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Keywords of this article:  injection molding 
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During plastic molding, moisture has big influence on the quality of plastic products. It may cause cracks, whitening, air bubbles, and even degrade the plastics. Therefore, it is necessary to control the moisture in plastic materials based on the requirements and conditions during molding process, so that high production efficiency and high quality products can be achieved.

Effectively control the moisture in materials is crucial in plastic molding process.
Effectively control the moisture in materials is crucial in plastic molding process.

Control the moisture of plastic materials effectively

There are many factors affecting the moisture in plastic materials in the process of drying, including the drying temperature, airflow and time, dew-point temperature and environment, according to Shini Plastics Technologies, Inc.

Drying temperature is critical for complete drying of the plastics and varies with different materials. Insufficient temperature will lead to poor drying effect or prolong the drying time, while too high of the drying temperature will decompose the material and decrease the mechanical properties.

Although the greater the drying airflow, the better the drying effect, it requires higher consumption of energy. Besides, there is slight difference between actual minimum airflows required by different materials for drying.

Dew-point temperature ≤-20°C can completely dry ordinary plastics, and the lower the dew-point temperature of the dry air, the better the drying effect.

To control the moisture in plastic materials effectively, it is necessary to choose suitable drying equipment. In the market, the common dryers include hot-air dryers, partial hot-air dehumidifying dryers and hot-air dehumidifying dryers. As the moisture required by different plastics is different, the choice of drying equipment should be made in accordance with the requirements of plastic materials and products.

For plastics without strong hygroscopicity and have no requirement for the moisture, such as PP, PE, PS, PVC and PPS, the common hot-air dryer can be adopted to reduce the cost. For medium hygroscopic plastics that require the moisture ≤0.05%, such as ABS, PMMA, PC, POM and PBT, the partial hot-air dehumidifying dryers can be adopted. For strong hygroscopic plastics that require the moisture ≤0.02%, such as PA, PET and PES, the hot-air dehumidifying dryers are required for complete drying.

It is also important to prevent plastics from moisture regain after drying. When the floor mount drying equipment is used, the dried materials require low dew-point drying hot air for the conveying, and shut-off suction box has to be installed. The materials should be left in pipes with heat insulation, as materials are not easy to become hygroscopic when they are at high temperature state.

When selecting the storage hopper to mount on the injection molding machine, choose the hopper based on the output, and don’t store the materials in the hopper too long as to avoid the moisture regain.

Mounting the hot-air dryer can also prevent moisture regain due to material temperature decrease. For occasions that require less moisture amount, the hot-air dryer has to pair with the hot-air recycler or replenish the dehumidifying air for the drying.

Maguire VBD-600 Vacuum Dryer.Vacuum dryers equipped with energy consumption monitoring functions

Maguire offers vacuum drying systems with throughput capabilities ranging from 30 to 1,000 lb. (15 to 450 kg) per hour. An energy-monitoring capability is now also available with all VBD models.

Among the models, new, intermediate-range VBD-600 dryer achieves throughputs up to 600 pounds (275 kg) per hour. This capacity is suitable for the manufacture of sheet, large automotive parts, and many other extrusion and molding applications.

 The VBD-600 dryer includes a new controller display that monitors energy consumption and enables processors to track consumption over time. This is the latest addition to an intuitive control system which enables operators to manage all drying parameters simply by means of a touch screen.

As drying polymer can account for as much as 15% of total process energy cost for a molding operation, the substantial reduction in energy consumption made possible by the vacuum dryer can yield a rapid return on investment for the equipment.

The new energy consumption display makes the radical energy efficiency of vacuum drying immediately measurable, according to the manufacturer.

As said, in comparison with desiccant dryers, the VBD vacuum dryer consumes up to 80% less energy, dries resin in one-sixth the time, and substantially reduces the heat history to which polymer is exposed. The speed with which the VBD system removes moisture makes properly dried polymer available for production only 35 minutes after a cold start.

 VBD vacuum dryers use gravity to move material through three vertically arranged stages of the drying process, with the discharge of material from one stage to the next controlled by high-speed slide-gate valves with an accuracy per dispense of +/- 4 grams.

The vessels for the three stages are, from top to bottom: a heating hopper that brings resin to a target temperature; a vacuum vessel in which vacuum reduces the boiling point of water, causing moisture within the pellets to volatilize and be forced out of the pellet into the low-pressure environment surrounding it; and a heavy insulated material retention hopper with transparent shroud to protect dry material. For hygroscopic materials, a membrane purge option is available.

 The vacuum vessel and retention hopper are mounted on load cells that monitor the weight of material at two critical points, making possible precise control over material consumption and documentation of process conditions for certification to customers.

Rotor within pellet dryer.New pellet dryer design reduces cost of wear

A newly designed pellet dryer for use with Nordson’s BKG pelletizing systems meets the challenge posed by the steady growth in use of glass-filled and other abrasive materials.

The new design reduces the number, complexity, and cost of the dryer components that are subject to abrasion and simplifies the maintenance and replacement, therefore reducing production downtime and making it possible for one person to carry out maintenance tasks.

While the new dryer has much the same overall appearance and footprint as a standard BKG dryer and provides the same throughput, there are substantial differences in the configuration of components subject to wear.

 The design enhancements for reducing wear or simplifying maintenance involve pellet inlet, rotor and pellet outlet. As a result of a new design made in accordance with flow simulations, the inlet now enters the dryer housing tangentially, reducing the impact on all parts in the lower part of the dryer, and allowing quick access to all remaining wear parts.

Due to the optimized pellet inlet, the rotor has been simplified, especially in the lower area, and wear is minimized by the reduced impact of abrasive pellets. Other measures taken include a new cover plate design and countersinking of screw heads. As introduced, rotor disassembly and reassembly take ten hours for standard dryers, while the new design reduces this time to about three hours.

 For pellet outlet, new design includes fewer wear parts, and these are more accessible. Disassembly or reassembly time is reduced from 6 hours to a range of 1 to 4 hours.          

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