HIPS vs PP – Composition & Shrinking Characteristics and Details

Comparing HIPS (High Impact Polystyrene) vs PP (Polypropylene): Get into their distinct compositions and understand their shrinkage behaviors. Learn the key differences and considerations for these widely-used polymers.

What is HIPS and What is PP? 

HIPS (High Impact Polystyrene):

Definition: HIPS is a type of polystyrene that has been modified with polybutadiene rubber to improve its toughness and impact resistance.


  • Amorphous in nature.
  • Improved toughness compared to general-purpose polystyrene (GPPS) due to the addition of rubbery polybutadiene.
  • Exhibits good dimensional stability and is easy to fabricate.
  • Typically used in applications where a balance between rigidity, impact strength, and aesthetic appeal is required.
  • Common applications include protective packaging, toys, and product housings.

Shrinkage Properties of HIPS: HIPS typically exhibits a shrinkage rate of 0.4% to 0.8%. The exact rate can depend on processing conditions, part geometry, and other additives or fillers present in the polymer blend.

Factors Influencing Shrinkage: The degree of crystallinity, cooling rate, and mold design can influence HIPS shrinkage. For instance, faster cooling rates generally lead to more shrinkage due to higher residual stress in the material.

PP (Polypropylene):

Definition: PP is a semi-crystalline thermoplastic polymer made from the monomer propylene. It is one of the most widely produced plastics globally and is known for its versatility across numerous applications.


  • Semi-crystalline structure provides good chemical resistance and a high melting point compared to other standard plastics.
  • Resistant to fatigue.
  • Low density which results in a lightweight material.
  • High clarity in its unstabilized and uncolored form.
  • Used in various applications, ranging from packaging to automotive components, textiles, and consumer goods.

Shrinkage Properties of PP: PP typically exhibits a higher shrinkage rate than HIPS, which can range from 1.2% to 2.5% or even more depending on the specific grade and processing conditions.

Factors Influencing Shrinkage: The degree of crystallinity in PP is a significant factor in determining its shrinkage. Higher crystallinity generally leads to more shrinkage. The cooling rate, mold temperature, part geometry, and the presence of additives or fillers can also impact shrinkage.

While both HIPS and PP are versatile and widely used plastics, their distinct properties and characteristics make them suitable for different applications.

HIPS vs PP – Difference Between High Impact Polystyrene (HIPS) and Polypropylene (PP)

High Impact Polystyrene (HIPS) and Polypropylene (PP) are two commonly used polymers in various applications due to their distinct properties. Their behavior, especially when it comes to shrinkage, is influenced by their respective molecular structures and how they cool from a molten state.

HIPS vs PP Comparison Details:

CharacteristicsHigh Impact Polystyrene (HIPS)Polypropylene (PP)
Degree of ShrinkageComparatively LowerComparatively Higher
ConsistencyHigh Consistent ShrinkageLess Consistent Shrinkage
WarpageLess prone to warpageMore prone to warpage
UsesPrecise Dimensions – Tools and Spares etcFG, Chair base etc
Table: HIPS vs PP Comparison
  • Degree of Shrinkage: PP generally has a higher shrinkage rate than HIPS. This is attributed to its semi-crystalline nature, where crystalline regions contract more upon cooling compared to amorphous regions.
  • Consistency: PP’s shrinkage can be less consistent due to its crystalline nature, making it more sensitive to processing conditions. This makes PP parts somewhat more challenging to design for tight tolerances compared to HIPS.
  • Warpage: Both materials can warp if cooled unevenly or if there’s a non-uniform wall thickness in the part design. However, due to its higher shrinkage and semi-crystalline nature, PP might be more prone to warpage issues than HIPS.

For applications where precise dimensions are critical, it is essential to consider the shrinkage characteristics of the material and possibly do some trials to determine the exact behavior in the specific processing conditions. Adjustments in mold design, processing parameters, and the selection of polymer grade can help in mitigating undesirable shrinkage effects.

Pros and Cons of High Impact Polystyrene (HIPS)


  • HIPS has better impact resistance than general-purpose polystyrene.
  • HIPS can be easily molded and extruded.
  • Exhibits lower shrinkage compared to some other plastics.
  • Excellent material for thermoforming processes.
  • Generally less expensive than specialty polymers.


  • Has a relatively low melting point which may not be suitable for high-temperature applications.
  • Susceptible to some solvents.
  • Less recyclable, especially when mixed with colorants and other additives.
  • Less durable than some high-performance plastics.

Pros and Cons of PP (Polypropylene)


  • Offers excellent chemical resistance to acids, bases, and many solvents.
  • Higher melting point than many standard plastics, making it suitable for microwave-safe containers and high-temperature applications.
  • PP has a low density, making it lightweight yet robust.
  • Highly resistant to flexural fatigue.


  • PP can degrade when exposed to prolonged UV light, unless stabilized.
  • PP surfaces can be challenging to adhere or bond to without surface treatments.
  • Due to its semi-crystalline nature and differential cooling, PP parts can sometimes warp if not molded properly.
  • Achieving a clear, glass-like finish is more challenging with PP compared to some amorphous plastics.

Among HIPS vs PP, Which is better and why?

Determining whether HIPS (High Impact Polystyrene) or PP (Polypropylene) is “better” depends largely on the specific application and requirements. Both materials have their unique advantages and limitations. Let’s compare the two in various categories:

Mechanical Properties:

  • HIPS: Offers good rigidity combined with improved toughness (over general-purpose polystyrene) due to the addition of polybutadiene.
  • PP: Generally has a higher tensile strength and is more resistant to fatigue than HIPS. PP also offers excellent resistance to flexural fatigue and is known for its hinge-forming ability.

Thermal Properties:

  • HIPS: Lower melting point and thermal resistance than PP.
  • PP: Higher melting temperature making it suitable for applications that require increased thermal resistance, such as microwave-safe containers.

Chemical Resistance:

  • HIPS: Provides good resistance to many chemicals but can be affected by solvents.
  • PP: Outstanding chemical resistance to both acids and bases, and is also relatively resistant to solvents.


  • Typically, PP has been more economical than HIPS, but market prices for raw materials can fluctuate based on various factors like oil prices, demand, production capacities, and geopolitical factors.


  • HIPS: Easy to process and mold, suitable for thermoforming applications.
  • PP: Can be processed through various methods including injection molding, blow molding, and extrusion, but might need specific mold designs or treatments due to its semi-crystalline nature.

Environmental Considerations:

  • HIPS: Less recyclable compared to PP, especially if it contains colorants or other additives.
  • PP: Widely recyclable and used in various eco-friendly applications.


  • HIPS: Commonly used for signage, displays, product housings, and certain types of packaging due to its appearance and formability.
  • PP: Extremely versatile with applications ranging from packaging, textiles (as fibers), automotive components, containers, and many other consumer and industrial goods.


Neither material can be categorically considered “better” than the other in all respects. Instead, the selection between HIPS vs PP should be based on the specific requirements of the application, such as mechanical properties, thermal resistance, cost, appearance, and environmental considerations. Understanding the needs of your project will help in selecting the most appropriate material.

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