Definition: 1. rapidity in movement, 2 rate of motion or progress, 3 to cause to move.
Injection speed is the speed of which the plastic material is injected into our mold or tool. The machine is capable of being set to a particular speed of injection, up to its maximum rating. This can be in inches, mm, cubic, or other scale, which can be set on the machines control panel or impute screens. It could be as in some shops a flow control valve on the side of the press with green, blue red stripes on the handle to set the speed setting. Most press manufactures today will list the capabilities for their presses in cubic measurements of styrene. An example may be 12 cubic inches per second. The injection speed is typically rated on screw movement, and than measured given the units previously mentioned.
With a hydraulic press, it is the setting of the fluid flow to our cylinder, (actuator) which since it is coupled with the screw will push it at the rate of speed that is programmed into the machine. "this is only if you have sufficient pressure(see previous article)" Understand that the pump only produces fluid at so many gallons or litters per minute, that this does not change, unless a variable pump is employed. What is being done is setting a control valve to limit flow up to our maximum. Flow is the rate of fluid per unit of measure, this is much like a faucet on the sink in that how fast you fill a glass of water is dependant on how open the faucet is. In newer presses this is via a screen setting and the older ones a flow control valve. Thus for example our cylinder (actuator) has a stroke of 12" and a volume of 10 gallons, for each inch of stroke we need 10/12 or 0.834 gallons of fluid to enter the cylinder. Another way to look at it is that to get 12" of stroke we need 10 gallons of fluid. If it takes 1 minute to put 10 gallons of fluid into the cylinder than the rate of speed is 12" per minute. If a speed of 6" per second was wanted than there would be a need of 5 gallons of flow per second or a 300 gallon per minute (gpm). Again the pump on the machine is typically pumping at a set rate.
Now 300 gpm is a lot of flow, and not many machines will have this, so to accomplish this they would use a system that is set up with accumulators. First off an accumulator is that, it accumulates fluid and that is all. The pressure that is behind this fluid is no greater than maximum pressure that the machine can produce. In most system the utilization of a nitrogen bladder is what will supply the force. So envision a series of tanks that have bladders which when compressed (nitrogen is compressible) with fluid will tend to flatten out and the nitrogen is compressed in the bladder and possibly an additional tank.
Now after compressing the gas, the tanks are full with hydraulic fluid (at system pressure); the machine now can utilize this fluid as extra gpm flow. When the machine injects, the system allows the combination of this fluid with flow of fluid from the pump, so that now there is a boost to the standard flow of fluid. This than allows the screw to inject faster than what the standard flow of the pump is rated at. Again this will work as long as the pressure needed is within our limits. Accumulators do not increase pressure available only fluid at machine pressure limits.
With electrics the speed is set and that is that as long as there is sufficient pressure made available.
This is when the use of fill time comes into play; in the question of how long is it taking to move our screw forward. This was discussed previously. Ideally all have calculated the barrels into giant syringes and now know that each inch or mm of stroke is equal to so many cubic inches or cc's of material. So that now a setup can say to inject 6 cubic inches of material into the cavity in 1 second or 2 seconds. (See notes below for conversions). If nothing else there is at least a shot weight at this point in time.
The injection speed (rate of fill), in most cases is influenced by the fact that the parts are not allowed to have blush, jetting, and or some other cosmetic effect. Basically it means that there may be a large range of speed settings that mean nothing because the cosmetics of the part determine how fast it can be filled.
Getting to speed is crucial. What this means is that the percentage of barrel used must be looked at. Is the length of stroke of the injection screw long enough to reach its setting or due to a short stroke it does not reach that speed. For those familiar with drag racing this is like trying to run the quarter mile in 50 yards. The car is still accelerating when it is told to shut down. The screw on the machine is doing the same. It is still ramping up in speed when in fact it reaches the position that tells it to shut down. As parts become thinner and longer flow lengths become the norm the issue is getting to speed. This can be studied if the molder plots screw speed setting, against actual results and will come up with the linearity of the screw. Basically is the screw injecting as it is asked to do?
The die cast industry has the same problem but they have overcome it with a brake on the screw is the easiest way to explain. Basically a die cast machine can load the cylinder with max pressure, when injection starts it has maximum pressure there and accumulators to assist in volume of oil to push it forward. The machine is able to inject the die cast materials into the mold extremely quick. Currently there is only one or two injection molding machine manufactures that offer this to the plastic industry.
Case study;
A molder had a multicavity tool in which the rate of rejects was high. It was determined by blocking off cavities that a quicker fill time produced acceptable parts. This is based on the volume of material capable of being injected into the tool per second. The pressure necessary to fill was no where near maximum that the press could deliver, it was only the speed of delivery. Adding an accumulator to the press to increase the injection speed allowed for the mold to run all cavitations.
Basically if there are 4 cavities and the fill time is 2 seconds, as that is what the machine is capable of it equates to 2 seconds per cavity to fill it. Blocking off one of the cavities eliminates 25% of the material so the other 3 cavities would now fill in 1.75 seconds. The machine did not change speed as it was at maximum it just changed volume needed to push into the tool.
The speed of injection can be set but only monitored on the machine by looking at the results for fill time. As previously discussed the fill time variation should be kept to a minimum of +/- 0.04 seconds. What the molder or process engineer need do is to look at the stroke and setting and see if by dividing the stroke by the resulting time is it the same as the setting or at least close to that setting as it will always be a bit slower due to ramping up on speed.
Further it may be found when doing a linearity study of the press for that mold that any speed setting above a certain point does not influence the part due to the fact the machine is not reaching that speed. This is an important fact in setting up the mold in another press, and another reason fill time should be ultimate result for a setting.
