Material Handling, continued

In the previous two parts the material has been brought into the plant and than into the production staging area. Now it is up to what further steps if any need to be done prior to processing.  Are materials to be added to the virgin material such as regrind, color or any other additives?  Do the materials and or additives need to be dried?

With the addition of other materials to the virgin material, mixing, and or blending these components together in a consistent blend is very important.  With the progression that has taken place in the auxiliary equipment market there are many varieties of mixing units from which to choose. Mixing could be all manual with the drum on the floor or on the rotating stand, adding by the cup full or by use of scales to account for the blend. Mixing could also be the latest proportional units that do it by weight and shot size. Whichever one is used, records of mix / setup and let down ratio need to be used. Automatic equipment should be calibrated and checked occasionally to assure the proper mix of materials is occurring. This can be done by collecting what is being pumped out and than weighing it to confirm the mix ratio. The other way is the accounting method; by recording production
usage of the raw materials. The bottom line is when the amount of colorant /additive used is divided into the plastic material used does this equate to the let down ratio / percentage used.

Let down ratio is the ratio of the additives to virgin materials. I.e. 25:1 would be 25 pounds of virgin material is mixed with 1 pound of additive.  In looking at the let down ratios for additives, please note that sometimes lot to lot changes can occur and the let down ratio changes.  Note that let down ratio is really the usage of the additive to the mix. If for instance we have a 50:1 ratio, we are adding less additive than at the 25:1 ratio. This is very important in costing of the job. The let down ratio is also important in the outcome of the finished product when compared to how much of the barrel are we using for the mixing of the additive. If we are using only 10% of the barrel per shot, the mixing capacity of the machine is diminished as compared to using 80% of the barrel. Thus, the let down ratio of the additive may come into play, i.e. how uniform of a mix are we capable of getting.  Also when considering this look at the pellet size of the additive. In most cases, pellet size is of no concern. Nevertheless, if the shot size is such that we do not have a lot of mixing / screw rotation occurring, the combination of let down ratio and pellet size is very important for a uniform mix.

The main thought here is that in adding of the regrind, or additives to the materials, document what is going on and use what ever checks you can put into the system.

Thanks for the time, to be continued.

Steven L Silvey
Sr. Technical Service
General Polymers Division
Ashland Distribution