1. Flow Rate
First, flow rate must be established. How is this usually done? Is it even studied at all? The answer is “It isn’t studied.” The sad fact is that it is so easy to do. The injection molding machine itself is the most available and easiest rheometer there is to use.

Start with at least 90 percent of the machine’s injection rate. Then decrease the fill speed and document the viscosity change. What you end up with is a shear rate curve (based on shear during fill). At a certain point the viscosity takes a sharp turn upward. This is where you do not want to be. At these flow rates the process becomes super sensitive to machine fill speed changes. When the viscosity changes it is amplified in pressure loss more than if you were on the correct side of the shear rate curve.

 2. Flash
Knowing this, what should you do if you are flashing the mold just because you are filling it fast? (Of course, you cannot hit the end of the cavity as that will result in flash). If the mold flashes simply because the viscosity is lower due to the high shear rates, it is a mold problem; not a process problem. You fix the flash. Failure to do so results in process failures later down the line because the flash never gets better.

  The wizard on the floor then does the best finessing he can do only to come back after the viscosity has changed and find that he has made shorts. Or the flash suddenly got worse. The typical situation is that they think they have made it go away only to find out that a couple of bad parts got to the customer. Wouldn’t it be better to push the mold in the beginning and know its weaknesses? If you correct them up front, they won’t come back.

 3. Pressure
What if when you filled the mold your injection pressure reached maximum? This is the worst situation you could be in because you are no longer in control of flow rate. Flow rate happens to be the primary process control variable. If this happens you either find where the loss is maximum inside the mold or you move to a machine with a higher intensification ratio.

 Slowing down may be an option so long as it doesn’t put you on the wrong side of the shear rate curve. If the viscosity increases by ways other than flow rate, the machine will need to use more pressure so you have to make sure you are not close to the pressure difference between the flow control valve and the pump.

 As for pressure, there must be adequate pressure to offset the effects of static pressure loss during pack. The issue with this is if the tool is not instrumented with cavity pressure sensors, it is impossible to truly quantify pressure losses. It is considered negligence to not look inside the cavity for this information. Yes, you can pack the part to a small short and measure the loss from the injection pressure.

 For example, if a small short occurs at 1000 psi, it is safe to assume that there is a 1000 psi loss from injection to end of cavity, but if you add another 1000, it does not mean there is 1000 at the end of cavity. Why? Because of static pressure loss. The long polymer molecules do not transmit hydrostatic pressure very well. There is a certain point in pressure loss where things really get critical.

 For example, if there is 12000 psi at the injection unit and 4000 at the end of fill, this is an 8000 psi loss. If the viscosity is increased by 10 percent (not uncommon at all) then you could take away 10 percent from the pressure loss. This is 800 psi. Subtract that from the original 4000 and your new end of fill pressure just dropped to 3200 psi.

  This is a 20 percent change in pressure from a 10 percent change in viscosity. What if your processing window was between 4000 and 3200 psi? You have a problem. So, during sampling, you want to know those pressure losses and try to minimize them by finding where the most loss is occurring. The critical point is about 50 percent of the packing pressure at the end of fill location. For example, if our end of fill pressure was increased by reducing pressure loss to 6000 then the change would be 10 percent at the end of fill.

  4. Temperature, Cooling Rate and Time
How do you optimize plastic temperature, cooling rate and time? You start with the manufacturer’s minimum and maximum limits and set them right in the middle. From there you start the process and try not to have to manipulate this variable. If it becomes necessary to change them, stay within the limits of the supplier’s recommendations. If you have to vary outside that range, identify what the limiting factor is and attempt to correct it. Because temperature changes affect cooling rate and the time needed to cool, try valiantly to not change this variable.

  At this point the cooling time is set at whatever it takes to make the part rigid enough to be ejected; however, oftentimes there are other issues that drive the cycle up. It has been found that to meet quoted cycles breaking the rules is often required. It is a very last resort to be out of the specifications; however, it does occasionally happen. The focus here is to reduce normal variation in all phases of the cycle.

  You may ask the question, “Does it really matter if I am out of the supplier’s specifications as long as I am making an acceptable part?” Yes and no. If you have a problem later down the line and it is big enough to get the customer concerned, the first thing that happens is the material supplier shows up and points out what you are doing wrong. Even if it is explained why you had to go around the problem, it won’t be good enough in the customer’s eyes.

  Surprisingly, it is probably something they felt they couldn’t live with that caused it to happen that way in the first place. Hence, it is so important to be able to explain the potential problems with any adverse action to the process and get an upfront agreement (in writing of course) that it could be an issue later down the line. Wouldn’t it be better to not let it get to that point?          

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