Published in Automotive Manufacturing International
Quantity Before Quality
In the product development process, it seems that prototypes are always required yesterday. But, that is never the only criteria. The prototypes also need to be inexpensive and delivered to specific quality standards.
Rapid prototyping and rapid tooling have fueled new alternatives for the creation of prototypes. With this new wealth of options, the difficulty in selecting an appropriate method is increased.
To assist in process selection, four broad classes of prototyping methods are evaluated.
- Rapid prototyping (RP)
- RTV molding (RTV)
- Rapid tooling (RT)
- Machined aluminum tooling (MT)
Selection of the right prototyping method requires an evaluation of critical project parameters. Time, money and quality specifications must be reviewed and understood.
To begin method selection requires an appreciation for the cost of quality. In general, as quality increases there will be a disproportional increase in both project expense and delivery. If production materials, tolerances and texturing are required, expect cost and delivery to be in line with a production tool.
It is easy to declare that all production related constraints must be satisfied. Challenges arise when evaluating acceptable levels of compromise. Yet, compromise is required, and often appropriate, when achieving drastic reductions in time and expense. For proper selection, project constraints must be realistically defined.
The selection process can be a difficult and arduous task, when quality specifications are identified in the beginning of a project. To make the process more manageable, begin by identifying the number of parts required.
In prototype development, the unit cost and production rate decrease as tooling prices increase. This results in very clear break even points, in both time and expense, between methods. The quantity decision will immediately identify the cost and time to complete prototype development in each of the four classes. With this information, the selection process can be narrowed to one or two methods that fit within budgeted time and money.
With a narrower range of processes to review, it is easier to manage the evaluation of quality constraints.
Time and Money
Size has a dramatic effect on project cost and delivery. Exceedingly small or excessively large parts will change the relationship between quantity, time and money. Therefore, the evaluation of processes assumes that the prototype is an "ordinary" size, fitting within an envelope of ten in3 to 1000 in3.
RP fabricates a prototype by growing the component one layer at a time. The additive nature of RP allows for rapid replication of complex geometry. Speed is also enhanced since no tools or molds are required.
RP delivers prototypes in one to seven days. Additional quantities can be delivered with minimal impact on time when multiple parts can be processed in the same run of the RP equipment. RP loses its time advantage when the component size combined with desired quantity forces multiple machine runs.
When compared to the other three prototyping methods, RP has a very high unit cost. Typical expense will range from $100.00 to $5,000.00. But, RP has no tooling related expense. The net effect is that RP is often the cost leader for quantities that range from one to ten.
RTV is a pattern-based tooling method. A pattern, frequently RP generated, is encapsulated in liquid rubber. After solidifying, the rubber is split along the parting line and the pattern is removed. A castable thermoset is poured into the cavity within the rubber. The urethane solidifies, and the part is removed from the rubber tool.
The use of a soft rubber as the tool medium eliminates many concerns and steps in tool design and creation, making RTV molds available in five to ten days. Prototypes are produced (cast) at a daily rate of two to 20. An order of 25 prototypes can be completed in ten days to 3 weeks.
The expense for RTV is comprised of three elements: pattern, tool and cast parts. The pattern cost is similar to that of an RP prototype. RTV tools range from $500.00 to $2500.00. At $35.00 to $250.00, cast urethane parts are significantly less expensive than RP but two to three times the part price of rapid tooling. The complete price for an RTV solution will range from $1,500.00 to $7,500.00.
The attractiveness of RTV is diminished when desired quantities exceed the typical RTV mold life of 25 parts. Larger quantities will necessitate the purchase of multiple RTV molds, increasing both delivery time and expense.
RT is frequently a pattern-based tooling method. Still coming into its own, RT is somewhat nebulous in definition. The one commonality amongst RT solutions is that they deliver prototypes in production materials (i.e. ABS and polycarbonate) faster than machined tooling and at a fraction of the expense.
RT solutions deliver shot ready tools in 1/2 to 3/4 of the time expected of machined tooling. The time decrease is most frequently achieved through the expedience of casting the tool medium against a pattern.
Built for speed, RT will frequently exclude automated action in a tool. For example, an undercut may be formed by a loose insert (hand load) that ejects with the part. These hand loads are removed from the prototype and manually reinserted in the tool. For this and other reasons, the injection molding cycle time is significantly greater than that of a machined tool. The resulting production rate is usually between 50 and 250 parts per day. Combining the lead-time for tool creation and part production, expect delivery in a two to five week range.
When compared to RTV, RT will be much faster for larger quantities since the tool creation time is offset by the higher daily production rate. For these same reasons, even larger quantities of parts will make machined tooling the most attractive solution.
Expect tool prices to be between $2,500.00 and $15,000.00. Unlike RTV, it is unusual for multiple RT tools to be required since tool life ranges from 100 to 2000 shots. Individual parts will be 1/2 to 1/3 of the cost of cast urethane parts. The purchase of a single tool with the reduced piece price gives RT the cost advantage when 50 to 250 prototypes are needed.
Machined Aluminum Tooling
Typically, MT will deliver a tool in four to twelve weeks. Designed and constructed for low cycle times, MT can easily deliver a thousand parts per day. It is this production rate that overcomes the delay in tool construction to make MT more attractive that RT for large quantities.
Expect to see a continuing decrease in MT delivery times. As the competitive threat from RT increases, suppliers are likely to continue process improvements to retain a competitive position. Also, technology improvements in CAM software and the adoption of high speed machining will assist in lead-time reduction.
Anticipate tool prices to be between $10,000.00 and $50,000.00, three to five times that of RT. Conversely, individual parts will be only 2% to 10% of RT parts. Due to the drastically lower part price, MT is frequently the cost-effective solution for more than 500 prototypes.
For the right application, each method can deliver the utmost in quality. But, quality is a relative term. It can only be defined within the scope of a specific project, assembly or component.
Quality is also time sensitive. The parameters for measuring quality will vary with the progression of the product development process.
Available materials and the additive, layer-by-layer nature of the process will affect RP quality.
RP processes offer a limited material selection with no ability to match the exact specifications of an injection molded thermoplastic. When using RP, there will be a sacrifice in material properties. The level of deviation in properties will depend on the specific RP technology and its available materials.
Without benching, RP system cannot match the surface finish of an injection molded part produced from a polished tool. This is an outcome of the additive nature of the process. The discrete layers that form the prototype yield small steps on all surfaces that are not coincident with X, Y and Z axes of the RP equipment's build envelope.
RP accuracy is as much about the resolution and repeatability of the process as it is about the people who will hand-finish the parts. Accuracy claims of 0.0025" to 0.005" are easy to come by, and some systems may be able to deliver. However, the benching process requires skilled labor to finish the surfaces of each prototype. As a result, RP should be expected to deliver ± .010 in. to ± 0.030 in. deviations from nominal dimensions.
Stability over time is also an issue. Some RP processes provide prototypes that will alter dimensionally over time. Residual stress, humidity and temperature can adversely affect the quality of the prototype.
The pattern used to create the RTV tool affects quality elements such as surface finish and accuracy. Since RP is frequently used to generate the pattern, the issues described above will affect the results of an RTV tool.
One advantage of RTV tooling is that the RTV rubber and the castable urethane demonstrate very low shrinkage, in the order of 0.001"/in. The result is that the cast urethane prototype is nearly identical to the original pattern.
There is a broad range of castable thermoset urethanes appropriate for RTV molding. This breadth is expanded by a supplier's ability to custom formulate (blend) materials to exacting requirements. Clear, rigid, semi-rigid and flexible parts are all available for functional, form, and fit analysis. The limitation is that thermoset materials can not match all mechanical, electrical and thermal properties of a thermoplastic material.
Like RTV tooling, the pattern affects quality elements such as surface finish and accuracy. Additionally, accuracy may be sacrificed through the reversal process required to develop the 'negative' from a 'positive' pattern.
With few exceptions, RT solutions yield the loosest tolerances. Beyond pattern and reversal considerations, accuracy is impacted by the unpredictable behavior of the thermoplastic as it is being injection molded. A thermoplastic's shrinkage is dependent on factors that include part geometry and molding parameters. Specified as a range of values by the resin supplier, construction of a RT tool requires an educated guess as to the rate of shrinkage. Should the shrinkage vary by as little as 0.001"/in, a ten inch part deviate by an additional 0.010".
Due to the time constraint placed on delivery of RT solutions and due to the impracticality of accurately machining the tool, the tool design practice of allowing for 'metal-safe' conditions is not appropriate. There is little opportunity to manipulate the tool to compensate for unexpected outcomes. One consolation is that RT can assist in predicting the true shrink rate when it is time to build a production tool.
With today's process technology, the biggest advantage of RT is that production intent materials are delivered quickly and cost-effectively. Production intent materials provide the ability to fully evaluate a new design with a confidence that it will be predictive of the production item.
Machined Aluminum Tooling
CNC and EDM combine to make machined tooling the highest quality and most accurate of the four processes. Commonly used in prototype runs, machined tooling is the baseline upon which all others are measured.
Tight tolerances, mold texturing, breadth of material selection are all provided by MT. An additional benefit is that the prototype tooling can be used to bridge the gap to production tooling.
These advantages are gained through the increasing cost of quality. Longer delays and higher expenses should be expected when pursuing this course of action.
Quantity before quality is a technique to simplify the selection of the right solution for prototype development. In the end, quality will be the priority and the ultimate decision-making factor. But to start with quality and to hold unreasonable quality expectations will only serve to complicate selection and drive up cost and delivery.
There will be exemptions to all of the presented information. But as with every rule-of-thumb, the overall principles and guidelines will be applicable in every selection process.
Once a method of prototyping has been selected, it is time to repeat this process on a narrower scale. Use quantity before quality to evaluate all the techniques within the chosen class of prototyping methods.
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