The Materials Analyst, Part 58: Frequently asked questions (Part 1)

Despite the fact that injection molding and the associated disciplines of mold design, part design, and material selection are making the transition from art to science, the reality is that very few people involved with injection molding are trained scientists. We tend to be hands-on types who develop rules of thumb through experience.

But when pressed to explain why a particular approach works or does not work, many of us are at a loss. We are like cooks who follow a recipe but have no real understanding of the role of the individual ingredients. We are fine as long as the recipe works. But if suddenly it fails to produce good product, we lack the fundamental understanding that may be essential to fixing the problem.

Analysis of problems with plastic materials and parts is part of the science that is supposed to contribute to this fundamental understanding, but it often takes us into unfamiliar territory. The scientific terms that material scientists use are unfamiliar and the explanation provided by the analyst may appear to be totally disconnected from the real world where product is made.

Often this arises from the fact that the analyst understands even less about manufacturing than manufacturing people do about analysis. The efforts of the manufacturing people to understand the analysts lead to some questions that come up repeatedly.

This series of articles will highlight some of these questions, illustrate the basic misunderstandings behind the questions, and hopefully make the whole process of solving production problems less of a mystery.

Importance of Molecular Weight
One of the most important fundamentals of plastic materials is the relationship between physical properties and the molecular weight of the polymer. This relationship was first articulated more than 80 years ago, and the theory was not well received at the time. But the wonderful thing about science is that eventually the truth emerges through experimentation and observation, and within a decade chemists were inventing new materials using the principle of high molecular weight as their guiding tenet.

Unfortunately, there are still many processors operating today who do not grasp this essential relationship. In more than 90 percent of cases, when a product is unexpectedly brittle, at least part of the root cause is an excessive reduction in molecular weight. This may occur due to poor process control, or it may arise because an alternate material with a lower molecular weight was substituted to reduce cost or improve the manufacturability of the product.

In the world of analysis, we seldom make a direct measurement of molecular weight. Instead, we measure the viscosity or the melt flow rate of the material and relate that measurement to a relative average molecular weight. Because differences in performance are so often tied to molecular weight, we frequently write reports that refer to parts made with a polymer of higher or lower molecular weight. The question that comes up more often than you might think is this: If my part is made from material with a lower molecular weight, why didn’t my part weight drop?

It is a fairly common practice to use part weight as a quality control metric. This approach is not without merit. It can be used to verify gate seal during the pack-and-hold phase of mold filling, it can detect problems with nonreturn valve operation, and it can even be used to detect contamination. But it will not distinguish between different average molecular weights of the same polymer. To understand why there is no connection, it may help to consider the process by which a polymer is made.

Polystyrene Sheds some Light
Let’s start with a simple and well-known polymer, polystyrene. Unless an impact modifier is added, polystyrene is a hard, brittle, transparent material made from a clear liquid known, appropriately enough, as styrene. Styrene consists of eight hydrogen atoms and eight carbon atoms arranged in the particular configuration shown in Figure 1. If we refer to a periodic table we find that the atomic weight of a carbon atom is approximately 12 g/mole; the atomic weight of a hydrogen atom is approximately 1 g/mole, and therefore the molecular weight of the styrene molecule is 104 g/mole.

Manufacturing polystyrene involves linking hundreds or even thousands of these individual styrene molecules together into long chain-like structures. This chemical process is known as polymerization. While most polymerization reactions require considerable effort, styrene will polymerize while simply sitting on the shelf unless it is chemically stabilized and protected from sunlight. For this reason styrene liquid is always kept in dark brown bottles and contains 1 percent of a material called hydroquinone, which hinders the tendency for polymerization.

But if we remove the stabilizer and store the styrene in clear glass so that ultraviolet light can get in, the styrene liquid will begin to increase in viscosity. Eventually it will turn into a clear solid. It will look the same until you open the bottle and try to pour out the contents. The conversion from liquid to apparent solid is the result of polymerization. Each polymer chain may be composed of as many as 2000 to 3000 individual styrene molecules, and the molecular weight of those chains will therefore be much higher than that of the original molecules.

But if we weigh the contents of the bottle before and after, we will find that the weight of the material in the bottle has not changed. We are simply taking the same amount of material and rearranging it on a molecular scale.

Roy Plunkett, the DuPont chemist who accidentally discovered the material that has become commercially known as Teflon, found the same principle in action. Teflon is made from a gas known as tetrafluoroethylene (TFE). Plunkett had been hired by DuPont to work on fluorine chemistry in the field of refrigeration and had just started some experiments for the purpose of studying the properties and behavior of Freon. An initial step in this research involved preparing TFE gas, which was then stored overnight in pressurized cylinders.

Upon returning the next day to start the experiments, Plunkett and his assistant found that no gas would come out of the cylinders and the pressure gauge on the cylinder read zero. But when the cylinders were placed on a scale, they weighed the same as they did when they were first filled with the gas. When the cylinders were cut open, a white solid coated the inside surface. Analysis showed the mystery solid to be polytetrafluoroethylene (PTFE). The elevated pressure in the cylinder had caused the gas to polymerize spontaneously.

As with the styrene, individual molecules had linked up to produce long chains with a very high molecular weight. But while the molecular weight of the substance within the cylinder had increased by several orders of magnitude, the weight of the actual material in the cylinder had not changed at all. The higher molecular weight of the substance was balanced by the fact that there were far fewer molecules of polymer than there had been of the original gas.

This illustrates the lack of a relationship between the molecular weight of the polymer and weight of the part that is fabricated from that polymer. Part weight is determined by three factors: the solid-state density of the molding compound; the volume of the cavity into which it is injected; and the effectiveness of the process in ensuring that the cavity is completely filled with that material, or packed out, as we say in the business. Let’s look at each of these factors.

Determining Factors
It is obvious from the Teflon story that the process of polymerization does change the density of a material. After all, a compressed gas turned into a solid. And polystyrene polymer has a density of 1.04 g/cu cm while the density of styrene liquid is .906 g/cu cm. But once we have grown a molecule to the point where it has achieved the status of polymer—a molecular weight of approximately 10,000 to 12,000 g/mole—density does not vary significantly with molecular weight.

If you do not believe this, simply look at data sheets for a family of materials that are distinguished primarily by molecular weight such as polypropylene or polycarbonate. A fractional melt polypropylene and 60-melt material both have a density of .90 g/cu cm. A blowmolding grade of polycarbonate with a melt flow rate of 1.5 g/10 min and a CD or DVD-grade material with a melt flow rate of 80 g/10 min both have a density of 1.19 to 1.20 g/cu cm.

The second factor, volume in the mold cavity, can be considered constant provided that we do not blow the mold open. As for the third factor, a well-controlled process should maintain a constant set of conditions for pressurization in the cavity. Therefore, there should be no expectation that part weight will vary significantly when the molecular weight of the polymer changes.

If we are looking for a measurable effect from declining molecular weight, then we should expect that as the molecular weight of the polymer decreases the part weight might increase slightly. The reason for this comes from the third factor that we just discussed, the distribution of pressure in the cavity. As molecular weight decreases, viscosity decreases, and viscosity differences are greatest at low shear rates.

In most properly constructed molding processes, maximum cavity pressure is achieved during the early stages of pack and hold, when the screw is barely moving and the shear rates are very low. Therefore, at this point in the process, a polymer with a lower molecular weight will have a measurably lower viscosity. The ability to transmit pressure through a polymer melt is inversely related to viscosity; the lower the viscosity the easier it is to pressurize the cavity.

Since the pack and hold stages of most injection molding processes are pressure controlled, a lower-viscosity polymer will be forced into the cavity with less resistance, resulting in a part that is slightly more packed out and therefore heavier. Studies that have used simple standard shapes like tensile bars and plaques have shown that the part weight increases are in the range of 2 to 3 percent.

So there you have it. If you evaluate two parts from the same mold made with the same material that have different properties and you find that the good part is molded from material with a higher molecular weight, don’t expect to put it on a scale and find that it is heavier. A fixed volume of material made with larger polymer chains simply means that it takes fewer chains to fill up that volume. And, as was pointed out more than 80 years ago, that is where the improved properties come from.