In design process of injection molding mold, selection of moulding materials, layout of runner system, cooling scheme and design of ejection scheme are closely related to nature of plastic itself. Although internal structure of plastic is relatively complicated, it is difficult to systematically grasp its performance. However, for general mold designers, some basic understanding of plastic properties, such as: fluidity, mechanical properties, physical properties, Chemical properties and molding processes, etc., will be of great help.

1 Classification of plastics

Plastic we often say is a general term for all kinds of plastics. It is widely used, so classification methods are different. According to application, it can be divided into two categories: general plastics and engineering plastic material. General-purpose plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), modified polystyrene (eg SAN, HIPS), polyvinyl chloride (PVC), etc., which are the most widely used material everyday, with low performance requirements and low cost. Engineering plastic material refer to some industrial quality plastics with mechanical parts or engineering structural materials. Its mechanical properties, electrical properties, resistance to chemical environment, resistance to high temperature and low temperature are superior, and it can even replace some metals or other materials in engineering technology. Commonly used are ABS, polyamide (referred as PA, commonly known as nylon), polycarbonate (PC), polyoxymethylene (POM), plexiglass (PMMA), polyester resin (such as PET, PBT), etc., the first four are the fastest growing, and are recognized as the four major engineering plastic material in the world.
According to process performance during heating, plastics can be divided into two categories: thermosetting plastics and thermoplastics. Thermosetting plastic is solidified into a network or a body shape after being heated, do not soften even after heating. This material is characterized by a hard texture, good heat resistance, stable size and insoluble in solvents. Commonly used are phenolic resin (PF), epoxy resin (EP), unsaturated polyester (UP) and so on. Thermoplastic softens and melts under heated conditions, is shaped after cooling, can be plasticized repeatedly, and undergoes physical changes during processing. Commonly used are polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), polystyrene (PS) and its modified varieties, ABS, nylon (PA), polyoxymethylene (POM), polycarbonate (PC), plexiglass (PMMA), etc. Such plastics are relatively simple to mold at a certain plasticizing temperature and appropriate pressure, their plastic products have different physical properties and mechanical properties.

2 Classification of thermoplastics and related basic concepts

2.1 Classification of thermoplastic materials

We are now in contact with thermoplastics, which can be divided into two broad categories: crystalline plastics and amorphous plastics. Crystallization is property of polymer to be regularly rearranged from unordered state of molten state to solidified state. A plastic having this property is called a crystalline plastic. On the contrary, it is called amorphous plastic. Crystalline material has a relatively obvious melting point. When processing temperature enters melting point, a viscous flow state occurs, viscosity of polymer rapidly decreases and irreversible plastic deformation occurs. Amorphous plastic is heated from solid state at normal temperature until it softens to viscous flow state, there is no obvious melting point in the middle. As a method for discriminating crystalline plastics and amorphous plastics, generally opaque or translucent is a crystalline plastic such as polyethylene, polypropylene, polyoxymethylene, polyamide, polyester, etc. Transparent is amorphous plastic, For example, polystyrene, polycarbonate, polymethyl methacrylate (plexiglass), polysulfone, and so on. Of course, there are exceptions, such as ABS is amorphous plastic, but opaque.

2.2 Related basic concepts

Different forms of thermoplastics have different process properties, shrinkage properties, physical and mechanical properties.
In general, for a crystalline plastic, when processing temperature is higher than its melting point, fluidity is good, cavity can be quickly filled, and injection pressure required can be small. Amorphous plastics have poor fluidity, so injection into cavity is slower and injection pressure required is greater. Therefore, in plastic mold design, reasonable size of runner system can be designed according to fluidity of plastic. On one hand, it can avoid that size of runner system is too large and waste materials, and at the same time, injection molding cycle is prolonged. And on the other hand, it is avoided that size of runner system is too small, which makes filling and holding pressure difficult. Of course, there are exceptions. For example, although polystyrene is amorphous plastic, its fluidity is very good. Indicators reflecting liquidity usually have a melt index (MFR) and an apparent viscosity. MFR refers to mass of melt flowing out of standard capillary every 10 minutes under a certain temperature and load in a melt flow rate meter, its unit is g/10 min. For high molecular polymers, under normal injection molding conditions, their flow behavior is largely inconsistent with Newton's law of flow. They are non-Newtonian fluids. Ratio of their flow shear stress to shear rate is called apparent viscosity. Apparent viscosity is not a constant at a certain temperature and can vary with shear stress, shear rate, and even some with time. Thermoplastics undergo different degrees of volumetric shrinkage from molten state to solidified state. Crystalline plastics generally exhibit greater shrinkage than amorphous plastics and are more susceptible to molding process. Shrinkage of crystalline plastics is generally between 1.0% and 3.0%, while shrinkage of amorphous plastics is between 0.4% and 0.8%. For crystalline plastics, post-shrinkage should also be considered, because they can continue to shrink after being demolded at room temperature, and amount of post-shrinkage depends on thickness of product and ambient temperature. The thicker shrinkage, the thicker.
Schedule 2-1: Mold shrinkage of common plastics

Plastic name	Shrinkage (%)	Plastic name	Shrinkage (%)
HDPE	1.5~3.5(2.0)*	POM	1.8~2.6(2.0)*
LDPE	1.5~3.0(1.5)*	PA6	0.7~1.5
PP	1.0~3.0(1.5)*	PA66	1.0~2.5
GPPS	0.4~0.8(0.5)*	SPVC	1.5~2.5(2.0)*
HIPS	0.4~0.6(0.5)*	TPU	1.2~2.0(1.6)*
ABS	0.4~0.7(0.5)*	PMMA	0.5~0.7(0.5)*
PC	0.5~0.7(0.5)*	PBT	1.3~2.2(1.6)*

Note: Parameter with * is recommended value. Rheology of a polymer refers to relationship between stress, deformation, rate of deformation, and viscosity during processing. This involves effects of temperature, pressure, time and molecular structure, molecular weight and their distribution on these factors. According to rheology of plastics, plastics can be further divided into shear sensitive materials and heat sensitive materials. The greater dependence of viscosity on shear rate, the faster viscosity decreases with increasing shear rate. This plastic is a shear sensitive plastic. Common shear sensitive plastics are ABS, PS, PE, PP, POM, and so on. If melt viscosity is more dependent on temperature, the faster viscosity drops with increasing temperature, this plastic is a heat sensitive plastic. Common heat sensitive plastic PC, PA, PMMA, etc. For high molecular polymers, shear rate has an effect on viscosity of above two materials. Increase in shear rate can reduce viscosity of melt to varying degrees, which can cause shear to become "shear thinning". phenomenon. Therefore, when designing runner system, it don’t mean the larger runner size, the smaller pressure drop. Appropriate small runner size can increase shear rate of melt to lower viscosity and further reduce pressure drop. Shear sensitive material is more obvious. Small gate size allows for increased melt shear rate, a large amount of frictional heat, a significant increase in melt temperature, a decrease in melt viscosity, and increased fluidity. Therefore, use of small gates is often successful for shear sensitive plastics. However, when wall thickness of product is thick, gate size should be appropriately increased in consideration of pressure holding to prolong solidification time of gate. Factors affecting performance of product are also orientation effects of plastic melt during flow. Macromolecules of plastic melt are stretched by external force and arranged parallel to each other along flow direction. This arrangement forms an orientation effect by freezing in the solid product before the plastic is cooled and solidified. Orientation effect may impair integrity of article, manifesting as inconsistent physical and mechanical properties in various directions, and may also cause uneven shrinkage in all directions, which may cause warpage of article. According to form of macromolecules in the melt and nature of action, it can be divided into "flow orientation" under shear stress and "stretch orientation" under tension. Conditions for controlling orientation are as follows:
(1) Decrease in melt temperature and mold temperature enhances orientation effect;
(2) Increased injection pressure increases shear rate and shear stress to enhance orientation effects;
(3) The thinner thickness of product, the stronger orientation effect;
(4) Larger gate sizes will enhance orientation effect.
Some special measures are sometimes taken to enhance orientation effect, improved tensile strength and bending strength in orientation direction. Such as stretch film, hinges and so on.

3 Polyethylene

Polyethylene (PE) is the most widely used in plastics and the most common one in daily life. It is characterized by softness, non-toxicity, low cost and convenient processing. Injectable material is milky white particles. Formula is:
Since main chain has a C-C bond structure, no side groups, good flexibility, and molecules are arranged in a regular symmetry, it is a typical crystalline polymer.
Polyethylene is relatively easy to burn. It burns with paraffin burning taste. Upper end of flame is yellow, lower end is blue, it melts and drip. It can continue to burn after leave fire.
At present, there are two main types of PE materials used in large quantities, namely HDPE and LDPE.

3.1 Basic Performance of HDPE and LDPE

HDPE (low-pressure high-density polyethylene, commonly known as hard soft material) has fewer branches in molecular structure, relative density is 0.94g/cm3~0.965g/cm3, and crystallinity is 80%-90%. Its most outstanding performance is excellent electrical insulation, good wear resistance, water impermeability, chemical resistance, almost insoluble in any solvent at 60℃; good low temperature resistance, still soft at -70℃. Main disadvantages are: poor resistance to quenching and heat recovery, poor mechanical strength, and low heat distortion temperature.
HDPE is mainly used to make hollow products such as blow molding bottles, and secondly used for injection molding, making turnover boxes, cocks, small load gears, bearings, electrical component brackets, and so on.
LDPE (high pressure low density polyethylene, commonly known as soft material) has more branches between molecular structure, density is 0.910g/cm3~0.925g/cm3, and crystallinity is 55%~65%. Easy air and moisture permeability, excellent electrical insulation and chemical resistance, softness, elongation, impact resistance, light transmittance better than HDPE, slightly poor mechanical strength, poor heat resistance, not resistant to light and heat aging .
It is widely used as extruded packaging film, sheet, packaging container, wire and cable sheath, soft injection molding and extrusion parts.
HDPE and LDPE have same performance:
1. Low water absorption rate, no drying treatment before molding.
2. Polyethylene is a shear-sensitive material, and viscosity is more affected by shear rate.
3. Shrinkage is large and direction is obvious, product is easy to warp and deform.
4. Since polyethylene is a crystalline polymer, its degree of crystallinity directly affects distribution of product density. Therefore, cooling water arrangement of mold is required to be as uniform as possible and density is uniform, size and shape accuracy of product are ensured.

3.2 When designing mold, please pay attention to:

1). Polyethylene molecule has an orientation phenomenon, which causes shrinkage in orientation direction to be larger than shrinkage in vertical direction, warpage, distortion, and influence on the properties of article. In order to avoid this phenomenon, mold design should pay attention to determination of gate position and choice of shrinkage rate.
2). Polyethylene texture is soft and smooth, and it is easy to demould. For products with shallow grooves on side wall, it can be force demolding.
3). Due to good fluidity of polyethylene, depth of exhaust groove should be controlled below 0.03mm.

4 Polypropylene

Polypropylene (referred to as PP, commonly known as 100% soft rubber) is polymerized from propylene, molecular formula is:
It is a crystalline polymer with light weight, non-toxicity and odorless characteristics. It also has characteristics of corrosion resistance, high temperature resistance and high mechanical strength. Polypropylene resin for injection is a white, waxy granule.
Polypropylene is easy to burn, upper end of flame is yellow, lower end is blue, a small amount of black smoke is emitted, melted and dripped, and it can continue to burn after leave fire, giving off a petroleum smell.
Polypropylene is roughly classified into a single polypropylene homopolymer and an ethylene-propylene interpolymer having improved impact properties. Copolymerized polypropylene article has improved impact resistance compared to homopolymer polypropylene.

4.1 Main advantages of PP performance:

1). Because it has good fluidity at melting temperature, wide molding process and less anisotropy than PE, it is especially suitable for making various shapes of simple products, surface gloss, dyeing effect, and scratch marks of product are superior to PE materials.
2). Among general-purpose plastics, PP has the best heat resistance. Its product can be boiled and sterilized at 100℃, suitable for making tableware, kettles and medical instruments that requiring high temperature sterilization. Heat distortion temperature is 100℃ ~ 105℃, can be used at 100℃ or more for a long time.
3). High yield strength and high bending fatigue life. Living hinge made of PP can withstand 70 million times of folding and bending without major damage when thickness is appropriate (such as 0.25~0.5mm).
4). Density is small, and it is one of currently known varieties of plastics with the lowest density. Density range of common plastics is shown in Table 2-2.
Table 2-2: Common Plastic Density Ranges

Plastic Name	Density Range	Plastic Name	Density Range
HDPE	0.941~0.965	POM	1.41~1.43
LDPE	0.91~0.925	PA6	1.12~1.15
PP	0.90~0.91	PA66	1.15
GPPS	1.04~1.06	SPVC	1.16~1.35
HIPS	1.04~1.05	TPU	1.2
ABS	1.04~1.06	PMMA	1.17~1.20
PC	1.2	PBT	1.26~1.30

4.2 Main disadvantages of PP performance:

1. Since it is a crystalline polymer, molding shrinkage ratio is larger than that of an amorphous polymer such as PS, ABS, PC, or the like. Size of molding is easily affected by temperature, pressure and cooling rate, warpage and deformation of different degrees may occur, thickness of turning portion is liable to cause bumping, so it is not suitable for manufacturing products with high dimensional accuracy or prone to deformation defects.
2. Insufficient rigidity, it is not suitable for mechanical parts. In particular, gap in the product is very sensitive to stress, so design of sharp corner notch should be avoided.
3. Weather resistance is poor. It is susceptible to ultraviolet radiation in the sunlight to accelerate aging of plastic, causing product to harden crack, fade or migrate.

4.3 Plastic mold design:

1). Mold shrinkage rate is large. When selecting gate position, melt should be filled in the cavity with a more balanced flow order to ensure uniform shrinkage of product in all directions.
2). Hinged articles should pay attention to choice of gate location, requiring flow direction of melt to be perpendicular to axis of hinge.
3). Due to good fluidity of PP, depth of exhaust groove should not exceed 0.03mm.