Molding pressure is largely classified into injection pressure (primary pressure) and holding pressure (secondary pressure). Primary pressure is required for pouring the molten resin into the mold. Secondary pressure facilitates the transfer of molten resin in the mold onto the surface of the mold by decreasing the pressure in the cavity, and is also used to prevent shrinkage and deformation of the products.
- The injection molding process consists of mold close, injection, cooling/metering and mold open/ejection. Mold Close
The core, a moving part of the mold, moves to the cavity, a fixed part, and the mold is closed. Injection
The screw moves forward and the molten resin flows into the mold through the nozzle.
Consistent pressure is applied until the molten resin in the mold hardens. Cooling/Metering
The molten resin in the mold is cooled and hardened by the cooling water flowing through the cooling channel inside the mold. It is also melted and metered by the rotating screw. Mold Open/Ejection
The mold is opened and the hardened product ejected out of the mold by the ejector pin.
- Short Shot: Failure to completely fill the mold due to insufficient force applied to the mold during molding.
- Sink Mark: Shrinkage of the surfaces of rib and boss areas due to a cooling difference after molding the rib and boss areas.
- Weld Line: A visible line or lines formed where molten resin from different directions flows into the mold and meets.
- Flash/Burr: Molten resin flows out from a parting line of the mold due to excessive force applied to the mold.
- Flow Mark: Wavy surface appearances on the finished products caused by improper molding conditions (temperature/pressure).
- Silver Streak: Silver streak marks on the surface of the finished products due to a decrease in the heat stability of molten resin.
- Mold Sticking: A finished product is stuck onto the inner surface of the mold due to excessive holding pressure or an insufficient draft angle.
- Jetting: A winding shape like a snake on the surface of the finished products caused by the improper design of the gate.
- Crack: Cracks on the surfaces of the finished parts caused by excessive internal stress.
- Peeling: The surfaces of the finished products peel, which is caused by an excessive injection speed.
- Black Streak: Black streaks on the surfaces of the finished products caused by insufficient gas vents or a decrease in the heat stability of the molten resin.
There are many types of troubleshooting items for injection molding, as in the above. In order to solve these defects, it is essential to identify and understand their root causes.
The plastication process is the process of heating and melting plastics. At this time, two heat sources are largely used, namely heater band and frictional heat (occurs between the rotating screw and the cylinder wall). It is known that the frictional heat generated by the friction between the rotating screw and the cylinder wall affects 70% of plastication. The plastication time must be shorter than the cooling time, and the holding time in the cylinder for materials that lack heat stability must be shortened by decreasing the plastication time.
The plastic injection molding procedure is as follows. A plastic resin is heated, melted, poured into a mold, hardened and formed into the desired shape. This molding method is suitable for the mass production of products with complicated shapes, and is commonly used along with extrusion molding. Thermoplastic resins are commonly used for plastic molding, but thermosetting resins, rubber and foaming molding materials can also be used for injection molding. Various molding devices and molding structures are under development, taking into consideration the types of molding materials, the shapes of molded products, and production. Two types of injection machines, namely the hydraulic and motorized types, are used for injection molding, and this is classified according to the force (hydraulic, motorized) applied during molding. The motorized injection molding machine is chiefly suitable for products required for accuracy. The hydraulic injection molding machine is suitable for molding large products. The molding processes of plastics are largely classified as mold close, injection, cooling/metering, mold open/ejection. Polypropylene does not need a drying process, but plastic materials that have a polarity, such as PC, ABS and Nylon, require a drying process to remove all moisture properly. Currently new injection technologies, such as Mucell, foaming molding, MmSH (E-Mold), Heat & Cool, IMD and 2-shot molding, are being developed and used.
A BOPP film is Bi-oriented Polypropylene film / Oriented Polypropylene film. The molten film is extruded in the die, which has a straight line shape slit, and is tightly placed onto a cooling roll. After it cools and hardens, it is bi-axially oriented in MD/TD and then the final film is produced. BOPP film improves upon various mechanical properties by arranging the molecular structure of PP in a consistent direction through biaxial orientation. It exhibits the advantages of transparency and gloss and is thus widely used as a packaging material.
An IPP (Inflated Polypropylene) film is produced with the following procedure. A molten film is extruded in the die, which has the thin cylindrical shape of slit, and is inflated by blowing air into it. Bubbles are formed and the film
A CPP (Cast Polypropylene) film is a non-oriented film and is widely used as packaging film with various types. CPP film is produced in the following way. A resin fed from the hopper is pushed into the feed section, compression section and metering section and is melted. The molten resin is extruded from the T-die in a consistent shape, and the extruded resin is cooled and solidified without a stretching process. The film that is produced then goes through Corona processing, using an electrical device in order to give polarity to non-polar film. The film obtained from these processes is rolled and marketed in the desired sizes.
Raw materials such as polypropylene and polyethylene, which are thermoplastic resins, are fed into the extruder and are formed into continuous workpieces with the consistent shape of a cross section, and are pushed out of the mold. This molding method is called extrusion molding.
Extrusion molding is used to produce tube pipes, gutters, films, plates, adhesive tape, mono filaments, products with unique designs, wire covering and plastic nets. These make up a large proportion in the consumption of plastics overall. Moreover, an extrusion molding machine consists of an extruder, a mold and a receiving device. A granulated or powder form of a plastic is fed into the hopper of the extruder, and it is heated, softened and melted in a heating cylinder made from steel. The molten plastic is mixed and compressed by the rotating screw and is passed forwards. The uniform molten plastic flows into the small holes of a circular metal plate and is refined. It is formed into the desired shape, and is extruded from the opening part of the mold. Lastly it passes through the cooling tank and becomes the final product.
Measurement methods in the data sheets for each product conform to ASTM (American Society for Testing and Materials) standards. The ASTM was founded in 1898 in the USA and is an international standards organization that studies, develops and publishes technical standards for testing, inspecting and analyzing the basic materials for the metal, petrochemical, construction and nuclear power industries. The ASTM establishes terms, products and testing methods for all engineering materials, and maintains 11,000 standards which are commonly used around the globe.
Polypropylene is environmentally friendly and light, and has high stiffness, excellent heat resistance and chemical stability. Therefore it is used as a structural material in various industries. Polypropylene is a polymer of monomers and forms a spiral structure. For this reason, it forms crystals easily and has an opaque characteristic. In order to make it transparent, a great deal of technical know-how is required since its optical transmittance must be increased by lowering its crystallinity.
PolyMirae has been developing new high clarity polypropylene products with many advantages by controlling the structure and crystallity of polymers such as nucleating agents. PolyMirae has been using metallocene catalyst technology, assessed as the best technology for this purpose, to develop polypropylene products that have high purity, high clarity and excellent properties overall. New products will gradually be released this year.
Furthermore, Clyrell products are being produced using a new technology which applies new comonomers to polypropylene. These are new PP products that satisfy both transparency and impact resistance, and are expected to expand substantially into the food packaging and industrial markets. Clyrell products as transparent materials for films have low gel and fisheye and thus they are drawing a lot of attention from customers in the multi-layer film field, such as retort films, decorative films, industrial films and protective films for electrical appliances.
The market for new concept polypropylene materials like this is expected to grow continuously, and they are expected to replace conventional polypropylene materials and other expensive transparent plastic materials. Therefore they are assessed as products that have a lot of potential to overcome the limitations of the conventional PP market.
PolyMirae will strive for the early release of new levels of products by focusing on the development of these new products intensively.
It would be impossible to do so, due to a difference in shrinkage rate. PP compounds suitable for the shrinkage rate of the existing ABS or a new design mold suitable for the shrinkage rate of PP must be used. Polymer chains move in disorder in a molten state and the volume increases. When the polymer solidifies and its temperature cools down, it returns to its intrinsic arrangement gradually, and thus the chain length decreases. This characteristic is expressed as shrinkage rate. The shrinkage rate varies depending on the type of polymers, and crystalline polymers tend to shrink more than noncrystalline polymers. ABS exhibits a shrinkage rate of between 4/1000 and 5/1000, whereas normal polypropylene exhibits a shrinkage rate of between 14/1000 and 16/1000. If normal polypropylene, therefore, is injection molded in the ABS mold, it shrinks more.
Impact strength is one of the most important characteristics and represents the mechanical properties of a resin. It refers to impact resistance when an object is impacted. In general, the greater the molecular weight and the higher the glass transition temperature (Tg), the lower the impact strength becomes. The strength of a specimen against impact is defined as the total energy used for breaking the specimen or the absorbed breaking energy per unit length. It varies depending on the shape and size of the specimen, and the testing methods. The Izod and Charpy methods are commonly used. The order of universal plastics in terms of impact strength is; LDPE > HDPE > PP (Block copolymer) > PP (Random copolymer) > PP (Homopolymer) > PVC > PS.
The melt characteristic of polypropylene is one of the important characteristics of resins, and affects both the processability and the properties of products. In general, the higher the MI, the better the processability. However, properties such as stiffness and impact resistance tend to decrease. Elements that affect the MI are molecular weight and molecular weight distribution. In general, the MI and the molecular weight are inversely proportional to one another.
The measure of melt characteristic is called MI or MFI (Melt Flow Index). It is defined as the mass of resin flowing in for ten minutes through a capillary under a constant load and at a constant temperature. An MI is measured according to the specified load and temperature for each resin.
An MI of polypropylene is measured using the method described in the ASTM D1238L standard. A PP resin is heated at 230
