a. General-purpose plastics
Refers to plastics with the widest range of daily use, which have low performance requirements, low cost and are easy to obtain. The most widely used ones are PE, PP, PS and PVC.
b. Engineering plastics
These are some plastics with industrial quality that can be used to manufacture mechanical parts or engineering structural materials. They have excellent mechanical properties, electrical properties, tolerance to chemical environments, tolerance to high and low temperatures, etc. They can replace certain metals and other materials in engineering technology. Common ones include ABS, PC, POM, PMMA, PBTP, PETP, PSF, PEI, etc., among which the first four are four major engineering plastics recognized internationally.

a. Thermoplastic materials
Molecular structure: linear polymers produced by binary polymerization or branched polymers produced by partial ternary polymerization.
Material characteristics: using heating and cooling to produce reversible flow and solidification states, which is a physical change.
The longer molecular chain or the larger molecular weight, the higher melting point; and the more branches in molecule, the lower melting point. In process of plastic synthesis, branches will be avoided as much as possible. Too low a melting point will cause plastic to chemically decompose before it melts to a flowable state and cannot be processed for injection molding. Therefore, too low a melting point has no practical value.
b. Thermosetting plastics
Molecular structure: Tertiary polymerization reaction produces a network polymer.
Material characteristics: During molding process, heating causes molecular structure to combine into a network; once combined into a network polymer, it cannot soften even if heated again, showing an irreversible change, which is a chemical change.
c. Thermoplastic elastomer (TPE)
Also known as "thermoplastic rubber", it is a newer plastic that can replace other plastics or traditional rubber in some cases.
It has some properties of traditional or vulcanized rubber, has simple and fast processing performance of thermoplastics, and can be reprocessed and has a variety of colors to choose from.

When most metal materials are cooled, they exist in a crystalline state, that is, metal atoms are arranged in a three-dimensional manner in an orderly and regular manner. Plastic materials, however, are very large molecules, and it is impossible for them to be arranged like metal atoms when cooled. They can only exist in a semi-crystalline or amorphous state.

Molecular structure of crystalline materials is generally chain-like and regularly arranged, while molecular structure of amorphous materials is branched and irregularly arranged.

 

Plastics that are easy to crystallize (crystalline materials)
PE, PP, PA (nylon), POM, PPS, TPX
Plastics that are not easy to crystallize (non-crystalline materials) PVC, PS, PC, ABS

  

Tm - melting point of crystalline materials
Tm - solidification point or crystallization start point of crystalline materials
Tg - glass transition point of amorphous materials
Tm = melt temperature
Tg = glass temperatyre
For crystalline materials, when temperature gradually rises to Tm, crystal structure is destroyed and becomes amorphous (molten). On the contrary, when temperature gradually drops from molten state to Tm, crystallization shrinkage occurs (with a clear melting point). This characteristic determines difficulty of thermal processing of crystalline materials, because material directly changes from solid to liquid during temperature increase process, and there is no plasticity range. (eg: PP)
For amorphous materials, there is no clear melting point during temperature increase process. Amorphous materials produce Tg due to different molecular motions. When temperature is above Tg, polymer chain has freedom to move and flow. Since amorphous materials have a clear plasticity range at temperatures above Tg, thermal processing is easier. (eg: PC)
Comparison table of melting points and glass transition points of various materials

 

Understanding crystallinity of plastics is of great help in product design, material selection, injection molding methods and prevention of plastic cracking.
Shrinkage
Non-crystalline materials have less shrinkage, while crystalline materials have greater shrinkage.

 

Some crystalline plastic parts (eg: POM, PP) need a long time to stabilize their size after demolding. This is because these plastics have high crystallinity and do not have time to crystallize during molding. Therefore, crystallization shrinkage will continue after demolding and during storage. We can accelerate crystallization shrinkage process of such plastic parts through heat treatment.
Other factors that affect shrinkage of plastic parts
①Mold temperature:
The higher mold temperature, the greater molding shrinkage rate;
②Material flow direction:
Generally, shrinkage rate of materials in flow direction is greater than shrinkage rate in vertical flow direction (but opposite is true for glass fiber reinforced plastics);
③Gate size:
The larger gate, the smaller shrinkage rate;
④Molded product wall thickness:
The thicker molded product wall, the greater shrinkage rate.
Molding shrinkage of various materials (%)

 

b. Thermal processability
Before amorphous materials are heated from room temperature to liquid, they have a range of plasticity, while crystalline materials have no or only a very small plastic processing range. Therefore, amorphous materials are easier to thermally process (such as hot stamping, ultrasonic welding, etc.) than crystalline materials. Therefore, if design engineers want to use crystalline plastics for above processes, they must be careful and should try to avoid using them to avoid producing many defective products.
c. Chemical resistance
Crystalline materials have good chemical resistance and are more resistant to solvents, but pad printing or screen printing is more difficult. Generally, special chemical liquids, corona or flames are used to treat the surface before pad printing. (Because non-crystalline plastics are easily corroded by solvents and crack in the future, it is necessary to be very careful when selecting materials for products. At the same time, no oily chemicals should be involved during injection molding, any production and processing process).

Fluidity of plastic refers to ability of plastic to fill mold cavity under certain temperature and pressure, which is often expressed by flow index. Flow index refers to: amount of resin that comes out of a certain mold in 10 minutes under certain temperature and pressure (kg/min or cm3/min).

 

1-Removable load 2-Insulation 3-Upper reference line 4-Insulation 5-Lower reference line 6-Steel cylinder 7-Die 8-Insulation board 9-Control thermometer 10-Die baffle

 

Note: If conditions not listed in table above are needed in the future, for example, for new thermoplastics, only loads that have been used can be selected, and temperature must also be selected from table. If it is really necessary due to characteristics of new thermoplastics, a new temperature can be selected. In this case, ISO/Tc61/SC5 should be requested to include this new condition. If approved, a suitable character code will be temporarily issued, and standard will be revised in 5-year revision.

 

a. Fluidity of plastic is related to chemical molecular structure of plastic. The longer molecular chain, the higher melting point and the worse fluidity. (It is not easy to fill cavity and requires higher injection temperature, injection pressure and cavity temperature), but because molecular chain is longer, it has good mechanical strength.
The higher fluidity, the better. Although high-fluidity plastics are easy to fill mold, they are also easy to penetrate into cavity venting gap or parting surface to form flash.
b. Comparison of fluidity of various plastics.
① Difference in fluidity
Good fluidity - PE, PP, PS, PA
Good fluidity - PMMA, POM, ABS.AS, HIPS
Poor fluidity - PC, hard PVC, PSF, etc.
②Problems related to fluidity
eg: If mold originally used for PC injection molding is used to inject PP, there will be problems? Because PP has good fluidity, precision and finish of mold are very high, otherwise tiny texture problems on mold surface will be reflected on injection molded product, causing surface defects of product. PC has poor fluidity, so requirements for mold are not so strict, but injection molding machine is required to have a higher injection pressure to inject qualified products.

Viscosity is an important measure describing flow behavior of plastic melts. The higher viscosity, the worse fluidity, and the lower viscosity, the better fluidity.

Temperature: The higher temperature of material, the greater fluidity.
Pressure: As pressure increases, fluidity increases. (But not all plastics are Newtonian fluids. Some plastics (such as PC) are non-Newtonian fluids. Increasing pressure is not very effective in increasing flow rate, but effect of increasing temperature is significant.)
Mold: Form and size of mold gate and runner will affect actual fluidity of melt in mold cavity.

a. Generally speaking, same type of plastic has different grades of fluidity, which should be considered when selecting. Supplier's product catalog indicates fluidity level of plastic. For example, PS material produced by BASF has three fluidity levels in catalog: easy-flowing grade 143E, normal-flowing grade 165H, and slow-flowing grade 168N.
b. If injection molded part requires high mechanical strength (high impact strength and high tensile strength), a grade with lower fluidity should be selected.
c. If injection molding has a thin wall or a very smooth surface, a grade with good fluidity should be selected.
d. Different processing and molding methods of plastics have different requirements on their fluidity. Take PP as an example:

 

 

Melt fluidity of circular runners
For circular runners, flow rate of melt in runner can be reflected by following formula:

 

Where: K-constant V=K
m-constant, generally varies in the range of 1.5 to 4
R-runner radius
L-runner length
It can be seen from above formula that when runner diameter increases, melt fluidity will increase exponentially, but when runner length increases, flow capacity will also decrease exponentially, so generally runner is required to be short in length and large in diameter to increase fluidity.

When plastics are stored before being formed, they absorb moisture from air and environment. Water absorption rates of various types of plastics are as follows (24-hour submersion test)

 

a. Water absorption of raw materials before molding
During processing of plastic materials, water absorbed by raw materials in air will turn into steam and remain inside or on the surface of parts, affecting quality of products, including:
Appearance: silver lines, bubbles, pitting, loss of surface gloss, etc.
Mechanical strength: For materials such as PC, ABS, and nylon, water will degrade plastic polymers at high temperatures, making materials brittle and reducing their strength.
Main method to solve problem of water absorption of raw materials is "drying and baking materials".
b. Water absorption of molded parts
Some plastic parts (such as nylon, especially nylon 66) still have a high water absorption rate after molding, storage and use environment of parts must be considered. If storage and use environment contains more water (eg: humid and submerged environment), size of parts will expand and cannot match other plastic or metal parts, which will produce extremely serious defective products (for example: nylon nozzles are matched with PC boxes, nylon will expand after absorbing water and cause nozzles to fall off). However, sometimes, we can deliberately soak nylon products in water to enhance their toughness and improve brittle cracking.
2. What kind of plastics should be used in following applications? Give examples of their application in our company's products.
Used as housings, covers, containers;
Low friction 10g bearings;
Stress mechanical components;
Heat resistance and chemical corrosion resistance.

 

Physical properties:
-Milky white translucent (sometimes transparent) waxy solid, specific gravity less than 1, can float on water.
-Melts after exceeding softening point (can be heat-welded)
-Good cold resistance.
-Excellent electrical insulation.
-Impermeable to water vapor, air, etc., but highly permeable to carbon dioxide and organic odors.
-Good processability (can be formed into a film without adding plasticizer and has flexibility).
-Poor adhesion using adhesives and poor printability, but printability can be improved after surface is treated with high voltage.

Chemical properties
-Excellent water resistance.
-Good chemical resistance. But easily corroded by halogens and strong oxidizing substances.
-Poorly soluble in organic solvents.
-Highly flammable.
-Will slowly oxidize in presence of light and oxygen.
-Non-toxic.
a. Advantages:
Can be sterilized in boiling water or steam at 100℃ to 110℃ without stress cracking.
Can resist most solvents and chemicals (but above 81℃, it can be dissolved by aromatics and swell at low temperatures).
Cheap price (so parts that used to use PC and ABS are now switched to PP).
b. Disadvantages:
① Large shrinkage rate, easy to deform.
Shrinkage rate 1.8 to 2.5%.
85% shrinkage in 24 hours, 90% shrinkage in the first week, and remaining shrinkage takes 3 months;
Using boiling water as heat treatment can speed up shrinkage to 1 hour, and deformation can be tested in the future.
② Poor impact resistance, especially at low temperatures;
③ Low viscosity during injection molding, easy to produce flash;
④ Low hardness, easy to scratch and cannot be ground and polished;
⑤ High-temperature grade PP must have antioxidants, which slowly seep out to form a "white gray film" a few weeks after injection molding, affecting appearance;
⑥ Can be decomposed by copper-containing substances, so insert must be electroplated before injection molding and should not be in direct contact with copper screws and corns.

Characteristics: Characteristics of PVC are greatly affected by degree of polymerization of molecule and amount of "plasticizer" added. PVC raw materials without plasticizer are called "unplasticized PVC" (English: UNPLASTICISED PVC). PVC is a material with slow fluidity and a limited processing temperature range. After adding plasticizer, its fluidity (DIN K value) decreases. It has many uses. Soft PVC parts are very easy to be contaminated (eg: color powder particles in injection molding workshop will be adsorbed on (PVC) wires or wire ears). Main reason is that plasticizer contained in PVC causes its surface micropores to be coarse, which is easy to adsorb color powder particles.

Advantages:
① Due to presence of chlorine atoms, it has excellent fire resistance and will automatically extinguish after adding fire;
② Depending on amount of plasticizer added, it can be very hard or very soft;
③ High chemical resistance. (But it reacts to some concentrated oxidizing acids such as sulfuric acid and nitric acid);
④ It can be transparent;
⑤ Low shrinkage (0.2 to 0.6%).
Disadvantages:
① During injection molding, it will decompose at a slightly higher temperature, releasing HCL, corroding mold and barrel of injection molding machine;
② It is easy to change color at high temperature;
③ Operating temperature is -15-55℃
④ It is extremely environmentally unfriendly. Harmful substances will be produced when waste is burned. It can only be incinerated at high temperature (above 750℃). HCL (hydrogen chloride) will also be produced after incineration.

Main advantages of polystyrene are:
-It is a hard plastic with high refractive index (1.59), is transparent and bright;
-It is completely tasteless, odorless and non-toxic;
-It has low specific gravity;
-It has excellent formability and can be used for injection or extrusion molding under a wide range of temperature and pressure conditions to produce products of various specifications and uses;
-Molded products have high precision, dimensional stability, hardness and low hygroscopicity;
-Stable and excellent electrical properties in a wide range of temperatures and high frequencies;
-Easy to color;
-Reusable (high thermal stability);
-High resistance to chemicals, especially to corrosive inorganic liquids;
-Uniform physical properties at thermal deformation temperature, and excellent toughness at low temperatures;
-Inexpensive.

b. Main disadvantages of polystyrene are:
- Product cannot be used for a long time at a temperature above 80℃;
- Poor impact resistance;
- Poor resistance to organic chemicals;
- Poor light and weather resistance;
- Surface is easily scratched.
a. Composition of ABS
ABS is abbreviation of acrylonitrile (A)-butadiene (B)-styrene (S) terpolymer, in which butadiene contains rubber components.
Characteristics of ABS
Advantages
①Various mechanical strengths such as tensile strength, bending strength, hardness, stretching, impact strength, creep resistance are uniform and balanced.
②Good heat resistance (heat deformation temperature 85~105℃).
③Good electrical and chemical properties.
④Small molding shrinkage, suitable for precision molding.
⑤Molding processing temperature range is quite wide and moldability is good.
⑥Excellent surface gloss.
➆Relatively cheap.
Disadvantages:
① Soluble in solvents;
② Opaque (some transparent grades);
③ Easy to change color (including color change during injection molding and color change during use).
c. Relationship between PS, ASN, HIPS and ABS:
PS is a copolymer of styrene (S);
ASN is a copolymer of acrylonitrile (A) and styrene (S), which has characteristics of high heat resistance, high tensile strength, and slightly yellow transparency;
When "butadiene" is added to PS, it becomes HIPS (high impact strength PS);
When "acrylonitrile" and "butadiene" are added to PS at the same time, it becomes ABS.

 

·Advantages:
① Extremely high transparency of about 92%, called organic glass:
② High rigidity;
③ Dimensional stability;
④ UV resistance, good resistance to environmental stress cracking, low water absorption;
⑤ Excellent electrical insulation;
⑥ Can be joined with chloroform solvent (common material for manufacturing fixtures).
Disadvantages:
① Easy to generate static electricity to attract dust;
② Plastic has slow fluidity, and it is easy to generate internal stress after injection molding, causing cracking.

Advantage:
①Excellent mechanical strength near room temperature;
②Excellent friction and wear characteristics;
③Excellent oil resistance;
④Has self-extinguishing properties (PA6, PA66);
⑤Composite effect of glass fiber, etc. is significant;
⑥Small oxygen transmission rate (PA6, PA66);
⑦ Long-term heat resistance ensures excellent temperature (PA6, PA66).
Disadvantages:
①High hygroscopicity, resulting in large dimensional changes;
②Poor acid resistance;
③Easy to change color during injection molding.

Advantages:
①Excellent mechanical strength, relatively balanced properties, resistance to creep and high elastic recovery;
②Fatigue properties are better than other E.P. materials;
③Excellent friction and abrasion resistance, suitable for rotating parts;
④Excellent chemical resistance;
⑤Good formability, good dimensional stability of formed products, suitable for precision parts.
Disadvantages:
①High crystallinity and large crystal shrinkage, so shrinkage rate during molding process is relatively large, which is not conducive to dimensional accuracy of molded product.
②Good chemical resistance, eroded by acids and ZnCl2, etc., especially when used in high-temperature lubricating oils, must be paid attention to.
③Due to high crystallinity, it appears white, and has low adhesion and printability.
④No self-extinguishing properties.

Advantages:
①Excellent mechanical strength, especially good creeping properties;
②Extremely high impact strength;
③Excellent electrical insulation and high frequency performance;
④Stable properties in a wide temperature range (-170-130℃);
⑤Excellent dimensional stability;
⑥Transparency;
⑦Flame resistance and self-extinguishing properties, good composite effect with glass fiber, etc.
Disadvantages
① It is hygroscopic and easily decomposes due to water when melted, which easily produces internal bubbles and silver lines;
② It is easy to produce residual strain;
③ Fatigue resistance and abrasion resistance are slightly low;
④ It has poor chemical resistance, solvent resistance, and grease resistance, and is easy to crack under stress.

·Advantages:
①High heat resistance (melting point 290℃, decomposition temperature 400℃, heat deformation temperature 240℃);
②High uniform strength;
③Very low water absorption and excellent dimensional stability;
④Excellent chemical resistance (insoluble in solvents below 200℃);
⑤Continuous use temperature: RYTON R4: 170℃
RYTON R8: 200℃.
Disadvantages:
①Generally a low-viscosity material, the viscosity is increased by adding filler materials;
②Expensive.

·Advantages:
① Transparent light amber, good appearance
② Good processability, easy to color and fill, can be metal-plated;
③ Excellent dimensional stability and latent resistance;
④ Continuous use temperature can reach 150℃ (UL), high heat resistance;
⑤ Excellent water vapor resistance and flame retardancy;
⑥ Excellent chemical resistance, non-toxic.
Disadvantages:
① Slightly higher viscosity;
② Water-absorbent, must be dried in advance.

a.PETP polyester
Advantages:
①High heat resistance;
②High elastic coefficient, small latent change, excellent mechanical strength, especially high toughness;
③Rich in elongation effect, suitable for fiber, film, screw processing;
④Excellent and stable electrical properties in a wide temperature range;
⑤Excellent chemical and oil resistance;
⑥Easy to produce stress cracking.
Disadvantages:
①Slow crystallization speed, relatively large heat treatment effect;
②Easy to produce hydrolysis in alkali, boiling water, and hot water.
b.PBTP polyester
Advantages:
①Lower melting point than PETP, especially good long-term thermal degradation resistance;
②Less water absorption, excellent mechanical properties and dimensional stability;
③Excellent chemical resistance;
④Excellent friction and abrasion resistance;
⑤Excellent weather resistance;
⑥Can withstand burning without losing its characteristics;
⑦Great composite effect, easy to improve characteristic values.
Disadvantages:
①It will decompose in hot water, high temperature and high humidity;
②Large V-notch effect of impact resistance (no filler grade).

·Advantages
①Excellent mechanical strength;
②High flame retardancy and low smoke generation;
③Excellent electrical properties and wide operating temperature range;
④Excellent chemical resistance such as aliphatic carbon hydride, acid, and dilute alkali metals;
⑤Excellent high temperature properties;
⑥High formability;
⑦High weather resistance.
Disadvantages
①Water-absorbent, molding material must be prepared and dried.
②High temperature molding is required.
③High price (15 times price of PP)

Advantages
①Heat resistance, melting point 240℃, heat deformation temperature 80℃ (transparent material) ~ 145℃ (opaque material).
②Excellent transparency. Visible light transmittance 90%, ultraviolet transmittance is the highest except for glass.
③Excellent electrical insulation, dielectric constant is the smallest among main plastic materials.
④Density is 0.83g/cm3, which is the smallest.
⑤Excellent chemical resistance, not attacked by drugs.
⑥Excellent safety in food hygiene.
Disadvantages:
Susceptible to UV degradation.
Comparison of transparency of several plastics

 

Bakelite is a thermosetting material. Bakelite powder is a mixture of linear phenolic resin and catalyst. Linear phenolic resin can be heated by an injection molding machine to make it flow, then injected into a heatable mold. Material in mold undergoes a synthetic reaction to produce a network of thermosetting polymers.
Advantages
①Hard and wear-resistant;
②Heat-resistant, flame-resistant, and solvent-resistant;
③Electrical insulation performance;
④Low hygroscopicity.
Disadvantages:
①Thermosetting material. Branches, runners, defective products, and waste products cannot be reused;
②There will be many unavoidable burrs and exhaust materials during molding, which need to be 100% removed by secondary processing. It is a disgusting job;
③Molding cycle is relatively slow, and it may be necessary to heat it in furnace for second time to ensure that polymerization reaction is fully completed, to ensure dimensional stability and sufficient strength.

During processing of some plastic materials, moisture absorbed by raw materials in air will turn into steam and remain inside or on the surface of parts, which will seriously affect quality:
-Appearance: Water vapor forms silver lines, bubbles, pitting, and loss of surface gloss.
-Mechanical strength quality: For materials such as PC, ABS, and nylon, moisture degrades plastic polymers at high temperatures, making materials brittle and reducing strength, affecting future service life.
-Purpose of baking: to drive away moisture in plastic materials.

Take PC material as an example:

 

Reason is: there are two forms of moisture in plastic raw materials. One is called "matrix absorption" and the other is "surface moisture".
-Matrix absorption: Nylon, PC, ABS, PMMA, etc. belong to category of matrix absorption. This type of raw materials will quickly absorb moisture from air, and after absorption, they can quickly penetrate into interior of plastic pellets. For this type of material, high temperature and long-term drying are required during production.
-Surface moisture: PP, PE, etc. belong to category of surface moisture. Moisture is only absorbed on the surface of rubber pellets, and permeability is very small. This type of raw material generally does not need to be baked. Even if there is a small amount of moisture in raw material, it will be driven away in reverse direction when entering front section of barrel.

If percentage exceeds following table, there may be problems with appearance quality and mechanical strength of product:

 

a. Responsibilities of injection molding technicians
-Must bake materials according to specifications shown in injection molding operation instructions;
-Each baking hopper must be marked with a "safety line" for amount of material used, and plastic must be loaded above safety line;
-If there is a problem with oven or temperature controller, repair it immediately.
b. Responsibilities of maintenance technicians
-Regularly compare thermometer of oven on machine with standard thermometer;
-Check whether heating tube is damaged.
c. Responsibilities of injection molding OC
-Audit work of injection molding technicians;
-Record baking oven temperature;
-Notify injection molding foreman if any problems are found.

a. Reasons to avoid using mold release agents
-Mold release agents are additional materials that indirectly increase production costs.
-Using mold release agents will reduce production efficiency because mold release agent must be applied and stability of the mold temperature will be affected.
-Using mold release agents will cause difficulties in later processing (such as pad printing falling off and ultrasonic welding difficulties).
-Some mold release agents will react chemically with plastic materials and cause cracking in the future.
-Mold release agents will reduce mechanical strength of plastic products.
-Using mold release agents may affect health and does not comply with food and drug regulations.
-Poor mold release should be improved by mold modification methods, and mold release agents should not be used.
b. Matters to be noted when using mold release agent in unavoidable situations:
-Do not use mold release agent that has not been tested in detail. It must be used after being verified by QC/OA department;
-Do not use excessive mold release agent for fog parts with high mechanical strength requirements, and do not rub it with hands. Only light spraying is allowed;
-Except for white mineral oil, other lubricating oils (especially pressure oil leaked from injection molding machines) cannot be used as mold release agents.
c. Responsibility for controlling use of mold release agents
-Injection molding department must prevent use of mold release agents that have not been verified;
-Injection molding department must control amount of mold release agent used;
-QC conducts aging tests on workpieces using mold release agents.