Plastic resins absorb moisture and water. Different resins (plastic materials) have different absorption rate and the amount absorbed is also different. Hence, some resins does not require drying. Presence of water in the material can cause various defects on the final product. The tell-tale sign that water is present is when silver streaks starts appearing on the surface of the finished product. Since water does not blend in with resins, the product’s impact strength is also compromised.
1. Moisture Absorption
Moisture is absorbed by plastics in two ways:
- Direct contact with air or water.
- By condensation – when cold and hot air combine to form dew.
The amount of moisture content allowed in most plastic resin is 1.5% or less. Excessive moisture can cause various problems during processing and molding. As a result, manufacturers take precautionary steps to prevent the material from having direct contact with air or water.
Some hygroscopic resins, tend to absorb moisture and will soak up dews that are formed on its surface. In order to remove moisture from the plastic resins, once the bag is opened, it is advisable to dry the plastic resin in the proper drying unit before processing it.
2. Drying Condition
- Moisture Content – Different types of resins have different moisture content that varies in range. The table below shows the permitted residual moisture in plastic pallets that will not affect the plastic resin’s properties during processing.
- Water Absorption Rate – The table below shows the rate of moisture being absorbed when exposed to air. The value given is in percentage by weight. This chart is applicable only to hygroscopic resins that are new (virgin material) – not applicable for Regrinds or Recycled Resins. Regrind or recycled resins are irregular in shape and has a tendency to absorb moisture faster due to the larger surface area.
- Drying Temperature – Depending on the type of resin, grade of resin and the relative humidity of the atmosphere, the resin will continue to absorb moisture until it has reached its saturation point. Drying temperature is the temperature required to drive moisture out of resin.
- Drying Time – This is the time required to dry the resin to its permitted residual level of moisture content. Depending on the type and grade, the value is given in hours.
3. Drying Methods
There are two principal methods for drying plastic resins:
- Melt Drying – Melt drying uses a vented barrel (shown below). These are specially designed plasticizing cylinders with an opening or vent in the barrel. It doubles as a compression screw that allow gas to escape during plasticizing (the process of melting plastic resins into molten liquid).
- Granules – Granules involve the drying of the plastic resin granules in a drying unit prior to processing.
4. Drying Equipment
Drying equipments are an important component to the injection molding systems. There are 2 kinds of drying equipments:
- Hot Air Dryer (Hopper Dryers) – Air is drawn from atmosphere and passes through a heater. The heated air is forced through the plastic resin in the hopper and then allowed to escape into the atmosphere again.
- Dehumidified Air Dryer (Closed Circuit System) – This type of dryer has a section for drying or moisture removal of the circulating air by the desiccant bed. Common desiccants are calcium chloride and silica gel. After several hours the desiccant which has become saturated with moisture is removed. It can be re-generated by placing in a high temperature oven that drives off moisture for future use. Another section is where the circulation pump forces the dried air through the plastic resin and absorbs its moisture. The air then flows back to the desiccant bed and releases its moisture onto the desiccant.
- Oven Dryers – Oven dryers originated back in the 1920s. These units consist of a series of trays mounted on a rack within a closed chamber. Plastic resins are spread over the tray while the chamber forces hot and dry air over the trays. As the dry air flows over the trays, it picks up moisture and transfers it to a desiccant bed. The dry air is returned to the chamber for another pass.
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