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Changeovers: Speed and quality can be allies

Updated: Jan 21, 2020

What’s happening here? A plant for a global rubber molding business that produces seals or “diaphragms” made from rubber and fabric wants to reduce customer complaints, shorten lead times, improve on-time delivery, and reduce internal scrap. How can this plant, which produces millions of parts that are small enough to fit in your hand, define what to solve first and how to solve it effectively?

The Situation as observed on the shop floor:

The diaphragms, which mimic the diaphragm that operates human lungs, have applications ranging from engines to pumps to medical devices. They are made in two ways. They can be molded either from a roll of fabric already coated with an uncured rubber, or from fabric and rubber components put together in the forming process. There are many types of these materials. The manufacturing process has four steps: cut the fabric to size, preform the fabric, mold the fabric and rubber, and trim the part. The fabric is pre-formed with heat. The rubber and fabric are molded with heat (300-500*F), pressure, and time (up to 5 minutes to cure the rubber). This business has three types of presses: the pre-form fabric presses are electric and compressed air; the molding presses are electric and hydraulic, connected to a central hydraulic system by pipes plumbed through the ceiling; and the trimming presses are electric. There are 60 total presses in the plant, and in one-third of the presses the tool stays permanently in that press. There are 70 employees on two shifts. Each employee makes only the specific parts he/she is trained to make because each part is made from different materials on different equipment and requires a certain artistic skill to manage the inconsistency in the process. The employees are given incentives to produce high quality parts. What’s happening here?

The Findings

With so many little parts, is it possible that just one or two process changes can make a massive difference? Indeed, how does one know where even to start? The first step in investigation is to rank the parts by volume, sales dollars, lost pieces, and lost time.  This probe of each “family” of parts can begin to bring some order and logic to the hunt by identifying the family of parts with the largest opportunity for improvement. Identification can also be done by observing the activity in the plant. The most “troubled family” can often be the one with the most presses, operators, floor space, and (of course) customer complaints.  We used both investigatory processes in this case. The family of diaphragms selected as most likely to produce high improvement had generated 15% internal scrap and 18 customer complaints regarding quality or delivery in the previous 12 months. Any defect in the diaphragm which causes the diaphragm not to seal renders the part useless/ scrap. The defect can be caused by: cured rubber left in the mold, loose strands of fabric, uneven coverage of the fabric, uneven pressure, fabric that has lost its shape, uneven thickness of rubber, to name a few. The mold needs to be cleaned and treated each time before being installed. Once the mold is set in place, it must heat up before the operator can make a good part and get approval to run. The time to set up a part is 2-3 hours.

The Solution

A shorter changeover shortens the lead-time – the time between customer order and delivery of the order -- and thus inherently should improve on-time delivery, certainly a primary goal. Yet intuitively, speed and quality don’t seem to be allies, and simply changing over a part in the shortest time seems the reverse of how one makes good parts. It is not the time saved that reduces the internal scrap or customer complaints, but what best practices are revealed in getting to the fastest time. Reducing the time for changeover to a different part improves the quality of the part! Art Stout revamped the set-up process, saving hours of lost time in critical equipment and lost product. Cleaning and treating the mold while cold and using stop blocks to repeatedly set a pre-heated mold in the exact same place in the exact same press increased consistency and speed. Each part in the family of parts had two or more pre-form tools and two or more mold tools. Only one pre-form and one mold is used now. Guards to restrict air and contamination from blowing on the mold were installed on the presses.

The Results

The changeover time is reduced from 2-3 hours to 18 minutes. The internal scrap is reduced from 9 bad parts to 4 bad parts per hour. 189 more parts are made each day, increasing the daily output from 742 pieces to 931 pieces. 

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