IMM 7/00

We are molding 12-mm-diameter polystyrene test tubes but have problems keeping the core perfectly straight. As a result, the tube wall becomes very thin-.5 mm. Any suggestions?

We have made several molds that have test-tube-like characteristics. One of the first things to watch for is the gate location. The gate should he on the top of the part to allow the material to flow uniformly down the walls of the part. This will minimize deflecting. The next thing to watch for is how well the core is retained in the mold. The core needs to have a minimal slip fit so that it resists sideward pressures. The core should also have plenty of bearing surface in the retainer plate. The retainer plate should he 1.5 times thicker than the shank diameter of the core pin.

Another consideration is the rigidity of the material out of which the core pin is made. The core needs to he made of steel that can be heat treated, preferably something that gets into the 55 to 60 Rockwell C range. We prefer S-7 heat treated to 54 to 56 Rockwell C. S-7 can be heat treated and still retain some toughness. Other steels such as M-2, A-2, and O-1 can reach into the low 60s and become very rigid, hut they also tend to become brittle.

Be careful about how large the waterlines in the core are. If the diameter of the hole drilled in the core is too large, it can compromise the strength of the core.
-R. Finnie, M.R. Mold & Engineering Corp., Brea, CA, (714) 996-5511, rick@mrmold.com

Core deflection generally can he traced to improper mold construction. As a molder of thin-wall tubes, jars, and containers, we have spent a lot of time on this problem. One central gate at the closed end of the tube with full perimeter venting at the open end parting line would he preferred; the second choice would be two suhgates 180º apart at the parting line, which would limit your venting capabilities at the closed tube end.

Core construction must provide stability and cooling. To obtain stability, the core should be one-piece solid construction with a shank diameter approximately three times its molded part diameter, and its bearing length should he a minimum of 1.5 times the shank diameter. To cool a small core as you describe, I would use beryllium copper material and hard flash-chrome-plate all external surfaces.

Lastly, don't forget process. You must keep the fill time to a minimum-just enough to avoid material heat and bubble problems.
-K. Gabrys, Tyco Healthcare, Crystal Lake, IL, (815) 455-4700, ext. 1124.

Here are two possible remedies to prevent core deflection:
1. Make the cores out of carbide, which is much stiffer than tool steel.
2. If the part design will allow, and if the gating is on the end of the part, put an inverted cone on the end of the core pin. This will center the core in the cavity with the incoming plastic force. As the core deflects to the side, the angle exerts force to center the core.
-J. Buss, Buss Precision Mold Inc., Clackamas, OR, (503) 652-5804, jbuss@bussmold.com

This is a problem that could be caused by gate location. I assume you are centrally gating the tube at the closed bottom. If this is the case, I would make the cores out of CPM 1OV. This steel from Crucible can be hardened to 58 to 62 Rockwell C.
-S. Jacobs, GE Industrial Systems, Bridgeport, CT, (203) 382-2049, steve.jacobs@indsys.ge.com

Add a valve gate to the end. The bearing length of the core should be equal to the molded part length. Clearance between the core and bore should be a minimum of .0002 inch. The stripper should be interlocked to the cavity, and components should be concentric.
- D. Tully-Bibber, Miniature Tool & Die, Charlton, MA, (508) 248-0111, mtd@miniaturetool.com

Use a very dense material such as Onviloy 1150 available from NSRW; call (800) 633-6290. This material has a density of 17.25. Tensile strength is 140,000 psi and yield strength is 125,000 psi.
-T. Jackson, GH Tool & Mold, Washington, MO, (636) 390-2424.

When molding a tubular-shaped product with a possibility of core deflection, the flow path of the resin must be concentric around the core. Pulling support pins can provide support to the core when the flow deflects a core pin, at a high cost. If the gate is at the closed end of the tube, the use of an AH Series valve gate (from Fisa Corp.) provides a positive, all-directional flow from the gate to fill the cavity evenly, preventing deflection of the core.
-B. Smith, Fisa Corp., Shelbyville, TN, (888) 684-3472, fisacorp@united.net

In many applications, our company has provided instrumentation for measurement of core deflection and pressure during the molding cycle. Taking these measurements can provide the molder with valuable information regarding when and under what conditions the core deflects.

Using pressure / deflection data, the molder can adjust process parameters (fill speed, pack pressure, and so forth) until the core deflection is minimized, and then set limits around the process variables so that parts with larger deflections are automatically detected.
-R. Wurst, RJG Inc., Traverse City, MI, (231) 947-3111.

I read with interest the possible solutions to core deflection in a test tube mold in the July 2000 issue of Injection Molding Magazine. I remember having a conversation with Jobst Gellert (Mold-Masters' president) about this topic 15 years ago. His comments made a lot of sense to me and none of the proposed solutions touched on the subject of symmetrical core and cavity cooling! I would suggest that the moldbuilder ensure that his cooling channels are balanced and symmetrical. If the cooling medium flow or direction is different from one side to the other, it probably does not matter how high a modulus the core material has; you will probably not prevent core deflection!

Another potential area that I speculate could cause the problem would be polymer temperature nonuniformity. This too could be responsible for unbalanced polymer flows in the mold and resulting asymmetrical forces on the core. If there is a hot runner system delivering polymer to the drop / gate, there is potential for temperature gradients in the polymer. Look around the area where manifold and mold plates meet. If it's a valve-gated system, look at the valve pad design. It should be designed to minimize heat loss from the manifold to the mold. Where are the manifold heater elements?

I must confess I have not bought a hot runner system in years and I am sure significant advancements have been made in their design. Nevertheless, temperature could be at the root of the problem.
S. Woloshyn, DuPont Canada Inc., Kingston, ON, (613) 548-5285, steve.woloshyn@can.dupont.com

I have seven years experience molding test tubes in styrene, propylene, and SB. It is my experience that the tool is the number one issue in producing even-walled parts. Processing is discussed after. The mold must be built to interlock all the cores / strip per bushings / cavities. It is much more expensive to build the tool that way, but otherwise, any mold with more than one cavity is going to have problems centering each and every core into each cavity.

The cores should be at least 2 inches in diameter in the core retainer plate. The nest in the core retainer plate should be snug fit, but not press-fit; you will want the core to float a little when the mold clamps up (by float, I mean the core must be able to rotate by hand while the mold is assembled, but open on a bench or press). The base of the core should be at least 4 inches long. There will be a taper at the end that goes through a stripper plate/stripper bushing. This angle should be large enough to allow for preloading without binding (I think 7º per side is what we used). To induce preloading, the core length in the base plate should be .002 inch longer than the core retainer plate.

The stripper bushings should be hardened H-13 to 54 to 56 Rockwell C. Otherwise, they will warp and crack. They should be tapered to fit the cores, with a slight over angle to allow shutoff at the base of the part. The mating side to the cavity will also contain a centering taper ring (all-one-piece bushing). This will cause it to nestle into the cavity. The bushing should be .001 inch larger in stack height than the stripper plate for preloading. The ring should rotate freely in the plate by hand.

The cavities should be generic, but have a nesting taper on the open end. The mold base should be all stainless steel 420. Cooling is regular closed loop from the tower (not evaporated water) at 7SF. A chiller is unnecessary if water volume is adequate. Use fountain tubes in cores to .200 inch from the tip, and use a spiral groove around the cavities. The stripper plate is not cooled. The part thickness should be about .032 inch, and can be tapered from top to bottom as necessary.

Please feel free to e-mail me or call if you have further questions.
-R. Mawhorter, Corona, CA, (909) 471-2898, romneym@hotmail.com