Shrink can be a tricky thing to navigate, and in essence is defined as the ratio of the component's designed dimensions to their post molded ones. There are some factors to consider when it comes to how plastic shrinks during and after molding. Hopefully we can shed some light for you in this article.
1. Material Type
The type of material will have its own shrink value. You can obtain these values from the manufacturers of the resin. Although you have this value, the classification of plastic is important. There are 2 main classifications of plastics that you must understand: Amorphous and Crystalline.
Common Plastics and their Estimated Shrink Values
Plastic Type Shrink in Inch per Inch
Acrylic Gen Purpose 0.020-0.035
Acrylic High Heat 0.003-0.010
Nylon 6/6 0.010-0.025
Nylon 6 0.07-0.015
Nylon 6/10 0.010-0.025
Nylon 12 0.08-0.020
PET - Amorphous 0.003-0.005
PBT 30% Glass Filled 0.001-0.002
Polyethylene Low Density 0.015-0.035
Polyethylene High Density 0.015-0.030
Polystyrene Gen Purpose 0.002-0.008
Polystyrene High Heat 0.002-0.008
Polystyrene High Impact 0.003-0.006
PVC Rigid 0.002-0.004
PVC Semi Rigid 0.008-0.025
PVC Flexible 0.002-0.006
Amorphous- The common plastics that fall into this category are Polystyrene, Polycarbonate, PMMA (Acrylic), ABS, and PVC. The amorphous group tends to not have an organized polymer structure and all of the shrinkage tends to occur due to thermal contraction. In other words, because of the nature of the molecular change, cooling within the mold will cause this material to contract. The contraction tends to be more uniform in nature and typically on the smaller side. For example, the typical shrinkage value for ABS is anywhere between .004-.007 inches per inch of part length. This means that a 1 inch part will shrink to about .996" to .993". This is relatively minimal in relation to crystalline materials which can have much higher shrink rates. Unlike crystalline materials, amorphous plastics typically have a very small dimensional change once after molding has commenced.
Crystalline- The common plastic types that fall into this category are Acetal, Nylons, Polyester, Polypropylene, and Polyethylene. Crystallines tend to have a more structured molecular organization scheme than the amorphous group. The shrinkage here tends to be greater due to the fact that thermal contraction and re-crystallization has to occur. These materials can shrink anywhere from .010" to .030"per inch of part length. This means that a 1 inch part can shrink to a final length of .970". It is also important to note that crystalline materials have a tendency to shrink at a higher rate in the direction transverse to the flow of material as opposed to the direction of flow. Crystalline materials will typically continue shrinking beyond molding. In fact, approximately 90% of the shrinkage occurs within hours of molding, and 10% occurs up to 48 hours after plastic injection molding.
Fiber filled materials typically show less shrinkage in the direction of flow.
Because there are other important factors at play, shrinkage values provided by material suppliers can only really be used as general guidelines.
2. Part Design
It is important to select materials that minimize your shrink deformities. Part design also has a large part to play in controlling shrink. For a plastic injected part, it is important, if possible, to have uniform wall thicknesses. The reason for this is that thicker sections of the plastic part will tend to want to shrink more than the thin walled sections. Overall part dimensions can also affect the shrink rate. When designing a part, it is important to keep in mind the tolerances for non-critical dimensions. It is standard to be able to hold some of the critical dimensions in a part, but it is good practice to tolerance in such a way that allows for "non-critical" dimensions to have room to shrink.
3. Tooling/Mold Considerations
Mold designs are important for ensuring that the material flows correctly within the cavity and that the part is able to fill. It is imperative to place the gate in an area where the flow of the molten plastic will fill the cavity efficiently before cooling prematurely hardens your part. Uneven cooling has a lot to do in determining how your part will shrink.
Cooling rate and mold temperature has a significant effect on shrinkage in plastic injected parts; however, it is particularly critical in crystalline materials. There are 3 types of cooling methods for exchanging heat: Radiation, convection and conduction. Hot plastic enters the mold and the heat moves by convection through the plastic until it comes to the surface of the mold. The heat is then conducted through the mold to the water cooling channels and then out of the mold. A considerable amount of heat also reaches the outside of the mold and is released to atmosphere through radiation. Approximately 80% of the cycle time is the cooling phase. The placement and design of the cooling lines are an integral part of the mold design and are critical for ensuring uniform cooling and shrinkage of plastic injected parts. Mold designers should understand the importance of this aspect of plastic injection molding.
In addition to cooling lines, a mold with a highly conductive steel will decrease shrinkage deformities while things like long flow paths, small gate areas, insufficient cooling, and low clamp force can contribute to increases in shrink rates.
4. Processing Considerations
Shrinkage values vary considerably with the level of packing. Longer cycle times constrain the part in the mold longer and reduces initial shrinkage. Typically, the longer the holding pressure and holding time is sustained, the more control can be maintained over the shrinkage. Unfortunately, minimizing the cycle time is more cost effective for manufacturing, so a happy medium is where plastic injection molders like to land. Melt temperatures of the material also need to be monitored to maximize the efficiency and uniformity of cooling. The packing and holding pressure phases of the molds cycle time are used to compensate for shrinkage, and most often the plastic injection molder will be able to determine the best packing and holding time to minimize dimensional disturbances caused by shrink.
In short, it is important to work with a plastic injection molding company that knows what they're doing with regard to controlling shrinkage through scientific processing methods. Many people can operate a plastic injection molding machine, but few have the expertise to get your parts correct the first time. This is why it is important to do your homework when choosing a plastic injection molder that will work for you and your company.
For more information involving control of part dimensions in light of shrinkage considerations, please feel free to contact us and our expert molding staff would be happy to help.
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