The Troubleshooter, Part 37:Cold molding creates cracks

This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Bob Hatch is manager of technical service and customer support for Prime Alliance, the Des Moines-based resin distributor. Before his present assignment, Bob managed a molding operation for 25 years.

I got a call the other day from an old acquaintance about a new problem. He was getting cracking from the center of a part he was molding-out two directions to the outside edge of the part-and he was concerned because his company had been running this part for several years without any problems.

The material was one of the popular alloys that is made up of polycarbonate and one of the polyester family materials. It has always been a mystery to me how these material manufacturers can alloy two stand-alone materials together and get a new material that is better than the sum of the base materials. Anyway, this is what we had, and I needed to find out why molded parts that had been running fine in the past were now cracking.

The sprue, runner, and gates seemed to be OK-at least they weren't totally upside-down-and that says a lot for the toolmaker. I could see prominent knitlines and a textured surface that led me to believe this molder didn't have all the problems worked out with respect to getting rid of the air in the runner system, as well as the cavity areas of the parts.

I sent some of the broken parts off via next-day air to Marshtech, an independent lab in Minneapolis, MN, and waited for an answer. The next day I got a call from Erlene Marsh, who said "cold molding" was the problem. She said she could tell from the analysis that the part had been molded cold.

Defining the Condition
Cold molding is something I don't run into very often, so I was a little unsure as to what I should recommend to the molder as a corrective action. Sure I could tell him to raise the mold temperatures from 50F to 150F or something like that and walk away knowing I had helped him correct his problems, but I wanted to know a little more about this cold molding condition.

I called the material manufacturer and asked to speak to its cold molding expert and all I heard on the other end of the phone line was dead silence. The person I was talking to eventually told me they didn't know who that would be but they would find out and have that person call me. Well, I have been waiting more than two weeks for the call and haven't heard a word from anyone yet.

I called a senior technical representative from one of the companies I work with all the time and asked him what the big deal is with this cold molding condition. He said his company refers to it as a "cold flow" molding problem, which can come from either a cold mold, cold material, or both. He said the key is not going below the manufacturer's recommendations on material heats and mold temperature.

My take on this problem is that cold molding is a condition involving molded-in stresses resulting from someone trying to cool the material faster than it should be by running the mold at a temperature below that recommended by the material manufacturer. In this case, I noticed that the actual mold temperature of 50F was below the material manufacturer's recommended 90F.

Running lower mold temperatures has always been a productivity thing with many molders. Personally, I run barrel melt temperatures on the low end of the material manufacturer's recommendation and open the nozzle orifice and sprue diameter to remove flow restrictions-all of which help reduce pressure losses and shear points. Also, don't forget to open up the gates if needed.

I never tell anyone to run a cold mold to eliminate warpage problems. I tell them to warm up the mold to eliminate molded-in stresses. I prefer to think that the barrel heats cause warpage, not warm to hot molds.

Here we have a classic mistake. This molder tried to cool off the mold to get rid of warpage, but I would have warmed up the mold and lowered the material melt temperatures instead.

We were now at the point where I had done all I could with the information I had been given, so it was time to call the molder and find out what else was going on.

Not All Materials Are Alike
I made the call and got right through to the owner of the shop. I asked him about the cracking issues and what had changed in his operation that might have caused the cold molding problems that I discovered during the review of the parts he had sent to me.

He said he had been to a seminar recently and the seminar speakers had suggested running a cold mold to help speed up the cycle times on most of his molds. I asked him what materials the seminar speakers were talking about and he said the focus was on olefins. After reminding him that he had sent me polycarbonate/polyester alloy parts to review, I asked him why he thought running the mold temperatures cold on this material would be appropriate. He got real quiet and told me he thought it might work on all materials, not just olefins. I told him to remember that he shouldn't run the actual mold temperatures any lower than the low limit recommended by the material manufacturers, which in this case was 90F.

I asked the molder what mold temperatures he was actually running and he said 30F, which he picked because it is just at the freezing point of water. He felt his actual steel temperature would be more like 40F to 50F from heat buildup in each of the individual water circuits. This turned out to be true. The problem I have with this logic is that you should never have more than a 5 deg F heat differential between the inlet and outlet waterlines, let alone the 20 to 30 deg F increases he was seeing.

I complimented him on his logic and asked if he actually checked the temperature of the steel itself and of course he had not. He was relying on past experience to tell him what the mold water controller setpoint would settle in at for an actual temperature. I didn't want to tell him but even I cannot do that. Water circuits in these molds today are just too much of an unknown quantity for me to figure the setpoint, let alone someone else.

The Fix
I suggested to the molder that he raise the mold temperatures to around 110F, both front and back, and let me know how it worked out. I went on to review the little discrepancies in the nozzle orifice, sprue, runners, gates, and venting to give him some other things to do that would also help him get rid of the breakage and not end up with warped parts-you know, the little things that you need to run amorphous materials, such as big runners and big gates. For this mold, the nozzle orifice needs to be .290 inch feeding into a .312-inch sprue O diameter, with a .250-inch-diameter main runner coupled to a set of .225-inch subrunners.

The gate depth is 90 percent of the wall being gated into, with the gate width twice as wide as it is deep. Next, size the gate land so the length is half the depth, never to exceed .030 inch. All of this opened up his flow path to a more acceptable level so as to eliminate processing variables that work against us.

Venting on the runners should be .003 inch deep, as wide as the runner, with a .060-inch land, dropping into a .040-inch channel to atmosphere. Polish the vent lands to an A1 finish to make them self-cleaning.

Part vents should be only .001 inch deep, .200 inch wide, with a .040-inch land, dropping into a .040-inch channel to atmosphere. Draw polish the vent lands, as we did with the runner, to make them self-cleaning as well.

The customer closed out our conversation by telling me to put all these suggestions into writing and fax them to him so he could get with his toolmakers and start making these changes to the mold, which of course I did.

He called back a few days later and said he made the changes and couldn't believe how well the mold was running. That finished our cold molding review and, according to the results, we did better than just an acceptable job of getting the mold optimized and the process tweaked to achieve the kind of parts the customer wanted.

Editor's note: Bob Hatch's troubleshooting book, "On the Road with Bob Hatch," contains 100 case studies. It may be ordered from the IMM Book Club at www.immbookclub.com or by phone at (303) 321-2322.