Greetings!

 

Hallo!

 

Well the K is in full swing and if you made it great if not read about the fun stuff.  Myself I am enjoying what I can and visiting  those on the list.

 

this issue continues with the topic of  filling the mold, and mentions that variable in all machines the CHECK  RING. yes it is a variable and they have come a long way in providing repeatable seals, yet there are really only two types in the long run.

 

Enjoy, let us know your thoughts, and as always pass us along to share the knowledge and hopefully long term get some work.

 

 

Ta

 

Steven

 

Built in Variable "The check ring"

 

Definition: Injection: the act of injecting. Inject: to force into a passage, cavity, Variable: changeable, inconsistent check ring: to stop/ arrest / prevent

 

In part one, (previous note) it was describe as to how to fill the part and the various methods of transfer. In all molding machines there is a built in variable and that is the check ring, back flow preventer, ball check and or any other name associated with the tip area of the screw. Oh yes, if you are running PVC there may be a smear tip which has no real prevention to back flow and if you have a plunger design (piggyback/ or micro type) machine they do not have.

 

In reality a check ring of any type on the front of the screw comes in basically two types, those that leak, and those that are going to leak. So from that understanding what this means is that there shall be some variability of shot size on a shot to shot basis, though designs have come a long way, and along with material selection on the steels used are getting better. On that same note these components do wear, and should be on a maintenance schedule to be checked, inspected and replaced. They should not be on the schedule which waits till they do not work.

 

Fig 1.

 

Fig. 1 shows a 3 piece sliding check ring which screws onto the front of the screw. The red arrow indicates the direction of injection and the view shows the position of the ring when material is being injected into the cavity and the ring is sealed against the seal.

 

The illustration in Fig.1 shows a 3 part ring assembly and a simple explanation on the workings of the design are:

1. In injection mode the sliding ring slides back and seals upon the seal, this than creates a seal so that as the screw is pushed forward directs all the force onto the plastic material (shot) forcing it into the cavity.
2. Upon screw rotation the pumping action of the screw upon the material forces the sliding ring away from the seal and material flows over the seal and between the sliding ring and body. The sliding ring may travel to the front portion of its travel distance.
3. Upon completion of shot generation any residual pressure on the material may push the ring back upon the sealing ring but any suck back used shall keep the ring in the forward position.

 

Fig 2

 

Shown above is a simple 3 piece check ring which will wear the torpedo or body of the ring assembly. In a 4 piece assembly there is a bearing wear area between the ring and the body on the front side of the ring.

 

Fig 3

 

The variability of the system

When filling the part, one does hope that the check ring assembly is performing as designed, but the question remains is it consistent. The travel of the ring assembly is critical and must be compensated for. In many processes this is taken care of with the suck back of the screw on the machine. What is meant is that when creating a shot of material (this is the volume that is injected into the mold), at the end of the screw rotation/shot creation the sliding ring is in the forward position. If one were to inject at this point than any travel of the screw forward would 1st force material back upon the sliding ring forcing the ring back until a seal is achieved. The volume of material now in front of the sealed ring is different than that which was formed.

 

If after forming a shot suck back was employed to pull the screw back a distance that is equal to the travel of the ring plus 0.5mm than as the screw is pushed forward the sliding ring would move back and seat prior to coming into contact with the shot volume of material that is to be injected into the mold. That is the concept and thought behind it.

 

Here is the question; how much travel do the check rings on the various machines in your plant have? Are they all the same, and or of the same type?

 

This than leads to the next question in filling a mold and that is what is the shot size, and how much travel is there of the screw. While the author has experienced a screw travel of ¼ turn every other shot (not advisable) and also the use of intrusion (again not advisable) in both cases parts were produced which were acceptable but also many rejects were encountered until all understood exactly how things worked, and processes set to compensate for these extremes.

 

In most shops the check ring travel is not known along with what type and clearances there are with the units. Maintenance is poor in regards to check rings and or whether they are working properly at all. This all leads to issues with filling to a consistent percentage which then effects part quality and the effect of packing on the parts.

 

If using an extremely small shot size which corresponds to short screw travel, is there enough travel to seat the sliding ring?

 

In the injection filling stage, the goal is to fill the cavities as uniformly as possible, and the knowledge of how the check ring functions and its variability on the process is critical.

 

To be continued.

 

 

SLSILVEY

01102013.01

 

 

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