In this example of what 2 saved seconds of cycle time can do for the bottom line, this molder making proprietary parts can increase productivity by 21.3% without increasing machine operating hours. Increasing throughput rate offers the greatest cycle time improvement.

We all know that cutting cycle time raises our game, and that lower melt temperature cuts cooling time. In the first part of this four-part series, here’s the why and how-to of putting those together to pump up the bottom line.

Web-exclusive: A note from R.F. Dray This series of papers by leading industry authorities and educators will show the molder the way to substantially reduce cost by improving productivity and at the same time improve molded part quality. The most obvious way to reduce molding costs is to reduce cycle time. Melt temperature is the easiest way to reduce cycle time; lower melt temperature also improves part physical properties. The injection molding machinery manufacturers have provided injection units that are virtually in total disregard for proper melt temperatures and polymer quality. The screw designs, l/d, and rpm are at best going in the wrong direction. Example: Viscosity = shear stress/shear rate. Most machines today require high rpm to achieve required recovery rates. As the formula shows, increasing shear rate (rpm) decreases viscosity and thereby increases melt temperature. As will be shown, lower melt temperature and proper dispersion can be provided with modern screw designs, not with the short l/d no-purpose designs provided by many machinery OEMs. These papers describe the advantages of lower melt temperature and also show how, with a new injection unit design, molders can routinely: o Run regrind (100%) without contamination o Run polymers without drying, problem-free o Run mono or coinjection on the same unit without machine changes o Achieve part weight accuracies (.02g) that eliminate shorts and flash and therefore virtually eliminate scrap The first paper below, by Don Williams, CEO of Opti Temp Corp., shows the cost savings derived from lower melt temperatures and proper cooling methods. The second paper by Mike Sepe, corporate director of technology, Dickten & Masch Mfg., discusses how polymer physical properties are influenced by lower melt temperatures. The third paper, which is authored by John Bozzelli, CEO of Injection Molding Solutions, covers injection molding with lower melt temperatures. The fourth and final paper is by R.F. Dray, CEO of R. Dray Mfg. He will explore injection unit and screw design solutions for processing polymers at proper melt temperatures and quality.—R.F. Dray

The global economy and the increase in imports from countries with low labor and overhead costs make it more necessary than ever that U.S. molders increase plant efficiencies to be competitive.

In the injection molding machine cycle, about 85% of cycle time is consumed cooling the plastic from a molten to a solid state. This 85%, then, is the largest opportunity in the injection molding cycle for improvement.

One question arises instantly: Is it worth it? It certainly is. The most cost-effective way to increase profitability is to improve the productivity on existing injection molding machines by decreasing the cycle time on existing molds.

More parts or more time—it’s all good

Table 1 shows the inputs required to determine the costs and profitability for a 450-ton injection molding machine processing at a base rate of 100 lb/hr of high-density polyethylene, operating 24 hr/day, five days/week. The inputs are generally available from the machine setup charts and from the plant’s financial statements.

Table 2 shows the results of cycle time improvement. The example shown is a molder making proprietary parts. Because of the improved cycle time, he can sell 21.3% more parts to his customer without increasing the machine operating hours. An approximate 2-second cycle time improvement was made on this machine.

From the results, you can see that each second of cycle time improvement was worth almost $49,300 in increased annual operating profit, the total increased annual operating profit was about $96,600, and the annual increase in revenue was almost $204,000.

Another way of looking at cycle improvement would be a custom molder who has contracted for a fixed number of parts on the machine. Because of the reduced cycle time, the molder can provide the fixed number of parts in about 1000 fewer machine hours. These 1000 machine hours are available to be sold to another customer.

These profitability improvements will be different for each molder and each machine, and are dependent on machine size, utility costs, plant burden, operating expense level, machine throughput rate, and type of material processed. It is necessary for each molder to do this calculation for each machine in his plant.

Three steps to cool

Now that we have determined that increases in operating profit are significant when we improve productivity by decreasing cycle time, and that the cooling part of the injection molding cycle offers a significant opportunity, what are some methods that can accomplish this?

Reduce the melt temperature of the plastic. The amount that we can reduce the melt temperature depends on operating conditions and material. However, experience has shown that with some materials and screw designs, it is possible to reduce the melt temperature by as much as 100 deg F. If we take the same example of the machine operating above and can reduce the melt temperature by 100 deg F, we see from Table 3 (p. 92) that we have removed 6400 Btu/hr less heat from the material. If the load on the chiller is reduced by 6400 Btu/hr, then the molding cycle will be out of balance—i.e., with the chiller operating at the same temperature and flow conditions, the mold surface temperature will be reduced (less heat input to the mold).

Thus, we can speed up the molding cycle to increase the mold surface temperature to the original level, and bring the molding cycle back into balance. By decreasing the cycle to increase the heat input to the mold, we increase productivity 21.3% and decrease the cycle by about 2 seconds. From Table 2, this is worth an additional $96,600 in annual operating profit on the machine for a proprietary molder. Keep in mind that if we are heating the material to a lower temperature, we are also saving about 2 kW of energy per hour on the heating side of the process.

Ensure turbulent flow in all mold cooling passages. It has been the practice for many years in the heat transfer field to design heat exchangers so that turbulent flow occurs in the heat exchanger passages. One of the functions of a mold on an IM machine is to act as a heat exchanger. Thus, it is now widely accepted that the mold and cooling system be designed for turbulent flow in the cooling passages.

How can this be determined? Osborne Reynolds developed a dimensionless number, using the physical properties of the fluid in 1883 that describes the flow properties in channels based on the value of his number. This number came to be known as the Reynolds number. While there isn’t room here for a discussion of the Reynolds number, suffice it to say that this number should be between 5000 and 10,000 to be sure that turbulent flow is present in the mold passages.

So how can we determine that we have turbulent flow in the mold cooling channels?

• First, if ethylene glycol is being used in the mold, remove it from the mold cooling system and replace it with treated water and a corrosion inhibitor. At this time, all cooling channels should be cleaned to remove scale and other mineral buildup that causes resistance to heat transfer. Generally, running the cooling system with treated water (and a corrosion inhibitor) at 50ºF will provide better heat transfer and higher productivity than a system running at lower temperatures with ethylene glycol (to prevent freezing). Be sure that the freeze protection circuit in the chiller is set to shut off the chiller if the cooling fluid temperature drops below 45ºF.

• Determine the existing flow and required flow in the mold cooling passages. The existing flow can be measured by flow meter or weighed water test, and the required flow by calculation.

• Review the mold circuiting, so that turbulent flow can be obtained with a reasonable amount of flow and pressure, and be sure that flow and pressure are available from the mold cooling system. Make sure all hoses and supply lines are sized properly for the required flow, and are as short as possible.

• Check the fluid temperature rise across the mold. This should generally be less than 2 deg F. If the fluid temperature rise is too great, the plastic in the mold will see colder fluid at the entrance to the mold than what the plastic will see at the exit of the mold, which can cause stress in the molded part.

• A central chilling system in a plant is a network. When a machine is removed from or added to the network, it affects the flow and pressure of all the other machines in the network. Adding or removing machines from the circuit temporarily and checking the flow and pressure at other machines in the network can determine this. If flow and pressures do change at the individual machines, it may be necessary to correct the central system design. Methods of correcting this are to adjust the pumping, adjust the piping and sizing, or use isolators (true closed-circuit heat-exchange devices) at each machine to isolate the molding machine from the central system.

Design the mold for efficient heat transfer. It is important that cooling passages are properly sized and positioned such that the mold will remove the heat efficiently. Also, the mold must be designed for balanced cooling—about the same amount of heat must be removed from both halves of the mold. There are computer programs available for analysis of the heat transfer in injection molds.

If molders in the United States are to remain competitive in the global economy, it is important that they use all of the technology available to them to be sure the injection molding machine is running at optimal performance.

Donald Williams is CEO and founder of Opti Temp Inc. (Traverse City, MI; www.optitemp.com), a supplier of heat transfer solutions such as chillers, heat exchangers, and mold temperature controls. He can be reached at opti5@chartermi.com.