The Troubleshooter: A Troublesome family mold

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 technical programs manager for resin distributor Channel Prime Alliance. Before his present assignment, Bob managed a molding operation for 25 years. You can reach him at [email protected].

Parts of varying wall thickness should not be run in the same tool.

I was asked to stop in for a visit at a molding shop the other day. While touring, the molding guys gave me parts to look at that were running poorly. The ABS parts ran in a three-plate family mold and looked nasty—full of splay—and were hard to fill.These folks were not new to molding. They had tried everything they could think of to get these parts to fill and pack. This was a tough problem.

I asked everyone to leave me alone so I could examine the machine and auxiliary equipment. A quick check like this often points to a problem area, although sometimes it misses subtle clues as to what is causing a problem, but it was all I had time for.

Two Thick, One Thin

After this check, I looked at the mold, doing my thing with the nozzle, sprue, runner, gates, and vents. I could see that two of the three parts had wall thicknesses of .240 inch, and one part had a wall thickness of .125 inch. The cycle time to produce an ABS part with a .240-inch wall is roughly 60 seconds, while the cycle time of an ABS part with a .125-inch wall is 30 seconds.

This is what is so wrong with a family mold. The customer pays too much for the thin-wall part just because it is in a mold with the thick-wall parts. Family molds have other issues too, but I won’t go into those here. Just know that I don’t like family molds. I looked at the three-plate runner and could see that the gates were too small for the thick-wall parts and too big for the thin-wall part. The result was that the molder was overpacking the thin-wall part and underpacking the thick-wall parts. The feed point to the runner was also undersized. This meant that with four runners coming off the sprue, the sprue O-diameter would create a shear point and lose pressure through the runner system.

Another problem was that the ends of the runner drops were not fully tapered. The ends of the drops should be tapered all the way down to the surface of the part, which gives us about a 30° tapered land and an actual land length between .002 and .004 inch. The ends of these tapered drops should look like a retractable ballpoint pen with the point retracted.The drops had a flared version of what I call squared-off ends, which gave us a 90° land with an actual land measurement of .060 inch—a far cry from the recommended .002 to .004 inch. The drop diameter was .175 inch, partially tapered to a .105-inch gate diameter. This gate diameter should have been .105 inch for the thin-wall part and .200 inch for the two thick-wall parts. Each of these gate sizes would then have to be fully tapered down to the surface of each part.

Of course, we would have to open up the drop diameters by .075 inch so we could increase the gate diameter at the bottom of the drops to .200 inch; this would accommodate the increase in gate diameter for both thick-wall parts. Then, we’d need to re-taper the entire length of each drop so no drops stuck in the mold.

The feed point or sprue O-diameter that the heated sprue bushing orifice fed into was only .200 inch to feed four trapezoidal runners of .240 inch deep each. Since four runners came off this short sprue, the sprue O-diameter should have been 30% larger than the .240-inch runner depth. This would put our new sprue O-diameter at .312 inch. This meant the sprue O-diameter was .112 inch too small. Also, at .150 inch, the heated sprue bushing orifice would need to be opened up to at least .290 inch, but it was not something we wanted to do until after the sprue O-diameter was enlarged.

This mold makes a strong case for buying a heated sprue bushing with a flow tube diameter that is larger than you think you’ll need. The molder had to find out if the heated sprue bushing in this mold could be opened up this much. Some can and some cannot.

Make it Match

Another important point is that the machine nozzle orifice needs to be opened up to match the flow tube diameter of the heated sprue bushing, which, in this case, would probably be .375 inch. Check the published specs for the heated sprue bushing being used to find out what this flow tube dimension really is, and then match that diameter with the nozzle orifice diameter.

A mismatch at this point will cause extra shear points, pressure loss, and dead material areas that drag out degraded or previous color materials into the new material’s flow stream. This can cause quality issues and also makes it difficult to change colors. Sometimes, if you do not make these changes, it takes as much as 24 hours of running production to get the old color cleaned out of the heated sprue bushing flow tube; the same is true with a hot runner system. This is a high price to pay to clean out the flow tube between colors or when changing material. It’s much better to make the suggested dimensional changes. If these changes are made correctly, more ABS can flow through the heated sprue bushing in the same fill time, fill and pack will occur more easily and quickly, and a faster cycle is possible.

Another issue I should mention is very important to toolmakers: Sucker pins should not be placed directly over the drops, as they were with this runner system. These poorly placed sucker pins restrict the flow of material to the drops, possibly causing pressure losses in the drops. Put the sucker pins just outside of each of the drops. I could tell right away that this had to be an inherited mold. The guys in the shop were sharp; the other molds being run were close to the dimensions needed for the big five—nozzle, sprue, runner, gates, and vents. I am happy to report that the toolroom made the changes I suggested and within a few days they had the mold back in the machine and had good parts by the third shot.