Good parts come from a stable process. Let’s assume you have been running for a while and then one of the scenarios mentioned causes a shutdown.

Look at your residence time. Just to keep away from the algebra of calculating the density differences from the barrel capacity (which is rated in styrene) and any other material, let’s assume you have a 6-oz barrel and a 20-second cycle. Your shot weighs in at 1 oz of styrene. This means there are six shots in the barrel and a 2-minute residence time (6x20=120 seconds= 2 minutes). In the most general case, plastic soaking at full heat will begin to color shift or degrade after 5-10 minutes. If you’re shut down longer than that, it’s appropriate to purge at least half the barrel’s capacity. On the other side of the machine you’ve got the mold cooling. Under production conditions, the cavity surface is usually hotter than the cooling temperatures because of the temperature gradient between hot plastic and a relatively cooler mold. Again, this takes a few minutes to come to equilibrium.

With all that said, if it’s a stuck part that only takes a few minutes to remove, you can probably start right up. After you’ve taken enough shots to empty the barrel, you should be back to a stable process.

Power outages are a problem because the barrel will probably be at full temp while you’re starting up all the machines. As soon as the power comes up, pull the carriage back, move the ram to the full forward position on all machines, and bank the heats down to around 250-300°F before beginning a startup. Most materials can heat soak at that temperature for several hours and not degrade. Since most people overcool their molds, if your chillers don’t have an automatic shutdown after a power outage, turn them off until you’re ready to start the machine in production. Otherwise, you will have an extremely cold mold and a difficult startup. It’s a lot easier to start up 10 machines after an outage when you don’t have frost on the mold and charcoal in the barrel.

Purging is only required when you are changing material or if you’ve burnt the material in the barrel. At full temperature, heat-soaking nylons, acetyls, and PVC for more than 10 minutes will cause (literally) an explosion of nasty fumes and burning material flying everywhere. If you’re molding these materials, and you know you’re not going to get around to these machines in less than 20 minutes, ideally you should fill the barrel with purging compound and let it heat soak until you can get around to starting it up.

While dimensions will change in the first 24 hours after molding, the first 80-90% of shrinkage will likely occur in the first 30 minutes. The exception to this rule is nylon, where it will grow as it absorbs water. Make note of the dimension of a good part hot off the press that you know will ultimately produce a good part after it shrinks; once you can achieve this hot dimension with three consecutive shots, the process is probably stable enough to resume production. How many shots that will take depends on the mold design, part thickness, part temperature, and a host of other variables unique to each part.

This answers the “How many parts do you throw out?” question. If you’ve shut off the cooling and had to bank the temperatures because of a power outage, you’re essentially starting from scratch. But remember, if you turned things down/off, you need to turn them back on again when you start up. This is the most common problem when a power outage occurs: First, people run around to lower all the settings and shut off the cooling. But many forget to recheck all the settings when the power comes back on.

So how do you recover from a disaster? It’s simple. First, have good documentation. You can’t get back to where you were if you don’t know where you were. Second, get to a stable process. When your machine produces the first good part, don’t be silly enough to believe the next hours of production will be the same. Hang around for several more shots and make sure everything is stable.

Which machine do you start up first? It’s fairly easy to train everyone to initially run around and bank the heats, reset the mold cooling, and purge barrels if you lose power. Have the knowledge of what parts are needed by your customers and what shipments are critical. Start those machines first and the others later. Starting up every machine quickly and spending the next 12 hours troubleshooting scrap isn’t a good investment of your talent.

Consultant Bill Tobin is a regular contributor to IMM. You can sign up for his e-newsletter at www.wjtassociates.com.