Definitions: Hopper: 1-a bin mounted over the feed opening that holds a supply of material. Feeding from the hopper is ordinarily by gravity. 2- dry- a sealed bin which allows dry air into the bottom section of hopper and extracts the moist humid air at the top.

The following are the principles to dry materials:

Dryers are used to remove moisture from plastics materials prior to processing. These yield a part that is both cosmetic and physically correct since moisture in plastics while processing may alter both. The physical process of drying is achieved by creating a vapor pressure differential between the moisture in the pellet and the atmosphere around it. In short, heating the pellet and lowering the dew point of the air around it, than using the airflow to sweep away the moisture.

There are four factors involved in drying materials.

1)                  Air Flow- CFM (cubic feet per minute) should equal the throughput of the material in pounds per hour processed on the machine.

2)         Dew Point- this is the dryness of the air and refers to the temperature at which moisture forms, i.e. the lower the better.

3)         Air Temperature- the temperature of the air as measured at the inlet to the hopper and specified by the manufacture per material.

4)         Residence time- the time the resin must be at temperature to dry properly.

The hopper design is critical to drying and the flow of material into our process. If in fact the hopper is not designed properly than the flow of material from the hopper is not even and we well end up with material not properly dried. This pertains to #4 above in way of residence time.

The key to a proper hopper is the cone of the hopper, and the best way to describe this is that if we poured our resin onto a table or floor and created a single pile of material as feed from a single point (as if creating a mountain) the angle that is created from the sides of the pile would equal the angle of our cone. This is the angle that should be in our hopper at the cone section. This angle than would allow the flow of material in a uniform manner from above. If incorrect we would get flow from the center of our hopper and possibly have many hammer marks on the sides of the hopper the result of being hammered on to get material to flow.

A simple test of material flow from a hopper involves the placement of different colored pellets onto the top of the material in the hopper and timing the amount of time it takes to produce parts of that color. These pellets should be place in the top center of material in the hopper.

Example:

Hopper capacity is 100 pounds

Shot size is 0.5 pounds

Cycle time is 1 minute

Residence time in hopper should be 3.3hours

Results:

 100 pound / 0.5 pounds = 200

 200 times 1 minute = 200 minutes

200 minutes divided by 60 minutes/hour = 3.3 hours

If our parts change color in 2 hours than the flow in the hopper is not uniform and this material did not see our 3.3 residence time in the hopper.  If in fact it is 3.3 hours than our hopper has correct flow.

In too many cases it is assumed that since all eliminates to drying are correct that the material should be dry. If upon processing it is noticed that the material started out dry and than as time went on it appeared to be wet, this is an indication of through put and though there are many things to check, firstly is the hopper capacity correct, the second would be throughput on material in our hopper.