Designers are faced with a number of considerations before tooling up for new composite parts. Even before they begin specifying and fabricating new parts, attention must be given to the following areas: critical performance parameters; material to meet performance requirements; finished part dimensions, contours and integrated functions; number of parts to be manufactured; time frame from prototyping to production and cost effectiveness. Tool costs encompass scrap, maintenance, repair and replacement, cycle time and labor. Due to a designer's time factors - which impact the design cycle - additional options to fiberglass "soft" tooling and traditional metal tooling should be considered.
A number of tool or moldmaking technologies are available for open or closed molding. The majority of short-run or prototype tools are made of castable materials, such as epoxies or urethanes.
As indicated previously, designers must think through a number of considerations before specifying a tooling material. In fact, the molding material cure cycle - along with special part requirements (such as CTE) - designates the type of material needed. For example, if the cure cycle is room temperature, any material can be used. However, if the cure cycle is at a temperature and pressure higher than room temperature and atmospheric pressure, then metals become the production standard in tooling material for production runs.
Mold cycle requirements generally determine the material type. If the material being processed is a 350°F cure graphite composite with aerospace dimensional requirements, a high nickel alloy such as Invar will typically be chosen to match the coefficient of thermal expansion of the composite being molded. If the material being molded is premixed with a catalyst - such as most bulk molding compounds - or if it is processed at a low temperature with some pressure, then aluminum tooling is acceptable for medium to high production requirements. If the temperature and pressure are high, the material of choice is P20 with Beryllium copper inserts for thermal control.
Current Moldmaking Technologies
Traditionally, molds or tools made of metal are either cast or machined. Casting is the most popular material of choice for "forming" tools (i.e., forming of thermoplastics). Machining is used for open or closed molding of materials that require greater strength. An all-metal tool is normally machined from a solid ingot of metal. Cost considerations are driven by several factors:
- Metal (aluminum is rather inexpensive while Invar is more costly).
- Time taken to machine (aluminum is easier to machine than steel or Invar).
- Time taken to get the tool to the finish required to meet the surface finish specified for the part being molded.
These traditional toolmaking processes are currently accepted because they have been used for hundreds of years. Additionally, the users of tools are typically forced to accept high costs and long turn-around times. For example, a tool/mold that moves from the CAD drawing to the actual part fabrication may take six months or more. The expense of the tool/mold has many variables, which becomes important in prototyping or rapid tooling requirements.
If there is a design change, the time to change the tool dimension is comparative to the time it takes to make another tool, which could add an additional six months to the project.
Prototype and short-run tools for composite parts are generally limited to fiberglass tooling that does not provide the durability of metal. Today, a new, patented toolmaking method - called Customer Moldmaking Process Technology (CMPT) - provides a cost-effective alternative to these traditional toolmaking methods. CMPT is a "hard" metal toolface manufacturing option available to composite part manufacturers at relatively low investment cost, while offering distinct advantages compared to traditional tooling methods - opened and closed.
A "rate" tool refers to one that can extend the rate of production beyond a prototype or initial tool or serve as a backup to an original tool. CMPT is designed to produce rate tools easily and less expensively. It is made by robotically applying metal alloy over a master model that represents final part contours and dimensions. The application speed and pattern is computer programmed and robotically applied. Similar to the lay-up of a composite part, the weaving of the metal over the splash is done in a warp-and-fill, warp-and-fill manner. Lay down speed - depending on the alloy used - is 3/8 inch in 24 to 48 hours over a 20-square-foot toolface surface area.
Tools' faces typically range in thickness from a 0.38-inch minimum to a four-inch maximum and can withstand pressures up to 1,500 psi. Cure temperature for a composite part layed-up and cured on a CMPT tool can range from ambient to 350°F and may go as high as 750°F depending on a specific design with high-temperature operation. Parts can be cured on a CMPT tool with or without vacuum as well as in an oven or autoclave.
CMPT offers designers:
- Enormously decreased time to fabricate the tool and the finished weight is much lower compared to machined steel or Invar tooling.
- Lower cost over total part development.
- Flexibility in terms of toolface material, coating options, backup structure and the molding method.
The multi-directional weaving pattern enables the tool to have a uniform thickness and density comparable to conventional metal tooling. For instance, a conventional aluminum tool can require reinforcing ribs to hold surface dimension, but the ribs tend to create hot and cold spots on the mold face.
Traditional fiberglass tools operate at a temperature as low as 250°F and high-temperature fiberglass tools can handle 350°F. However, these tools often suffer damage in handling, may lack vacuum integrity and may need frequent repair. CMPT enables faster heat-up - translating to faster production cycles and durability up to 10 times that of fiberglass tools.
An all-metal CMPT tool can be made of almost any alloy or hybrid alloy in as little as six weeks from a CAD drawing to master model to the programming of the robots to a net-net all metal tool ready for production. In some instances, the tool can be made from the prototype or plastic part itself. A custom tool with contours demonstrates a savings of around 60 percent or more, depending on the tool configuration, metal alloys and the reduction in turnaround time (six weeks versus six months).
CMPT is unique compared to other processes. A custom tool can incorporate features that can enhance or help control the cure process. For example, if the molding material is considered corrosive, the custom tool can have a stainless steel surface. If the need is to accelerate part production or control heat-up and cool-down, the tool can have conformal lines for coolant or hot oil. The need for thermal control can be achieved by the use of copper in selective layers or areas. Thermocouples can be embedded at critical areas within the tool to monitor temperatures. A permanent release coating can be added to the surface to eliminate the need for non-permanent release agents. An aluminum tool can be created and a surface enhancement added, which converts the surface to a hardness of up to 60 Rc for increased wear resistance and permanent release.
The CMPT toolface is cost effective as it duplicates all of the properties of a solid metal tool without the high cost, high weight, long leadtime and lengthy finishing time. For example, a five-foot square, three-foot thick Invar billet weighs 35,000 pounds and generally takes four months before it is ready for conventional fabrication (CNC machining) at a cost of some $20,000 - even prior to machining and finishing. However, a five-foot square CMPT toolface with a three-foot contour takes only about seven days to make, and is ready for production after being fitted onto a flex-contour holding table. The cost reduction is at least 60 percent.
The Boeing Company (Seattle, WA) has been using the CMPT toolface because it was capable of meeting initial and final time and performance requirements for a carbon fiber/epoxy aerospace component. In only seven weeks, a seven-foot by five-foot toolface with a two-foot height contour, made from 0.5-inch thick Invar was created. In comparison, a cast and machined Invar toolface may have required six months leadtime to produce the ingot, about two months to CNC machine and about two months to hand polish.
Johnson Controls Interiors (Holland, MI) - an automotive parts supplier - built a one-foot by one-foot by one-inch thick CMPT steel toolface section with a 3/4-inch copper layer integrated onto the backside of the 1/4-inch toolface for heating and cooling. The process was done to demonstrate the dimensional capabilities of CMPT tooling. Currently, Johnson Controls plans to implement a vacuum-forming tool for manufacturing a vinyl door panel bolster. Johnson Controls will construct the metal back-up structure for the tool.
Companies have shown that there is a need for rapid tooling for short- and long-term production runs. Designers have the ability to receive an all-metal CMPT tool in as little as six weeks, which requires no work or modification after it is received. A cost-savings will be seen since there is a price differential between electroformed nickel tools or one made of a cast "soft" material.