Material selection for an injection molding application can sometimes prove to be very challenging. What happens if you identify a material then find that it can be supplied as a homopolymer or a random copolymer. Is there a difference? The answer is YES. The choice made for your project can affect part quality.
The Homopolymer:
A homopolymer has the same base unit which causes the molecular chain to have a high degree of consistency and size. However, length can vary depending on how long the polymerization process is allowed to occur.
The high degree of consistency in a homopolymer creates a high degree of regularity. When many of these changes flow and combine, they are able to create a very tight entanglement and when they cool and shrink, they also have a high degree of crystallinity which increases shrink.
The Copolymer:
A copolymer, as shown in the image above, has more than one base unit and each base unit is a different size. There can be more than two base units. Due to the variation in size of the base units, the copolymer chains will be spaced much further from each other and have a higher degree of irregularity. And similar to the homopolymer, the length of the molecule will depend on how long the polymerization process is allowed to occur.
The high degree of irregularity does not allow the polymer chains to form a tight structure, leaving a lot of space between the molecular chains. Therefore, when the polymer flows, there can be alignment but there will be more irregularity and not as tight of a structure which prevents excessive shrinkage.
When comparing the two types of polymers, assuming each is the same length (same molecular weight, per se) the homopolymer will be much more organized and structured therefore creating more mechanical strength and chemical resistance but have high shrinkage. The copolymer will have more random orientation which will create space between the molecules allowing for easier chemical attack and less mechanical strength and also have lower shrinkage. Of course, we could discuss these comparisons in much more detail but we will stick to the basics for now.
As material selection relates to injection molding, the properties of the material is a crucial factor.
The major properties when comparing homopolymers to copolymers are:
- shrinkage
- chemical resistance
- mechanical strength
Each of these properties must be considered with regards to the outcome of part quality.
For example, when injection molding a thick part using a homopolymer, the part will have:
Pro’s:
- chemical resistance
- mechanical properties
Cons:
- A high degree of shrinkage which may cause internal stress, cracks or even voids.
If the same part is injection molded with a copolymer, the part will have:
Pro’s
- Less internal stress and possibly no voids.
Cons:
- Reduced chemical resistance
- Reduced mechanical properties
Knowing the difference between the two polymers is critical with regards to the impact on final part quality.
Fortunately, flow analysis simulation can identify the shrinkage variation as well as the flow characteristics between the two types of polymers. Because the characteristics of each type of polymer is different, the injection molding process for each will also be different.
A skilled analyst can develop an optimized injection molding process for either of these types of polymers and provide critical insight into the part quality with respect to each.
Without the critical insight of the analysis, the potential for reduced part quality or even part failure exists.
For more information, support with your Tool Design, assistance Troubleshooting any of your Plastics Injection Molded projects, or Autodesk Moldflow Training contact the Team at Bozilla Corporation.
Bozilla Corporation’s Plastics Injection Molding Team has over 20 years of experience analytically and on the floor. We specialize in optimization, consulting, engineering, troubleshooting and Autodesk Moldflow software training. Additionally, our plastics engineers have a full understanding of polymers and how they influence an injection molded part. Your success is our success. Our skilled Team is focused upon meeting the goals and timelines of our customers.
www.BozillaCorp.com, 800-942-0742, info@BozillaCorp.com
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