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Summary: Helmets whose outer shells are bonded to the foam by putting foam beads and the shell in the mold together to be steamed while the foam expands are strong and use all of the available space in the shell. This can be a superior technique for making helmets. It permits larger vents, a mixed blessing. It is also known as co-molding or molded in the shell. We decided to use inmold without the hyphen because it is simpler.
Manufacturers seem to have as many descriptions for the process as there are terms. Many of them are confusing, so we put up this page.
The process all these terms describe is essentially the same: a thin piece of plastic is placed in a mold, and conforms to the surface. Beads of polystyrene are then added to the mold, it is bolted together and then injected with steam under pressure. The steam expands the polystyrene beads, forming them into the shape of the mold, essentially making a completed helmet. The plastic piece was heated by the steam too, and it conforms very tightly to the expanded polystyrene (EPS) foam. It not only adheres to the foam, but leaves no gaps, filling every available bit of space with foam. You can feel this with your thumb, finding no "beer can effect" when the helmet is inmolded.
If all other things are equal, this process can produce a helmet with superior strength. The fusing of the EPS and plastic shell gives the helmet great resistance to cracking from an external blow, and the full-thickness foam liner maximizes the impact management for any given thickness of helmet. Because of the heat, the plastic has to be a higher grade such as GE's Lexan polycarbonate, since the plastics used for helmets with glued-on or taped-on shells would melt in the mold. In addition, a manufacturer can add internal reinforcing in the form of inserts of nylon, plastic or even metal to further strengthen the helmet. All of these innovations make the process much more expensive. Design and labor costs rise and the procedures involved reduce the number of helmets per hour that a single mold can make while sometimes increasing spoilage and rejects.
In a competitive market where few consumers are interested in paying for the safest helmets (as opposed to the sexiest) and fewer still are informed about helmet manufacturing techniques, the additional cost for producing a sophisticated inmolded helmet is difficult to recoup in the selling price. So manufacturers tend to reserve those techniques for their more expensive helmets. And the top of the line in recent years has been devoted to bigger vents, slimmer lines and lighter weight.
As the material between vents gets slimmer, manufacturers have had to increase the hardness (density) of the remaining foam to provide impact protection. The result is harder foam and smaller surfaces in contact with the head. There is no test in any US bicycle helmet standard for what we call point loading, but it stands to reason that localized loads from impacts are higher with the thinner, harder ribs. That makes no difference at all to a magnesium headform, but to your skull and brain it may. The Australian standard does have a test for localized loading, but we have been unsuccessful in promoting one for the ASTM or CPSC standards used in the US.
In addition, manufacturers dare not advertise that a given helmet is much more protective than their other models. They will be sued if they do, either because the rider was injured in one of their less protective helmets and obviously could have been better-protected, or because the rider was hurt in the most protective model and did not expect that after reading the advertising. That's why you never see such claims in helmet ads in the US.
The result of those competitive, marketing and legal factors is that helmets are not inmolded to provide the consumer with superior protection. They are made that way to permit the manufacturer to open up larger and larger vents, and to keep the helmet from breaking up in testing even though the material left between the vents is slim. A helmet with huge vents can be sold at a premium price. A safer helmet cannot.
The period of the safest-ever inmolded helmets probably occurred in the early 1990's while designs were still very conservative. The inmolded helmets of that period were round, smooth, had reasonable vents and only moderately dense EPS. In subsequent years they have evolved into helmets with very large vents and much less foam.
What does all this mean to a consumer? We think you should know that there is no guarantee that an inmolded helmet is safer than a cheaper model with a taped-on or glued-on shell. Both will be designed to the same impact standards. In fact, the thicker, softer foam and smaller vents required if the shell is not molded in may make for a safer helmet in low-impact falls or if you are a senior citizen with a more fragile brain. We would like to be more definitive about that, but we do not have any test data to back up our analysis. In short, our advice is that as long as all the helmets you are looking at meet the same CPSC standard, you can buy whatever fits you well, and whatever you think looks and feels good on your head, with the vents you require, regardless of the construction techniques used. But do check out our page on helmet ratings by Virginia Tech and Consumer Reports.
If you still have enough patience for more on what you should be looking for-but may not find-check out our page on the ideal helmet and our page on helmet construction.