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PS Injection Molding Processing Guide

Table of Contents

PS polystyrene, is a polymer synthesized from styrene monomer by free radical polymerization reaction.

It is a colorless and transparent thermoplastic with a glass transition temperature above 100°C. Therefore, it is often used to make various disposable containers that need to withstand the temperature of boiling water, as well as disposable foam lunch boxes.

PS material classification and characteristics

Polystyrene (PS) including ordinary polystyrene, expanded polystyrene (EPS), high impact polystyrene (HIPS) and inter-regulated polystyrene (SPS).

Ordinary polystyrene resin is non-toxic, odorless, colorless transparent particles, like glass-like brittle material, its products have a very high transparency, light transmission rate of more than 90%, good electrical insulation properties, easy coloring, good processing fluidity, good rigidity and good chemical resistance, etc..

The shortcomings of ordinary polystyrene are brittleness, low impact strength, easy to stress cracking, poor heat resistance and not resistant to boiling water.

PS plastic properties

1. General-purpose polystyrene is a thermoplastic resin, a glossy, transparent bead or granular solid. Density 1.04~1.09, transparency 88%~92%, refractive index 1.59~1.60.

2. Under the effect of stress, it produces birefringence, which is called the stress-optical effect. The melting temperature of the product is 150~180℃, the thermal decomposition temperature is 300℃, the thermal deformation temperature is 70~100℃, and the long-term use temperature is 60~80℃.

3. After annealing at 5~6℃ lower than the heat deflection temperature, the stress can be eliminated and the heat deflection temperature can be increased.

If you add a little alpha-methyl styrene in the production process, can improve the general polystyrene heat-resistant grade.

4. It can be soluble in aromatic hydrocarbons, chlorinated hydrocarbons, aliphatic ketones, and esters, etc., but only in acetone swelling. It can resist the action of certain mineral oil, organic acid, alkali, salt, low-grade alcohol, and its aqueous solution.

5. Low water absorption, in a humid environment, can still maintain its mechanical properties and dimensional stability. Optical properties are second only to acrylic resins.

6. Excellent electrical properties, volume resistivity and surface resistivity are very high, and not affected by changes in temperature and humidity, and not affected by corona discharge.

7. Irradiation resistance is also very good. Its main disadvantage is brittle and easy to crack, impact strength is low, heat resistance is poor, can not withstand boiling water, only at lower temperatures and lower load use.

8. Poor sunlight resistance, flammable. When burning, black smoke, and a special odor.

PS material main properties

1. Mechanical properties: high strength, fatigue resistance, dimensional stability, creep is also small (very little change under high-temperature conditions);

2. Heat aging resistance: enhanced UL temperature index of 120~140℃ (outdoor long-term aging is also very good);

3. Solvent resistance: no stress cracking;

4. Stability to water: easy to decompose in contact with water (high temperature, high humidity environment need to be used with caution);

5. Electrical performance:

a. Insulation performance: excellent (moisture, high temperature can also maintain the electrical properties of stable, is the ideal material for the manufacture of electronic and electrical parts);

b. Dielectric coefficient: 3.0-3.2;

c. Arc resistance: 120s

6. Molding processability: common equipment injection molding or extrusion.

Due to the fast crystallization speed and good fluidity, the mold temperature is also lower than that required by other engineering plastics. When processing thin-walled parts, it only takes a few seconds, and for large parts, it only takes 40-60s.

PS material molding process

Injection molding

Polystyrene has good fluidity, good processing properties, easy coloring, and good dimensional stability. Available injection molding, extrusion, blow molding, foam, thermoforming, bonding, coating, welding, machining, printing and other methods of processing into a variety of parts, especially for injection molding.

In general, the material can be used directly without drying during injection molding. However, in order to improve the quality of the product, it can be pre-dried in the blast oven at 55~70℃ for 1~2h.

Specific processing conditions are roughly: barrel temperature of about 200 ℃, mold temperature of 60-80 ℃, injection molding temperature of 170-220 ℃, 60-150 MPa, compression ratio of 1.6-4.0.

After molding the product in order to eliminate the internal stress, can be in the infrared lamp or blast oven at 70 ℃ constant temperature treatment 2 ~ 4h.

Extrusion molding

Extrusion molding, the general use of the screw L/D ratio of 17 ~ 24, air cooling, extrusion temperature of 150 ~ 200 ℃.

Blow molding

Blow molding, injection molding and extrusion can be used to blow mold the desired products. Blow molding pressure is generally 0.1~0.3MPa.

PS material injection molding performance

General PS melting point of 173 ℃, the decomposition temperature of 330 ℃; PS injection molding temperature adjustable interval is large. When injection molding, the general use temperature of 180 ℃ – 240 ℃.

Because it is an olefin plastic, it does not absorb water, production, do not need to coax dry, but for product quality, available 60 ℃ temperature drying 1hr, to drain the floating water;.

Polystyrene melt viscosity is small, the flow length ratio is large, generally do not need a large gate and runners can be filled with products;.

However, PS is hard and brittle, the product design, the amount of sharp corners should be avoided, pay attention to the release slope, otherwise, the product will be cracked.

The products are easy to carry static electricity and the surface is easy to absorb dust. The shrinkage rate is 5‰; the overflow value is 0.03mm.

What are the process conditions for PS injection molding?

1. Drying treatment: Usually no drying treatment is required. If drying is required, the recommended drying conditions are 80C, 2~3 hours.

2. Melting temperature: 180~280C. For flame retardant material, the upper limit is 250C.

3. Plastic Injection Mold temperature: 40~50C. Injection pressure: 200~600bar.

4. Injection speed: fast injection speed is recommended.

5. Runners and gates: all conventional types of gates can be used.

6. Chemical and physical properties: Polystyrene has poor chemical stability, can be dissolved by a variety of organic solvents, will be strong acid and alkali corrosion, is not resistant to grease, easy to change color after exposure to ultraviolet light.

7. Polystyrene is hard and brittle, colorless and transparent, and can be mixed with a variety of dyes to produce different colors.

8. It is resistant to water, and dilutes inorganic acids, but can be corroded by strong oxidizing acids such as concentrated sulfuric acid, and can swell and deform in some organic solvents.

The typical shrinkage rate is between 0.4~0.7%.

What are the precautions for PS injection molding?

PS raw material has a very small moisture absorption of about 0.02% to 0.03%, so it does not need to be dried before the injection molding process.

PS raw material molten material flow performance is good, so the shape of very complex plastic molded products are also easy to injection molding, but in mass production will appear flying edge defects, only need operators to use a knife to deal with it.

PS raw material is amorphous polymer and no obvious melting point, from melting to decomposition temperature range is very wide and suitable for molding plastic mold temperature range is also wide.

PS as raw material injection molding production of plastic mold products, the molding of three conditions are as follows.

Temperature control of the injection molding machine barrel

PS plastic raw material has a wide range of injection molding process, at 120 degrees Celsius when it starts to become molten, at 180 degrees Celsius has good fluidity, PA raw material has good thermal stability, its temperature exceeds 300 degrees Celsius when it starts to decompose.

General injection molding machine barrel temperature can be controlled in the range of 140 to 260 degrees Celsius, injection molding machine nozzle temperature is generally 170 to 190 degrees Celsius.

PS injection molding plastic mold products when the steel mold temperature

The temperature of the steel mold of PS plastic injection molding should be as uniform as possible, and the temperature difference is not more than 3 to 6 degrees Celsius range is the best, the temperature of the steel mold can be controlled in the range of 40 to 60 degrees Celsius.

Different plastic mold manufacturers specific mold temperature should be decided according to the plastic mold products to be injected, raw materials and different situations of injection equipment.

Injection pressure conditions of injection molding machine

The injection pressure of PS as the raw material for injection molding plastic mold products is generally between 30 and 120MPa, using screw type injection molding machine is 20% to 30% lower than the plunger type injection molding machine.

Selecting the right injection pressure for the injection molding machine will help to reduce the internal stress and brittleness and make the mold easy to release.

Advantages and disadvantages of PS materials

Market supply PS plastic is a transparent glass-like material, specific gravity of 1.05, and water is basically the same, hard and brittle, knocking, metal-like “ding dang” sound, crisp sound, commonly known as “loud rubber”. Non-toxic, tasteless. Pure PS products, dropped on the ground, after a crisp sound, it will immediately shatter.

Due to the good fluidity of PS plastic, high decomposition temperature, and melt specific gravity stability, it has become an injection molding machine to determine the plasticization efficiency of the index parameter.

The world’s injection molding machines are used to calibrate the capacity of the injection molding machine PS.

Combustion characteristics: flammable, can continue to burn after leaving the fire, the upper end of the flame is golden yellow, burning will soften and blister, no liquid drops, and emit a thick black column of smoke, at the same time, emitting styrene monomer “sweet smell” taste.

Advantages of PS materials

1.Polystyrene resistance to acids and alkalis and low-energy alcohols, erosion by many hydrocarbons, ketones, advanced fatty esters and softened, soluble in aromatics.

2. Excellent electrical insulation, is a good high-frequency insulation material, has excellent and arcing.

3. Polystyrene is a very transparent material with a high surface gloss. Easy to print.

4. Polystyrene can be freely colored, odorless, tasteless and non-toxic, does not cause the growth of bacteria.

Disadvantages of PS material

Poor mechanical properties, hard and brittle, susceptible to solvent erosion and stress cracking; low surface hardness, easy to scratch; poor heat resistance, low heat deflection temperature.

Application areas of PS materials

PS is widely used in the optical industry due to its good light transmission, which can be used to make optical glass and optical instruments, as well as transparent or brightly colored ones, such as lampshades, lighting appliances, etc.

PS can also make many electrical components and instruments that work in high-frequency environments, etc. Because PS plastic is a difficult inert surface material, industry lamination, the need to use professional PS glue bonding.

PS alone for products, brittle, and a small amount of other substances added to PS, such as butadiene can significantly reduce the brittleness and improve the impact toughness, this plastic is called impact PS, its mechanical properties greatly improved, the use of this plastic to produce many mechanical parts and components with excellent performance.

Electronic appliances

Connector, switch parts, household appliances, accessories parts, small electric cover or (heat resistance, flame retardancy, electrical insulation, molding processability);

Automobile

External parts: mainly corner Gusan, engine heat release hole cover, etc.;

Internal parts: mainly internal mirror brace, wiper bracket and control system valve;

Automotive electrical parts: automotive ignition coil stranded tube and various electrical connectors, etc.

Machinery and equipment

Videotape recorder belt drive shaft, electronic computer cover, mercury lamp cover, iron cover, baking machine parts, and a large number of gears, cams, buttons, electronic watch housing, camera parts (heat-resistant, flame retardant requirements).

PS material characteristics and comparison with other materials

1. Mechanical properties: high strength, fatigue resistance, dimensional stability, creep is also small (very little change even under high-temperature conditions).

2. Heat aging resistance: enhanced UL temperature index of 120~140℃ (outdoor long-term aging is also very good).

3. Solvent resistance: no stress cracking.

4. Water stability: easy to decompose in contact with water (caution is needed when using in high temperature and high humidity environment).

5. Environmental protection: recyclable, no harmful substances, no odor, no pollution, green.

Summary

This blog introduces the types of engineering plastics PS, its properties and injection molding process, the selection of equipment, product modeling and mold design considerations in actual production, as well as the solution of common defects.

In the actual PS material selection and injection molding product production, it is recommended to choose an experienced injection mold manufacturer and injection molding product supplier to recommend a suitable PS material in terms of product use and functional requirements, appearance, etc. to ensure the smooth implementation of the project.

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