Spotlight: Back-injection moulded carbon composites

Materials World magazine
1 May 2018

Richard Gregory, Quality and Engineering Director of the composites facility at motorsport and advanced engineering group Prodrive, UK, explains the process of back-injection moulding of carbon composite parts.

The process?

Back-injection moulding adds moulded plastic features onto the back surface of carbon composite parts, eliminating the need for conventional bonding of clips, fasteners, threaded inserts, and hinges. It provides easier, more robust assembly with lower costs while maintaining a high-quality exterior finish.

It is a reaction moulding process that, unlike conventional injection moulding, requires no heating of the tool. The mould tooling is made from polyurethane material, chosen for its ease of working, speed of machining, and convenience of handling, that is readily compatible with the low reaction temperatures generated (below 100°C).

Prior to injection, the carbon composite part and any additional inserts or fasteners are located in the mould to ensure correct orientation. A patented process is used to prepare the back of the composite for secure bonding. Once the mould is closed, the injection process is completed within cycle times typical of a normal injection moulded part. The material injected is typically a two-part polyurethane resin, which may be glass or carbon filled to meet specific load requirements.

Why it was developed

Cost is a major barrier to the increased use of carbon composites. The addition of those features necessary to mount or retain a composite panel, or carry hidden services such as wiring, historically require separate and labour-intensive operations that increase the cost of the finished part. The exposed ‘A’ surface of a composite panel is highly finished and subject to accurate dimensional control. The rear ‘B’ surface is relatively rough and subject to dimensional variation, which makes attaching features by conventional bonding difficult, requiring lengthy preparation of the surface to ensure adequate bond strength and sufficient accuracy of position. Back-injection moulding was developed to overcome these limitations.

What it can achieve

By moulding onto the finished composite, the process combines the premium visual properties of a carbon composite with the convenience of an injection-moulded plastic part, at a much lower tooling cost. Adding the more complicated features by moulding also means the visual parts can be made using more efficient production methods such as press moulding, instead of manual methods like hand lay-up. The improved dimensional consistency from the automated process permits additional automation during downstream manufacturing operations, such as trimming.

Functionally, parts produced using the back-injection process show a greater resistance to delamination between the composite and the moulded features than bonded parts, when subjected to environmental and climate testing. The presence of the back-injected resin also protects the raw ‘B’ surface of the composite from exposure to moisture during environmental testing, which could otherwise degrade the part. This not only increases durability but also eliminates deterioration that could otherwise be visible when viewed from the ‘A’ side.

Composite parts with back-injection are dimensionally interchangeable with non-composite parts, allowing manufacturers to produce limited edition variants of existing models by substituting surface panels without making changes to the underlying mountings. 

Alternatively, when designed-in from the outset, back-injected parts can offer sealant benefits through improved acoustic absorption or the convenience of moulding integral stiffening ribs onto the back face of the component – the ease of tooling modification makes the fine-tuning of such features quick and economic.

The addition of suitably positioned ribs can also prevent any distortion of a back-injected composite part that is subsequently subjected to low-bake paint plant temperatures as part of the vehicle finishing process.

Typical applications

Existing applications include a range of interior and exterior parts for premium and niche vehicles, often as replacements for painted ABS parts – panels and enclosures are particularly suitable. Future automotive applications are expected to extend the use of back-injection into more structural cases, such as bumper assemblies. One advanced study is considering the feasibility of an instrument panel crossbeam, normally produced as a magnesium casting, because the back-injection process can combine all the necessary fixings and supports with the weight saving of carbon composite.

Cabin components for other industries, such as the executive jet and luxury marine sectors, are also potential applications, as they have similar volume requirements to the niche automotive market