Big Parts Are Not Difficult To Mold

By Bob Hatch

All it takes is the right sprue and nozzle sizing to correct cosmetic problems.—Bob Hatch

Since I am in the process of setting up an office for my new business, I had to dig through parts to see what was interesting enough to write about this month. The part I found jumped out at me as if it wanted me to tell its story.

	The part is a cover for something about the size of a student desk at home or in your local school classroom. The material appears to be ABS but it could also be PC/ABS or straight ASA, or even acrylic or impact acrylic. In this case I have provided the corrective action as if it were a PC/ABS; this will cover the bases for all the materials I mentioned. It appears the problem with the part is one of cosmetics, not warpage, which is usually the complaint with a big part like this one. There’s a heat bloom around the center gate in a circular pattern and splay further out from the gate but still radiating in a circular pattern from the center gate. This tells me that the molder is using way too much heat in the heated sprue bushing and machine nozzle to get the material to flow well enough to fill and pack this really big, flat part. The question I have to answer now is why the molder is using the extra heat in the processing setup.
A heat bloom at the gate on the front side (top) and splay radiating outward to all four corners on the underside (below) of a large PC/ABS part pointed to high processing heats. By matching the nozzle orifice and sprue bushing at the correct size, the molder could reduce the temperatures and shorten the cycle.	The problem is seldom caused by the molder. Nine out of 10 times the mold is the culprit and the molder just raises heats to get the material to flow easier. The mold then must be cooled to offset the cycle time issues caused by the higher barrel temperatures, and injection speed must be slowed to give the trapped air time to get out of the mold to avoid burns on the part.
	Sprue bushing Let’s break it down into two areas. First, we have the heated sprue bushing/nozzle orifice to troubleshoot, and then we need to look at the single center sprue gate being formed by the orifice in the heated sprue bushing. I know the problem is not in the barrel because the surface splay and heat blooms are only near or around the gate area. If the problem were in the barrel, we would have cosmetic issues all over the part instead of just at the gate. What could be wrong with the heated sprue bushing? I look at the orifice size and shape and then check the molding machine nozzle orifice to see if it has been sized to match the flow tube or bore diameter of the heated sprue bushing. Of course, I cannot determine the nozzle orifice size from the part, so all I can do is mention this to the molder to encourage examination of the sizing

For PC/ABS we want the flow tube diameter of the heated sprue bushing to be 10-14 mm. When working in English I recommend that the flow tube diameter be at least 1?2 inch for materials like PC/ABS and also for straight polycarbonates, especially the stiff-flow grades.

I worked on a project in Canada several years ago and we got by with a 10-mm flow tube diameter for a 2-3-lb part. The 10-mm size made the flow path just about .400 inch, which works well for easier-flow grades of PC or variations of PC. I like to err on the side of being too large rather than even slightly too small when it comes to diameters. Being on the larger size gives you the opportunity to correct the cosmetic defects as well as lower the barrel heats to the low side of processing temperatures recommended by the material manufacturer. When the heats are lower, you can usually cycle faster and, to most of us, this means extra profit.

Nozzle
Next, I check the orifice size and shape coming out of the heated sprue bushing. It’s tapered, similar to one in a cold-sprue-gated part, and smaller, but still of a size that requires operator clipping or flush cutting. This is a standard in our industry and appears to be a good choice for this part. A sprue-gated, heated sprue bushing would be a good choice here and would eliminate any secondary work. The valve gate should be big enough to function properly.

The second part of this tapered sprue review is to determine the diameter of the heated sprue bushing orifice at the point where it is attached to the molded part. Also, we need to know the diameter at the small end of this little sprue. According to my verniers, the tapered sprue diameter is .175 inch where it attaches to the part and about .090 inch at the tapered end of the sprue.

For a part with a nominal wall of .150 inch, these sizes are too small to fill and pack the cavity. The diameter of the small end of the tapered sprue should be at least as big as the nominal wall thickness of the part. Then apply the taper—usually .017 inch for each 1-inch length of the sprue—and you have the diameter of the sprue where it attaches to the part.

This short sprue is only 1?2 inch long, so the diameter of the sprue at the small end needs to be the same as the nominal wall, or .150 inch. The tapered end should be .150 inch, but I would break the rules here and make the large end of the tapered sprue more like .190 inch. Why not follow the rules? In some cases you just know from experience that the taper needs to be more than the rules dictate; this seems to be one of those times.

Cycle
When the nozzle and sprue bushing orifice sizes are correct, you should be able to run the heats on the front zone of the barrel, the nozzle, and the heated sprue bushing all at the same temperature, which will be 475°F for PC/ABS.

Once this is accomplished, we should see the cosmetic defects, heat blooms, and surface splay go away. If we have sized the flow path large enough, we should be able to reduce the barrel and heated sprue bushing heats to somewhere around 475°F. By lowering the heats, we can look at speeding up the cycle anywhere from just a little bit to as much as 25% faster.

The target cycle on a part like this is about 200 or 250 times the wall thickness, which gives us a target cycle of 30-38 seconds plus 5 or so seconds for the screw recovery and platen movement speed. This means we should achieve a total cycle time of 35-43 seconds once the machine factor is figured in. If the present cycle time is in the 1-minute range, which parts of this size usually are, then we have helped this molder earn more profit.

July, 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.