THERMOPLASTIC

Thermoplastic are prevalent due to characteristics which make them highly suitable for injection molding, such as the ease with which they are may be recycled, their ability to soften and flow upon heating and it is safer.

Thermoplastic injection molding is a manufacturing process that creates fully functional parts by injecting plastic resin into a pre-made mold. Thermoplastic injection molding has several sub categories, such as rapid injection molding, which is best utilized in fine tuning prototypes prior to a product being given the go-ahead for production. Another sub category, production injection molding, is best utilized for full product runs. Developers utilize the thermoplastic injection molding process for many applications, as it can produce anything form car door panels to cell phone cases with good accuracy and surface finish.

Injection molding thermoplastic’s process works are, when thermoplastics are moulded, raw material is fed through a hopper into a heated barrel with a reciprocating screw. Upon entrance to the barrel the thermal energy increases and the Van der Waals forces that resist relative flow of individual chains are weakened. This reduces its viscosity, which enables the polymer to flow. The screw delivers the raw material forward through a check valve and collects at the front of the screw into a volume known as a shot. Shot is the volume of material which is used to fill the mould cavity, compensate for shrinkage, and provide a cushion to transfer pressure from the screw to the mould cavity. When enough material has gathered, the material is forced at high pressure and velocity into the part forming cavity. Often injection times are well under 1 second. The packing pressure is applied until the gate (cavity entrance) solidifies. Once the gate solidifies, no more material can enter the cavity; accordingly, the screw reciprocates and acquires material for the next cycle while the material within the mould cools so that it can be ejected. This cooling duration is dramatically reduced by the use of cooling lines circulating water or oil. Once the required temperature has been achieved, the mould opens and an array of pins, sleeves, strippers, etc. are driven forward to demold the article. Then mould closes and the process is repeated.

Thermoplastic injection molding can produce parts with very good accuracy, which thereby makes the process an ideal one for both prototyping and manufacturing runs. However, in order to produce the best possible parts, product design has to meet minimum and maximum requirements regarding thickness. Specifically, the thermoplastic injection molding process is able to create parts within 0.2 mm tolerance. Furthermore, the process can produce more advanced parts with tolerances as precise as 5 micrometers regarding diameter and linear features. Surface finish accuracy is typically anywhere from 0.5 to 1 micrometer in accuracy.

     Figure 1: Thermoplastic Injection molding machine

The figure 1 shows thermoplastic injection molding. In thermoplastic injection molding, the molded parts retain their shape after they cool below their melting point. Therefore, the primary objective of the thermoplastic injection molding process is to get the resin up to its melt point as quickly as possible (which is one reason for the tapered screw; see Resin Heating), and then to get the resin down below its melt point once it's in the mold (which is why the mold is cooled). A secondary objective is the thorough mixing of the resin pellets with the colorant beads. This is another reason for the tapered screw; it does a superb job of mixing the resin and colorant as they are compressed together more and more moving up the taper.

OBJECTIVES

·         To explain about injection molding process for fabrication of thermoplastic material for polypropylene.

·         To describe the mechanical, physical and chemical properties of polypropylene.

DISCUSSION

a)      Inherent physical and chemical properties of polypropylene

1.      Low density

All types of natural or unfilled polypropylene have the same very low density of 0.90 g/cm3 that is the lowest of all commonly available thermoplastics. Parts molded from polypropylene are lighter weight, and therefore more parts can be molded on a part per-weight basis.

2.      High-temperature resistance

The relatively high melting point of 334°F (167°C) for polypropylene allows continued at 220°F (104°C). The resin begins to soften at about 250°F (121°C), but nevertheless can be used intermittently at this temperature. To extend polypropylene’s useful temperature range and service life, an antioxidant system is incorporated. However, any environment (such as moisture) that tends to extract the antioxidants may lead to a more rapid breakdown of polypropylene, especially at elevated temperatures.

3.      Chemical resistance

Polypropylene, like most polyolefins, is highly resistant to solvents and chemicals. With few exceptions, inorganic chemicals produce little or no effect on polypropylene exposed to temperatures up to 250°F (121°C) for a six-month period. Polypropylene is quite resistant to polar organic chemicals but is subject to swelling and softening by non-polar solvents, such as benzene, toluene, carbon tetrachloride, etc. Suitability for use in these environments should be determined by testing. Compatibility data with common chemicals are available.

4.      Stress-crack resistance

Polypropylene has excellent resistance to environmental stress-cracking. Embrittlement that occurs with other plastics in the presence of oils, detergents, and other stress-cracking agents is not observed with this resin. Generally, only very potent oxidizing agents produce stress-cracking in polypropylene.

b)     Mechanical properties

Polypropylene has excellent mechanical properties. The numerous homopolymer and copolymer grades offer various combinations of stiffness and impact strength to meet the specific requirements of many injection molding applications.

1.      Stiffness

Stiffness is defined by the measurement of the flexural modulus on a molded specimen. Of the polypropylene family, homo-polymers possess higher stiffness than both the random and impact copolymer varieties. Polypropylene resins are intermediate in stiffness to that of polystyrene and high density polyethylene (HDPE). High impact copolymers and random copolymers are similar to the flexural modulus (stiffness) of HDPE, while homo-polymer polypropylenes can be stiffer than impact modified polystyrene.

2.      Impact strength

The impact strength of polypropylene can be measured in several ways. The most common methods of measurement are the impact strength as determined by a pendulum type apparatus striking a notched specimen (lzod and Charpy) and the drop weight impact strength as determined by a failing weight on a molded specimen. The impact strength reported is greatly dependent on test temperature. For many applications, polypropylene homo-polymer provide adequate impact strength at or above room temperature. However, for applications with requirements for low temperature impact resistance, impact copolymers are recommended. These grades not only improve the impact properties but also reduce the brittleness temperatures of molded parts.

c)      Molding Process

1.      Design of gates

Gate design is a major decision in mold construction. Gate location, size, and type will influence ease of molding, part dimensional stability, toughness, appearance and the need for trimming. Problems relating to venting, core deflection and “reweld” (weldlines) can be prevented or solved using the proper gate and location.

2.      Gate location

Because the gate area is often highly stressed, it should be located so that the product’s properties and appearance are not adversely affected.

Gate location should:

• Ensure a balanced flow (rapid and uniform filling) in the cavity so that no areas of the part are overpacked

• Gate in the thickest section and direct material flow from thick to thin sections whenever possible

• Ensure mold fill under realistic temperatures and pressures

3.      Cooling

For faster cycles, mold cooling requirements must be considered from the start. The cooling system should balance the heat flow from the part to ensure uniform part cooling and minimize residual stresses, differential shrinkage and warpage. Cooling the mold below the dew point should be avoided because it will cause condensation and molding problems.

4.      Ejection systems

Parts made with polypropylene release readily from molds. A properly designed ejector system will facilitate part removal as long as the following design principles are adhered to:

• A matte mold core surface (grit blasted or draw polished) facilitates stripping

• Ejector pins must be strong enough to withstand forces encountered in part ejection

• Ejector pins should be positioned to evenly distribute the load across the part

• Minimum clearances between pins and mold should be used to eliminate flashing around the pins

• Ejector pin location should be consistent with providing aesthetically pleasing part surface finish

• Ejector pins should not be located near part wall intersections to allow cooling channels to be located where they will be most effective

Ejection of molded parts is not limited to the use of ejector pins. The use of a stripper plate is another common ejection system for circular parts or where the ejection force is evenly distributed across the part surface.

5.      Optimizing mold cycles for maximum output

The molding cycle is comprised of the following steps, each having a time component in the overall cycle time:

·         Injection fill

·         Mold pack

·         Hold

·         Part cooling

·         Mold open

·         Part ejection

·          Mold close

CONCLUSION

Like all thermoplastic injection molding resins, polypropylene has its own special characteristics. These characteristics not only affect the properties of the finished moulded pieces, but they also determine optimum moulding conditions. Available as homo-polymer, random copolymer or impact copolymer types, polypropylene is offered in a broad range of grades and types which have properties that are fully outlined in their respective product data sheets.