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9/1/2010 | 5 MINUTE READ

Injection Molding: Time Now for a Safety Review

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For months many of us have been consumed with the Gulf oil pipeline blowout.

For months many of us have been consumed with the Gulf oil pipeline blowout. In a sense, what happened there can easily happen at your molding plant. Some of our machines work at five times the pressures that caused that oil pipe to blow. It’s time to take a break and review some safety items that apply every day to your job.

Safety is not a hot topic in our industry. There have been a fair number of articles about injuries and deaths at plants, but few on safety and prevention. Technology has changed your life and the equipment in your plant. One of the problems you must combat is that in your hectic workday you get lulled into a routine just like those workers on the Gulf platform. What are the safety issues, old and new? What you don’t know can hurt you—or someone else in your plant. It’s time we got proactive and started a Safety Review.

Let’s begin with the basic concept of what is happening within the equipment you work on or walk by every day. Nearly all plastic processing equipment has to melt and convey plastics. Melting means high temperatures—250 F to 800 F—and conveying means injecting or extruding at high pressures. The minimum pressure in injection molding machines is usually about 16,000 psi, and the maximum can be 30,000 to 50,000 psi.

Let’s put that in context. Think of the Gulf oil spill. Down a mile under the ocean, the water pressure is 2000 psi at the site of the pipe break, and the well is spewing oil and gas at even higher pressures. What would happen if a hydraulic hose broke releasing 1500 to 3500 psi of warm oil? Is it possible for the melted plastic at 250 F to 800 F and 16,000 to 50,000 psi to break loose and reach out and touch you? Those temperatures and pressures produce so much force that if even if a fraction of it hits your body you could lose a limb or your life. Unfortunately, it happens more often than you know. Ask some the processors if they ever saw a hopper get blown off the feedthroat into—or right through—the ceiling. Are there any telltale pockmarks in the ceiling of your plant?

These are a few points that I feel are important. Take them as a starting point, and email me any additions you might have to suggest. I will add them to the list and the updated version will be on my website (scientificmolding.com) under the publications tab for you to download and discuss with your crew and co-workers. If you find any points below that raise real issues for your workplace, establish a plan of action...now.




  1. Who is responsible for your safety? You are. Sure, the company might have to pay the hospital bills, etc. But it’s too late then; the point is to prevent you or anyone else from getting hurt.
  2. Does management allow anybody to tamper with or bypass safeties on the machines? If so, get your resume out and look for another job. Do not stay at that plant—it will be out of commission in two or three years.
  3. Is there a designated meeting area in case of an evacuation? Have you had a practice evacuation?
  4. Is there a mechanism in place to account for all those in the plant if any emergency does occur?




  1. Realize that most process equipment, an injection molding machine, for example, is top-heavy. If it going to be moved, make sure everyone involved understands this point and makes appropriate accommodations for lifting.
  2. Is the machine kept clean and oil leaks managed? With any new machine, install itwith an eye to making it easy to clean. Keep water lines, etc. off the floor so that it’s easy to get a broom, vacuum-cleaner wand, or mop underneath.
  3. Do all the safeties work; are they checked periodically?
  4. Are there visible bubbles or cuts in the hydraulic hoses? If so stop and replace them immediately. These are often signs the hose is degraded and perhaps ready to fail. If a hose busts, there is the possibility that the hot oil sprays you or the nearby machine barrel. The hot oil will knock you down with a thermal shock, and the loose hose could whiplash and cause a severe injury. If the hot oil contacts a machine’s injection barrel the heaters may ignite it. If the oil catches fire there is a good chance the entire building will burn down. Hydraulic oil is a huge fuel source, and if it spreads the plastic pellets will catch fire too. Oil and plastic pellets on fire are very difficult to extinguish.
  5. When installing a screw and check ring after cleaning or repair, before you bolt on the end cap, bring the screw to its forward or zero position. Then insert a piece of solder, Plastigage, or Silly Putty on the screw tip, and bolt on the end cap. Now remove the end cap to make sure there is clearance of around 0.060 in. between the screw tip and the end cap by measuring the thickness of the solder or putty. The screw tip should never touch the end cap.
  6. As the barrel is brought forward for the nozzle to mate with the sprue bushing, note whether the injection unit moves up, down, or sideways as it mates with the sprue bushing. If it does, it means it’s not aligned and you risk the possibility of a stream of molten high-pressure plastic coming at you.
  7. Are there platforms or safe means for working on machines or do workers have to climb on the press to work on it?




  1. Does your plant handle any fine powders such as colorants, fillers, or other additives? These may be dangerous if inhaled and can also contaminate bearings, oil, etc. on the machine.
  2. Some polymers are not compatible and can chemically react with one another. Slight traces of one can catalyze spontaneous degradation of the other. That degradation may generate gas, which can build up pressure that can blow the barrel apart, the end cap off, or the hopper into the roof. It is commonly known among experienced processors that switching from acetal to PVC or vice versa can be problematic. When changing over from one of those to the other, your best bet is to clean the barrel and screw completely. Several other incompatible combinations must be recognized for having this problem, too. Here are some that I know about:

  •  PVC and soft-touch materials such as olefinic TPV.
  •  Acetal and a number of soft-touch resins such as TPV.
  •  Alcryn TPE and TPV or acetal


Do I have any of these wrong, or do you know of others? Get in touch if you have anything to add.


About the Author

John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS, and other specialties. E-mail john@scientificmolding.com or visit scientificmolding.com.


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