PS Injection Molding：You Should Know

Table of Contents

1. What is PS Injection Molding?

2. Why is Polystyrene (PS) Used in Injection Molding?

3. PS Material Properties and Grades

4. PS Injection Molding Applications

5. PS Injection Molding Process

6. PS Injection Molding Design Guidelines

7. Summary

8. FAQ

1. What is PS Injection Molding?

PS injection molding is a plastic processing method that uses polystyrene (PS) plastic as raw material. The mixture is heated and melted using an injection molding machine, then injected into a mold, and finally cooled and solidified to obtain the finished product.

2. Why is Polystyrene (PS) Used in Injection Molding?

Polystyrene (PS) is widely used in injection molding due to its excellent rigidity, smooth surface, and ease of processing. Its low melting point and good flowability make it an ideal material for producing precision parts. PS comes in various grades, such as general-purpose polystyrene (GPPS) for transparent, rigid applications, and high-impact polystyrene (HIPS) for products requiring higher toughness. This versatility allows manufacturers to efficiently and economically produce a wide range of consumer goods, electronics, and medical products.

3. PS Material Characteristics and Grades

3.1 PS Material Characteristics

Polystyrene is a well-balanced and cost-effective thermoplastic:

1. Transparency and Gloss

General purpose grade polystyrene (GPRS) has extremely high light transmittance, resulting in crystal-clear products with a high surface gloss, making it an ideal material to replace glass for transparent packaging and display stands.

2. High Rigidity and High Stability

PS material has good rigidity and strong resistance to bending deformation. Its molding shrinkage is low (approximately 0.4%-0.7%), and its water absorption is extremely low, ensuring that products maintain precise dimensions and shapes even in complex environments.

3. Ease of Processing

Polystyrene has excellent melt flowability, easily filling complex molds. It has a wide molding temperature range and short processing cycles, making it particularly suitable for high-volume, high-efficiency production methods such as injection molding and extrusion.

4. Lightweight

With a density of only 1.04-1.09 g/cm³, far lower than glass and most metals, it helps reduce product weight and transportation costs, offering significant advantages in packaging and daily necessities.

5. Electrical Insulation Properties

Polyssessing excellent electrical insulation properties, and with dielectric properties unaffected by temperature and humidity changes, it is suitable for manufacturing high-frequency insulating components and electronic and electrical components.

6. Low Cost

As one of the most commonly used general-purpose plastics, PS raw materials are inexpensive. Combined with its efficient processing characteristics, it provides manufacturers with a highly competitive overall cost advantage.

3.2 PS Material Grades

Based on their properties and application areas, PS is mainly divided into the following two grades:

Characteristic Dimension	General Purpose Polystyrene (GPPS)	High Impact Polystyrene (HIPS)
Main Characteristics	High transparency, high gloss, good rigidity, brittle	Good toughness, high impact strength, opaque
Transparency	High transparency (88%-92%)	Opaque, milky white
Mechanical Properties	Good rigidity, but low impact strength, easily broken	High impact resistance, good toughness
Thermal Properties	Heat deflection temperature 70-100°C, limited continuous use temperature	Heat deflection temperature slightly higher than GPPS, generally moderate heat resistance
Processing Methods	Injection molding, extrusion, blow molding	Injection molding, extrusion, thermoforming
Typical Applications	Transparent packaging boxes, lamp covers, optical parts, disposable cutlery, toys, hangers	Housings for household appliances (TVs, air conditioners), computer monitor casings, toys, food packaging containers, daily necessities

4. PS Injection Molding Applications

Polystyrene (PS) is a commonly used material in injection molding due to its good flowability, fast molding speed, and low cost. Its main applications include:

1. Packaging Industry: Transparent beverage cups, yogurt cups, disposable lunch boxes, cutlery, and transparent flip-top packaging boxes.

2. Consumer Goods Industry: Children's toy shells, transparent clothes hangers, picture frames, stationery pen barrels, combs, and disposable lighter shells.

3. Home Appliance and Electronics Industry: Air conditioner panels, television shells, router shells, battery case covers, and transparent protective covers for dashboards.

4. Medical and Pharmaceutical Industry: Disposable petri dishes, syringe tubing, test tubes, and medical trays.

5. Hardware and Tools Industry: Screwdriver handles, level shells, saw handles, and hardware parts boxes.

5. PS Injection Molding Process

Polystyrene (PS) is one of the easiest plastics to process in injection molding due to its good flowability, fast molding speed, and low cost. This section provides a systematic process reference, following the order of molding process → core parameters → defect countermeasures.

5.1. PS Injection Molding Process

PS injection molding is a cyclical process, with the complete cycle including the following five stages:

1. Raw Material Preparation

• Inspect Packaging: PS has extremely low water absorption (<0.02%), and can be directly machined if the packaging is intact.

• Drying Treatment: If the packaging is damaged or stored in a humid environment, it needs to be dried in hot air circulation at 70-80℃ for 1.5-2.5 hours to remove surface moisture and prevent silver streaks from appearing on the product.

2. Plasticization (Heating and Melting)

• PS granules are sheared by the screw and heated by the external heater in the barrel, changing from a solid state to a molten state.

• Temperature Gradient Distribution: Low at the hopper end and high at the nozzle end, ensuring uniform plasticization of the melt.

3. Injection Filling

• The screw advances forward, rapidly injecting molten PS into the mold cavity.

• Injection speed and pressure need to be precisely set according to the product wall thickness and structure to ensure the cavity is completely filled.

4. Holding Pressure and Cooling

• Holding Pressure Stage: After injection, the screw maintains a certain pressure to replenish the melt that has shrunk due to cooling and prevent shrinkage cavities.

• Cooling Stage: The product continues to cool and solidify in the mold. Cooling time accounts for approximately 50%-80% of the entire cycle.

5. Demolding

• Once the product has cooled to sufficient rigidity, the mold opens, and the ejector mechanism pushes the product out.

• After demolding, the next cycle begins.

5.2 PS Injection Molding Key Process Parameters

1. Temperature (°C)

Part	GPPS	HIPS	Notes
Barrel Rear	140–180	150–180	Prevent premature melting; ensure uniform material conveyance.
Barrel Middle	170–210	180–220	Gradual temperature increase; uniform plasticization.
Barrel Front / Nozzle	180–230 / 170–220	190–240 / 180–230	Ensure melt flow; prevent drooling.
Mold	20–60	30–70	Higher mold temperature reduces internal stress and improves surface gloss.
Thermal Decomposition	~290	~290	Flame-retardant PS limit 250°C; avoid long residence times.

2. Pressure

Parameter	Range	Notes
Injection Pressure	60–150 MPa	Thin-wall: high; thick-wall: low; excessive pressure may cause flash and internal stress.
Holding Pressure	50–70% of Injection	Just enough to compensate shrinkage; too long increases demolding resistance.
Back Pressure	5–20 MPa	Moderate back pressure helps colorant dispersion; too low may cause air entrapment and bubbles.

3. Speed & Time

Parameter	Range	Notes
Injection Speed	Medium–High	Thin-walled parts require fast injection; HIPS should not be too fast to protect rubber phase.
Screw Speed	0.8–1.2 m/s	Improves plasticization efficiency; ensure sufficient cooling.
Cooling Time	t = (1.5–2.5) × thickness⊃2; (s)	Wall 1 mm: 6–10 s; 2 mm: 15–25 s; 3 mm: 30–45 s.

5.3 Common Defects & Solutions in PS Injection Molding

1. Brittle / Stress Cracks

Causes: High internal stress; uneven molecular weight distribution

Solutions:

• Increase mold temperature to promote uniform cooling.

• Reduce injection pressure to minimize melt stress.

• Slow down injection speed to avoid rapid filling stress.

• Anneal finished parts (70°C hot air for 2-4 hours).

• Use materials with uniform molecular weight for consistent plasticization.

2. Silver Streaks / Bubbles

Causes: Moisture in raw material; degradation; air entrainment

Solutions:

• Thoroughly dry raw material (moisture <0.02%).

• Lower barrel temperature to prevent thermal degradation.

• Increase back pressure to expel trapped air.

• Clean hopper and screw dead spots regularly.

• Ensure proper storage of materials to avoid moisture absorption.

3. Flash / Burrs

Causes: Insufficient clamping force; high melt temperature

Solutions:

• Increase clamping force to ensure complete mold closure.

• Reduce barrel temperature to prevent melt overflow.

• Lower injection pressure.

• Inspect mold parting surfaces and repair any gaps or wear.

• Ensure alignment pins and guide posts are functioning properly.

4. Black Spots / Burn Marks

Causes: Local overheating; excessive screw shear

Solutions:

• Clean screw and barrel dead spots to avoid material burn.

• Reduce screw rotation speed to minimize shear heat.

• Check temperature control system for hot spots.

• Minimize residence time in the front barrel.

• Use heat-stable, high-quality PS material.

5. Flow Marks / Waves

Causes: Injection speed too low; low mold temperature

Solutions:

• Increase injection speed for smooth melt flow.

• Raise mold temperature to prevent rapid cooling.

• Enlarge gate size to reduce flow resistance.

• Optimize gate position to shorten flow paths.

• Adjust holding pressure and time for uniform cooling.

6. Sink Marks / Depressions

Causes: Insufficient holding pressure; uneven cooling

Solutions:

• Increase holding pressure.

• Extend holding time to fully compensate shrinkage.

• Optimize cooling channels for better uniformity.

• Lower barrel temperature to reduce volume change.

• Add ribs or supports in thick-wall areas.

6. PS Injection Molding Design Guidelines

6.1 Optimizing Wall Thickness

For optimal performance and manufacturability, the wall thickness of PS parts should generally be between 0.04” and 0.12” (1.0 to 3.0 mm). Maintaining consistent wall thickness is crucial to avoid issues such as sink marks, warping, and uneven cooling. Avoid abrupt transitions between thick and thin sections, as this can lead to stress concentrations and poor material flow. Thin walls should be reinforced with ribs where possible to maintain strength without increasing mass.

6.2 Incorporating Draft Angles for Easy Ejection

Apply draft angles of 0.5°–1° on vertical walls to facilitate smooth ejection from the mold. Insufficient draft can cause parts to stick, damage mold surfaces, and increase cycle times. Adequate draft angles reduce friction and improve part release, helping to maintain mold longevity and production efficiency.

6.3 Designing Proper Radii

Avoid sharp corners in PS parts to minimize stress concentrations, which can lead to cracking or warping. A minimum radius of 25% of the wall thickness is recommended, while for enhanced strength, use 60% of the wall thickness. Rounded corners also improve melt flow, mold filling, and part durability during injection molding.

6.4 Ensuring Accurate Part Tolerances

PS parts have typical commercial tolerances of 0.1–0.3 mm for parts under 160 mm. For smaller parts (≤100 mm), fine tolerances of 0.05–0.1 mm are achievable with proper mold design and process control. Design realistic tolerances to reduce mold adjustments, rework, and production costs.

6.5 Managing Undercuts with Core and Slide Mechanisms

Undercuts in PS parts can complicate mold design and increase tooling costs. Whenever possible, minimize undercuts in the design. For necessary undercuts, use core pins or sliding mechanisms to handle complex geometries efficiently. This ensures smoother mold operation and reduces the risk of defects or part damage.

6.6 Additional Considerations for PS

• Material Choice: Choose GPPS for transparent, rigid applications and HIPS for high-impact or durable parts.

• Cooling: Ensure uniform mold cooling to reduce internal stresses and improve surface finish.

• Gate Design: Optimize gate size and location to minimize flow lines, sink marks, and weld lines.

• Shrinkage Compensation: Account for typical PS shrinkage (0.4–0.7%) in the mold design.

• Surface Finish: PS can achieve high gloss surfaces directly from the mold; avoid over-polishing, which can cause flash.

7. Conclusion

PS material, with its excellent processing performance and wide applicability, has become one of the important materials in the injection molding field. In practical applications, high-quality injection molded products can be achieved through reasonable process control and defect prevention measures.

Alpine Mold possesses advanced equipment and rich production experience, providing customers with comprehensive services from mold design to mass production. Our professional team is familiar with the properties of PS material and proficient in PS injection molding processes, providing customers with the highest quality injection molds and molding services to meet your various needs.

8. RFQ

8.1. What is PS molding?

PS molding refers to the process of shaping polystyrene (PS) plastic into parts using techniques like injection molding, extrusion, or thermoforming. It allows mass production of precise, rigid, and lightweight components.

8.2. What is PS material?

Polystyrene (PS) is a thermoplastic polymer available in two main types:

• GPPS (General Purpose PS): transparent, rigid, brittle, high gloss.

• HIPS (High Impact PS): opaque, tough, and resistant to impact.
  

8.3. What is the processing temperature of PS material?

• GPPS: Barrel 140–230°C, Mold 20–60°C

• HIPS: Barrel 150–240°C, Mold 30–70°C

• Exceeding 250°C (especially for flame-retardant PS) may cause degradation.

8.4. Can polystyrene be injection moulded?

Yes, PS is commonly injection molded. It flows well when heated and can produce detailed parts with good surface finish, precise dimensions, and consistent quality. Proper mold design, wall thickness, and cooling are essential to prevent defects like warping or sink marks.

8.5. What are the 4 stages of injection molding?

• Clamping: The mold is closed and secured.

• Injection: Melted plastic is injected into the mold cavity.

• Cooling: Plastic solidifies, taking the shape of the mold.

• Ejection: Finished part is removed from the mold.