Home » Plastic Injection Molding Basics » Injection Molding Shrinkage
Injection Molding Shrinkage
Shrinkage values for polypropylene (PP) molded parts can range from approximately 0.008" to approximately 0.025" per inch. This broad range of possible shrinkage values have been calculated from a multitude of injection molded parts having various thicknesses. Linear mold shrinkage is mostly dependent on the particular polypropylene resin type, the processing conditions used to make the plastic parts, the part design, and or including the material flow path direction that's partly imposed by gate location(s). In summary, shrinkage is a function of all of these variables. Thicker parts shrink more so, anisotropically, inch/inch, than do thinner parts. However, packing more weight of polymer into the geometry of an injection mold / molded part can generally yield slightly less shrinkage. Exceptions to conventional shrinkage rules lie with filled PP. Filler-modified PPs, i.e., calcium carbonate, talc, glass, etc., will not necessarily shrink proportionally or equally in relationship to any given weight % of filler that might be contained within the polypropylene material.
Rules-of-thumb:
Other significant control options include: material temperature, injection, mold and gate temperatures, time in the mold, product temperature at time of ejection. Premature ejection can cause high shrinkage rates and can cause warpage. The longer the cycle, the greater control and dimensional predictability.
Tradeoffs can occur between low to high mold temperatures, higher mold temperatures provide reduced stresses and better stability after forming. But, it results in longer cycle times. Colder mold temperatures result in faster cycle times. But, greater stress inducement is a resultant factor. The higher stresses can cause uncontrollable shrinkage as well as cracking (usually in the flow direction). Many of these factors are prevalent in thermoforming as well as injection molding.
Rules-of-thumb:
- General purpose PPs, including Homopolymer Polypropylene, Random Copolymer, Polypropylene, Impact Copolymer Polypropylene, HPP, RCP, ICP, respectively, shrink slightly slower than nucleated or clarified PP.
- Nucleated PPs shrink faster and slightly more than general purpose PPs.
- Clarified PPs (C-HPP & C-RCP) shrink the quickest in injection molding operations; to approximately 98 % of final dimensions and slightly more so in finished dimensions. Generally speaking, over time, shrinkage in relationship to the various components of PP can result in different shrinkage amounts (inch/inch).
Other significant control options include: material temperature, injection, mold and gate temperatures, time in the mold, product temperature at time of ejection. Premature ejection can cause high shrinkage rates and can cause warpage. The longer the cycle, the greater control and dimensional predictability.
Tradeoffs can occur between low to high mold temperatures, higher mold temperatures provide reduced stresses and better stability after forming. But, it results in longer cycle times. Colder mold temperatures result in faster cycle times. But, greater stress inducement is a resultant factor. The higher stresses can cause uncontrollable shrinkage as well as cracking (usually in the flow direction). Many of these factors are prevalent in thermoforming as well as injection molding.
Post a Comment:
You may also like:
Category
Featured Articles
How to Choose a Good Chinese Injection ...
It is very important to choose a good mold manufacturer, because they not only can provide you with satisfactory molds, but also ...

Precision Injection Mold Technology in ...
The development of precision injection molds, precision injection molding was first produced in the 1970s. With the development ...

Plastic Injection Molding Process ...
The propositions behind injection molding process simulation are very basic. First, that although our plastic materials and ...

Visit injection molding factory in ...
Not being in the injection molding industry, I was looking for tips on how to judge a company or factory on their capability or ...

Injection Mold Cooling Design
The design of the injection mold cooling system is very important. The cooling time takes up 70% to 80% of injection molding ...
