Even considering all the many conditions that impact the life of your mold, the main enemy may be wear caused by differences in mold temperature and/or general mold alignment, and temperature differentials could be a conspirator in this plot.—Karl Szanto and Jerry Seidelman, Tech Mold Inc.
So your customer has sent you a new mold to run, or perhaps you have made the major investment in tooling for your own product; then comes the moment of truth. You take delivery of an absolutely beautiful mold and get ready to put it into production. It starts up without a problem—wow!
Now, just how can you be sure that the part quality you started out with will last over the long term? How long will it be until you start seeing dimensional problems, flash, damaged components, or even a mold that won't run anymore? Temperature variation or improper mold alignment may contribute to any or all of these problems.
When beginning the concept or design of a new mold, some of the most important factors to consider are the production process, the equipment that will be used, and the mold conditions required to run a perfect part consistently. It is at this point that consideration of methods used to establish thermal stability and alignment within the mold during production becomes so important.
First, let's look at the mold and items that affect cavity/core alignment:
• Guide pins and bushings. These provide the primary alignment of the mold halves.
• Parting line interlocks. These provide the precise centerline alignment between the A, B, and stripper plates.
• Interlocks for moving plates. Long "bar" interlocks can ensure centerline alignment of all moving plates.
• Interlocking A and B inserts. There are three elements:
1. Interlocking angles—These angles between the cavity/core components generally create the final alignment of the stack components.
2. Steel and hardness selections—The proper selection of compatible materials and hardness differentials in interlock situations is key to improving the longevity of any mold.
3. Cavity/core stack float—This is a method that allows interacting components to align themselves as the mold closes. If temperature differences between the A and B halves are anticipated, then this would be a very good option to consider.
Next, how do we overcome the effect of temperature change in stationary and movable mold halves?
• Mold cooling, including the temperature differential between mold halves. Remember, steel grows approximately 0.0000065 inch for every degree Fahrenheit of temperature differential. That doesn't sound like much, but when A and B halves vary by as little as 20 deg F and the mold is 30 inches long, the difference in plate lengths is about 0.004 inch. That is enough to start wearing of the alignment components. To extend mold life, try to keep the all of the mold plates (mold halves) at a consistent temperature.
• Hot runner system heating. Since these systems are driven by electric heaters, they can be a major contributor to mold temperature variations if proper cooling is overlooked.
• Insulator plates. As production begins, insulator plates (top and bottom of the mold) will create a temporarily controlled closed environment. But any heat generated within the mold will eventually migrate through the insulating materials into the press platens and affect the alignment of the mold halves.
Once all mold-related thermal and alignment issues have been considered and addressed and the mold is put into production, a whole new set of potential problems arise, all related to the molding machine and the mold installation process.
Hanging the mold. It is important that the mold is leveled when installed. This allows all moving components to operate with equal weight loading. It also allows the molded product to fall in the most predictable way. Integration of robotic part removal is also simplified.
Clamping the mold. Use enough clamps, equally distributed and tightened to the correct torque value. If one half of the mold slips, misalignment is immediate and your troubles are just beginning. Direct bolting is many processors' preferred method (no mold clamps used). This will most likely create a mold that is wider than the conventional frame that requires mold clamps.
Press knockout rod length. Make sure that enough KO rods are used. Make sure that they are all of equal length. This will eliminate uneven pushing of the ejection system and ensure minimal wear on ejector system components.
Mold weight. Make sure that large molds are properly supported. Remember, platen sag can occur on the stationary half of the press, too.
Worn platen bushings or frame support pads. This is often overlooked and can be the root cause of misalignment. No matter how good the mold interlocks are, they cannot counteract worn, ineffective platen-to-platen guiding components.
Effect of press platen temperature to mold alignment. Heat will naturally migrate from the mold to the press platens. Thermal growth and a natural misalignment will be the result.
Misalignment can be easily detected by simple observation-even when the mold is in production. Look to see if there is scuffing or galling on any of the alignment surfaces. Listen for a clunking sound as the various components come into contact during mold closing. Feel the machine frame to detect a distinct vibration when the various mold components come together. Excessive lubrication of areas that do show wear will only mask the problem and delay the consequences.
Mold misalignment can occur over a long period of time, or it can happen very quickly. Either way, any misalignment endangers the life of the mold, alignment components, delicate shutoffs, and the quality of the parts being produced. The benefits of running a trouble-free mold are obvious; increased productivity and profitability are just the beginning. Good care, an aggressive preventive maintenance program, and a vigilant production crew are the keys to running a mold that will provide a long, trouble-free production life.
Whether it's a complete mold or something as basic as leader pins, bushings, or parting line interlocks, always remember: You can't go wrong buying a high-quality mold and good-quality components from a dependable source.
Karl Szanto (firstname.lastname@example.org) is VP of operations and Jerry Seidelman (email@example.com) is sales and marketing manager at Tech Mold Inc. (Tempe, AZ).