Rapid Prototyping Of Small Composite Parts With Complex Shapes

 

Rapid high-Performance Manufacturing (RAPM) is a research and development program for the rapid, low-cost and agile manufacturing of small, complex-shaped composite parts. RAPM is divided into three different material and process routes: resin infusion, thermoset prepreg and thermoplastic prepreg. Some details of the development progress are given below.

 

Resin Injection Precast Design

 

RAPM Deep-Draw Challenge Part and Preform Element

 

The geometry of the part was inspired by an actual part from a difficult-to-manufacture production platform – a tray assembly with two deep-draw sections and spacers around the deep-draw sections and access holes (see above).

 

The part was manufactured using NCF- non-crimp fabric, resin infusion/resin transfer molding (RTM). The NCF- non-crimp fabric preform was broken down into sub-elements: skin, deep-draw sections, and machined access holes and lay-ups around the deep-draw sections.

 

All fabrics were 400 g/m² ±45° biaxial, but the 3-ply layup was 154 g/m² UD-unidirectional tape. The elements were laid up differently to achieve the designed panel thickness.

 

Even using 12 different NCF- non-crimp fabric preform elements, a 2/3 piece reduction was achieved compared to the 66 cut pieces of the current part using prepreg. RAPM estimates that this also reduced the labor for the part (primarily cutting and layup) by 90%.

 

12 NCF Panels are 2/3 Less Than The Original Prepreg Panels

 

Resin Injection Modeling

 

Flow Analysis Model

A model of 12 preform elements was created for resin flow analysis using low pressure RTM (30 bar max) to determine the best strategy for infusion, resin outlet positioning and flow channels. The model assumed a fiber volume fraction of 50% for the base preform ply and 54.8% for the reinforcement ply.

 

The resin outlet was initially located at the center of the bottom edge of the part (see image below). Flow modeling indicated that at 95% resin fill, the resin flow path was towards the bottom right corner and there was a high chance of forming a dry spot along the bottom right corner of the part “By moving the outlet to where the resin wants to go, the chance of creating a dry spot can be greatly reduced”.

 

 

The original resin outlet position (left) resulted in a resin flow pattern (right) with a high risk of a dry spot in the lower right corner. The proposed solution was to reposition the resin outlet to the predicted dry spot (right)

 

Flow modeling was performed by Huntsman (Basel, Switzerland), the supplier of the FAF2 two-component epoxy resin used, using PAM RTM software (ESI Group, Paris, France). Flow modeling showed that: weak and strong flow channels should be used to ensure uniform preform filling and reduce the risk of channel dry spots; the resin distribution channel main feed lines should be shortened (to slow resin flow); and fast flow was predicted to occur along the edges of various preform elements.

 

 

The “channel” is the gap between the preform and the mold that allows resin to flow over the top of the preform and affects the flow during infusion.

 

The ‘channel’ is essentially the gap between the preform and the edge of the mould. “In flow modelling of the part, a small channel means preventing runaway” This is because with a small gap between the preform and the mould, resin can flow over the top of the preform and once the gap is filled, the resin is forced down into the preform.

 

However, having a strong channel would mean a larger gap and, for complex geometries, too fast a resin flow.

 

These results influenced the preform and tool design of the part to ensure that dry spots did not occur. With these modifications, flow modelling indicated that the mould should be filled in 120 seconds at a flow rate of 10-40 g/s at a maximum pressure of 30 bar and a tool temperature of 130-150°C.

 

Note that initial resin injection trials indicated dry spots at the bottom of the rectangular deep draw section and fibre deformation near the resin exit due to lifting of one of the skin preform layups. To address this, the resin inlet and exit positions were reversed. Flow modelling confirmed this.

 

Tsotsis, “Process Modeling for Improving Resin Infusion Tool Design and Part Quality.”

 

Preforming and Infusion

 

Preform tools for rectangular and V-shaped deep-drawn details (top left, top right) produce preformed elements that are then assembled to a full-part preform tool (bottom left) and heat-set into the final part preform (bottom right).

 

The preforms are formed using low-cost tools made from polyurethane Raku Tool from RAMPF Tooling Solutions (Grafenberg, Germany). These tools are very easy to grind, provide extremely fine surface structures and excellent dimensional stability, and are heat-resistant up to 110°C, which is sufficient to heat-set the preforms.

 

The rectangular and V-shaped deep-drawn sections are preformed to near-net shape by preheating and pressing into their respective preform tools. The details are then assembled using the full-part preform tool. Although preforming is done manually in RAPM, it can easily be automated for series production.

 

 

The design of the machined polyurethane preform tool and aluminum infusion tool (lower right)—including thermal analysis (lower left)—is designed to work together to enable low-cost, flexible processing.

 

The assembled preform is then loaded into the aluminum infusion tool. The preform and infusion tool are designed to enable a robust yet cost-effective process. Thermal performance modeling of the infusion tool was completed before the tool was finalized and machined.

 

Results showed that the difference between hot and cold spots was within tolerance, at 5.6°C for the top tool and 4.3°C for the bottom tool.

 

After infusion and initial cure at 130°C, all parts were post-cured at 180°C and then machined to net shape. The parts were of similar or better quality than prepreg-based production parts.

 

Part trials on the RAPM resin infusion process showed that the method can be used to manufacture high-quality parts for four different preform fabrics and two different resins.

 

 

Finished parts manufactured by SGL Composites showed similar or better quality than prepreg-based production parts and were sent to Boeing for a comprehensive evaluation

 

Prepreg Parts

 

Most of the details of the RAPM thermoset prepreg part trials, including a discussion of the spring frame and double diaphragm forming (DDF) process used. This process is shown below:

 

RAPM Parts Molded Using Thermoset prepregs

 

However, there is one achievement in this section of the RAPM that deserves to be detailed here. All three of the above parts were made using three different epoxy prepregs based on Solvay (Alpharetta, GA, USA) resins:

 

⚫CYCOM 5320-1 Out-of-Autoclave (OOA-) resin

⚫ CYCOM 970 – solvent and hot melt prepreg options

⚫ XEP-2750 – a new aerospace system, now commercialized as EP-2750, optimized for press molding.

 

One of the features of EP2750 that is optimized for compression molding is its fully impregnated nature, while CYCOM 5320-1 is optimized for edge venting required in vacuum bag-only Out-of-Autoclave (OOA) prepreg processing.

 

CYCOM EP2750, although only slightly higher in resin content, 40% for CYCOM 5320-1 and only 36% for CYCOM, better maintains hydrostatic pressure during compaction and curing in the mating metal tool cavity of the stamped part, reducing the risk of dry areas, inconsistent cured part thickness, wrinkles and poor surface finish.

 

 

Compression molded parts using CYCOM 5320-1, even with an additional layer of prepreg, showed wrinkles and porosity (Figure 16). However, repeatable, high-quality parts were produced by adding Transformer Film prior to compression molding (Figure 17)

 

However, Solvay did develop a method to achieve the same results with CYCOM 5320-1. It is a patented Transformer Film that is applied to the part layup prior to compression molding and increases the resin content to maintain hydrostatic pressure during compression molding. The initial TS-RAPM-003 curved C-channel part made with CYCOM 5320-1 showed wrinkles and also had porosity. An additional layer of prepreg did reduce the wrinkles and porosity, but did not eliminate them.

 

As shown in Figures 16 and 17, the wrinkles and porosity were eliminated by using Transformer Film. As Alejandro Rodriguez, Senior Application Engineer at Solvay, explains, “Repeated production of C-channel parts using compression molding with CYCOM 5320-1 3K 8HS prepreg and Solvay’s Transformer Film demonstrates the quality and robustness of this high-speed manufacturing process.”

 

Thermoplastic Prepreg Molding

RAPM also uses compression molding with thermoplastic composites. Pictured below is one of the first parts to be trialed using this process. It uses a single-step process to mold a blank made from 12K carbon fiber-reinforced polyetherketone (PEKK) 12-inch-wide unidirectional (UD) tape into the final molded part.

 

Ribs Molded From UD Tape Ribs Are Molded From UD Tape Blanks In A One-step Process

 

The tool developed uses a thin aluminum bladder, pressurized with an inert gas at high temperature, which expands during the stamping process to apply uniform pressure to all surfaces of the part. This makes it possible to maintain horizontal hydrostatic pressure on the vertical flanges of the part when using a press that lacks horizontal hydraulics and controls, thus acting only in the vertical direction.

 

 

Bladder Sealing Gasket

 

The aluminum bladder is milled from a single billet of 2024 aluminum and is sized 30% smaller than the final part thickness to accommodate the bulk of the thermoplastic unidirectional tape in the unconsolidated billet. Aluminum bladders do not require superplastic forming. Bladders can be made from magnesium and require superplastic forming.

 

Just below the aluminum bladder is a high temperature graphite sealing gasket that spans the entire perimeter of the aluminum bladder. The tool setup works by causing the press hydraulics to bottom out on this gasket and then apply force through the gasket.

 

This force then seals the argon gas inside the aluminum bladder. The press hydraulics do not apply pressure to the material laminate itself—the pressure is applied by the inflated bladder. To pressurize the bladder, a pressure inlet was added to the top steel tool. This allows argon gas to be supplied from a cylinder and pressurize the bladder cavity while the press hydraulics seal the bladder and hold the tool closed.

 

The top and bottom steel tools are 410 stainless steel, along with a removable insert in the lower tool to facilitate part removal. When the stamping cycle begins, the upper tool is lowered along with the aluminum bladder to slowly press the bulk stackup into the cavity of the lower tool. This is where the additional 30% volumetric factor in the aluminum bladder at room temperature is key to fitting the entire stackup into the tool without interference.

 

The pressure applied by the bladder to the thermoplastic is critical to achieving proper consolidation as the thermoplastic undergoes volume changes and shrinkage due to its coefficient of thermal expansion (CTE- coefficient of thermal expansion). This shrinkage is overcome by the pressure applied by the aluminum bladder perpendicular to the horizontal and vertical surfaces of the part.

 

However, parts produced using this method showed porosity and variation in cured part thickness. This is difficult to control using an aluminum bladder that only presses vertically (moves in only one axis). Forming experiments successfully demonstrated that a thin aluminum bladder can apply enough sustained pressure to consolidate a polyetherketoneketone (PEKK) thermoplastic composite part with acceptable nondestructive inspection (NDI) results.

 

However, this method requires more analysis to be successfully applied to this complex geometry stiffener, which features a pad in the center of the web and vertical flanges on all four sides.

 

More success was achieved with a magnesium bladder tool for thermoplastic fabric prepreg blanks of the consolidated curved C-channel. The process was completed using a Boeing St. Louis PtFS workstation. The consolidated blanks were then stamped into the final part shape by the ATC manufacturing department. The two processes used together overcome the limitations faced by stiffeners and show that stamping can achieve complex geometries for complex-shaped composite parts that may not be possible with standard one-step compression molding.