Before you pick up the phone to complain to your material supplier about inconsistency in the resin, check your press.
Raw material receives more than its share of blame for processing issues. Poor cosmetics, dimensional instability, problems with consistent filling, lack of toughness in the molded part, and unexplained failures early in the product life cycle are often attributed to poor quality of the raw material. But if the analysis of a problem focuses only on the material, the real solution is often missed.
This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure. Michael Sepe, our analyst and author, is an independent materials and processing consultant based in Sedona, AZ. Mike has provided analytical services to material suppliers, molders, and end users for 20-plus years. You can reach him at [email protected].
One of the most common complaints related to processing involves the need to adjust process conditions in order to fill out the cavity properly. The process will be running along with no apparent problems when a new lot of material hits the floor. Soon a process technician is standing by the press making adjustments to correct for short shots, flash, or a sudden change in a critical dimension. This is often viewed exclusively as a material issue and the phone starts ringing at material suppliers or their distributors. But there is usually more to it, and a knowledgeable processor can conduct most, if not all, of the analysis at the machine.
Viscosity control: Temp
Fundamentally, most successful process control as it relates to mold filling is about control over melt viscosity. While this sounds like a simple statement, there are several components to this control.
The first is melt temperature. All commercial polymers have an appropriate temperature range over which they can be molded. Some are very wide, such as polyethylene, and others can be quite narrow, as is the case for PVC or PBT polyester. The appropriate temperature for a given application depends on the interaction between the geometry of the flow path in the mold, the molecular weight of the polymer, and the capability of the molding machine.
Temperature is not just about the setpoint of the controls for the injection cylinder’s heating zones. The thermocouples that provide feedback to these controls are buried in the wall of the injection barrel and are some distance from the actual melt in the cylinder. In addition, much of the thermal input into the material does not come from the heater bands, but instead derives from the mechanical work provided by the screw. This in turn is a function of screw and barrel design, the condition of this hardware, and process setpoints like screw rotation speed and backpressure.
These not only determine the actual temperature of the melt, but also govern the thermal distribution through the mass of molten material. I have seen unmelted pellets in molded parts running on a machine where the barrel settings suggested that the material temperature was nearly 50 deg C (90 deg F) above the crystalline melting point of the resin. This clearly points to a lack of homogeneity in the melt, a problem that plagues most processors, goes largely undetected, and is often attributed to “bad material.”
The other major factor that determines melt viscosity is shear rate. Two things govern shear rate: the geometry of the flow path and the velocity at which the melt travels. Since geometry is a constant, the one variable a processor can control to change the shear rate is melt velocity, often called injection speed.
Plastic materials are non-Newtonian in their behavior. This means that the viscosity of the material changes as a function of the rate at which it flows. The faster it flows, the less resistance it provides, and therefore the more easily it should fill the mold. But the process must be able to control the material’s flow rate.
Ask most processors about the injection speed associated with their process and they will refer to the speed setpoint on the control. How this looks depends on the machine control. On many, unfortunately, it is expressed as a percentage. The number on the screen or the setup sheet for first-stage injection will read simply “65%” or “85%,” which begs the question, “Of what?” This question almost always brings a blank stare. For those machines that actually deal in a numerical value, the setpoint is expressed as a rate of linear travel such as 2 in/sec or 50 mm/sec. This may seem like an enhancement of the percentage expression, but it still presents problems because it does not address the true flow rate of the plastic.
The flow rate that appears in equations for calculating shear rate is the volumetric flow rate. It is related to the linear velocity of the screw by the diameter of the screw in the injection cylinder. Consider for a moment that the setup sheet that calls for an “injection speed” of 2 in/sec pertains to a press with a 2-inch screw diameter. Now let’s assume that due to scheduling limitations, the mold is placed into a press that has a 1.5-inch-diameter screw and the setup sheet is faithfully replicated with an injection speed setting of 2 in/sec. Has the volumetric flow rate been kept constant?
Of course not. The smaller screw contains nearly 45% less material per unit length than the larger screw. Therefore, if the speed of linear travel remains the same, the volumetric flow rate has decreased in the smaller machine by 45%. Therefore, the shear rate and consequently the viscosity of the material have changed.
The degree of change in viscosity depends on the polymer. For an example, refer to the plot of viscosity vs. shear rate for polypropylene above. This plot shows the relationship for these properties at three different temperatures: 204°C, 227°C, and 249°C (400°F, 440°F, and 480°F). Assume that we have selected the midpoint temperature of 227°C and that the shear rate for the process in the larger machine is 10,000 sec-1. In the smaller press, the shear rate will have dropped to 5625 sec-1 and the melt viscosity of the material will have increased by 50% even though the technician will insist that the machine is “set up the same way.”
Some molders will place a call to the tech service rep for the material manufacturer or the distributor right then and there to complain about the poor consistency of the material, but most processors will make appropriate adjustments to bring the parts into specification and then dutifully write up a new setup sheet for this second press because for some reason the material just does not seem to run the same in the second machine.
Achieving consistent fill time
The real constant that needs to be maintained is the fill time, defined here as the time required to fill the mold cavity 95-99% full by volume. If the same volume of material is always injected into the mold over the same time interval, then the shear rate is constant and the viscosity of the material should also be constant. The linear velocity setpoint on the machine control is simply a means to an end, not an immutable process constant.
If we assume that a molder is sufficiently knowledgeable to properly document the fill time associated with the process, there are still practical considerations in setting up the molding machine so that this process parameter is consistently maintained. The most important consideration in maintaining a constant fill time is the ability of the process to run with an abundance of first-stage pressure. This means that the available pressure on the machine must exceed the pressure required to fill the cavity 95-99% full.
There are typically two reasons that this condition is not met. First, the injection molding machine lacks the needed pressure. On a hydraulic machine, the real available plastic pressure is the product of the maximum hydraulic pressure multiplied by an intensification ratio. This intensifier is a function of the geometry of the hardware, and can vary anywhere from 6:1 to as high as 50:1.
Most processors accustomed to working on hydraulic presses do not think in terms of plastic pressure. They focus on the machine control, which gives the setpoints in hydraulic pressure. However, it is still possible to determine whether or not the process is pressure-limited. If the first-stage pressure setpoint on the machine is set at maximum, and with each shot this pressure setpoint is achieved or very nearly achieved, then the process is pressure-limited.
Even if the machine is capable of generating the required pressure, it may be artificially limited by the technician setting up the process—our second reason. The actual first-stage pressure may be set up at, for example, 1500 psi of hydraulic pressure. If each time the machine cycles, the control shows that this setpoint is being achieved, then the process is also pressure-limited, even though the machine may be inherently capable of exceeding this setpoint.
Under these conditions, small fluctuations in the viscosity of the material will result in changes to the fill time. Viscosity is a measure of resistance to flow. The units associated with viscosity are the product of a force (Pascals or psi) and time (seconds). If the process is capable of maintaining the same fill time, it will do so by calling for more pressure in response to an increase in melt viscosity or less pressure if the melt viscosity declines.
By maintaining a constant fill time, the press automatically controls one of the key variables that determine the real viscosity of the material. However, if the process is pressure-limited, either by machine design or processing strategy, then an increase in melt viscosity will produce an increase in the fill time because the pressure needed to maintain a constant fill time is not available. If the fill time increases, the shear rate decreases, and the change in melt viscosity becomes amplified.
Check your machine
Most processors judge their raw material consistency according to measurements such as the melt-flow rate (MFR), melt viscosity, or intrinsic viscosity. These are all measurements designed to quantify the consistency of the raw material. MFR is the most common of these measurement parameters and any material produced to a target nominal MFR will be manufactured to within a certain tolerance range around that nominal value.
But the conditions used to measure MFR value have nothing to do with the process conditions associated with injection molding. A properly established process in a capable injection molding machine should be able to tolerate normal lot-to-lot fluctuations in MFR. If the molding machine seems to flinch every time a new lot of material hits the floor, then there is a fundamental problem with the way in which the machine is controlling the melt viscosity of the material.
In Part 2, we will review how to determine if the machine is capable and how to treat the root cause: poor control.