Lower process temps cut cycle time. That’s a no-brainer: Change the barrel temp setpoints. What? You say make the process consistent and fill time constant? You’d better explain that.

The first two installments in this series showed that there are advantages to using lower processing temperatures, such as minimal polymer degradation, better color stability, and faster cycles. As we begin our discussion on processing, let us first establish what lower melt temperatures mean.

If you’re thinking all you have to do is change the barrel temperature setpoints on the control screen to 10, 25, or 50 deg F lower, you’re mistaken. Reduced melt temperature means that the actual melt temperature is within the accepted range as stated by the resin manufacturer and that the melt is uniform in temperature and viscosity. Rarely can this be achieved via a standard general-purpose screw design. One of the main reasons many molders use higher temperatures than necessary is that the melt quality is poor, consisting of both melted polymer and partially unmelted (solids) polymer.

In processing, it is important that we know the melt temperature and that there is melt uniformity. It’s easy to say, “measure the melt temperature,” but hard to do. It’s astounding that we can put a man in space and establish some 3900 colleges and universities in the United States, and yet neither has provided us with a way to measure barrel melt temperature without interrupting the cycle.

Considering this industry is the fourth-largest manufacturing base in the United States and this is a critical processing parameter, the SPE or SPI should work to resolve this. In the extrusion industry, 99% of all extrusion processes measure melt temperature constantly and monitor melt uniformity by watching temperature and extrusion torque variations. This can be done with the shot-pot ICU design previously covered (May 2003 IMM, immnet.com/articles/2003/May/2133 and February 1999 IMM, immnet.com/articles?article=726).

But for measuring melt temperature on a typical molding machine, the best we can offer is two methods, and both have problems:

• Preheat the probe to a higher temp than your best guess for the actual melt, interrupt a cycle, and make a purge patty.

• With an infrared device that has the appropriate wavelength, a peak pick mode, and a full target area visibly defined, interrupt a cycle and take the melt stream temperature as it is being purged.

For melt uniformity, the test is straightforward: As you are running a color, stop feeding the color and note the uniformity of the natural color. Or if running natural, add color and check uniformity. Count how many shots it takes before the color shows in the part for an actual measure of residence time. Then inspect the parts for color dispersion. If you see streaks of color, you do not have melt uniformity.

Keep fill time constant

Once the melt temperature and melt consistency have been properly established, processing at a lower temperature or really any temperature will follow the standard guidelines:

First stage, sometimes called boost or high pressure, is used to fill the part 99% by volume. At the end of first stage, the part is not packed out and may appear short or contain sink marks. The last area to fill does not see much pressure at the end of first stage, as the cavity is not full. This may be different from the way you currently mold but it is a critical point in developing a stable process. Know what the part looks like at the end of first stage by bringing second-stage, or hold, pressure to near zero. It should be slightly short.

Understanding the polymer’s flow characteristics during cavity filling is critical in developing first stage. Plastics do not flow like oil or water, which do not change viscosity as a function of flow rate. Plastics change viscosity as injection rate changes. This can be demonstrated through moldfilling analysis, capillary rheology, or using the injection molding machine as a rheometer.

Figure 1 is a typical viscosity vs. flow rate relationship for most polymers. It shows how viscosity changes with flow or shear rate (temperature effects are not shown). Because of this phenomenon, we must keep flow rate (injection velocity) constant. If flow rate changes, viscosity changes; if viscosity changes, parts will be different! Therefore, our first strategy of a consistent process is to fill all cavities identically each shot. To do this, we need to keep flow rate (shear rate) constant. We can measure injection velocity by the ram’s velocity or by fill time. Since not all machines have velocity measurement capability, we will use fill time. Bottom line: Fill time for a part that’s 99% full must be constant shot to shot and run to run.

This velocity or fill time control is similar to cruise control on a car. Note that we are not worried about a specific machine velocity setting. We have established a universal number based on a plastic variable that will work on any machine: fill time.

Know when to ease up

The next question is, how do we keep fill time constant while running production as lots or temperatures change? Most hydraulic machines have a flow control valve that regulates oil to the hydraulic ram (Figure 2). This can be a manual valve, servo valve, proportional, cartridge, and so on. These valves are adjusted manually or electrically, closed loop or open loop, to regulate the flow of oil to the ram; usually the setting on the control is in mm/sec or in/sec.

For these valves to function correctly, they require one common variable to be set correctly: a pressure differential (delta P) across the valve. That is, first-stage pressure must be set higher than the maximum pressure required to push the plastic to fill the part 99% full. If first-stage set pressure on the pump side nears or equals the pressure in the hydraulic ram, the injection speed will slow down. If the ram slows down, then the viscosity of the plastic will change (get stiffer) and process variations occur. Bottom line: for velocity control you must operate your machine with a delta P across the flow control device and with abundant pressure on the pump side.

Caution must be exercised in setting this delta P, or the amount of abundant pressure. This is often called first-stage high-pressure limit. We need an adequate delta P to control velocity. We also must protect the mold from the situation that arises if it is a four-cavity mold and one of the cavities blocks off due to metal contamination or unmelt. Then we would be driving four cavities’ worth of plastic into only three cavities. If there were slides in the mold, we could flash them and damage the tool on the next shot.

This presents a dilemma. Without delta P, we have no process control on fill, and parts will vary thanks to viscosity changes (such as temperature, lots, colors, and percentage of regrind). With too much pressure, we run the risk of overpacking the mold and may damage it. The ram must be taken off cruise control before the last area of the part fills out, which is why we end first stage at about 99% full. Otherwise, the mold will likely flash. It’s like driving a boat into a dock; you must cut the power before you hit the dock.

A well-built mold should be able to withstand a certain amount of excess pressure. Overpressurizing the mold is likely to happen for a number of reasons throughout its life. The question is, how much extra pressure is needed to gain velocity control without damaging the mold? The machine should not be set to full system pressure unless required for the mold.

Securing the pressure differential

Methods to find the minimum delta P are available, but not practiced by many molders. One way to find this pressure is to raise the hydraulic pressure limit on first stage as you are making short shots until the fill time stops decreasing and peak pressure during first stage stops increasing. Then you can measure how much you are using to drive the plastic into the part during first stage vs. the first-stage set pressure limit. This is the pressure differential, or delta P. Delta P is the difference between what the hydraulic ram uses and what you have set for first stage (first stage must be higher). First-stage setpoint limit is a relief pressure.

You must maintain this delta P as lots change or if you change temperatures. You must set the machine with enough first-stage pressure to control velocity for the resin’s typical viscosity range. The ram will use whatever pressure it needs to control velocity, providing there is sufficient delta P. Easy-flow or hotter plastic will require less pressure. With stiffer lots or cooler temperatures, it will use more pressure. This is velocity control, so viscosity is constant shot to shot and run to run.

Every shot should take a little different pressure for first stage. Lower temperatures in processing will require higher pressures, and the machine must be capable of providing adequate plastic pressure. If you do run out of pressure, you can downsize the barrel or shot cylinder for the ICU and the intensification ratio will increase. This has an added benefit of providing a longer stroke distance for the shot size, which is better for velocity control. Ideally, I like to see a shot volume of 25-65% of the shot capacity.

Another method of finding the required delta P is to ask what the machine manufacturer recommends. It designed and built the press; it should understand this concept fully. What is the manufacturer’s recommendation for setting first-stage pressure limit?

As a general rule, set first-stage pressure limit to 200-400 psi higher than the highest pressure during first stage for hydraulic machines (1500-2500 psi higher for electric machines). But each machine’s hydraulic architecture is different and required pressure differentials can vary. Also, be sure to find the correct delta P and verify that the mold can withstand the possible overpressurization.

Part 1 of this series covered cycle time savings with lower melt temperatures (June 2006 IMM, immnet.com/articles/2006/June/2901); part 2 discussed the advantages of lower melt temperature to eliminate polymer degradation (August 2006 IMM, immnet.com/articles/2006/August/2943). In December, we conclude the series with an exploration of the injection unit and screw design solutions for processing polymers at proper melt temperatures and quality.

John Bozzelli is the founder of Injection Molding Solutions (Midland, MI) and initiator of Scientific Injection Molding. You can reach him at john@scientificmolding.com.