Choosing the Right Polymer Grades for Injection Molding

Purchasing the wrong polymer grade for injection molding can directly result in product failure, high rejection rates, or field performance issues. Different applications require vastly different material characteristics. There are different properties needed for a food container than for an automotive dashboard. The performance requirements of these applications differ significantly in terms of mechanical strength, thermal resistance, and regulatory compliance. Failing to recognize the need for such differences can lead to a production halt, a costly quality problem, and a reputation-damaging field failure.

What Is a Polymer Grade?

Each polymer grade may define a certain composition that will yield specific polymer properties of interest. A polymer grade defines a specific formulation within a polymer family that delivers targeted mechanical, thermal, and processing characteristics. Each composition can be modified for a given application by changing the molecular weight, the additives, the fillers, and the processing of the material to yield a different grade. Grades are intentionally engineered by adjusting molecular weight, additive packages, fillers or reinforcements, and intended processing conditions. Polypropylene by itself can be hundreds of commercially available grades, each optimized for a specific balance of flow behavior, stiffness, impact strength, and thermal performance

Important Considerations for Raw Injection Mold Buyers

Focus on the following attributes when assessing the available choices for injection molding materials:

• Melt Flow Index (MFI): MFI indicates the flow behavior of a polymer under standardized conditions. Higher MFI grades flow more easily into thin or complex mold geometries, but this improved processability typically comes at the cost of reduced mechanical strength due to lower molecular weight.
• Tensile strength: Tensile strength, modulus, and elongation together define how a molded part responds to applied loads, stiffness requirements, and deformation before failure.
• Impact resistance: Critical for products that will be subjected to physical stress and/or be dropped or exposed to cyclic or sudden mechanical shock during service life.
• Heat deflection temperature: Indicates the temperature at which a polymer deforms under a specified load and serves as a comparative indicator of thermal resistance rather than a maximum service temperature.
• Chemical resistance: Critical for the products that will be used with solvents or cleaning agents to avoid stress cracking, swelling, or long-term material degradation.

Polymer Grades That Are Commonly Used for Injection Molding

The injection molding raw materials can be sourced through multiple polymer families with varying characteristics.

ABS has a wide range of grades that can be used to balance strength and ease of processing.These grades may also vary in impact modification, heat resistance, and surface finish.Polypropylene can be flexible and can also be more rigid. It is frequently modified with mineral fillers to control stiffness, shrinkage, and dimensional stability.

Polyethylene can be primarily HDPE, and in select cases LDPE or LLDPE, depending on stiffness and impact requirements for injection-molded parts..

Polycarbonate has the added quality of optical transparency and exceptional strength to resist impact. It also offers excellent dimensional stability for demanding applications.

The Negative Effects of Choosing the Wrong Material when Producing Injection Molded Products

Incorrectly selecting the plastic material to be used when producing an injection-moulded product can have extremely negative effects. 

Parts can become excessively warped during the cooling phases. This is often driven by incompatible shrinkage behavior or filler orientation relative to mold design.The flow characteristics will be poor, and this will lead to incomplete filling of the cavities, and visible defects will remain. 

Production runs will be completely scrapped because material properties were not aligned that the grades would be compatible with the specific mold design and processing parameters.

Choosing Appropriate Polymer Grades

• Before purchasing any polymer grade, identify and document your product’s functional requirements.This should include mechanical, thermal, chemical, and regulatory needs.
• Review technical data sheets from at least two resin suppliers. 
• Before making large purchase commitments, ask the suppliers if they can conduct a testing or evaluation of the polymer on your behalf. 
• Engage your injection molder and technical polymer supplier to discuss filler and plastic material selection associated with the processor and injection molder design and processor specifications. Effective grade selection requires alignment between material behavior, mold design, and processing conditions.

Ensuring Consistent Quality Control

Quality suppliers of industrial plastics in India, whether based in India or elsewhere, utilize effective quality control and assurance when producing polymer resins. Be as specific as possible regarding the granular grade of plastics you need when crafting the purchasing agreement with the supplier. With every shipment you receive, ask for a quality control certificate. Key parameters to monitor include MFI tolerance, filler content, and moisture levels where applicable. Gradual quality control consistency ensures less risk in production and a more predictable outcome. Stay with the suppliers for whom you have developed a consistent outcome over time.

Conclusion

When a company can understand and adequately convey the correct selection of polymer grades to their suppliers, they have a clear advantage over their competitors. The company will reduce the waste created during the manufacturing process, improve the quality of the end product, and improve the time to market of the product.