Speed a word that can describe how fast someone goes or a machine operates. Maximum speed would be the term used for the maximum that a machine or object may achieve. An example is how fast a certain motorcycle can go, say 140 miles per hour. In molding we may say a machine can inject at 1000mm per second or 6 inches per second. It may also be stated as so many cubic centimeters per second or cubic inches per second. 

Now the issue with all this speed discussion is how much distance does it take to get to speed? As in our example our motorcycle is not instantly at 140mph, it takes some time and distance to achieve this speed. Is this a quarter of a mile, or 2 city blocks? It is the same with injection molding machines, especially hydraulic types. There is a distance that is necessary so as to get up to speed.  This can be calculated and must be taken into consideration for our mold (factory) requirements. In injection molding the speed result is Fill Time; the time it takes to fill a part to a % full and measurable unit of weight per time, also measurement of distance traveled from start to transfer on the press. 

In hydraulics we have a pump, which develops both pressure and volume, and both come into play when we are looking at the speed function. Our hydraulic cylinder has a volume which when filled pushes onto the piston which moves the shaft and in turn is speed or movement of that shaft. Thus a cylinder with a bore of 10" has a volume of 75.54 cubic inches per 1 inch in length. Thus to move the piston or shaft, / screw 1 inch forward we need 75.54 cubic inches of fluid. Since there are 231 cubic inches per gallon we would need 0.32 gallons of fluid to move our shaft/screw 1 inch in distance. 

The issue not discussed above is that pressure is also needed. The pressure has to be greater on the supply side than on the work side. Basically we need to over come the resistance to movement, in this case the stiffness of the plastic material, screw friction, and or any loss in pressure within the architecture of our hydraulic system. The flow control valve regulates the gallons/liters per minute that is supplied thus the speed of movement. 

Taking everything into consideration we need a distance of travel to get up to speed. What is this distance? That has to be determined for each and every press. A rule of thumb is that we would like to use at least 25% of the barrel capacity, so that some sort of control was available, both for the speed and operation of the process. 

To calculate speed on a press there are numerous software programs out there. Also some which used in conjunction with other testing can give results of the linearity of the screw. What this means is that if speed is set at 1000 mm per second is the machine actually getting there. Mathematically this is an easy calculation; since by dividing the distance traveled by the fill time thus for example a distance traveled of 500 mm in ½ second fill time equals 1000 mm per second, while a distance of 500 mm in 2 seconds fill time equals only 250 mm per second. (based on one speed setting) 

Why is this important?

An example would be that having produced good parts on a 1 cavity prototype tool using a fill time of 1 second.  The production tool is now a 16 cavity tool, with identical runner/gate pattern. The press is much larger, as is the barrel. It is found that the volume is such that the setup is using 10% of the barrel and that with all controls maxed out the fill time is now 2 seconds resulting in a non fill of the parts.  

Issue, speed was not achieved with the bigger press, though in reviewing the specification for the press it is found that it has a higher speed capability than the smaller press. The problem is that we need some stroke, distance to get to speed. This is proved out when cavities are blocked off, and good parts are produced. 

In conclusion our speed is calculated by how fast we go, in this case inject into a mold which can be measured by the fill time on that mold (factory). If it is established that a certain fill time is necessary to produce those parts, in that mold (factory) than that has to be maintained, and it becomes an external issue in that is the press (environment) capable of delivery the required speed, not only per the manufacture but also real world, and the question becomes what distance is minimum to achieve said speed if at all. 

Note: On electric presses we still need to get up to speed, though response may be quicker than with hydraulics. Also the use of accumulators can get to speed quicker due to larger volume of oil, and also some presses may have a break which allows them to lock the screw/cylinder while building up pressure thus allowing for quicker time to speed. 
 

To be continued.

 

SL Silvey

silveysplastics@hotmail.com