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Dick Hussey's Tech Corner

Shrinkage (and Other Plastic Problems)

The term "shrinkage" often brings to mind one of the more memorable Seinfeld episodes. But in the case of Allantés, the issue is front and rear bumper covers. The thermoplastic resin used to make these bumpers was a blend of polycarbonate and polybutylene terephtalate called Xenoy. This was perhaps not the best choice. Over time, we are seeing several different problems as a result of the use of this material. It is brittle, too easy to break when installing and handling, and also it is prone to shrinkage.


The use of reprocessed material when these parts were being molded could have added to this problem of physical properties. In many plastic molding operations, defective parts are chopped up and the material is run back through as part of the resin mix to mold new parts. But there can be a problem with this. Reprocessed material usually does not have the same physical characteristics as virgin or first extrusion material.


GM found this out the hard way a few years ago with pulldown motor housings. Without warning, they switched from clear plastic to a black plastic for their pulldown housings. Some bean counter probably determined by doing this they could shave a few cents off the cost using recycled plastic rather than first extrusion material. In our experience these new parts made from recycled resin only lasted a couple cycles before they disintegrated or exploded. We had ordered twenty of these housings. By the time we learned of the problem, five had been used. All five suffered the same fate. The cost savings program probably did not end well. We never did get credit for the five broken ones, only the 15 unused ones we returned. At the time we returned them, no one else had reported a problem so I don't think they understood what was going on.


Rear bumper, very common


Front bumper


Rear bumper

Now 20 plus years after these plastic bumpers were made shrinkage is

showing up in several ways. On the front bumpers, we are seeing cracksthrough behind the headlight washer cutouts. With the cutouts, there is a natural weak spot behind the cutout. If the bumper cover shrinks enough, crack can develop behind the cutout. It is a difficult area to patch, as there is little room for repair materials on the underside of the cover on top of the bumper reinforcement.

Rear bumper shrinkage for some reason has been even worse. In the extreme case, we have seen several rear bumper covers that actually split into two pieces, down through the exhaust cutout. That has been a rare occurrence though. The biggest problem with rear bumpers has been on the radius on the corners. Many of them have vertical cracks there. Both my '91 and my '92 developed this problem AFTER they were painted, which was frustrating. But then I realized something else: we were not seeing these corner cracks on 1987 and 1988 rear bumper covers, only on the later years. That brought back something I had learned the hard way years ago: 1987and 1988 rear bumper covers were different than the later years. In my mind that made them less desirable, and I focused on the later years unless someone needed an '87 or '88 rear cover to be compatible with an early bumper reinforcement. Analyzing the inside of the cover and reinforcement when assembled showed why the early ones don't crack. The reinforcements are shorter and have rubber cushions inside the ends. My '91 is now sporting an '87 rear bumper assembly. Maybe next year the '92 will get one also.



If you have an '89 and up with original rear bumper, and if you are having it painted, there is something you can do to minimize the chance of the bumper cover later on developing a crack in the corner. Looking at picture E which shows the inside of the rear bumper assembly from an '87 Allanté, you can see the clearance and rubber mount which tends to insulate the cover from the reinforcement. By contrast, picture F shows the inside of the rear bumper from a 1989 and up Allanté. As you can see, there is no clearance between the bumper cover and the reinforcement. The cracks occur on the outside in line with the end of the reinforcement. The good news is that this issue can be easily corrected, and without buying another bumper,

although it takes some effort. Remove the bumper from the car, and then remove the bumper cover from the bumper reinforcement. Then cut off 2"

from the end of the reinforcement. Then grind a radius on the outside edge of the bumper reinforcement.

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