Abstract: This article presents classification of plastics, properties, advantages and disadvantages, and processing requirements for commonly used plastics.

Key words: plastics, thermoplastic, thermoplastic, shrinkage, density, properties, processing temperature.

1. Classification of plastics:

1. By appointment:

1.1 Ordinary plastic: This is plastic with widest range of daily use, low performance requirements, low cost and easy production, such as PE, PP, PS, HIPS, PVC, etc.

1.2 Engineering Plastics: Generally refers to certain industrial grade plastics that can be used to make mechanical parts or engineering structural materials. Its mechanical properties, electrical properties, chemical resistance, high and low temperature resistance, etc. have excellent performance, and can replace some metals such as copper, aluminum, zinc and some alloy steels in machinery or other materials common are ABS, PA, PC, POM, PMMA, PU, ​​PXU, PPO, PTFE, etc. Among them, first four are fastest growing and are top four internationally recognized engineering plastics.

2. By heating power:

2.1 Thermosetting plastics: Refers to plastics that are not softened by heat after being heated and cured, and are molded repeatedly. They are characterized by a hard texture, good heat resistance, relatively dimensional stability, and are insoluble in solvents. General There are PF (bakelite), UP ( unsaturated polyester), EP (corrosive resin), PUR (polyurethane) and so on.

2.2 Thermoplastics: Refers to plastics that can soften when heated repeatedly and harden when cooled, and have low heat resistance.

Secondly, shrinkage when molding plastics:

Type

Shrinkage rate﹪

PE

HDPE, LDPE

1.5-3.0, 1.5-3.5

PP

1.0–3.0

PS

Universal, heat-resistant, hardened

0.2-0.8, 0.2-0.8, 0.2-0.6

ABS

Punch-proof, heat-resistant, 30% fiberglass

0.5–0.7, 0.4–0.5, 0.1–0.15

PA

66, 6, 610, 1010

0.1-2.5, 0.7-1.5, 1.0-2.5, 5-4.0

PC

0.5—0.7

Note. Shrinkage rate formula: S = A—B/B*100﹪ (S is shrinkage rate, A is mold size, B is part size)

Thirdly, density of commonly used plastics:

Plastic name

PP

EDPE

PND

ABS

PS

Relative density

0.90–0.91

0.91–0.925

0.941–0.965

1.04—1.07

1.04—1.06

Plastic name

PMMA

PC

PA1010

PVC (soft)

PVC (solid)

Relative density

1.17—1.20

1.20

1.04—1.06

1.16–1.35

1.35–1.45

Fourthly, properties of widely used thermocouplesflippers:

1. Polystyrene, also known as polystyrene, PS, GPPS, hard adhesive, is a general purpose light transmitting material, and its characteristics are as follows:]

1.1 Optical performance is good, light transmission reaches 88-92%;

1.2 Excellent electrical performance, volume resistance reaches 1018 ohms;

1.3 Good color rendering;

1.4 The coefficient of thermal expansion is large, which easily generates internal stress. It is advisable to use high material temperature, mold temperature and low pressure. Increasing injection time is useful to reduce internal stress and prevent shrinkage and deformation (especially for thick wall plastic parts). However, if material temperature is high, it is easy to get silver lines, and if material temperature is low, transparency will be poor;

1.5 The biggest drawback is brittleness, and impact strength is low: 83.3-98 MPa;

1.6 The heat resistance temperature is low, and maximum continuous use temperature of its products is 60-80°C;

1.7 Poor acid resistance

2. Modified polystyrene (HIPS), commonly known as "non-breakable adhesive", has higher strength, impact resistance and greater elasticity than polystyrene.

3. ABS, overall performance is as follows:

3.1 High mechanical strength;

3.2 The impact resistance is high, and it will not fall off quickly at low temperature;

3.3 Notch sensitivity is better;

3.4 The creep resistance is good, and it will not fall off quickly when temperature rises;

3.5 Has a certain surface hardness, scratch resistance, good wear resistance and low coefficient of friction;

3.6 The product has a good texture;

3.7 Good electrical performance, less effect of temperature, humidity and frequency changes;

3.8 Generally, heat resistance can reach 90°C, and heat resistant type can also be used continuously at 110-115°C;

3.9 Low temperature resistance down to -40℃;

3.10 Resistance to acids, alkalis, salt, oils and water;

3.11 Non-flammable;

3.12 It can be spray applied, printed and electroplated;

3.13 It can be blended with many other thermoplastics or thermosets to improve processing and performance of these plastics;

3.14 Disadvantages: not resistant to organic solvents, poor weather resistance, strong hygroscopicity.

4. Polyethylene (Polyetheren), called PE, Marix, is most massive and most commonly used plastic. Its characteristics:

4.1 Soft, non-toxic, affordable and easy to process;

4.2 The shrinkage rate is larger than other plastics (1.5-3.5%), and directionality is obvious, easy to deform and deform;

4.3 The tendency to crystallize is higher than other plastics (macrystallization material);

4.4 Fluidity is excellent and fluidity is sensitive to pressure changes;

4.5 It is advisable to use high-pressure injection, injection speed must be high, pressure must be sufficient, and temperature of material must be uniform.

5, polypropylene (polypropylene), referred to as PP, commonly known as "100% adhesive", its properties are as follows:

5.1 Crystallized material, excellent fluidity, good formability, surface gloss, coloring effect, injury resistance and other aspects are superior to polyethylene;

5.2 has highest temperature resistance (100°C) among general purpose plastics;

5.3 High tensile strength, half of room temperature tensile strength is still maintained at 100°C;

5.4 High yield strength and high flexural fatigue life;

5.5 The shrinkage rate is slower than that of HDPE, and degree of deformation, such as warping and curling of product, is relatively small;

5.6 High cooling rate;

5.7 The surface hardness is higher than HDPE, and scratch and wear resistance is better;

5.8 It has good resistance to stress cracking, and parts will not crack too easily after being installed in accessories or after tightening screws;

5.9 The density is low (0.90–0.91), which is one of lightest types of plastic currently in use

5.10 It has good resistance to chemical corrosion, withstands inorganic acids, alkalis, salts and many organic solvents at temperatures below 80 °C, and has little water absorption;

5. 11 Good electrical performance, low dielectric constant (2.2-2.6);

5. 12 Impact strength depends on temperature, which is higher than low-temperature embrittlement temperature of polyethylene;

5. 13 Not suitable for products that require high dimensional accuracy or are prone to deformation defects. Mold design should pay special attention to structure Safe enough

String;

5. 14 Insufficient rigidity, not suitable for mechanical components;

5. 15 finishing and assembly limited;

5. 16 Poor fire safety;

5. 17 Poor weather resistance;

6. Nylon (NYLON, POLYARHIDE) is abbreviated as PA, which is a crystalline material, and its characteristics are as follows.

Benefits:

(1) High mechanical strength, good toughness and high compressive strength.

(2) The fatigue resistance is outstanding, and original mechanical strength can be maintained after repeated bending.

(3) Surface is smooth, coefficient of friction is small, wear resistant.

(4) Resistant to corrosion, alkalis and most salts, as well as weak acids, engine oil, gasoline.

(5) Non-toxic, inert to biological erosion, with good antibacterial and antiviral properties.

(6) Temperature resistance, wide temperature range, can be used for a long time at 45-100°C, short-term temperature resistance can reach 120-150°C.

(7) It has excellent electrical properties and good electrical insulation.

(8) Details are light, easy to paint and mold.

Weaknesses:

(1) Easily absorbs water

(2) Poor lightfastness

(3) Not resistant to strength, oxidizers

(4) Design requirements are strict

Processing requirements: molding conditions are generally suitable: low mold temperature, low material temperature, long time and high injection pressure,

7. Abbreviated as PMMA (POLYMETHYL METHOCRYLATE), also known as ACRYL or PERSPEX, it has following characteristics:

(1) Best transparency (initial transmittance 92%)

(2) Low water absorption.

(3) Excellent weather resistance, suitable for making various outdoor appliances,

(4) Excellent insulating performance and higher surface resistance than most plastics.

(5) Drawback: fragile texture

(6) High-pressure injection should be used. Provided that there are no defects, high material temperature and mold temperature should be used to increase fluidity, reduce internal stress, improve directionality, and improve transparency and strength.

8. Polycarbonate (POLYCARBONATE), referred to as PC. Commonly known as "bulletproof rubber", its complex properties are as follows:

(1) Mechanical strength is best of all thermoplastics

(2) High temperature resistance and weather resistance, thermal distortion temperature is 135-143°C, and long-term working temperature is 120-130°C

(3) High molding precision and good dimensional stability.

(4) Good optical performance, light transmission reaches 75%-89%

(5) Good coloring

(6) Fast Cooling

(7) Low water absorption and excellent electrical insulation.

(8) Resistant to dilute acids, oxidizing agents, reducing agents, salts, oils, fats.

(9) Disadvantages: High processing requirements and high requirements for mold design.

(10) Processing requirements: injection pressure should be high, mold temperature should be controlled within 80-120°C, material temperature should be slightly higher, and cooling time should not be too short due to higher mold temperature .

9. Polyoxymethylene (POLYFORMALDEHYDE), called POM, commonly known as "Saigan", characteristics are as follows:

(1) Crystallization material, melting range is very narrow, melting or solidification rate is fast, crystallization rate is fast, temperature of material is a littleBelow melting temperature, crystallization occurs and fluidity decreases.

(2) Strong heat sensitive and easy to decompose. The decomposition temperature is 240°C, but it will decompose if kept at 200°C for more than 30 minutes, releasing irritating and corrosive gases.

(3) Medium fluidity, insensitive to changes in temperature, but sensitive to changes in discharge pressure.

(4) High degree of crystallinity, large volume change during crystallization, large molding shrinkage range, and fast shrinkage rate.

(5) Low friction, high elasticity, shallow side grooves can be forcibly removed from mold, and surface of plastic parts can have a wrinkled pattern, but it is easy to get surface defects such as wrinkles, pinch lines, shrink holes, dents, and other shortcomings.

(6) Processing requirements: In general, temperature of material should not exceed 200 ° C. It is suitable for use of high pressure and high injection speed, and cooling time can be short. A slightly higher mold temperature (75-120°C) helps improve product appearance. The nozzle opening should be large.

10. Polyvinyl chloride (PVC) has rich raw material sources and low price. Its molding characteristics are as follows:

(1) Low hygroscopicity, but should be dried first to improve fluidity and prevent bubble formation.

(2) Poor fluidity, easy to decompose, especially in contact with steel, copper and other metals at high temperature, decomposition temperature is 200°C, irritating and corrosive gases are generated when decomposed.

(3) The molding temperature range is small, and temperature of material must be strictly controlled.

(4) The nozzle opening and cross section of mold gate should be large, there should be no dead corner stagnation, and mold surface should be chrome plated.

5. Various plastic injection molding processing temperatures

Plastic name

Glass transition temperature

Melting point

Processing temperature range

Decomposition temperature (in air)

PS

ABS

LDPE

PND

PP

PA66

PA6

PMMA

PC

90℃

95℃

-125℃

-125℃

-20℃

50℃

50℃

105℃

150℃

225℃

225℃

110℃

130℃

164℃

225℃

265℃

180℃

250℃

180-260℃

180-250℃

160-240℃

200-280℃

200-300℃

260-290℃

260-290℃

180-250℃

280-310℃

250℃

280℃

266℃