Soluble Cores

Using FDM Soluble Cores to Produce Composite Parts

Overview

Composite parts are manufactured by winding, wrapping, molding and laying up various combinations of materials and resin systems on molds, bucks, patterns, cores and mandrels. Producing hollow composite parts that trap the pattern can present a manufacturing challenge. One way to overcome the challenge is to use clamshell tooling to lay up two halves (and bond) or lay up a single piece by working from the inside of the tool’s cavity.

Application Outline

In the past, most companies that use clamshell tools to produce hollow composite parts used computer numerical control (CNC) machines to produce patterns from polyurethane board that were then used to produce the molds. The cost of this approach is high and lead times are long, particularly if the pattern is machined by a subcontractor. For these reasons, many companies have switched to producing patterns using Fused Deposition Modeling (FDM). FDM technology is an additive manufacturing process that builds plastic parts layer by layer, using data from CAD files. FDM offers substantial time and cost savings by enabling patterns to be produced in much less time and at lower cost.

But even after improving pattern-making, companies that make hollow composite molds face a time–consuming layup process. After the pattern has been produced, it typically takes about three days to build molds and lay up parts using this method. Typically 12 to 15 hours of direct labor is required, and the rest of the time is spent curing.

A new approach provides substantial improvements for low–volume production by replacing the mold with an FDM soluble core. Soluble cores substantially reduces delivery lead time and labor expenses by eliminating the need for making a mold and also reducing the time required for laying up the part. Instead of the tedious process of laying up the two mold halves, then laying up the part in each half, and then bonding the two halves together, the composite cloth can be wrapped around the soluble core. After the part has cured, the core is simply dissolved.

The nature of additive manufacturing makes it possible for FDM to produce much more complex geometries than are possible with other cores. In many applications, greater design freedom generates performance improvements and cost reductions. FDM soluble cores are strong enough to withstand the loads of composite manufacturing processes. And there is no risk of damaging the part during core extraction because the core simply melts away as it soaks in a liquid bath.

Process Overview

Manufacturing FDM soluble cores requires two changes to the standard FDM process. First, the core is designed to make its internal structure mostly hollow. Second, the strong thermoplastic of a standard FDM part is replaced with a soluble material that is normally used for the construction of a part’s support structures. The core can be designed in two different ways. One way is to create a solid 3D model and use the sparse fill option, while in Insight — the Fortus build preparation software — to automatically create an internal structure that minimizes the internal volume of the core. The second approach is to create (in CAD) an internal structure that keeps the core stable under the temperatures and pressures of composite molding while promoting flow of the solution to accelerate core removal. Both of these approaches minimize material consumption, build time and washout time.

It’s relatively simple to integrate FDM soluble cores into the manufacturing process. No modifications to the process are needed prior to composite curing and core removal. The cure cycle is also unchanged, but temperatures must be limited to avoid distortion. In general, composite parts with FDM cores must be cured at temperatures below 250 °F (121 °C) and at pressures less than 50 psi (345 kPa). The only process change is that, after the composite part has cured, the core is removed by dissolving it in a solution bath. To do so, the part is placed in the Stratasys WaterWorks soluble support system.

Customer Story

After each race, Joe Gibbs Racing (JGR) engineers have just three days to diagnose a problem, find a solution, and implement it before the car ships to the next race. JGR’s ability to go from concept to production part has helped lead it to three championships and position it as one of the most competitive teams on the NASCAR circuit.

One Sunday, a tire blew out in a JGR car and engineers identified a problem with the duct outlet supplying air to cool the tire. In the past it would have taken over a week to develop a new duct outlet concept design, build and evaluate an FDM prototype, build a mold using an FDM pattern and lay up a composite part. This process could not be completed in time to produce a new part before the next race.

JGR used an FDM soluble core to substantially reduce the time required to redesign the duct and build a production part. On Monday, a JGR engineer designed a new duct outlet to deliver air over the tire bead exactly as needed to keep it cool. Then the engineer built a concept model in only four hours using the Stratasys Fortus system. After completing several iterations on the concept and confirming its fitment on the car, the engineer produced an FDM soluble core. The final carbon fiber part was laid up on the composite core. After the part had cured, the soluble core was dissolved away. The new part was ready on Wednesday, in time to be bolted onto the car before it was shipped to the next race.

How Did Using FDM Soluble Cores Compare to Using Clamshell Tools for JGR?

Method

Cost

Time

Labor

FDM Pattern and Clamshell Mold $350 3 days 15 hours
FDM Soluble Core $90 1 day 2 hours
SAVINGS $260 (74%) 2 days (66%) 13 hours (87%)