Injection Molded Part Quality
Are Today's Injection Molded Parts Achieving Their Intended Physical Properties?
The general purpose reciprocating screw is going on 60 years old, and while it is a design we have grown used to, some think it is also a design that too easily allows molders to abuse their material. Further, the consequences of such abuse can lead to parts falling out of spec, or outright product failure.
Amazingly, for the majority of molded parts, the answer to the headline of this page is no. Many molders, unfortunately, disregard physical properties in molded parts. In many molding operations, the main concern is to fill the cavity, and the easiest way to do this is to increase melt temperature. This makes fill faster and easier; if the part flashes, pack and hold is adjusted. The problem is that increasing melt temperature has a myriad of consequences that are easily and often overlooked, or rarely considered:
- Material degradation
- Increased residence time
- Part specification violation
- Mechanical or performance failure
Thermal Sensitivity of Polymers
A physician named Walter Bortz wrote a book about aging called We Live Too Short and Die Too Long. In it he discusses the phenomenon of aging in human beings. One of his major points is that we fail to react collectively to the early signs of physical deterioration because the human body is so robust that symptoms do not appear until about 50% of our initial capabilities have been lost. Unfortunately, we go downhill very quickly once problems have become evident, and by the time 80% of our initial ability to function is gone, it is too late.
In this respect, plastic materials are similar to people: Both are composed of long-chain molecules that contain primarily some combination of carbon, oxygen, hydrogen, and nitrogen. These long-chain molecules, when damaged, form free radicals that accelerate the deterioration process.
The actual chemistry of degradation depends on the polymer, but the net effect in most cases is a reduction in the material's average molecular weight, a condition that is readily documented by a simple melt-flow-rate (MFR) test.
The Greatest Enemy: Excessive Heat
While some polymers are degraded by the presence of excess moisture during processing, thermal degradation is something all thermoplastics have in common. Even the moisture sensitivity of resins like polycarbonate and polyester is driven by heat.
PC containing .1% moisture can exist indefinitely without deterioration at room temperature. It is the heat of melt processing that permits the chemical reaction known as hydrolysis. If we consider only the effects of thermal degradation, we find that not all polymers are equal. Some resist the rigors of melt processing better than others.
The chart below provides a relative rank order of common melt-processed polymers in terms of their thermal stability in the melt. This ranking is not based on the maximum temperature a material can withstand. Rather, it focuses on the processing window, which is the difference between the lowest possible temperature at which molding can occure and the temperature at which significant degradation begins. It also considers the rate at which degradation occurs once the upper temperature limits are exceeded.
Given this, it is not surprising that a material like PBT polyester has a limited processing window, and that the consequences of exceeding the upper melt temperature limit are severe. This is shown in Figure 1, which plots the MFR of molded parts as a function of melt temperature.
This graph shows that the MFR retention was controlled if the melt temperature was maintained between 455 F and 490 F. Once the melt temperature exceeded 490 F, the integrity of the material declined rapidly.
This process was also plagued by another common feature of many molding processes: extended residence time. The concern over elevated melt temperatures is one of time as well as temperature. Even a melt temperature in the acceptable range may produce degradation if the time the material spends in the melt state is prolonged.
Of course, this is PBT polyester, a material near the bottom of the thermal stability scale. But even a material like polypropylene, at the opposite end of the spectrum, shows vulnerability if the melt temperature is elevated.
Fig. 2 shows the increase in MFR for a 4-melt-flow-index virgin material after repeated processing. Material processed at a melt temperature of 400 F maintains excellent retention of MFR even after six passes through the molding process. However, material processed at 480 F undergoes as much of a change in one pass through the melt process as the material run at 400 F undergoes after six.
The Minimal Impact of Moisture
Even in materials where hydrolytic degradation is a concern, it is possible to provide a wider processing window with the same moisture content if the melt temperature and residence time are controlled. Figure 3 shows the results of an experiment that studied the interaction of melt temperature and moisture content on the retained viscosity of a glass-fiber-reinforced PET polyester and the associated physical properties of the parts.
Destructive tests were run on the product to determine the point at which performance dropped below a minimum specification; these results were correlated to changes in melt viscosity relative to the raw material. Melt viscosity is the reciprocal of MFR, so it declines rather than increases as the polymer degrades. The presence of the glass fiber also influences the measurements. Nevertheless, it is still possible to determine the role of melt temperature and its interaction with the moisture content of the resin.
In our tests, parts were molded at 530 F and 570 F, both temperatures that fall within the range recommended by the material supplier. Moisture contents between 100 and 900 ppm (.01% and .09%) were used.
While moisture content clearly drives the degradation process in PET polyester, note the difference in the point at which this moisture content becomes a deciding factor in the destruction of the polymer. When molded at 530 F, parts still function satisfactorily at a moisture content of 350 ppm (.035%), almost twice the maximum allowable moisture content of 200 ppm (.02%) listed in the material supplier's literature. However, when molded at 570 F, a moisture level of 150 ppm (.015%) is sufficient to cause degradation that results in part failure.
The solution to correct polymer processing is simple and cost effective. The easiest way to reduce the cost associated with high melt temperature is to use available technology from the extrusion industry. (Lower cost may also be realized by using regrind, masterbatches for colors and additives, foam, coinjection, and venting.) This extrusion technology can provide shot weight accuracy (fewer rejected parts) while using lower melt temperature (shorter cycles). The problem is finding existing injection units to incorporate it.
A new injection unit called the ICU (Injection Control Unit) is now available in new and retrofit versions for all sizes and makes of machines. The ICU incorporates all extrusion processes and provides less part variation than existing equipment. It consists of an extruder, an inject accumulator, and a spool-type valving system with a small secondary accumulator named the ICS (Injection Control System). The screw is about half the normal diameter and can rotate at all times except during inject. Shutoff is incorporated and decompression is essentially inside the mold, providing greater accuracy in preventing drool. The ICS, actuated by the carriage, provides better accuracy in pack and hold. the ICU achieves part weight accuracy since the screw provides uniform viscosity and the ICS has positive closure (no non-return valve).
We have looked at the following consequences of increasing melt temperature:
- Material degradation
- Increased residence time
- Part specification violation
- Part mechanical or performance failure
It is hard to fault the mold technician or operator for these problems. Melt temperature measurement, in most machines, is only possible by inserting a pyrometer in the purge. This provides a lower reading than insertion in the melt stream and is normally done at startup rather than in production when melt temperature increases. To further limit the technicians's capabilities, the screw design is likely what is referred to as general purpose (or "no purpose"). These designs found their genesis in the 1950s and have not been used in the extrusion industry for 30 years.
Interestingly, the extrusion process offers valuable insight into tackling the problem. Injection screw L/D is typically around 20:1 in injection molding. In extrusion, the norm is 30:1 or greater. Virtually all extrusion screw designs are either barrier or barrier with mixer. The reason these longer L/Ds and screw designs are used is simple: They provide low melt temperatures, uniform viscosity, and high throughput without reducing the physical properties of the polymer.
We may ask why the extrusion industry has changed to these modern designs, while the injection molding industry, for the most part, has not. Are the processing requirements different? Are the molding machines and molds incapable of handling higher viscosities? Is the molder able to determine damage to the polymer during molding? The answer to why the extrusion industry has modern designs is simple: They save money and improve product quality.
Processing Requirements and Screw Design
One of the processing requirements - the melting of polymers - is the same for molding and extrusion. We must take a solid and make it into a viscous liquid. This is accomplished mostly by friction, shear, and, to a lesser degree, by conductive heat transfer. The percentage varies with the size of screw, as the watt density is greater in a smaller machine.
The reason general purpose screw designs are not used in extrusion is the phenomenon called the "breaking up of the solids bed." This occurs in all non-barrier screw designs. In the transition section of a screw, the melt pool on the forward side of the flight constantly increases as the solids bed decreases during melting. As the solids bed decreases to a point near the end of the transition section, the increasing pressure penetrates the remaining solids bed, causing it to disperse into the metering section. The metering section does not have the ability to effectively melt these remaining solids. At best this creates viscosity variations; at worst the molder can have unmelted pellets in the molded part. and stress concentration areas that may cause failure.
Mixing sections, if designed correctly, melt the remaining solids. Barrier designs separate the melt from the solids, keeping the solids from entering the metering section. Good designs provide a larger processing window and a better ability to process a variety of viscosities.
High Melt Temperature Costs
Melt temperatures in the extrusion industry are normally 50 to 100 F lower than in injection molding. In many cases, the molds and injection units are not capable of handling the higher viscosities. Therefore, they are forced to use more energy for heating and cooling than is necessary, and live with longer cycle times.
Injection units normally do not provide the molder with any way to determine damage to the molded part because direct melt temperature measurement is not available, and in many cases the melt temperature exceeds recommended requirements in order to help fill the part or to "process around" a flawed mold or part design.
Although resin suppliers provide recommended processing temperatures for their polymers, they rarely show the damage a polymer can suffer from being run at excessive temperatures.
WHAT IS GAINED?
It is necessary to eliminate the destructive polymer processing that is the norm in the injection molding industry. To continue this type of processing is expensive and impedes progress. Scrap can be reduced or virtually eliminated, energy and resin costs can be cut, and more competitive parts can be molded using existing processes from the extrusion industry.