Plastics and Molding Process
2.2 Processing properties of plastics

After plastic part is taken out of mold and cooled to room temperature, dimensions of each part of plastic part are smaller than original dimensions in mold. This property is called shrinkage.
Forms of molding shrinkage
(1) Linear dimensional shrinkage of plastic parts
(2) Direction of shrinkage
(3) Post-shrinkage
(4) Post-processing shrinkage
Basic factors affecting shrinkage
(1) Plastic type
(2) Plastic part characteristics
(3) Form, size, and distribution of feed port
(4) Molding conditions
Calculation of shrinkage:
Molding shrinkage value of plastic parts can be expressed by following formula.

 

Where:
-actual shrinkage (%)
-calculated shrinkage (%)
a-unidirectional dimension of plastic part at molding temperature (mm)
b-unidirectional dimension of plastic part at room temperature (mm)
c-unidirectional dimension of mold at room temperature (mm)

Ability of plastic to fill cavity under certain temperature and pressure is called fluidity
@Common plastics can be divided into three categories according to their fluidity
Good fluidity, medium fluidity, poor fluidity
@Plastics with small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, small apparent viscosity, and large flow ratio have good fluidity.
Main factors affecting liquidity:
@humidity
The higher material temperature, the greater fluidity, but different plastics also have differences.
@pressure
As injection pressure increases, molten material will be subject to greater shear and its fluidity will also increase.
@Mold structure
If temperature of molten material is lowered and fluidity resistance is increased, fluidity will be reduced.

So-called crystallization phenomenon is a phenomenon that when plastic changes from a molten state to a condensed state, molecules stop moving freely and are in a slightly fixed position, and there is a tendency for molecules to be arranged in a regular pattern.
Generally, crystalline plastics are opaque or translucent. Amorphous materials are transparent (such as plexiglass, etc.). Exceptions: Poly(4)methylpentene is a crystalline plastic but has high transparency. ABS is an amorphous plastic but is not transparent.
Requirements and precautions for fin-crystal plastics in mold design and injection molding machine selection:
Heat required to raise material temperature to forming temperature is large, so equipment with large plasticizing capacity should be used.
Heat released during condensation is large, so it must be fully cooled.

Thermal Sensitivity - Sensitive to heat. When exposed to high temperature for a long time or cross-section of feed port is too small, or when shearing effect is large, temperature of material increases and it is prone to discoloration, degradation, and decomposition. This property is called thermal sensitivity.
Water sensitivity - Some plastics (such as polycarbonate) will decompose under high temperature and pressure even if they contain a small amount of water. This property is called water sensitivity.

Hygroscopicity - plastic's affinity for water
Based on this, plastics can be roughly divided into following two types
Hygroscopic and adherent to water - polyamide, polycarbonate, ABS, polyphenylene ether, polysulfone
Non-absorbent and not easy to adhere to water - polyethylene, polypropylene

Stress cracking refers to fact that some plastics are sensitive to stress, and are prone to internal stress during molding, making them brittle and easy to crack. When plastic parts are cracked under action of external forces or solvents, this phenomenon is called stress cracking.
Melt fracture refers to phenomenon that when a polymer melt with a certain melting index passes through a nozzle hole at a constant temperature and flow rate exceeds a certain value, transverse cracks occur on melt surface. This phenomenon is called melt fracture.

Compatibility refers to ability of two or more different types of plastics to not produce phase separation in molten state.

Melt spinning molding
Injection molding
Film blow molding
Extrusion molding
Calendering molding
Blow molding
Vacuum and pressure molding

 

Manifestation, influencing factors and calculation methods of shrinkage of thermosetting plastics are basically same as those of thermoplastics.

Flowability of thermosetting plastics is usually expressed in Raschig flowability (in millimeters). The larger value, the better flowability.
Each type of thermosetting plastic is usually divided into three different levels of flowability:
Lasig flow value is 100-130mm: suitable for pressing plastic parts without inserts, simple shapes and medium thickness;
Lasig flow value is 131-150mm: used for pressing plastic parts of medium complexity
Lasig flow value is 151-180mm: can be used to press thin-walled plastic parts with complex structures, deep cavities and many inserts or for injection molding.
Factors affecting flowability:
(1) Plastic type
Generally, resin has a small molecular weight, filler particles are fine and spherical, water, plasticizer, and lubricant content are high, and flowability is high.
(2) Mold structure
Smooth surface of mold and simple shape of cavity are conducive to improving flowability.
(3) Molding process
Use of pre-pressing and preheating, increasing molding pressure, and increasing molding temperature (under conditions below plastic hardening temperature) can improve flowability of plastics.
Impact of fluidity on molding:
@ Too much fluidity: too much overflow, loose filling, loose plastic parts, uneven mixing of resin and filler, easy sticking to mold, difficult demolding and cleaning, and early hardening;
@ Too little fluidity: insufficient filling, difficult molding, and increased molding pressure.

Specific volume - volume occupied by one gram of plastic (measured in cm3/g)
Compressibility - ratio of volume or specific volume of plastic powder and plastic parts (its value is always greater than 1)

During molding process of thermosetting plastics, resin undergoes a cross-linking reaction, molecular structure changes from linear to stereoscopic, plastic changes from being both fusible and soluble to being neither fusible nor soluble. In molding process, this process is called curing (maturation).

@Source:
(1) Left over from plastic production process, absorbed during transportation and storage before molding.
(2) It is a by-product of chemical reactions during molding process.
@Effects: (flow, shrinkage, bubbles, corrosion, odor, etc.)
@Treatment method. (Preheating and drying, opening exhaust grooves, adding exhaust steps)

Thermosetting plastics:
When heated, they quickly change from a solid state to a viscous flow state, and this state exists for a very short time;
Chemical action changes molecular structure from linear to a network, molecules stop moving, and plastic hardens into a hard solid.

 

Film Blowing Molding
Used for blowing PP (polypropylene), HDPE (high-density polyethylene), LDPE (low-density polyethylene), LLDPE (linear low-density polyethylene) and other plastic films, which are transparent, glossy, airtight and non-toxic, and are widely used in clothing, food, textiles, daily necessities, and other commercial or industrial packaging.

 

Extrusion molding is a molding method that uses extrusion effect of a screw or a plunger to force heated and molten polymer material through die under pressure to become a continuous profile with a constant cross-section.

 

Plasticized polymer material close to viscosity flow temperature is passed through a series of horizontal rollers rotating in opposite directions, so that material is subjected to extrusion and extension, becomes a thin sheet product with a certain thickness, width and smooth surface. It is mainly used to produce sheets, films, artificial leather, etc.

 

 

Sheet cut into a certain size and shape is clamped on the frame of mold, heated and softened at a suitable temperature in a highly elastic state. Sheet is heated and stretched at the same time, then pressure applied makes it close to mold surface to obtain a shape similar to mold surface. After cooling, shaping and trimming, product is obtained.