Definition: Mold Qualification. 1) To qualify for use 2) a set of procedures/steps which lead to the approval for production of a mold Short shots: 1) parts which exhibit a non-fill of their shape or form 2) used in mold trial to visualize fill pattern for single or multicavity tools
Though there are many steps to qualify a mold, and dependent on the industry and product there can be many rigorous tests, and steps that are already established. One which is a benefit for the processor and tool maker and or designer is a short shot test.
*** it is very important to understand the tool and to know whether or not a short shot can be produced in the mold safely and well eject properly not damaging the tool and or requiring the tool to be pulled from the machine to dig out the parts.
Some pre-qualifying steps involved with running a short shot test, are:
1- determine that it is safe
2- Determine that parts are easily eject-able and stick to proper side of mold.
3- Have at hand note paper/book, camera and or table and marking of the parts to view the progression of the short shots. Also needed is a scale to weigh parts.
4- Understand the machine and settings effect
a. This is key as one needs to know when a pressure is decreased for example on the pack and or hold that the pressure is 0 and that the press does not behave in a manner which because one has decrease pressure to 0 but has time on the pack and or hold than applies pressure to reach a set point.
b. Know the effects of injection forward time on the system.
5- Cycle time remains same as when manufacturing production parts.
Running a short shot test shows the progression of fill at the preset fill speed and pressure and a short is produce by only decreasing the volume of material to be injected into the tool. Thus the only change is the volume of material by shot size, and shot size is the only change that it modified once the press is set up for running the test.
Step 1: Weigh and record the current full part/parts and or shot. **when recording the data include; number parts, note weight, transfer and shot size, and if able pressure at transfer.
Step 2: Turn the pack and hold pressure to 0 as the short shot is only concerned with filling. As stated previously understand the machine, and how it reacts to various settings. When turning pack and hold to 0 than one may see screw bounce, and no pressure reading once transfer is made between fill and pack.
Step 3: produce a part and record the part/parts as fill to transfer. These parts show what the fill is to the point of transfer, and are representative of actual production transfer point parts.
Step 4: decrease shot size
Step 5: record the new shot data and save as it.
Step 6: proceed with the steps until one has created enough shots to see the fill progression for the cavity / cavities.
One should end up with parts that are 90% or more full down to parts which may only be 5% full if necessary.
The calculations and results:
The results should be progressively shorter part / parts for the mold. Though most may look at the parts and say this one appears 90% full and this one 75% full this is not a correct observation and the parts should be weighed and compared to a full part. This weight of the short part versus the full part now can be calculated as a percent of full, this than give an correct comparison cavity to cavity and can be used even in a family mold of different volume parts as a comparison of fill.
By observing the fill pattern one can than see if there is hesitation within the mold which may show why a sink is occurring or some other cosmetic or defects shows up. The author has observed in one family tool that both cavities were filling at the same rate until 40% fill point in both parts (2 cavity tool) than cavity one continued to fill while cavity 2 stopped filling and then picked up filling again only after the cavity one was full. This lead to issues both in packing out and finished part size in both of the cavities.
In another example a multicavity tool had some issues, and upon performing a short shot test it was observed that 2 of the cavities were full and packing while the other 6 cavities were less than 20% full.
The short shot test is simple, and if performed correctly gives excellent results. The main issue the author has noticed is that many well adjust both speed of fill and pressure available to create a short shot, which it does but it is not representative of what the production process is producing. It is critical to only change the volume of material by just that the volume available for injection into the mold.
SL SILVEY
www.silveysplasticconsulting.com
27012013
