Polypropylene (PP) is everywhere—car bumpers, appliance housings, packaging, and medical devices. It’s light, tough, and inexpensive, but molding it right isn’t as simple as you think. Its semi-crystalline nature and variable shrinkage mean that small mistakes in design or processing can lead to warpage, sink marks, or brittle parts.
This guide gives you the essentials: which PP grades to use, the processing parameters that matter most, and how to avoid the defects that waste time and money. Therefore, you can get consistent, high-quality parts from the first shot onward.
Polypropylene is a semi-crystalline thermoplastic with a balance of stiffness, toughness, and chemical resistance. It’s one of the most versatile materials in injection molding. Choosing the right type of PP and understanding how it behaves during molding can save you from costly redesigns and process headaches later.
The stiffest and most rigid form of polypropylene, made from a single polymer chain structure. It’s ideal for parts that need maximum strength without adding weight—such as appliance housings, rigid packaging, and automotive interior trim.
Because PP-H has higher crystallinity, it holds its shape well and resists many chemicals. But in low-temperature conditions, it can lose impact strength and become brittle. If your product might face cold storage or outdoor winter use, you’ll want to either modify the formulation or consider a block copolymer alternative.
Mold design for PP-H needs to account for a shrinkage rate of about 1.5–2.5%. Getting this right at the tooling stage prevents undersized parts and costly adjustments later.
Random copolymer polypropylene is modified with small amounts of ethylene distributed along the polymer chain. This change in structure improves clarity and impact resistance, making PP-R a good choice for applications where appearance and low-temperature toughness matter—such as clear lids, food containers, and certain medical disposables.
It has lower stiffness than homopolymer grades, so designs that need structural rigidity may require slightly thicker walls or reinforcing features. On the plus side, PP-R offers better flexibility and reduced brittleness in cooler environments, extending the range of conditions where the part can perform reliably.
Shrinkage is similar to PP-H but can vary slightly depending on the ethylene content. Testing with the exact grade before finalizing mold dimensions is the safest way to avoid rework.
Block copolymer polypropylene contains larger ethylene blocks in the chain, giving it the best impact resistance of all PP types—especially in cold weather. It’s the material of choice for heavy-duty or outdoor products like automotive bumpers, storage crates, and power tool housings.
While PP-B is tougher, it has slightly higher density and may require more careful control of melt flow to fill complex parts cleanly. This toughness also comes with a modest trade-off in stiffness, so designs needing very high rigidity might require added fillers or structural ribs.
Shrinkage rates are generally within the 1.0–2.5% range, but because PP-B is more ductile, post-mold dimensional stability can be influenced by cooling uniformity. Balanced cooling channels and consistent mold temperature are key to keeping parts in tolerance.
| Property | PP | ABS | HDPE |
| Density (g/cm³) | 0.90–0.91 | 1.04 | 0.94–0.97 |
| Tensile Strength (MPa) | 25–40 | 40–50 | 20–37 |
| Impact Resistance | Medium–High | High | High |
| Shrinkage (%) | 1.0–2.5 | 0.4–0.9 | 1.5–3.0 |
| Heat Deflection Temp (°C) | 95–110 | 85–100 | 75–90 |
| Chemical Resistance | Good–Excellent | Fair–Good | Excellent |
| Molding Temp (°C) | 200–250 | 210–280 | 180–230 |
| Typical Applications | Automotive trim, packaging, containers | Appliance housings, interior parts | Pipe fittings, chemical containers |