Do It Right the First Time

Finding the cause of failing hinges and cracked makeup jars.

By Victor Suben, P. E.

The development of the right packaging for cosmetic and personal care products must carefully balance function, price and aesthetics. The consideration of one aspect to the exclusion of the others will almost always result in a design that fails on some level.

Because beauty products have complex formulas, basic compatibility between contents and container is one of the first questions to be answered.

What’s Wrong with that Compact?

A certain cosmetics manufacturing firm produced a pressed powder face product that was sold in round compacts. These were shiny compacts with a hard surface to resist scuffing. The compacts had a tortoise shell appearance that was created by mixing pigmented and unpigmented resin in the hopper of the injection molding machine. To achieve the shiny surface finish and to meet price constraints, the compacts were made of general purpose styrene (GPS).

The mass market product was one of the best selling compacts of this type, but despite the popularity of the pressed powder, there was a problem. Each month, there were complaint letters that described how, after a few uses, the compact’s hinge would break. Rather than discard the package, the consumer often said she continued to use the compact by holding the halves together with tape or rubber bands. These complaint letters were especially troubling because they were far more numerous than the complaints for any of the company’s other products. Clearly, something had to be done.

Finding the Cause

At the time, we did not think to review the face powder’s ingredients. After all, it was only a powder designed to be applied to the face. It couldn’t possibly have any ingredients that could adversely affect a plastic material, right?

We attributed the problem solely to the compact. These compacts were produced in several tools, each of which had six cavities. Our first step was to carefully measure compact covers and bases from each cavity of each tool to make sure they still met the specification’s requirements. These measurements were made with an optical comparator to ensure that the measurements were made as precisely as possible. We found that for the most part, the dimensions met the specifications.

After measuring the compacts, our next step was to attempt to determine whether any of the cavities was producing parts with inherently weak hinges. We had special fixtures made to mount the compacts on a Universal Test Stand. By subjecting the compact hinges to tensile stresses on a cavity-by-cavity basis, we found that their strength at failure was fairly consistent. The data also showed that the material was brittle. Instead of stretching and ultimately tearing, the styrene would reach a point at which it would just snap. Failure occurred at fairly high forces in the 40 – 60 pound range.

Clearly hinge failure could not be attributed to either dimensional inconsistency or weak hinges. Although many of the complaint letters stated that hinge failure occurred after just a few uses of the compacts, we though the problem might be fatigue of either the plastic or the metal hinge pin.

We designed and had built an apparatus to automatically open and close the compacts. This pneumatically operated device could open and close about 10 compacts at a time. It was connected to a counter so that we would know how many cycles a compact went through before the hinge failed. Because we wanted answers in a hurry, we initially set a fairly high cycle rate. It should be noted that we tested empty compacts.

When failures occurred at the high cycle rate, we examined the cause of failure and found that the plastic had melted as a result of heat built up due to friction. So we reduced the cycle rate and resumed testing. After days of testing at the reduced cycle rate, we had not induced any hinge failures. We thought that perhaps failure could be induced by performing the testing at an elevated temperature. After days of testing at elevated temperature, we were still not able to induce any failures.
At this point it was suggested that we make the compacts out of acrylonitrile butadiene styrene (ABS). This idea was initially rejected because of cost implications. Various configurations of hinge pins were tried. None of these had the desired effect of reducing the customer complaints. Finally, without determining exactly why the compacts failed, the issue was resolved by switching to a resin that was a blend of ABS and SAN.

Loose Powder Sinks Jars

In another episode, one of our divisions sent in some SAN jars containing loose powder for evaluation. When they were received, the jars were severely cracked. Initially, it was believed that the jars were cracked as a result of either handling or imperfections induced during the manufacturing process. With this belief, no further action was taken.

When the contract filler undertook to fill a second set of jars, it was found that these jars too were cracking. We then reviewed the ingredient listing of the loose powder and found that the powder contained esters. Knowing that SAN is prone to cracking when in contact with esters, we concluded that the jar material needed to be changed.

Eureka!

I now realize that the earlier problem of the failure of the GPS compact hinges was probably aggravated by some of the ingredients in the powder.

Both of the above case histories are examples of what happens when packaging materials are selected based solely on price and aesthetics. These anecdotes illustrate the importance of considering the formula’s ingredients together with the chemical compatibility characteristics of the proposed packaging materials.

We developed a solution to the problem of the cracked jars very quickly because, by that time, we had a procedure in place to review new product formulae.

Final Testing Is Critical

Although it is important to review new product formulations early in the development phase, it is also important to review the proposed packaging materials. And, before full production begins, it is important to fully test the final package with the final formula.

There have been many cases in which the colorant of the final package leached from the package onto the product, or where a product caused discoloration of the package.

A new fragrance product was thoroughly tested with a proposed glass bottle and a proposed mechanical spray pump. Although all the materials that were tested were the ones that would actually be used with the product, they were not the final versions. The glass bottle was inert, and caused no problems. The proposed pump had a colored actuator with a colored insert. The insert incorporates the spray nozzle, and fits into the actuator. When the fragrance is dispensed, the insert is in direct contact with the product.

The initial testing was performed with a white insert, but for production, the insert was to be colored. The package testing showed no incompatibilities. Production began with actuators and inserts molded in the correct color.

But, when the first few units were subjected to quality assurance audits, it was found that the product that was sprayed was the same color as the actuator and insert. This led to an immediate shut-down of production, and a mad scramble to find an alternate method of coloring the resin that would not leach into the product. This effort was constrained by the fact that the new colorant had to be the exact same shade as the original.

Thorough testing of the final formula in the final package before full production will always be less costly in terms of dollars and precious time than having to stop manufacturing to find and fix a packaging problem.