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This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

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Check Ring Test on a 2-Shot Machine…

It is always best to isolate the barrel so the check ring is the only thing being tested, and outside influences are minimized.
Question:
With a 2-shot machine, how do I perform a check ring capability study on my 2nd shot?
My Response:
You want to run the shot by itself if possible. Using both barrels will result in a less reliable number. If you were forced to use both shots, the fill and pack the first shot and then subtract that weight from the final weight.
Additional Thoughts: 
When using a two shot machine, have a way to segregate the parts from the two cores. If the check ring shows high variation, you can run the numbers again separately. 
For example if the combined variation is 5%, but the variation in shot 1 and shot 2 may only be 1% by themselves. In such a case, the ring is fine, but the tooling needs help.
Adversely if a high variation is present in one or both of the shots when calculated alone, then the check ring is inconsistent.
-Andy

Flash, with Lower Part Weight…

Just because the mold is flashing, does not mean the part weight has increased…

Question: Hello, we have a part which is flashing, but the part weight is lower than before. Does this make sense?
Answer: When this is the case, it is usually caused by too much material entering the mold during injection. As the mold becomes full, the pressure builds in the mold cavity. When this process transfers from high pressure injection to low pressure packing, the screw bounces back causing material to exit the mold cavity. The high high pressure may cause flash, but the screw bounceback loses material which can not always be compensated for with packing. The result is often a lower part weight. Beware, that it is common for sinks or voids to also be present in such a part. This is a great example of why you always want to document the final part weight as well as the short-shot weight (without packing) for all your processes.
Additional Thoughts:  The best solution for such a problem is to have the process transfer before any of the mold cavities are full. This allows you to use packing pressure and time to more accurately control the final part weight.
-Andy

Screw Retraction During Packing or Recovery…

Many older electric molding machines (and a few new ones) have a programming glitch which causes the screw to retract if the packing or back pressure is set too low.

Question: When trying to make a short shot, I set my packing pressure to 0 and the machine pulls the screw back during packing. How can I make an injection-only short shot?
Answer: Turn packing time to 0 (or the minimum allowed time) and use additional cooling time (if necessary) to maintain the same overall cycle time.
The Cause: The screw is retracting due to a programming error on the machine. The machine senses that the pressure in front of the screw at the end of injection is higher than the set packing pressure (0) and it begins to retract the screw. The same thing often occurs during screw recovery. This misbehavior also happens on some machines when the mechanical resistance to movement is higher than the pressure setpoint. If the machine is not well lubricated and maintained, the minimum packing pressure and back pressure necessary to prevent screw retraction can often be very high. In such cases you should consider having a service representative review the condition of the machine and the software to determine if something can be done to rectify this situation.
-Andy 

Optimize Your Clamp Movements

Part removal is the easiest part of the injection molding process to optimize and there is no excuse for basic clamp movement optimization.

Situation:

While walking the recent NPE 2015 trade show in Orlando, many of the processes were dialed in beautifully where the mold quickly opens and closes while the part is ejected on the fly. Unfortunately, some machine manufacturers had their molds slowly opening and closing, pausing 1 or 2 seconds before starting ejection, and a couple molds even cycled the ejection system multiple times. Many of these processes lost many seconds of cycle time to clamp movement. This poorly showcases a machine at the show, but loses a lot of productivity at your plant.

My Suggestion:

You should always optimize clamp movements You do not have to slap the mold shut or shake the machine violently as the mold whips open to improve your part removal process. A good basic strategy is as follows:

Mold Opening Fast : This takes place after the initial mold breakaway speed. After the mold components are disengaged, the opening speed should be increased since there are no obstructions. The optimal speed is the fastest speed the machine can safely handle without any awkward movements, machine vibrations.You may need a third, moderate final opening speed if the machine has difficulty maintaining a consistent final mold opening position for robotic part removal.

Final Mold Open : The final mold open position should only be enough to allow proper part ejection.

Eject on the Fly : If a robot is not being used, you should start part ejection before the mold is opened. The fact that the mold is moving away from the part tends to help the part drop downward. This feature typically reduces the total amount of distance the mold has to open when properly set.

Mold Close Fast : Just as with mold open fast, the mold should move quickly when the mold components are not engaged. It is critical that the mold closing speed slows down to a safe speed prior to the mold components making contact for the final mold closing.

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