Shot Size Procedure

[The Rookie]

Hi all!

I was wondering if I could get some feedback on a procedure that I was given. This is to be carried out on a new mold after the barrel and mold temperatures are at operating conditions. I attached the full procedure, but I am just curious as to what most people do for setting up the initial shot size.

The Short Shot Setup
At the end of this procedure, the parts should appear cosmetically perfect. The dimensions of the parts may not be optimum, but they should look complete.

Shot Size Estimations
1. Determine maximum stroke for the machine
2. Set shot size to 80% of this value
3. Turn off melt decompression
4. Determine maximum shot pressure for the machine
5. Set injection pressure to 50% of this value
6. Set pack pressure to 0 psi
7. Set hold time to 0 seconds
8. Set injection speeds to 50% of the machine maximum
9. Ensure machine is transferring on position
10. Set transfer position to 0.01 inches

Transfer Position Estimations
1. After purging procedure, prepare a shot
2. Manually move the unit forward
3. Manually inject the shot into the mold for 1 second
4. Record the cushion
5. With injection unit still forward, prepare next shot
6. Manually retract the injection unit
7. Open the mold and eject the parts
8. Repeat 3 times
9. Calculate average cushion

Transfer Position Adjustments


Part Observation
New Transfer Position


Very Short
Average Cushion - 20% of total shot capacity


Short
Average Cushion - 10% of total shot capacity


Okay
Average Cushion


Flash
Average Cushion + 10% of total shot capacity


Serious Flash
Average Cushion + 20% of total shot capacity



Attempt a Single Cycle
1. Single Cycle Machine
2. Observe Transfer Position
3. If the screw makes transfer, re-cycle and proceed
4. If the screw stalls before making transfer, switch to manual, make a shot, move the unit away, and eject the part. Add 10% of the total shot capacity of the machine to the transfer position and try again

Injection Pressure Setting
1. Adjust transfer position to achieve an 80% filled part
2. Set injection pressure to 30% of the machine maximum
3. Increase the injection pressure in increments of 5% of the machine maximum until the fill time does not increase

Re-couple the Cycle
1. Adjust transfer position to achieve 95-98% filled part
2. Set pack and hold pressure to 50% injection pressure

**Proceed to Viscosity, Cavity Balance, and Gate Seal Studies**

[nikom]

That sounds a little weird to me compared to what I did in school. The one time I did this in school they had us set a large injection stroke and large cushion, then decrease the transfer position until we got full parts.

The place I'm working now doesn't have any documented procedures or training so I'm making it up as I go. I made a spreadsheet that I enter the estimated shot weight and select the resin type and it will give me an approximate shot size needed on any of our machines (example 110 mm screw position on a 250 ton machine). I typically increase this position by an appropriate value (in this case probably 15 mm) and then set a large cushion (maybe 30 mm) to ensure a short shot, I'll set hold pressure to zero but keep hold time to something I feel is likely for the part. From here I'll decrease transfer until I get the fill only look that I think is appropriate. I'll then add hold pressure of 50% peak and find a minimum hold time (gate seal study). Then depending on the resulting cushion I'll shift all positions to get a cushion of 3 to 6 mm. Our parts are typically highly cosmetic and with complex geometry so injection speeds tend to be slower and often profiled so I typically don't do a viscosity study.

I typically do all this in auto or semi-auto unless I'm afraid the part won't eject or it has complex hydraulic/side actions.

[nemanja]

Hi,

Depends from molding product, dimensions are sometimes more important than cosmetics (at least in my case).
Im making around 20ccm of cushion plus 98% of shot volume (that is determent during fill study). After i add hold till part is filled.
After i just move cushion to be around 20-30% of shot, depends of total volume.
Injection speeds and pressure profiles depends from mold mostly. Most of my parts have complex geometry with overmolding something inside, so i usually need to have few speeds and pressures.
After is just optimization of process, to be faster, without impacting on stability.
It looks nice, but my personal feeling that you wont be able to do all steps in all cases.

BR.

[JayDub]

Interesting procedure, but why? Myself, I pick up the phone and ask the mold designer to pull the sprue/runner/cavity volume off the CAD model for a pretty good estimate of shot volume.

[chrisprocess]

Should have access to total estimated shot weight, from this you can calculate total shot size. Underfilling a mold is fine, overfilling a mold can be very problematic