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Repairing Hoods Made of Metton – Case Studies with Polyvance’s R&D Team

Repairing Hoods Made of Metton – Case Studies with Polyvance’s R&D Team

Posted By: Kurt Lammon.

One hot summer day, a dump truck pulled into the lot at Polyvance. The truck’s owner had heard from a local farmer friend that the company repaired broken plastic and wanted to see if they could help him with a problem he had on his hood. 

The problem he was experiencing was with the horseshoe pockets, where the hood guides bolt onto the hood of his Kenworth T800. They were both broken out. As a result, the hood didn’t sit correctly and was vibrating. The driver was not only annoyed by the noise but was sure something else was going to break on the hood if he didn’t fix it soon.

The hood on his T800 is made of Metton® , a trade name for a material called dicyclopentadiene, or DCPD. Metton is a thermoset liquid molding resin built on DCPD chemistry. It has a lot of advantages over other materials for making large parts with lots of contours and deep draws. Metton is stiff and has good impact properties. 

Compared to metal, a part made of Metton is much lighter. Light weight means better fuel economy, which is important for over-the-road trucks. However, since Metton is a thermoset material, it cannot be broken down into its constituent chemicals, melted, or otherwise recycled. So these large parts end up in the landfill where they will remain for hundreds of years. 

Metton, as experienced in this driver’s hood, is unlike fiberglass or SMC, because Metton does not have any fiber reinforcement throughout the resin, making the material relatively weak in tension, particularly when subjected to vibratory stress. As a result, the material seemed to routinely fail at the horseshoe pockets, where the hood guides mount. 

Kenworth does not make a repair kit to address this problem. The only  solution is to replace the hood. The cost of a new hood is $5,000, or  you can purchase the internal panels for about $1,800. This price does not include the eight hours of labor required for installation. Unfortunately at the time the driver came to Polyvance for assistance, an alternative solution was not available, since a band-aid approach wasn’t going to solve his problem. 

The analysis of this driver’s Metton-made hood at Polyvance however has shed light on the part’s repairability. Despite requiring a two-sided mold, thanks to the liquid resin’s low viscosity, very large parts can be filled out reliably with a cure time that is very fast – only 60 seconds – making for a part cycle time of only about five minutes. Furthermore, the injection pressures are very low (only 50 psi), meaning that the mold can be made of aluminum or even composites for short runs. 

The main problem with the hood was that the hood guides put a point load onto a thin section of the hood, and the hood itself is subjected to a lot of engine and road vibration. In order to create a reliable repair to address the problem, Polyvance engineers decided that the loads from the hood guides had to be spread out over a larger area to reduce the point load concentration at the thin section where the guides bolt through the hood. The horseshoe pocket revealed itself as the ideal structure to bond a reinforcing panel to, so Polyvance created molds to fit into the horseshoe pockets. 

After extensive testing of adhesives, Polyvance’s R&D team introduced the 2510 PlastiFix two-component methyl methacrylate adhesive as the optimal bonding agent for securing the reinforcing casting and plates to the Metton hood. 

It took over six months of development and design work to complete the kit, which was introduced in 2015. Unfortunately, the gentleman with the dump truck who originally informed us about the problem had to replace his hood. However, the insight he provided opened our eyes to the problems endured by heavy duty truck owners with respect to their Metton hoods and in general challenges with plastic repair. 

Many large trucks have hoods made of Metton, and perhaps there were others out there with the same sort of problem. It didn’t take long for our phones to start ringing about the similar Kenworth W900 models. These had the same problem but with a different geometry to the pockets. During the development process it was discovered that Kenworth has two configurations for the W900 pockets – round and square. 

Subsequent to the introduction of the W900 repair kits to the market, we started receiving calls about the T880 hoods. This problem was different – instead of the hood guide mounts, the T880 hoods were breaking where the damper struts mounted. Apparently, when the damper struts wear out, the hood slams open, putting excessive stress on the hood where the struts mount. The root of the problem was the same; the unreinforced Metton material was subjected to a repetitive point load, causing a crack that continued to propagate with each repeated vibration. 

Due to the more complex geometrical structure of the T880 hood, where the struts mount, Polyvance 3D scanned the area, giving the CAD software a perfect rendition of the surface where the reinforcement cap would be applied. The reinforcing casting conforms to the contours of the hood, spreading the shock load over a much bigger area of the Metton to restore the broken hood’s functionality, keeping potential repair costs down and the hood itself out of the landfill. 

Using Polyvance’s kits to repair damaged Kenworth hoods provides a triple benefit – they save the vehicle owner money, they create a repair that is stronger and more durable than the factory original, and they keep another large piece of unrecyclable plastic out of the landfill. 

Polyvance will continue to develop repair solutions for the heavy duty industry as we become aware of problems that operators experience. For questions about available repair kits or potential solutions, please email Polyvance’s service team at info@polyvance.com or call at 1-800-633-3047