CLOSED MOLDING

Sandwich Cores for Closed Molding

Sandwich laminates are defined as a layered material with two thin, stiff and high strength outer faces bonded to a thick and low weight core material. Core materials enhance panel stiffness by dramatically boosting the Moment of Inertia as a result of increasing the panel thickness. In that respect the core acts much like the web of an “I” beam, separating the load bearing flanges. High-performance core materials enable strong and stiff sandwich constructions, thus reducing a part’s weight to the minimum. These types of solutions not only tie up fewer resources, but also consume less energy in their use phase (e.g. trains).

“Which sandwich core should I use for closed molding?” is a frequently asked question. Unfortunately, there is no simple answer. Choosing an individual product is based on a handful of constraints; structure and performance (mechanical properties), geometry, and lamination process. The mechanical properties and the lamination process will dictate the core type and density. The geometry and the process will dictate the core configuration or finishing option. Sometimes a combination of two or more materials or multiple configurations is best.

Benefits of sandwich construction in closed molding

Placing dry fabrics under the pressure of vacuum will cause them to compress. The reduction in laminate thickness will have a severely negative effect on the laminate’s stiffness. Using a core will overcome the change in thickness and increase the stiffness.

For closed molding processes, the majority of time and labor will be to place the dry materials in the part. Sandwich construction can greatly reduce the number of layers in a laminate. With the proper selection of fabrics and core, laminates can be “built” in as little as three layers; skin-core-skin. Using pre-cut kits for fabric and core will permit parts to be built that meet the entire definition of “lighter, cheaper, and faster”.

Wide variety of material options

While various core materials can meet the requirements for stiffness and strength, individual materials can offer additional application-specific advantages: the natural balsa products BALTEK® SB and SBC features unrivaled mechanical properties; AIREX® C70 and C71 PVC foam are chosen for their excellent ratio of performance to lightweight density and AIREX® T92 PET foam combines easy processing with cost efficiency and recyclability.

For more specialized applications, AIREX® R63 offers unparalleled damage tolerance and T90 excellent fire resistance, while R82 combines outstanding radar transparency, temperature resistance and fire properties. AIREX® PX plays its cards best where very high loads are required or to be able to hold a screw and other hardware.

Resin flow and proper wet-out

Most AIREX® and BALTEK® products are available with GP (rigid) or GPs (semi-CK). Special grooves and perforations (holes) in GPs facilitate fast, uniform resin distribution in any infusion process resulting in increased resin flow rate along with lower resin absorption and better cosmetics. Using GPs eliminates the need for disposable surface media or additional expensive interlaminar flow material. Resin can be accurately controlled with no voids or dry spots, and no weak spots due to resin-rich areas. These traits give GPs great advantages over typical rigid and contourable core sheets and thick fiberglass infusion/molding mats. Proper use of GPs can result in a dramatic decrease of part infusion time for faster production

Laminate weight and total cost

While a sufficient core-skin adhesion requires a certain amount of resin, any additional resin uptake on the core material’s surface represents both extra cost and weight, especially in resin flow methods (e.g. infusion). Resin absorption will also be dependent on the finishing option.

The new AIREX® T92 SealX technology constitutes a breakthrough innovation in further reducing the resin uptake by a stunning 50 %. AIREX® T92 SealX hence creates a completely new class of PET cores which translates into substantial reduction in resin weight and cost.

In addition, BALTEK® and AIREX® flexible sheets are available with knife-cut kerfs to deliver the lightest possible structure.

Tailored for ultimate performance

Such an extensive range of specialized materials offers vast opportunities for optimization. Our Hybrid Core Concept® is the toolset to leverage these opportunities into customer benefits. It allows optimization of each sandwich structure with regard to the concept’s three pillars:

1. Properties (weight, stiffness, strength, damage tolerance, heat resistance, etc.)
2. Cost (material, resin uptake, finishing options, ease of processing, life cycle costs)
3. Sustainability (CO2 footprint, recyclability, life cycle analysis, etc.)

Optimization of any of the above-mentioned criteria of the Hybrid Core Concept’s triangle can be achieved.

Intelligent combinations

In many applications the requirements vary throughout the entire part. Some areas may be more weight-sensitive, while maximum stiffness is required in others. Using a single material configuration for the entire part may be simple, but in most cases intelligent combinations of different materials will offer a better solution that optimally meets customer-specific target criteria.

The table below lists the 3A products most suitable for the specific closed mold process:

*SORIC may be purchased as a pre-preg fabric directly from the supplier

Below are the configurations (finishing options) which best fit a specific closed mold process:

*The proper amount of film adhesive must be unsed to ensure proper kerf fill