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Web-exclusive: Running a 24/7 injection molding operation

January 1, 2007

4 Min Read
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There are many ways to go from a five-day, three-shift (24/5) operation to running a full-time seven-day week. This is one of them.

A common practice currently is to run the traditional 15 shifts/week, then schedule some overtime for traditional workers and backfill the shortage with part-time workers. The drawbacks involved with this approach are quite acceptable if the production need is short term. To operate 24/7 in the long term, expanding capacity without purchasing additional assets, a more permanent solution is necessary.

Going to a seven-day week has one distinct advantage. Even though injection molding is inherently a non-continuous batch process, with so many parts produced per molding cycle, the production process itself becomes continuous. This avoids the headaches and wasted time of the Monday morning startups. Startups are still needed in the event of a full plant shutdown. Machines can be idle, but only for mold changes, maintenance, or from lack of work. Overhead expense can be spread over a seven-day week rather than a five-day week. All advantages also come with disadvantages. The primary disadvantage of a scheduled seven-day week is that there is no more ?weekend overtime? available when extra time is needed to fill backorders. The second disadvantage is maintenance: Machines don?t schedule themselves for breakdowns, thus your machine scheduling must allow for maintenance. Preventive maintenance programs now become mandatory. It is better to anticipate a breakdown than react to it.

When anticipating the switch to 24/7, gather data on machine use and machine downtime. Study the data to calculate how much machine time is available.

Making the Transition

Many industries run on a continuous basis. However, when running a foundry or electric utility, for example, only a single product is delivered. The literature is full of examples on how personnel are scheduled. The same fundamental approach applies to injection molding.

Under current practice, going to seven days/week would require scheduling 21 shifts/week, based on an 8-hour shift, three shifts/day, seven days/week. This is feasible for short-term production needs.

The new proposal is to run the operation with 14 shifts/week: four 12-hour shifts, two shifts/day, seven days/week.

The workers would be assigned to two teams: A or B. Each time is further split into two shifts: Days or Evenings. Shift changeover can be made either at noon and midnight, or at 6 a.m. and 6 p.m. The individual shift teams have their own first-level supervision. For upper and middle management, who work five-day weeks, a noon/midnight shift change allows contact with both the Day and Evening shifts.

The work week is shown in the table below. Paychecks are distributed twice a month, in this example on Wednesday and Thursday. By creating a worksheet with the calculations described below, a 3.5% savings in payroll based on the requirements of heads/machine/year is typical. Vacation time can be calculated at 40 of hours vacation for every 1000 hours worked, which is the equivalent of two weeks/year.

Implementing the 24/7 program requires educating workers. Absenteeism is costly for both the employer and employee. Causes include personal illness; staying home to take care of the children; medical, dental, legal, and educational appointments; or awaiting tradesmen for home repairs.

Each day worked includes overtime. Only the last four hours of the shift accrue overtime pay. Leaving early can mean loss of overtime pay.

Calling in sick on a three- or four-day work week means a reduction in pay greater than it would have for a five-day week. Since all workers get some non-weekend days off, appointments can be scheduled for those days.

In the event of personal sickness, the worker uses sick days or loses pay as they would under any other system, or they might be able to trade schedules with someone from the team. Having the changeover time at noon allows every team member half of the normal day for personal business.

Calculations

Business always comes down to the best economic investment for the best return. Set up a spreadsheet using the following variables as well as those that effect your company?s operation:

These numbers are generated on the assumption that the loading is one operator/running machine. Obviously those operations that are running as fully automatic as possible will have a much lower head count requirement/machine.

The machine running costs increase when running 24/7?electricity and other utility costs are a variable of the machine running, not the overhead costs. Overhead per machine (front office, rent, indirect personnel, etc.), machine utilization, and maintenance costs will go down because it is a fixed cost absorbed over a greater number of hours per year.

Calculations of people/machine and production hours/year must be done based on the specific geographic location as well as the need. Using the flextime headcount approach there are fewer people required. Your specific calculation should show that staffing a 24/7 operation in this mode costs less than stretching a 15-shift operation, even though both operations are paying overtime.

Bill Tobin is a frequent contributor and consultant who also publishes an independent monthly e-newsletter with tips on injection molding. E-mail [email protected] to request to be put on the circulation list.

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