October 1, 2005
Creative thinking solved a problem with breakage at the flow line. What can you solve with a little brainstorming?
Weldlines are like death and taxes—the only sure thing in molding. When a flow mass is split and then comes together it forms a weldline. This is sometimes called a flow line. Usually the presence of this line is not a problem. If the masses are hot enough, the pressure high enough, and the cooling is even, there is a sufficient amount of welding that the strength of the part is essentially unaffected.
However, we should keep in mind how material flows. Think of a mass of spaghetti on your plate. You push it to one side and then put a saltshaker in the middle. With your fork you carefully move the spaghetti around the shaker until it has again joined together. Look at what you've done (besides making a complete mess of the salt shaker, ruining the meal, and annoying your dinner partner). The spaghetti is tangled together on the side you originally placed it, slightly oriented on both sides, and around the saltshaker but not as entangled where the two “fronts” came together. Thus the weld joint is weaker than the rest of the material.
The same thing happens in plastic: The molecules aren't as entangled in a weldline as they are in the parent flow path. Studies have shown the strength of the weldline is weaker than the rest of the part.
A weldline always shows up parallel with the line of the flow path. Thus if you have a part with holes in it, each hole makes a weldline. It is critical that the flow of material keep the amount of parallel weldlines at a minimum. A part with weldlines in different directions has less probability of breaking than one with all of them in the same direction.
THE BROKEN PIPE
None of this should be earth-shattering news to anyone in the industry, but let me relate a story that happened to me in Saudi Arabia. I was teaching customer service engineers for the largest resin company in the country. Since farming and irrigation consume large amounts of plastic in the Middle East, there was a small group of engineers whose entire assignment was to assist customers with PVC problems. Two were Saudi engineers. One, who was highly experienced, was an expatriate from the UK. Agriculture consumes PVC in two flavors: Extruded pipe and molded pipe fittings, either threaded or solvent-weldable.
When the course came to weldlines, the gentleman from the UK immediately went to his office during the break and brought back a T fitting for pipe. This was for solvent-welded, 4-inch-ID pipe with heavy (.25-inch) wall stock. The solvent-weld portion to accept the pipe was similarly large and had a proportionate wall thickness. It was gated at the parting line.
The customer had complained that the part split on the other side across the top of the T. This split occurred when the part hit the chute of the molding machine or was dumped into a box. The engineer had broken the part cleanly in half. A perfect impact fracture (shiny, with waves of propagation) could be seen at the split. Where he broke the part, the surface looked like a fatigue fracture (rough like sand paper). In all his years in the UK he had never seen this failure and was at a loss as to how to advise the molder.
The problem here was the customer was rushing. When you have thick walls it is a common mistake to have very cold molds and cool material. With a small gate that froze off quickly, coupled with the inability to properly pack a large mass of material, a very weak weldline formed but was covered by the print of the parting line. The part was highly stressed already so when the cores pulled, the part was cold. The impact either at the chute or the packing box caused the fracture that caused a leak in the fitting.
The cure to this problem is fairly simple:
Increase the mold heat.
Check the material heat (which was probably too low, anyway) and set it to the middle of the melt point range.
Do a part weight study, varying the packing time and weighing the parts to find out when the gate closes. All your packing must occur during this time.
Open the mold after the same period of time as with your original process conditions and eject the parts if they are solid enough. Keep in mind that the interior of the part is probably still liquid. Instead of ejecting them onto a part chute, eject them into a large water bath.
Put a wire mesh conveyor into the bath to extract the parts.
The higher heats will make a stronger weld. The water bath will make up for the extra time you would probably need if you had to keep the part in the mold until it was fully cooled.
When I told the engineer this, he said he had seen the water bath trick in Europe but that it never had occurred to him here. His comment was, “They haven't learned this trick yet, have they? This is why I've never seen this problem.” Bingo!
Often the answer to a problem is right in front of us, but we don't see it. Why? The two generic excuses: 1) We always did it that way; 2) it's convenient. We don't read the books, we look for the easy answer, or we don't take the time to understand the cause of the problem and find a creative way to eliminate it. These problems constitute the “consultant's full employment act.” Many of my friends and I make a handsome living doing this when the industry's mission should be putting us to work on new problems or putting us out of business for lack of work.
My only advice to people facing problems is to keep in mind that most problems are not new. With a little research you can almost always find that someone else has solved it, how they solved it, and how to adapt the solution to your problem.
Consultant Bill Tobin of WJT Assoc. ([email protected]) is a regular contributor to IMM.
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