Plastic Injection Molding – Choosing the Right Kind of Resin (2018)

Choosing the right kind of resin for your plastic injection molding project is crucial and is considered plastic injection 101. In this article, we look at what you should consider when choosing the correct resin for your project.


The number of polymer compounds (plastic compounds) has grown the past 20 years significantly, and as a result, plastics are now being used in more applications than ever before. Modern-day compounding techniques have built a reputation for both strength and endurance.
These improvements combined with the advantages of corrosion resistance and aesthetic appeal put plastic components at the forefront of manufacturing of many industrial and consumer components. Due to their strength, versatility, cost and lightweight characteristics, plastics are playing a larger role in the medical, construction and transportation industries.


A Few Questions to Keep in Mind When Considering Which Resin to Choose:


  • The required strength application of the part, does it need to be flexible or rigid?
  • The final finish of the part, what appearance it will have including surface texture and transparency and color requirements?
  • Regulatory requirements for the region you are producing for, i.e. EU regulations differ from U.S. regulations, does it have to be RoHS, REACH, EN 1452 or FDA compliant?
  • What chemical or environmental elements will the part be exposed to, i.e. rain, the wind, snow, the sun? Plastics primer – Thermoset v.s. Thermoplastic
  • What is the life expectancy of the part?


Plastics fall into two basic categories – thermoplastics and thermoset plastics:


1. Thermoset Plastics – As the term implies these plastics are set or cured during processing from a liquid form and cooled into solid. When these plastics are heated, it creates a chemical reaction that “sets” the part into a permanent form. The chemical reaction is non-reversible, which means parts made using the thermoset process cannot be melted again or reshaped. These materials tend to be a challenge for recycling purposes unless a bio-based polymer is used (which is rarely the case but luckily becoming more prevalent.)  


2. Thermoplastics – or thermosoftening plastics are heated then quickly cooled in a mold to form a part. Once the part has cooled down, they revert to their original state and can be melted down and cooled again, unlike thermoset plastics. For this reason, thermoplastics are easier to recycle and reuse. The majority of manufactured polymer comprises of this type of resin today and are commonly used in the injection molding process.


Plastic Mold Design Checklist

Family and type categorize thermoplastics. They fall into three broad categories namely – commodity resins, engineering resins, and speciality or high-performance resins.


High-performance resins come at a higher cost and are the most expensive, and thus commodity plastic resins are commonly used for many everyday applications. Commodity resins are easily processed and relatively inexpensive, and you can find them in standard mass produced items like packaging and wrapping.

Resins used in engineering are more expensive but offer better strength and resistance to chemicals and environmental exposure.


Within each of these families, certain resins have a different morphology. Morphology describes the arrangement of molecules within a resin and falls into two categories, semi-crystalline and amorphous, and each has unique characteristics:


  • Semi-crystalline resins – tend to be opaque, offer excellent abrasion and chemical resistance, are less brittle and have higher shrinkage rates.  
  • Amorphous resins – shrinks less when cooled, better transparency, work well for tight-tolerance applications, tend to be brittle and lack chemical resistance.  


Available Resin Examples


plastic injection mold




An example of an amorphous resin is polystyrene (PS). Like most amorphous resins, it is transparent and brittle, but can be effectively used in high-precision applications. It’s one of the most widely used resins and can be found in nearly any restaurant or food court in the form of foam cups, plates or plastic cutlery.


Higher up the amorphous chain are the engineering resins such as polycarbonate (PC). It’s temperature and flame resistant and has electrical insulating properties, and can, therefore, be used in electronic components.


An example of a speciality or high-performance amorphous resin is polyetherimide (PEI). This resin offers strength and heat resistance, however, unlike most other amorphous materials it happens to be chemically resistant and therefore popular in the aerospace industry.



An inexpensive semi-crystalline commodity resin is polypropylene (PP), and like most semi-crystalline polymers it’s flexible and chemically resistant. The low cost of this resin makes it the obvious choice for an array of applications such as bottles and packaging.

A popular semi-crystalline engineering resin is polyamide (PA or Nylon). Nylon is chemical and abrasion resistant and has as low shrinkage and warp level. There are bio-based versions of this resin available making it an earth-friendly alternative to other resins.


The strength and endurance of the material make it a lightweight alternative to metal in the automotive industry.

Plastic additives to improve characteristics

A variety of resins have particular properties for which they’re known, and as we’ve seen in this blog, resin families (engineering, commodity, and high-performance/speciality) contain both amorphous and semi-crystalline alternatives. But, the higher the performance, the higher the cost will be. To keep manufacturing costs low, many plastic injection molding companies use fillers or additives to gain the quantities and qualities they need at a lower cost.


These additives can be used to improve the performance or convey other characteristics to the final product.



Here are a few of the most common additive applications:


  • Antistatics – Additives used to decrease the static electricity conduction, often used in sensitive electronics
  • Plasticizers and Fibers – Plasticizers make a resin more pliable, whereas fibres add strength and stiffness
  • Antimicrobial – Additives that are used in food-related applications or high-contact consumer products like toys
  • Optical brighteners – Additives used to improve whiteness
  • Flame Retardants – Additives used to make products flame resistant
  • Colorants – Additives that add color or special effects such as fluorescence


Final Selection

When selecting a resin to use for your molding project, you should start by choosing the family of material that will give you most of the properties you need. Once you’ve determined this, you can go ahead to select the appropriate grade of material.


Choosing the correct material for a project is one of the most important factors in creating the perfect plastic parts. The advances in polymer science in recent years have contributed to the development of a large selection of resins to choose from. It’s of utmost importance to work with an injection molding company that has experience with a wide variety of resins and applications including resins that are compliant with the standards required in the region your products will be used.


Do you have anything to add to this blog or questions you’d like to ask? Please feel free to leave a comment below and one of our experts will get back to you.



plastic mold design checklist

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