One of the most detrimental challenges extrusion processors face is the occurrence of Die Drool, also referred to as plate out, or die bleed. Your company may even have its own name for it. It is the accumulation or buildup of materials on the surfaces of an extrusion die, at or near the die exit. It can be hardened or loosely attached, or a combination of both. Any way if found evident, there is cause for concern with pending negative effects on the finished product.
Drool can occur in all types of extrusion dies, anywhere a compounded production resin is being used. In most systems, the likelihood increases exponentially the longer uninterrupted production runs. Depending on the type of die, the buildup of material may be easily visible and just as easy to remove by scraping the material away. However, in many cases it presents a hidden problem waiting to happen, that cannot be overlooked.
Severity of resultant defects can vary. Defects can include die lines, blockages or stress concentrators in the finished product. Die lines can result from the buildups drag effect on the product surface. Aside from a visual defect, a die lines can further compromise the mechanical properties of the finished good. Loosened die drool may break off at any time without warning. With the intermittent result of a conglomerate of now dissimilar material being lodged somewhere in the extrudate. Examples include a fully or partially blocked off channel or passage in a profile or tube. This conglomerate can also act as stress concentrator, again compromising product integrity. Whichever the resultant defect, it can lead to a pile of scrap parts, up to including the absolute worst case scenario, an in the field failure for the end use customer!
The underlying or root cause of Die Drool is the separation of material components within a compounded production resin. As the material exits the die, the (abrupt) changes in pressure and velocity the resin undergoes can cause a separation of the compound. As the separated components get pushed out of way, they begin latching onto surfaces, then onto one another, typically in the out of the way lower flow/lower pressure areas of the die.
A solid understanding of the production resin and the compounded components is essential. If the problem regularly occurs even during a short production run, the compound itself may need to be altered or changed. Working with the resin supplier, removing the incompatible components, adding a compatibelizer or changing the compounding procedure may be the best solution. This is always a reasonably good place to start, with consideration for any limitations a change in material can present.
Further, optimizing die design can aide significantly to prevention. Reducing the amount of abrupt change, the material undergoes at the die exit, can often be achieved by increasing land lengths. Eliminating any dead spots or areas where separated materials can tend to hang up needs to be considered. Depending on required geometry of the finished product, another option includes adding specialty low friction coatings to the metal surfaces in these areas. They will generally help to promote flow and minimize the ability of any separated material to latch on.
During the course of a production run, adjusting the die temperature in areas where die drool is present may help to reduce or eliminate build up. Further, brief routine or planned shutdowns to allow for operator inspection of the die are strongly recommended. The results of the inspection, will dictate the next steps to take. Which could include removal by scraping or air stripping, using a purge procedure to clean the system, and effectively removing any buildup, and the least productive but sometimes necessary option of shutting the line down completely, to thoroughly clean the die. For the sake of productivity, the later should be a last resort, only if the situation has been allowed to progress to a point of no return.
As an added countermeasure, it also possible to detect resultant defects using downstream detection devices. These could include vision systems, mass detectors or ultrasonic gauging systems. With an audible alarm, alerting the operator to a problem, which can be potentially traced back to die drool. Routine testing of mechanical properties (as required), can also potentially indicate a problem has occurred. As with anything, prevention is the best course of action, and detection is secondary to further protect the customer.
Die drool is an inherent issue all extrusion processors must contend with. It is possible to deal with it effectively, by taking into consideration the preventative measures explained.
Best wishes for your many productive, defect free production runs!