Tag: shrinkage

Homopolymer vs. Copolymer

By Bozilla
May 24, 2022

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:

homopolymer chain

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:

copolymer chain

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

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Can Gate Location Really affect Part Warpage?

By Bozilla
August 17, 2021

Based on part geometry, gate location(s) will determine how the polymer fills the cavity  If the cavity doesn’t fill in a balanced/uniform fashion, the internal stresses will be anisotropic- meaning non-uniform properties  So it is important to place a gate in a location such that the polymer flowfront fills the cavity at a uniform rate and reaches the end of the cavity at all locations, including weld line locations, simultaneously

With simple part geometry, identifying an ideal gate location may be possible by using experience and examining the part  However, with more complex geometry and gating limitations (cooling line interference, ejector pin interference, slides, etc), it is nearly impossible to determine the appropriate gate location(s) without using FEA(flow simulation)  Not only can FEA(flow simulation) produce actual deflection results(warpage), it can also provide data that is a precursor to warpage-such as volumetric shrinkage and frozen-in stress which is typically due to a response from forcing the material into the cavity while the material is trying to freeze

gate location and part warpage courtesy of sciencedirectcom

Gate location(s) will determine polymer orientation  Based on that location, it will ultimately determine polymer shrinkage  Also, different regions of the part will cool at different rates(regions of the cavity near the gate that were first to fill will cool before regions furthest from the gate)

 

Why is this important?  Because there are 3 major components that contribute to warpage:

 Polymer Orientation

Polymer Shrinkage

Cooling Effects

 

Shrinkage and orientation are both directly correlated to injection location(s) on a part as it relates to processing conditions  Warpage due to cooling effects is  based on the rate of how the polymer cools on one side of the cavity relative to the other side Non-uniform cooling through the thickness will create warpage

Because gate location(s) directly correlates to the contributors of warpage, gate location is therefore extremely important in the tool creation process and ultimately the quality of the part

The injection molding professionals at Bozilla Corporation have over 20 years of experience assisting OEM’s, Tier 1 & Tier 2 suppliers, and Tool Shops to create quality parts that meet timing and goals

wwwBozillaCorpcom

 

About the author

Chris Czeczuga President Bozilla Corporation

Chris Czeczuga is a Plastics Engineer, Injection molding expert, Military Veteran and the President of Bozilla Corporation He has proven success with many OEM’s Tier

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