a manufacturing plant that produced gasoline fuel dispensing systems—during which we toured the plant floor with a small group of engineers and managers. we stopped in front of a punch press, asked everyone to look at the operation and determine the percentage of value adding time (time when materials are being converted, altered by either machines or labor).
we then took out we stopwatch to time the operation. Two workers in front of the punch press proceeded to bend down and pick up a large sheet of thin stainless steel from the left side of the press. They placed the steel into the bed of the press. Then they removed their hands to press buttons outside the press, which indicated that their hands were out and clear of the press. The large press came down and formed the metal into a side of a gasoline pump. Then the two workers reached into the press, removed the formed sheet and placed the formed sheet at the right side of the press.
What was the value adding percentage?One engineer said, “100 percent, the workers never stopped working.” Another engineer said, “75 percent,” and another said, “50 percent.” we looked at our stopwatch and said, “Only 12 percent of the time was the process adding value.
Adding value is only when the dies are pressing against the metal to create the formed sheet, the rest of the time is waste.” Waste is defined as that which does not add value to the product—excess inventory, excessive motion, transportation, defects (producing scrap), waiting time (workers watching machines), overproduction, incorrect processing and the underutilization of people’s creative talents.
Then we asked, “What can you do to improve the value adding time?” An engineer immediately said, “You can place a table over here and put the raw inventory sheets on top of the table, so they wouldn’t need to bend down; they could just slide the sheets directly into the press.” Another engineer said, ”
We could install a leveler to automatically raise the sheet metal to keep it at a constant height.” A third engineer said, “We could put a spring into the back of the punch press to force the formed metal to leap forward after the stamping.” “Yes,” we said. “You all know what to do, so do it!”
This demonstrated the importance of stimulating others to think about how they could bring change to the production process without spending a lot of money.
Another punch press operation was observed. Now, we asked, “How long does it take you to make this changeover when you go from one product to another?” “Around two hours,” was the answer. “we want you to do it in less then 10 minutes,” said we. There was a lot of grumbling—especially from a worker who had been working on the press for the past four years and it always took him around two hours to do the changeover.
we proceeded to explain the process, “First, we want you to make a shim; cut out a piece of metal and attach it to the top of the next die that you are going to use. This shim will make each die the exact same height so that when you do the changeover, you will not have to adjust the cam.
You can then just pull out the old die and slide the new one in without any height adjustment. Second, when the previous process is complete and you are ready to do the next changeover, blow a loud whistle to attract the changeover team, time it with a stopwatch, chart the results, and post those results near the punch press machine. Third, put a metal block under the inventory so the sheet metal slides in quickly. If the thickness of the metal changes you can easily put in another metal block to meet that thickness.”
we discussed the value of examining each part of the changeover process to reduce the time it takes.
He explained external activities (outside exchange of die)—things you can do while the machine is running—including making sure all of the tools and inventory needed are ready and close by; checking to see that the next die is ready with all of the necessary parts; pre-heat the die if necessary, etc.; and internal activities (inside exchange of die)—things that can only be done when the press stops—including the use of cranes to lift the die or pneumatics to move the die around; change the clamping method to reduce the number of bolts and hoses, etc. we then requested to see the new changeover process. It was completed in 12 minutes.
First of all, it is important to point out that this method applies to a specific set of circumstances. Many molds changeovers can be optimized for maximum efficiency, but there are conditions that must be met before these techniques can be applied in the ways described. Here is a list of questions that must be asked during the development of fast changeover procedures:
Does the mold being used have a hot runner system? Hot runner molds do not completely adapt into the quick change-over system described, but with some modification to the procedures the change-over times can be reduced dramatically. The primary drawback is that not only do the screw and barrel require purging, the hot runner manifold itself must also be converted over to the new color.
Does the material being used require drying time? If so, what is the throughput used per hour? The methods described in this article cover the use of dried material, but part size and material throughput could affect timing and equipment needed for fast changeover.
Is the process being worked with stable? The primary goal of a process engineer is to develop a process that offers stability with any color. Stability is defined as a process that can be used to mold most if not all colors with minimal scrap and only small adjustments if any.
The injection mold industry has entered the era of transformation, low profit and shuffling. The management of the injection mold workshop needs to establish a “scientific, perfect, systematic, and standardized” operation management system to achieve a working atmosphere of “everyone manages everything and everyone manages” The department has the following measures and suggestions for improving the work efficiency and reducing manpower of each position in the injection mold shop.
Mold making is a challenging business. Not only is the process of injection molding incredibly complex from a technical perspective, but the business pressures are also significant. In a recent Tech-Clarity study, The respondents included a mix of mold makers that are part of a larger company (42% of respondents) and those that are independent job shops (58% of respondents). The research finds that while there are numerous challenges, there are also opportunities to stand out.
At many molders, once the press completes its last cycle, it typically is shut down and sits idle while the changeover crew takes out the finished mold and puts it away, locates and retrieves the next mold, stages any required auxiliary equipment, and finally conducts the set-up.
We recommend pre-staging the next mold changeover while the press is still in production with the current mold. Use a set-up cart that contains all of the tools necessary to tear down and set up the mold. The cart should include torque wrenches, impact wrenches, mold clamps and bolts, nozzles, cleaners, and any other equipment required. A member of the crew should go over the set-up cart during the pre-staging effort to ensure it isn’t missing any essential items. Pre-staging helped Donnelly shave as much as 33% off the average changeover time.
Scheduling and staging material in time is also critical. Many materials require pre-drying from 2 to 6 hr before processing. Failure to adequately schedule and pre-stage the material has the potential to idle a press much longer than the rest of the changeover activities combined.
Prioritize the set-up crew’s activities: Concentrate on getting the new mold up and running first, not on putting the just-removed mold into storage. There’s time for that after the changeover is complete and production resumes.
Sometimes mold set-ups are protracted simply because there are too few people in the shop qualified to execute a mold changeover. In even a small molding shop, it is not uncommon to have several machines due for mold set-ups at the same time. Yet many molders rely on just one or two specialists. That means presses may remain idle while specialists are bogged down during busy periods or difficult set-ups.
This can be avoided by training press operators to do mold changeovers. Donnelly has trained 40% of its hourly personnel for this task. Not only can all changeovers be done in a timely fashion, but set-up times have also been reduced since press operators are able to pitch in and help in all phases of the process. Shift supervisors now create ad hoc changeover teams of certified molding operators on a daily basis, rather than having only one person or one crew perform them.
I want to share a case:A trouble spot for many molders is mold storage. They often waste time finding and retrieving molds. Part of the problem is inefficient ways of storing and inventorying molds. Another part of the problem is storing molds too far away from the press. To improve mold storage and retrieval, Donnelly built what we call mold grandstands. They resemble high-school bleachers (see photo) and are located adjacent to the press. Every mold is assigned a place on a specific rack, and each mold is numbered. The most frequently run molds are stored where they are easiest to reach. So, when a mold is needed, one of the crew performing the changeover needs only to walk across the aisle to retrieve the mold. An overhead crane is used to transport the molds. We have separate grandstands for small and medium-size molds, while the larger molds are stored on a vertical racking system near the large injection machines.
Hot runners may be classified according to how they are heated. There two main types of hot runner systems:
Insulated runners
This system allows the molten polymer to flow into the runner, and then it is cooled to form an insulating layer of solid plastic along the walls of the runner.
Heated runners
For heated runner systems, there are two designs: internally heated and externally heated. The first is characterized by internally heating. The resistor is placed directly in the melt channel and heats the material from the inside. The second consists on manifold is heated with interior flow passages. The manifold is designed with various insulating features to separate it from the rest of the mould, thus reducing heat transfer.
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