At one point or another, we have all been in the position of standing beside the molding machine with a new mold hung in the press and it’s time to make that first shot. The process technician or engineer builds up a shot size and away it goes. As the screw moves forward, everybody is holding their breath wondering if too much or too little plastic was injected. Is the mold going to flash (Figure 1)? Will the parts come out of the mold? What if something else breaks and the mold must be pulled after only one shot?
Did you also found yourself wondering exactly how the shot size was determined in the first place? Did the process technician or engineer guess (Figure 2)? Did they simply extrude material out of the barrel and look at the puddle of plastic and say “that looks about right”?
This part of the process used to drive me crazy when working as a plastics engineer for a Tier II automotive supplier. I remember thinking, “We couldn’t be this blind (or lazy), could we?” We are dealing with volumes and we should be able run some simple calculations to determine what I now refer to as a “safe start-up shot size”. As it turns out, this can be calculated.
When dealing with a new mold, first determine the mold volume (parts + runners). The volume of the mold should be the easy part since most molds today are designed in some type of 3D CAD system. Once that is known, simply use a percentage of that volume (i.e. 80%) based on what feels comfortable. That percentage may need to vary from mold to mold depending on the part and mold design. Keep in mind you still need to be able to eject the part and have the mold run at whatever shot size percentage you choose, so use caution here. In order to translate the desired volume percentage to a shot size on the molding machine, it is important to know if the machine is rated in volumetric terms or linear terms. If it is volume, then you are all set…as long as the units are the same. If it is linear, then you have to understand the volume equation for a cylinder.
If you are dealing with a transfer mold or don’t know the mold shot volume, things get a little tricky. But hopefully the person who sent the mold also sent a sample shot, including the runner system if there is one. Convert the shot weight into a volume. I like to use the plastic’s solid density to do this because it is meant to be a “safe” start-up shot size.
Once you have your safe start-up shot size, don’t forget to account for the V/P switchover position for your actual machine shot size setting. Also, do not make any adjustments immediately after the first shot. Let the process stabilize before building the rest of your shot size, adding pack and hold pressures, etc….
Learning how to do these simple calculations will help reduce some of the frustration and nervousness during mold start-ups and save a substantial amount of money also. We also suggest that you include this shot volume calculation as part of your mold start-up specification sheet. If you don’t have a mold start-up specification sheet, then now is a good time to make one.