Posted by Roger West on March 20, 2003 at 10:55:19:
I enjoy reading the comments on SLOG. Sometimes, however, a comment is made that I feel needs correcting. I do not question the sincere intent of comments, so my comments should be taken as corrections to what I consider fallacious reasoning and not an attack upon a contributor. Recently, a contributor to SLOG, who recently received new panels, has had problems with them and has made comments on SLOG that I feel need to be corrected. The story starts when he received the panels and found the membrane tension too high. I sent him instructions on how to heat treat them, which he successfully accomplished. Within a couple of days the panels began exhibiting a series of stator insulation breakdowns, the cause of which will be a mystery until I can analyze them. The only clue we have is that the breakdowns are near where the stator wires are adhered to the horizontal ribs and are predominantly on the rear stator set (the panel half that the membrane is attached to).
My feeling that the heat-treating process might possibly be a candidate as a cause of the stator insulation breakdown problem was a speculation, not an accusation, as there were no other possibilities that came to mind. An event occurred two weeks ago that added another possibility. We had shipped panels to a customer and he experienced a similar problem. However, we were aware that the crates that the panels were packaged in were "flat-dropped" (panel back side toward the ground) from the bed of a truck just after being unloaded from the cold hold of an airliner. Several insulation breakdowns occurred very close to the junction of the ribs and the stator wires. I suggest that at the moment of impact the high inertial forces on the wires on the back half of the panel would momentarily cause the wires to bend downwards between supporting ribs. The sharp (90 degree) edges of the ribs would tend to concentrate great pressure on the stator insulation, which happens to be where the breakdowns occurred. Under these conditions it would be expected that damage to the stator insulation on the front panel half should be less because the initial impact would not push the wires toward their supporting ribs, but rather away from them, which is just the opposite case of the wires of the panel's back half. Quoting a comment taken from an email I received from the customer: "The first breakdowns happened to the rear stator tubes", which supports my suspicions.
The pernicious practice of flat-dropping crates has been a problem before on rare occasions. It is insidious in that the crate shows virtually no damage while the damage to the product can be considerable. I'm sure that the very cold temperatures of the cargo hold of a high altitude aircraft (it can be below -70F on international flights) could make the stator insulation more vulnerable to impact damage. It is very difficult to convince freight forwarding companies that it is their fault unless there was a witness. At this point, my feelings are that the damage in both cases was more than likely due to this mode of shipping abuse. It is not surprising that both we and the customer involved don't feel responsible for the damage. The breakdown may not be immediate if the impact does not fully break through the insulation as in the case of the customer's panels. It was more likely coincidence that the breakdowns started just after the membrane heat-treatment.
Back to the comments of the customer. He has made some erroneous speculations about the damage of his panels that I feel need to be corrected. First, he suggests that a surfacant (primer) has been used to treat the surface of the stator insulation so that the adhesive adheres to it. He mentions the presence of a white residue as proof of this. Fact: we do not use a surfacant. The white residue is a micro-crystallization of the adhesive (cyanoacrylate) due to application of an adhesive accelerator. Being a good insulator, it does not affect on the performance of the panel. The customer further states that the glue causes stress on the insulation and possibly damages it. Fact: the insulation has a low surface energy which makes it near impossible for cyanoacrylate to make a strong bond to it. This is actually a desired characteristic in our assembly as it eliminates any stress that could result from the adhesion process. The important point here is that if the adhesive doesn't bond to the insulation it cannot stress it. Furthermore, the adhesive is not a solvent of the insulation material and does not chemically attack it. In fact most acids won't even attack it. Continuing, the customer states that the stator wires would have less stress on them by placing notches in the ribs to hold them rather than using adhesive. Fact: The notches are designed such that the wires require a mild insertion force to install them. This force is not strong enough to invite damage to the insulation, but is sufficient to hold the wires in place during the manufacturing process. On the other hand, the adhesive approach is very similar in effect to the notched-rib approach as a consequence of the fact that the adhesive adheres well to the exposed rib surface between the wires but not to the insulation on the stator wires. Therefore, the adhesive forms a "notch" of precisely the same dimensions as the cross section of the insulated wire, hence there is no compressive force involved. The stator is held in place by a thin film of adhesive placed over the tops of the insulated wires at the point where they are adhered to the ribs. Thus, there is virtually zero stress on the insulation using the adhesive approach. Finally, the customer states that a destructive degree of force is caused by straightening out the mild curvature that occurs in the stator insulation as a result of taking it off of a spool. Fact: a mild curvature does exist in the insulation when it is de-spooled as would be expected. However, the material is quite flexible and the force required to straighten it is very minimal. I have demonstrated that the weight of a plastic ball-point pen is sufficient to remove the curvature of a 12" length of the insulation, hardly enough to cause significant stress. Furthermore, after the insulation has been threaded onto the wire over a reasonable time period the curvature relaxes and ultimately disappears. I won't quote some of the remarks that were made by people to whom this was demonstrated when I told them that someone had the opinion that the stress required to straighten the insulation is high enough to do damage.
People are certainly free to entertain their opinions, but when they are published, and we feel that they are misleading, it is necessary to correct them. Especially, if they are capable of damaging the confidence of our customers. With respect to the customer that has written his opinions to SLOG, we honestly do not feel responsible for the damage of his panels, otherwise we would have rectified it long ago. To my knowledge, in his communications to me and SLOG, he has not considered the possibility of hidden shipping damage, which I feel to be the most likely failure mode. I'm sure that had he been suspicious of this possibility he would have appealed to the freight forwarder for redress. Maybe, it's not too late. Nevertheless, we offered him our standard warranty repair. I might also mention that the customer gave a warning on SLOG not to heat treat a membrane if Sound Lab instructs them to do so. The heat treatment process is rather simple, but one must follow strict directions so as not to damage the membrane or stator insulation. We heat treat every panel we manufacture. It rarely needs to be done again. If it is required we are careful to make certain that the customer is capable of doing the job correctly. Otherwise, we suggest that the panel be sent to the factory to have it done. In the particular case of the customer, we recommended it as he certainly appeared to have the capability. In fact, according to his testimony, he accomplished the job properly.
The insulation material that we are now using has the finest characteristics of any that we have ever used. We have had over a year of full production experience with it and over three years of field experience with it. With the exception of the two cases mentioned and two cases where the insulation was externally cut, there have been no failures whatsoever. I have yet to find a failure that can be attributed to the insulation itself. It has demonstrated by far the highest reliability of any material that we have ever worked with. Concerning my remarks, if anyone wishes to visit our premises I will be most willing to demonstrate the substance of them.