Brittle Parts
IMM 9/99

My company has two 32-cavity molds that manufacture approximately 40 million of the same part, per year, combined. One mold is a three-plate design, and the other uses a hot runner system. The problem we are having is that both molds are using the same resin, same colorant, and making the same part, but the parts from the hot runner mold are extremely brittle by comparison and lower in tensile strength. The only difference, operationally, is that the hot runner mold makes parts at a much higher cycle time. Could the cycle time affect the tensile strength? Are we missing something obvious? Please help!

It sounds like the material's barrel residence time limits are being exceeded. The degradation takes the form of reduced melt viscosity, and this manifests itself most dramatically as reduced impact strength. Send a sample of your raw material and a part or two from each mold/process to a good plastics analysis lab for melt viscosity analysis. The good parts will have a melt viscosity that is between 60 and 80 percent of the virgin resin. The melt viscosity of the bad parts will be much lower relative to the virgin resin. You must reduce the cycle time. Consider the following alternatives:

• Reduce the melt temperature in the barrel and in the hot runner system to minimum acceptable levels.

• Increase the rate of cooling (e.g., higher flow rates will be better than lower temperatures).

• Run the hot runner mold in a machine with a smaller barrel. Your total shot size should not be less than ¼ the size of your barrel. Your resin supplier should be able to provide more detailed direction here.

While you are at it, verify that your hot runner system is properly sized as well.

If the previous issues are all within the resin manufacturer's specs, you may have to look into the resin's moisture content during molding. The use of overdried, redried, and/or recycled materials can result in similar phenomena.
-E. Parnagian, Hewlett-Packard Co., Andover, MA, (978) 659-2476.

My guess is that the cold runner, three-plate mold design uses up to 30 percent or more of the machine's barrel capacity with every shot. This will result in good molded parts, using the correct recommended melt temperature. When molding the same parts using a large hot runner manifold system and a longer cycle, the residence time in the system degrades the polymer. Hot runner systems also tend to have very small gates to prevent drooling, but they add to the shear stresses above and beyond a typical cold runner's three-plate design, further degrading the material properties. What I don't understand is why the hot runner system takes longer to mold the same parts than a cold runner system.
-R. DeBeer, Montell Polyolefins USA, Lisle, IL, (630) 960-118L


Hot runner systems do not run slower than three-plate systems unless they are improperly cooled. The very nature of molding with rnulticavity tooling often dictates that the runner will require more cooling than the parts, so eliminating the runner system should offer an immediate reduction in cycle time. Indeed, this is one of three major reasons for using a hot runner system. The others are elimination of regrind and reducing shear heating requirements / stresses by reducing the time the melt loses heat before it fills the cavity.

Do you have cooling channels located properly for the part and drops? If so, see that they're open and flowing to capacity. To verify, flow water through a pipe of the same diameter as the cooling lines into a bucket for 10 seconds (yes, you need a splash guard to keep dry and to capture the total output). Now do the same through the cooling lines exactly as they are hooked up. The output should be within 10 to 20 percent of the straight pipe to allow for bends and increased flow length. Greater differences need investigation.

Material degradation in a hot manifold/injection molding unit is the most likely cause of embrittlement, especially in some early manifolds that were prone to hot spots. Is the manifold sized for less than three, and preferably one, full shot? If not, this would be my first concern. How about the total capacity of the barrel and manifold in both shots and residence time? Are you running a larger cushion in the manifold tool? How do these pieces of information compare to the material supplier's requirements for processing? Some materials should be at processing temperature for only 10 minutes.

Are you using a material such as nylon that can be overdried? I have been involved with a couple of these situations where the molded parts needed to be shipped with wet sponges in the bags so they would absorb enough moisture to allow assembly without fracturing.
-R. Caufman, RC Marketing Inc., Warren, PA, (814) 726-7442.

Brittleness in parts that are molded in a hot runner tool often is caused by excessive temperature inconsistencies along the melt transfer surfaces, resulting in degraded material. Some resins are more forgiving than others. Commodity materials can usually survive the physical signs of material degradation. However, temperature imbalances can still exist in these systems, resulting in difficult mold startup, higher temperature settings, stringing, and material degradation. Engineering grade resins are not as forgiving and can display more severe symptoms, including gate freeze, uncontrollable drool, splay, discoloration, warpage, and other evidence of thermally degraded resin in molded parts. Thermal coordination can be optimized in an existing tool by reviewing mold design, material, and complete process conditions. Even hot runner systems that are thermocouple-controlled can have drastic temperature differentials as a result of their ability to sense only in the direct area of location. We recommend Duratherm Processing Systems Inc. in St. Charles, IL, which specializes in thermal coordinating systems and can offer a complete review and solution. Duratherm's phone number is (630) 443-9772.
-M. Frary, Thermal-Tech Systems, Geneva, IL, (800) 745-9350.

The properties of the processed plastic could be thermally damaged to a certain degree (even though degradation may not be visible). Any plastic/colorant will degrade if kept at the processing temperature for an extended period of time. You should check the design of the hot runner system, its runner size, volume, and flow length to see if the melt residence time in the hot runner system is too long. You also should consult with your material supplier about the thermal stability of the plastic/colorant, and the influence of residence time and processing temperature on melt integrity.

You mentioned that the hot runner mold has a much higher cycle time compared with the cold runner mold. Higher cycle time also means longer melt residence time, which might contribute to the material degradation, resulting in poor plastic properties such as low tensile strength and brittleness. You also need to check if the parts are properly packed out with the hot runner mold. Frequently, a hot runner mold requires more filling and packing pressure because of the higher pressure loss in a hot runner vs. a cold runner. A molded part will have lower tensile strength if underpacked.
-C.Meng, Mold-Masters Ltd., Georgetown, ON (905) 877-0185.

Parts from the hot runner mold that are both brittle and of measurably lower tensile strength indicate that the problem in the hot runner production is either moisture or temperature. Could the dryer on the hot runner setup be faulty? Alternatively, there may be a spot in the press with the hot runner mold, or perhaps the barrel size is larger, which leads to overcooking the material in the hot runner setup. Also, could there be a high-compression screw in the machine with the hot runner mold?

If no answer is found after checking the dryer for air flow, dew point, and proper residence time, look for some sort of temperature problem. With a hot runner system, the residence time of the melt in the barrel will be longer, possibly causing the problem.
-W. Foster, Tessy Plastics Corp., Elbridge, NY, (315) 689-2077.

Since the material is hotter for a longer cycle time, the polymer chains have longer to relax than in the three-plate mold, lowering the orientation and in turn lowering the tensile strength. As the temperature increases, the material volume expands, giving the polymer chains room to return to their random coil. If you want the two parts from different molds to have the same properties, you would need to balance the heat they both receive.
-D.Sweeley, BICC General, Montoursville, PA (570)368-3100 ext. 263.

Your questions raises many of my own, but I'll use the information provided. First, why si the hot manifold tool running a slower cycle? You need to identify if it is a cooling issue and make the necessary arrangements to correct it. Then, you need to find differences between the two molds, processes, and machines by breaking them down and analyzing the data. I would verify the actual melt temperature of the plastic in the hot manifold and barrel to ensure that the material is not over temperature and degrading.

Identify if certain cavities produced from this tool are brittle (repeat cavities). Check to see if the mold is balanced during filling, and if restrictions exist (metal damaged probes, and so forth); this will lead to unbalanced filling and packing of the parts. Generally, a machine running a hot manifold tool should be equipped with a nozzle filter. If this is a hot manifold to subrunner (cold runner) tool, make sure your gates are always sealed if required. There are many areas to troubleshoot, but this will get you started.
-E. Vandegrift, Calmar Inc., Lee's Summit, MO (816)246-3617.

Cycle time will affect materials in the same way that residence time will. With a longer cycle, residence time increases and you have a larger volume of material backing in the manifold. You might want to look into the possibility of using a press with a smaller barrel, or keeping your middle and rear zone temperatures on the low end to avoid degradation.
-F.Shaw, Gain Technologies, Sterling Heights, MI, (810)254-9000.

You are comparing a cold runner, three-plate mold with a hot runner for your 32-cavity molds. The brittleness and lower tensile strength you experience could be related to the hot runner type and design in your mold. Most likely your gate is too small. If it is a hot tip, the nozzle tip may be too far forward, causing too much shear heat. Send us the design for analysis.
-M. Hoffmann, Caco Pacific Corp., Covina, CA (626)331-3361.