/webtest/epaper_i.php?lang=en&ID=789 SHINI PLASTICS TECHNOLOGIES, INC.

Special Reports

2013 Quarter 4 Issue 17

Application of Plastics Auxiliary Equipment

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The general technological process of plastics molding is as follows: drying, conveying, mixing, injection molding, cooling molding, taking out and recycling of wastes. Plastics auxiliary equipment is used for improving production efficiency and product quality during this process. According to their applications, they can be divided into the following types: dehumidifying & drying, feeding & conveying, dosing & mixing, heating & cooling, auto robot and granulating & recycling, etc. The following will generally introduce them: 

Dehumidifying & drying

Plastics used for plastics molding are all hygroscopic, which means moisture will be absorbed on surfaces of plastic particles and inside. If using these plastics directly for injection molding, products will have quality problems, like air bubble and burr. Therefore, the first step of plastics molding is to remove the moisture in plastics. The traditional way of hot air drying just can remove the moisture on plastics surfaces. For removing the moisture inside plastics, it’s necessary to heat dehumidified air to dry, which means to use dehumidifying dryer to dry plastics. To get good dehumidifying and drying effect, dew-point temperature of dehumidified air usually should reach to -40℃. However, if for PET preform molding, it’s better for the dew-point temperature to reach to -50℃. 

Feeding & conveying 

Molding workshops are all equipped with conveying device or system to improve production efficiency. For large-scale factories, the conveying control of whole workshop is set in a control room for realizing to automatically convey material from storage tank to drying tank and then to the molding machine. For small-scale factories, set several feeding machines or a one-to-many feeding machine beside the molding machine to feed material. 

Dosing & mixing 

In plastics molding process, it usually will mix several kinds of materials, like master batch, additives and recycled material, to make the products qualities meet requirements, like requirements of color and hardness. Some molding occasions have high requirements toward precision of blending ratio for two reasons. One is to reduce the quality flaw to the minimum and the other one is to avoid wastes of material to minimize costs. For such molding occasions, gravimetric technology is usually adopted to mix materials in ratio basing on components with accuracy of ±0.1%. 

Heating & cooling 

For reducing reject rate, it’s helpful to preheat and keep temperature constant before molding. Therefore, manufacturers of plastics molding usually will set a mold temperature controller beside the molding machine. The mold temperature controller will preheat molds before first molding and then keep temperature constant for molds. Plastics are melted by high temperature which will increase the temperature of mold rapidly. The mold temperature controller will cool the molds to keep their temperatures constant. 

Plastics in molten state must be molded by cooling. It’s not that practical in industrial production to mold molten plastics by natural cooling, since the cooling period is too long. It’s necessary to shorten the cooling period as much as possible to increase the production efficiency. The usual way of cooling is to introduce cooling water into molds to cool the molds fast by principle of exchanging heat and thus to shorten the cooling period. 

Automatic robot

After molding, small plastic products can be pushed out by ejector pin or be taken out manually. The traditional manual process of taking out is of low production efficiency and may have potential safety problems if not done appropriately. Auto robot can take out products quickly and have high repeatability. It can work with all kinds of ends of arm tooling (EOAT) to replace manual process of taking out to greatly increase production efficiency and save labor cost. It’s best suitable for continuous and fast operation. For taking out large plastic products, it has absolute advantage over the manual way.

Granulating & recycling

Along with plastics molding, it will produce all kinds of wastes, like sprue, scrap and defective product. It’s quite necessary to make use of these wastes to reduce material cost to the lowest. Use granulator to recycle the wastes into regrind. Next, mix those regrind with new material according to proportion and then mold them to reduce consumption of raw material, master batch, additives, etc.