This chapter explains the properties of plastics and the characteristics of typical material in mold processing.
Property of Plastics
Plastic properties are classified by 5 criteria.
The properties mentioned here could be references for plastics selection. However, these properties are subject to change due to temperature or humidity changes.
Therefore, it is necessary to account for changes in environmental conditions on selection of plastics.
Mechanical properties
Mechanical properties refer to displacement or breakage of plastics due to some mechanical change such as applying some load.
Mechanical properties are dependent on the temperature, force (load), and the duration of time the load is applied. It may also be affected by ultra-violet radiation when used outside.
Thermal properties
Thermal properties include heat resistance or combustibility.
Thermoplastics has a larger coefficient of thermal expansion or combustibility and a smaller thermal conductivity or specific heat than other material such as metals.
Chemical properties
Chemical resistance, environmental stress crack resistance , or resistance to environmental change are referred as chemical properties.
When a plastics contacts chemicals, there is some kind of change. After having a plastics in contacted with chemicals under no stress for about a week, changes in appearance, weight and size of the plastics are examined. Such changes are referred to as chemical properties.
Electric properties
Electric properties are also referred to as electromagnetic properties. Electric properties include insulation, conductivity and electro-static charges.
Due to their good insulation property, plastics are often used in electric fields. However, plastics do have a defect; they are easily electrified.
Physical properties
Specific gravity, index of refraction and moisture absorption are called physical properties.
The specific gravity of the plastics is small, and it varies depending on the character of high polymer , or thermal and mechanical treatment of the plastics.
Characteristics of Molding Material
Here, characteristics, application, cautions, molding condition and physical properties of typical molding material (thermosetting plastics has only physical properties.) are described.
Molding material often changes into different states due to many factors. General features, applications, caution and molding conditions are presented as follows:
Polyamide (Nylon) (PA)
Characteristics
Excellent in impact resistance and chemical resistance as well as in electric property and low temperature.
Higher fusing point, good thermal resistance.
Due to its self-lubricant character, it is often used for bearings of mechanical parts.
Usage
Often used for moving parts of machines (bearing, gear, cam) or bolt.
Caution
Because of its water-absorption character, dimension accuracy may be affected and the material quality may change. So, it needs to be dry enough before molding.
Low dissolving (melting) viscosity- watery texture may cause development of flashes.
Molding condition
Resin temperature is generally set high. Control the mold temperatures for uniform cooling.
Polypropylene (PP)
Polypropylene (PP)
Lightest specific gravity among all plastics generally used
Excellent flow
Applied for various gates such as pinpoint gate , direct gate , special gate, etc.
No need to dry as it has little water absorption
Molding shrinkage rate changes according to mold temperature
Usage
Often used for extra large parts or extremely thin parts Due to its excellent fatigue resistance, it is often used for hinges that are subject to repeated bending
Caution
Due to its large shrinkage rate, it may experience deformation if the cooling of the mold is not sufficient. Temperature adjustment must be performed for molding that requires precise dimension. When using the mold that may cause sink marks or holes, injection pressure should be set relatively higher.
Molding condition
Molding temperature is usually 40 – 60°C
Injection pressure standard is 800 – 1200kgf/cm²; however, the highest pressure free of flash is appropriate. The molding temperature range appropriate is 200 – 300ºC, and it is better to set up within the higher range.
Polyethylene (PE)
Characteristics
There are two types of polyethylene: low density polyethylene and high-density polyethylene.
Low-density polyethylene is softer than high-density polyethylene. It is excellent for molding.
High-density polyethylene has excellent stiffness and impact resistance.
Excellent chemical resistance
No need to dry because it does not absorb water
Use
Low-density polyethylene is used for products requiring softness and flexibility. It is often used for complex-shaped plastics or packing material. Low-density polyethylene is also used to improve flow of molding materials.
High-density polyethylene is used for cylindrical containers, or for large plastics products such as containers.
Caution
Higher mold temperature results in the following: molding cycle becomes longer, impact strength is reduced, molding shrinkage becomes higher, and specific gravity increases.
Lower mold temperature causes the surface of the plastics to peel off or deform.
Molding condition
Higher mold temperature will improve brilliance and appearance of the part.
Higher injection pressure will result in uniform temperature of the molten plastics inside the mold. It also enhances the density and strength of the plastics.
It is advised to apply low holding pressure after filling the molten plastics.
Acrylonitrile butadienstylene(ABS)
Characteristics
Elastic and unbreakable
Since it is Amorphous plastics , it has poor climate resistance
Easy to maintain dimension accuracy, well-balanced material
Relatively easy to perform secondary processing (mechanical treatment or galvanized treatment, solvent sealing, etc.)
Use
Often used for home electric appliances or interior parts
Caution
Due to its moisture-absorption characteristics, drying is critical before molding. Otherwise, bubbles or cracks may appear on surface.
Thin plastics shapes, where it is hard to fill in the molten plastics, should be avoided.
Molding condition
Maintain the mold temperature relatively high, around 60 – 80°C to stabilize the temperature Set injection pressure high as Acrylonitrile butadienstylene has poor flow.
Polycarbonate (PC)
Characteristics
High melting point, high melting (fusing) viscosity
Relatively small molding shrinkage rate (0.5 – 0.8%), and not affected by the position of the gates
Does not soften below 150°C
Excellent impact strength
Usage
Used for the parts requiring strength or parts subjected to dynamic and severe loads
Caution
This plastics needs drying before molding due to its water absorption characteristics; otherwise, appearance or quality may be affected.
High mold temperature causes longer molding cycle Low mold temperature may cause deformation of the part If injection pressure is too high, the part may deform internally and be easily broken.
Molding condition
Mold temperature is appropriate between 85 – 110°C
Higher temperature will lead to better flow and glossy appearance(brilliance).
It also reduces deformation of the product. Injection pressure should be set high.
Molding temperature should be between 260 – 300°C
Polystyrene (PS)
Characteristics
Excellent flow
Less molding shrinkage, no water absorption
Due to its stable thermal character, thermal decomposition does not occur even at excessive molding temperature.
Excellent electric insulation and chemical resistance
Caution
If molding condition is inappropriate, deformation may develop and remain inside the part.
This will cause cracks upon removal from the mold.
Excessive injection pressure may affect mold removal or cause development of flashes.
Molding condition
In general, mold temperature is appropriate below 40°C To improve surface brilliance, 60 – 70°C is recommended.
Polyvinyl chloride (PVC)
Characteristics
High viscosity at the time of melting
Poor flow of molten plastics
Excellent chemical character such as anti-oxidant and anti-alkaline character
Poor climate resistance, weak against temperature difference
Caution
Temperature control is critical; if it is not properly done, the part may burn through thermal decomposition or corrode the mold.
For better transfer of injection pressure , sprue, runner and gate need to be thicker and shorter, and the thickness of the part made needs to be as thick as possible.
If injection speed is too fast, friction heat may develop and cause combustion.
Molding condition Molding should be performed within the injection pressure range of 800 – 1200kgf/. It is advisable to set the pressure a little higher. Molding temperature should be set lower, but in general, it is processed under 180 – 190 Reduce the injection speed
Phenolic resin (PF)
Phenolic resin (PF)
