers in the DSPC system.

The DSPC fabrication process involves three steps per layer. First, the ceramic shell model is 'sliced' to yield a cross-section of the ceramic mold. Second, a layer of fine powder is spread by a roller mechanism on a separate pass. Third, a multijet printhead moves across the layer, depositing binder in regions that correspond to the mold's cross-section. The binder penetrates the pores between the powder particles and adheres the particles together into a rigid structure. Once a given layer has been completed, the ceramic shell model is sectioned again at a slighdy higher position, the plain is lowered, and the process is repeated until all layers are formed. The DSPC mold is cleaned of excessive powder, fired and poured with molten metal (Figure 5).

A DSPC mold may contain integral ceramic cores, allowing a hollow metal part to Manifold be produced. A DSPC mold may also be made for highly complex parts of any size. Furthermore, virtually any molten metal can be cast in DSPC molds. Automotive parts have already been manufactured in aluminum, magnesium, ductile iron, stainless steel, as well as other alloys.

automotive cylinder heads - case study
The automotive cylinder-head case study is based on a time-compressed effort to develop a new DOHC engine. Similar results were observed in short-run production programs of cylinder heads, turbochargers, and transmission cases and race car intake manifolds. In this specific case, by combining DSPC and conventional casting practices the customer was able to test five different versions of that engine (block, heads and manifold designs), select the optimal design and receive 150 sets of the engine castings identical to the approved version. Using traditional methods, or even with the help of rapid prototyping, eight weeks would not have been enough time to achieve a functional engine, let alone any design iterations. Moreover, had the customer been forced to create five different sets of core boxes (Soligen conservatively assumed that the external geometry did not change), the cost of these changes would have substantially exceeded the original budget.

Table 1 and Figure 6 and Figure 7 illustrate a comparison between traditional sand casting assisted by rapid prototyping (for patterns and core boxes via take-off from RP masters) to produce temporary tooling and DSPC in producing a series of these new cylinder heads. For dynamometer testing, each design iteration requires three cylinder heads (two
Table 1: Cost and production lead time assumptions
DSPC Sand casting
Cost of patterns (one time) nocost $38,000
Cost of a set of core boxes nocost $42,500
Cost of each casting starting $15,700 declining to $8,750 $1,200
Average cost of each iteration $36,675 $53,700
Time to receive tooling no need for tooling l6 weeks
Time to receive a new set of core boxes no need for tooling 8 weeks
Time to cast a set of 3 cylinder heads 2 weeks 1 week
Dynamometer testing 4 weeks 4 weeks
(time between iterations)
Total time between Iterations 6 weeks 13 weeks
for each dynamometer testing iteration, and one as a machining setup piece). The company conservatively assumed that the external shape of the heads could remain unchanged, therefore each iteration only required new sets of core boxes and slight modifications of the patterns to accommodate for core prints. Had the company assumed that each version of this design required a new set of patterns and core boxes, the differences between DSPC and sand casting for the design verification phase would have ballooned. Another assumption was that the transition to production would not require a new set of production tooling since the production quantity (150 engines) was relatively small (Table 1).

Tooling for conventional casting dominates the cost and lead time required to produce a first article part. With DSPC, there is no tooling and the part's cost and delivery time are independent of the part's geometry. Figure 6 and Figure 7 show the cumulative time and cost differences between traditional casting and DSPC.


"Tooling for conventional casting dominates the cost and lead time required to produce a first article part"
DSPC enabled the engineer to make many more design iterations in a short period of development time. Had the program called for a change in alloy (such as from aluminum to iron), the lead time between iterations of sand casting parts would be drastically increased, as well as the program cost. With DSPC such changes would not affect the delivery time at all. It may, however, slightly affect the program cost (by the difference in the cost of the alloy).

Once the design had been verified and the first article batch successfully tested and approved, the company quickly moved to produce the short production run. The CAD file of the approved cylinder head was directed and a matchplate pattern and three core boxes were electronically created. Soligen used its DSPC technology to cast the matchplate and the core boxes. This process took 18 days, after which these tools were used to produce 300 cylinder heads (another 2.5 weeks). Parallel to the casting process, CNC code was generated to machine the heads. The process of converting the CAD data to files for the pattern and the core boxes was executed at no additional cost to the customer. The total Non Recurting Engineering (NRE) charges for the tooling and setup were US$50,000.

The transition from CAD to conventional casting proved seamless to the customer. The creation of the production tooling from the same CAD file as the cylinder heads, eliminated interpretation errors in the design process of the production tool. Similar results were observed in short-run production programs of engine blocks, turbochargers, transmission cases and race car intake manifolds.

Automotive intake manifold Using DSPC to produce the prototype and short-run production of cylinder heads and blocks in aluminum and ductile iron enabled the development team to eliminate the need for temporary tooling. The project, which induded five rounds of design iterations, was completed using only 50 per cent of the budgeted dollars, and within 20 per cent of the budgeted time. Savings from eliminating unnecessary engineering hours by eliminating the time normally spent on the design and production of prototype tooling should also be considered.

DSPC works directly from a CAD file to create the actual ceramic casting molds, complete with accurate integral cores. The result is a functional metal part, which can be tested in real circumstances, and then changed or approved as required. Changes simply become adjustments to the CAD file, and then a new part can be cast immediately. After a part is approved, production tooling can be made, using DSPC, from the same CAD file. DSPC elimmates a considerable amount of the time and cost associated with using the more traditional prototyping and tool creation methods for multiple design iterations.

The results of this case study confirm that, by combining DSPC and conventional casting and CNC practices, long lead times, product development costs, and production tooling costs are substantially reduced and the following advantages can be realized:

speed
Substantial reduction of time-to-market due to the elimination of the need for tooling at the design phase.

ease of making design iterations
Elimination of engineering and extensive CAD work in blending core geometries, draft angle and core prints design.

cost reduction
The ability to incorporate design changes without the cost and lead time associated with producing or modifying patterns or tooling.

design flexibility and versatility
The ability to make complex parts with integral cores just as easily and accurately as simple parts. Plus the ability to test an unlimited number of design iterations, including testing a design with different alloys.

seamless transition from cad to conventional casting
Design and fabrication of production tools can be made from the same CAD file as the approved part and the design is verified.

Yehoram Uziel
Soligen Technologies, Inc, USA

Originally published in Engine Technology International, November 1997