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The Importance of Mold Design to Productivity
   by John E. Haley, AQA Corp.
   Production   Fall  2009
  
The importance of mold design on the productivity of a tool is often overlooked in the design of a mold. Several areas in the mold design exist where the molder must work with the mold builder in order to optimize the productivity of the mold. A good standard for mold productivity is saleable parts out of the press per hour. Cycle time and part quality are the critical aspects of saleable parts per hour. The areas of design found to be most important for increased productivity are the sprue bushing, runners and gates, hot manifold, venting, cooling, and ejection. While each of these items is specific to the mold being built, good design for each can contribute to improved part quality and optimum cycle time.

Characteristics Critical to Mold Design
The first and most important area in mold design is the size of the sprue bushing, runners, and gates. This criterion is critical to ensure that the part can be filled stress-free, packed out, and frozen off to obtain the optimum molded part. With the use of mold flow analysis and the previous work of injection molding professionals, the correct size of the sprue bushing, runner, and gates for each material molded can be determined quite accurately. Following the guidelines for these sizes is most important, especially for the gate land length dimension, which should never be greater than .050" and should be as small as practical. Sprue bushings that are too small will create excessive heat and pressure in the plastic. A good starting point is to make the “O” (inlet orifice) diameter the same diameter as the main runner.

Another area that relates to the sprue bushing is the nozzle. Often, molders will change molds without ever looking at the nozzle size and making sure it is matched to the mold they are running. Many times, this will lead to excessive pressures and temperatures of the material. Always matching the nozzle diameter to the sprue bushing “O” diameter is imperative.

Too often the mold maker is left to decide the sizes of the sprue, runners, and gates and only when running the first samples does the molder learn that the sizes are not optimal. Much of this can be resolved beforehand by following the principles of runner and gate design found in the Injection Molding Handbook1, as well as other reference materials. Again, runners sized too small affect the heat and pressure of the plastic and runners too large may slow the cycle for cooling time and cause unnecessary regrind.

The following chart can be used as a guideline for runner sizes, depending on part size and the length of flow of the runner. If this data can be determined from mold flow, then that is a better place to start because it uses the rheology of the material being molded.

Gate location and gate size have a large effect on the fill of the part. Gate type and location should be the first determination when designing a runner layout. This is especially true in multi-cavity molds and large parts. Many different types of gates are available to the designer. The one selected should be determined by the part geometry, as well as the effect of the gate type and location on part dimensions, material flow, and part appearance. Most problems such as part appearance, warping, long cycle times, and cycle-to-cycle part variation can be attributed to some degree to the size and location of the runner and gate. A good rule of thumb is to start with the gate at 60 percent of wall thickness for crystalline materials and 75 percent of wall thickness for amorphous materials. Gate width should be two to three times the gate height when using a tab gate.

Hot manifolds, too often, are only available to the custom molder when the customer is willing to pay for them in the initial tool. However, the advantages of hot manifolds to the custom molder, including reduced cost of molding runners and sprues, as well as handling and remolding of regrind, usually pay for a hot manifold within a relatively short period of time. The hot manifold is usually of no cost benefit to the customer, given the way plastic parts are quoted. It should be used to increase productivity for the custom molder through the elimination of regrind. The tendency is to look at the capital outlay of adding a hot manifold in the mold design. Often this makes the tool quote non-competitive.

However, as is usually the case, there is a cost to not doing something. This cost can be determined as the cost of regrind. Regrind is the most expensive material molders use because they have already paid for it, molded it, reground it, and then have to reprocess it. Sometimes it is more difficult to process than virgin material. If the yearly volume of a job is high enough, it is a good investment for the molder to include a hot manifold for the mold, even if it is at the custom molder’s expense.

Venting is a topic that is often neglected by the mold builder or at best is an afterthought. Vents need to be included in the mold design and put into the mold so that they eliminate the hot gases generated during injection. The number of vents should be too many rather than too few. An accepted rule is one vent for every inch of part periphery. However, too often, vents have lands that are too long, not polished, and do not lead to atmosphere. So while the mold may look good, if it is not made properly it really isn’t doing much to eliminate trapped gases. Going to the trouble to install proper vents both for the part and the runners will pay off in increased part quality and decreased cycle times. Many times, parts can be vented at ejector pins, slides, and lifters. Following the raw material suppliers’ recommendation for vent sizes will help eliminate flash. Finally, vents can be neglected when performing mold maintenance; they should always be checked and repaired if necessary.

Cooling takes up the greatest portion of the injection cycle and is as important to part productivity as any other aspect of the molding cycle. The size and location of water lines are critical to optimizing part quality as well as cycle time. The best cooling for the part is to make it as uniform as possible in order to eliminate molded-in stresses and non-uniform shrinkage. In reviewing the mold design, it is important to determine if there are adequate cooling lines that are optimally located to provide the most efficient amount of heat transfer for the parts in the most uniform manner possible. The water line size should be large enough to allow flow that can keep the mold at a constant temperature during molding. The use of quick connects should be specified so that they don’t restrict the flow through the water lines. Making sure each mold half is correctly plumbed so that the mold isn’t simply turned into a water heater also is important. Looping A and B halves together makes it almost impossible to keep each mold half at the same temperature.

Following the resin manufacturers’ recommended mold temperature will help optimize both part quality and cycle times. Including a plan for water flow for each mold half is always helpful to the molder and should be required in the design.

Ejection is dictated by part geometry. This is an area where the part designer and the mold designer need to work together, if possible, to ensure that the part can be ejected easily from the mold. Unfortunately, this cooperation is not always possible. Adequate draft and the elimination of any unnecessary undercuts are both extremely important to ejecting the part without distortion or stress marks. Guided ejection also is important in mold design. This feature is worth the cost in savings of damaged pins and uneven wear on ejector pins.

In more complex molds, the relationship between the ejector pins and other moving parts, such as slides or lifters, must be carefully checked so that there is no interference. Using the largest possible ejector pins is recommended to alleviate stress marks. More complicated ejector systems, such as two plates and accelerated knockouts, are used often and with great success. What’s more, most of the mold base suppliers have components that make these systems fairly economical and easy to build. The most productive ejection system is one that operates automatically. However, this is not always possible. Through the use of automation, molders have been able to develop systems that interface extremely accurately with the ejector system and do not require operators.

So many more points can be made about the importance of good mold design in relation to part productivity. But if these basics are followed, mold makers will be well on their way to making a productive mold. It is always amazing to see the same mistakes being made when there is no good reason for it, and it always seems to be at the expense of the molder. All of the above items affect the size of the processing window for making good parts. By optimizing these design features, the molder is given the best conditions possible for productively processing quality parts.

John Haley is chief engineer for injection molds at AQA Corp., with over 27 years of experience in molding production and in-mold design, engineering, and manufacture. AQA Corp.’s mission is to support the U.S. molding industry by supplying quality molds from off-shore at substantially lower cost relative to local sources. AQA constitutes most of the links in the supply chain. For more information, call (734) 222-8700, email jhaley@aqa-us.com, or visit www.aqa-tooling.com.

Reference
1 Injection Molding Handbook, Rosato et al, 3rd Edition, Kluwer Academic Publishers