MATERIAL DRYING

This section will discuss moistur e levels in plastic ma terials, methods of dehu-midification, and how to measure the amount of moisture present.

Moisture Levels

Moisture is the number one cause of molded product defects. The moisture level of resins must be approximately 1/10th of 1 percent, by weight. If the moisture level is higher, then the moisture turns to steam as it travels through the heating cylinder of the molding machine (which is usually hotter than 300° F [149° C]).

The steam prevents plastic molecules from bonding together properly and weak

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parts are produced. In addition, the visual evidence of this steam (splay) is usu-ally not aestheticusu-ally acceptable.

There are basically two types of plastic ma terials: hygroscopic and nonh y-groscopic. The term hygroscopic refers to the tendency for the material to absorb moisture directly from the atmosphere, like a sponge. The most common of these materials are nylon and polycarbonate (although there are many others). Hygro-scopic material suppliers tell their customers that it is vitally important to predry the resin before molding and the y provide detailed inf ormation for drying the resin, what temperature to use it at, and the length of time required for process-ing. Although the suppliers of nonhygroscopic materials (such as polypropylene and polyethylene) claim their materials do not need drying, they too can provide drying information to those who ask for it. All materials should be dried before processing to control as man y conditions as possib le. Even though the pr imary resin may not be hygroscopic, fillers used for making the final compound may be hygroscopic. Predrying the ma terial provides a “jump-start” to pr ocessing and makes plasticizing easier. The more control a molder has of v arious processing parameters, the lower the cost to manufacture a product and the fewer the number of rejects produced. Therefore, an efficient injection molding operation will make the investment in equipment and time to predry all materials, regardless of whether or not they are hygroscopic.

How to Dry Materials

The two primary types of drying units are the hot-air dryer and the dehumidify-ing dryer. Either unit can be used f or nonhygroscopic materials. However, the dehumidifying dryer must be used for hygroscopic materials.

Because nonhygroscopic materials do not absorb moisture, but merely allow moisture to stand on the surf ace of the pellets, hot-air dr yers are sufficient for removing surface moisture. They work on the principle of evaporating moisture by applying heated ambient air to the plastic resin. When this heated air is circu-lated through the resin in a hopper, it absorbs the surface moisture and transfers it to a disposal area (usually a floor drain).

Since hygroscopic materials tend to absorb moisture throughout the material, they must be dried by a dehumidifying system. This is similar to the hot-air dryer but includes a dehumidifying bed tha t the heated, moisture-laden air circulates over before it is returned to the hopper. The dehumidifying bed contains an a b-sorbent material (usually silica) that removes the moisture and traps it in the bed.

After the bed is saturated (normally after 8 hours of drying), it is regenerated by exposing it to high-temperature air to drive off the moisture. The absorbent mate-rial is electronically charged with ions that have an opposite polarity to those in the moisture. The moisture is attracted to the absorbent material as the air circu-lates over the bed. This action is what pulls the moisture out of the plastic mol-ecules and traps it within the bed.

Drying systems are available as central units designed to dry a large volume of plastic. From this unit, plastic is fed to many indi vidual area units that dry smaller volumes and feed three or four molding machines. Drying systems also can be indi vidual units mounted ne xt to each machine and dedica ted to that single machine. There are many advantages and disadvantages to each system and these must be explored before making a final decision on which system to use. Most larger molding facilities incorporate some of each system. One thing to consider is that the farther a plastic must tra vel before it is in the machine hopper, the greater the chance of inducing moisture back into the plastic before processing it.

Regardless of w hat system is used f or dr ying the ma terial, a hopper dr yer should be mounted directly on the molding machine to maintain the dryness of the material being immediately processed. Do not accept claims that hopper dry-ers can actually dry the material while processing it. In some cases, this may be possible, but it is r isky. Hopper dr yer units should not be depended upon for primary drying, only maintaining dry material.

A note of awareness: plastic materials will only stay dry for 2-3 hours before they begin absorbing moisture again. For this reason, materials should be dried just before they are to be used. This applies to regrind too.

Measuring Moisture Levels

How can you tell if the material is properly dried? In general, plastic resins must have a de w point v alue of betw een –20 and –40° F (–29 and –40° C). As the material becomes dr yer, the de w point r eading decreases. This reading can be taken at the r esin discharge tube of the dr yer by using a de w point meter. The material supplier will provide data on what the acceptable dew point range is for a specific grade of material.

If a dew point meter is not available, the moisture level can be estimated using the Thomasetti volatile indicator or TVI. This test consists of making a sandwich of two glass micr oscope slides with thr ee pellets of r esin between them. This sandwich is then placed on a 525° F (274° C) hot pla te and allowed to heat up.

When the plastic pellets melt, they will form three puddles. If more than a single bubble appears in an y of the pud dles, it indicates moistur e is present and the material should therefore be dried.

Other indicators of moisture include a frothing of the resin as it exits the nozzle during purging, splay (silver streaking) on the surface of the molded par t, and brittleness of a molded product.

SUMMARY

Proper storing and movement of raw materials are important to the success of any molding operation. If materials are stored too long, or improperly, additional costs

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are incurred through the need for excessive storage space and potential contami-nation of the resins.

Most molders utilize some type of compounding, either to mix color materials (and/or other fillers) into natural resin, or for blending regrind with virgin mate-rials. This, too, is considered part of the storage and handling process.

When considering storage space, height should not be overlooked.

When using storage cells (of any style), it is wise to assign identification num-bers to each cell. Then, the material stored in each cell can be tracked and traced for inventory control requirements. The most common method is the use of alpha and numeric locator combinations, for example, A-1, A-2, A-3, etc., and continu-ing through the alphabet. After uscontinu-ing Z, the next alpha designator would be AA.

This provides for an unlimited number of identification combinations.

Next to moisture, contamination is the primary cause of defects in molded parts.

The moisture level of r esins must be a pproximately 1/10th of 1 per cent, by weight. If the moisture level is higher, then the moisture turns to steam as it trav-els through the heating cylinder of the molding machine (which is usually hotter than 300° F [149° C]). The steam prevents plastic molecules from bonding to-gether properly and weak parts are produced.

QUESTIONS

1. If materials are stored too long, or improperly, why are additional costs incurred?

2. What determines how much storage space is required?

3. What device is commonly used for storage?

4. When is it considered advisable to use a silo for storage?

5. What is the major disadvantage of using a silo for storage?

6. What is the desired moisture level in raw plastic material, by weight?

7. Which materials should be dried prior to processing?

8. What does control of process parameters offer the molder?