The Extruder - October 2010
Society of Plastics Engineers South Texas Section
Numerical Screw Position vs. Back Pressures
by Dane Kinchen, Blackwell Plastics Project Engineer
Do you know anyone that has suffered from sleep apnea or snoring? We have worked for years with a doctor that had an idea and vision to create a device to cure these ailments. He is convinced that sleep apnea or chronic lack of oxygen to the brain is a cause of many diseases that we are all familiar with. He is an evangelist for his idea and claims to have cured many people. We received a prototype tool from him that has run lots of production on this prototype tool well beyond its intended life.
The part is a small part with thin walls. The problem we encountered was short shots/non-fill back to back with flash. Some of the flash was due to the wear and tear of the prototype mold on the parting line. We decided it was more cost effective to move to a new mold instead of investing more money into the prototype mold that had run for years. This customer was in need of parts while we built their new mold. This required us to run the old mold in production until the new tool is produced and ready for production. The need for parts and timing for the new mold let us with only one option: to“run it as is”.
After hanging the mold in the press, setting the die height, connecting the thermolators, and getting the heats up, I began programming the settings into the molding machine’s data base from a “set-up sheet”. This set-up was supplied with the mold from the customer who was the previous molder of this part.
I shot the first shots and all went off without a hitch with the exception of a few minor changes that are not enough for me to mention. The parts seemed “fair” but not great.
Flashing of parts because of mold damage presents the molder with three standard options for resolution. The options are a secondary labor operation to trim off the flash, repair of the mold, and accepting the parts as is. Since the mold repair was not feasible, and the defects were unacceptable, this customer asked that we have an operator trim off the flash manually until the new mold is in production and damaged mold is retired.
After one hour of production our quality control staff started checking parts per protocol and noticed that some of the shots needed heavy trimming and some of the shots were short and had non-fill on the very tip of the corner.
Keep in mind that this is polyurethane material, and needs to have free flowing gates and a free flowing check ring to reduce the build-up of friction heat from screw slippage during recovery.
Knowing this from pervious experiences I began looking into the set-up to try to pin-point why I am having variances. One of the things that jumped out at me was the setting from the set-up sheet did not call out for pressure trips but instead only called out for screw position. This means that instead of pushing the screw to a numerical number and then holding the back pressure to a specific controlled psi for a set amount of time we were simply stopping the screw at a numerical position for a set amount of time.
After mentioning this to one of our senior process engineers he quickly pointed out that the free flowing check ring has a lot more movement than a standard check ring and could have movement as much as plus or minus 0.150. He also noted that there were no pressure trips in place to trigger the machine when the amount of controlled pressure that had been reached to ensure that the mold got filled equally during each shot removing variances from shot to shot. This method sends the screw to the numeric position and continues until the screw has reached the set back pressure for a set amount of time triggering the machine that the parts are filled with the same amount every shot.
Once we made the changes to our program adding the back pressure trigger, quality was improved along with scrap rates due to constant fully filled parts. We notice that shot to shot had virtually zero variances in part appearance and weight. Unfortunately the flash was not improved but we did eliminate short shots and non-fill. Although the flash was not eliminated, it was consistently the same.