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Since I first started working in food packaging, I have been asked, and sometimes told, how much more complex thin-wall molding is compared to other subsets in the injection molding industry. I always found it a bit comical because the laws of physics don’t change based on how thin a side wall is on a container. In fact, the one thing I like to point out in the food packaging industry is that polyolefin materials follow all the rules, unlike many types of engineering grade resins. This significantly improves our chances of predicting how process changes will impact part characteristics.

Thin-wall molding common rigid food packaging poses many potential processing pitfalls. Source: Netstal

I have run materials that cause the parts to increase in size after they have been demolded, and materials that if heated an additional 50°F will expand 1,000 times their size. At least with polyolefins, we can often know what we’ll get by increasing the barrel temperatures or decreasing the cooling, at least with a standard non-filled resin.

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Each of the subsets in our industry has complexities of its own, and each one requires learning and experience to master.

For this article, I thought it would be interesting to compare rigid food packaging to automotive interior molding, since they are the industries in which I have spent the majority of my 31 years in injection molding.

Cycle Time Rules All

The biggest issue is simple — cycle time — which makes complete sense. In rigid food packaging, we are running a 48-oz. container with an overall wall thickness of .024 inch out of polypropylene (PP) with a mold temperature of 50°F. It should be no surprise that the 6-second cycle time is just a fraction of the time we used to mold a steering wheel bezel with a .150-inch wall thickness out of PC/ABS with a Class A surface and requiring a mold temperature of 150°F.

Now the obvious point here is that it’s going to take a much longer mold-closed time to cool the thicker parts, but it also takes longer due to the mold temperature and the temperature of the plastic. “Less heat in/less heat out” is the adage I’m sure we’ve all heard at some point, and it is absolutely true. PC/ABS runs at hotter melt temperatures, so even if we were using it in the same mold, it would still take longer to cool the parts compared to a PP.

So usually if someone asks me the “what’s-harder-thick-vs.-thin” question, I go to the cycle-time answer. However, this is not me saying that thin-wall molding is more difficult or complex, it is just faster. I think it’s human nature to want to think that what you’re doing takes more ability or is more difficult than what others are doing. Ultimately, each of the subsets in our industry has complexities of its own, and each one requires learning and experience to master. Regardless of the market, we are all still focusing on the same four plastic variables: heat, speed, pressure and time.

As opposed to a mold for automotive interior trim that might have two or four cavities, rigid food packaging molds are usually multicavity stack molds. It’s easy for someone who has never run an interior trim automotive mold to assume that operating a two-cavity mold can’t be anywhere near as difficult as running a 96-cavity multilevel lid mold. That depends on what is going on inside that two-cavity mold. One of the most difficult molds I ever had to run was a single-cavity mold with so many moving components that the slightest mistake caused a crash.

I’m just making a point that injection molding, regardless of the industry or the customer, requires skill and critical thinking. You really can’t, nor should, generalize.

Most interior automotive trim components feature Class A surface finishes requiring forethought on design elements like gate location. Source: Engel

Feeling the Pressure

Thin-wall molding typically requires high injection pressures to fill those thinner walls, which can push many machines to the edge of being pressure limited. It’s not uncommon to have a fill pressure exceeding 25,000 psi with fill times that are under .25 second. Because of the thin walls, the flow length is much more critical than in thicker products. With thin walls, using too much hold pressure — or any hold pressure in some cases — can cause parts to warp because of molded-in stress. Every tenth of a second counts in this world. Fill speeds are increased to inject as fast as possible and produce a good part, and it’s not done for any other reason than to get as much time out of the cycle as possible.

Interior trim automotive molding with Class A surface finishes most often needs hotter mold temperatures in the cavities. This can help with hiding defects like gate blemishes, or if we are running glass-filled resin, it helps the plastic migrate to the surface to hide the fibers. Part design is critical for cosmetic reasons like hiding the gate and ensuring the heat from the plastic entering the cavity forming steel doesn’t cause a visual defect. Running melt temperatures that don’t impact the color is another factor. Too hot and a beige will turn slightly red, while too cold and it turns slightly green.

Plastic injection molding is a science, and that science is the same regardless of what end market you’re in or what part you’re molding.

Tangent: It’s impossible to adequately express my frustration when dealing with color and automotive parts. I once had a customer tell me that the black part I had just molded was too green. Never mind the part’s color, my face was red as I did my best to fight back my natural response after spending hours to eliminate a gate blemish due to the part being gated in a bad location. 

These are just a few examples of differences in the worlds of these two industries. They both have challenges that training can help with but require experience to understand and master. That said, plastic injection molding is a science, and that science is the same regardless of what end market you’re in or what part you’re molding.

About the Author: Robert Gattshall has more than 31 years of experience in the injection molding industry and holds multiple certifications in Scientific Injection Molding and the tools of Lean Six Sigma. Gattshall has developed several “Best in Class” Poka-Yoke systems with third-party production and process monitoring suppliers such as Intouch Monitoring Ltd. and RJG Inc. He has held multiple management and engineering positions throughout the industry in automotive, medical, electrical and packaging production. Gattshall is also a member of the Plastics Industry Association’s Public Policy Committee. He is currently the molding engineering manager at Amcor. Contact: 262-909-5648; rgattshall@gmail.com.