Controlling Shrinkage
By Douglas M. Bryce
What Is Meant By "Shrinkage"?
All materials have a specific "shrinkage rate" value assigned to them by the material manufacturer. The use of the term "rate" is actually a misnomer because it implies that the shrinkage occurs as a function of time, which is not true. Regardless of this inaccuracy we will use the term because it has become accepted throughout the industry. Shrinkage rate is a value that can be used to predict how much difference there will be between the plastic product when it is first molded and the plastic product after it has cooled to room temperature.
Everything (except water) expands when it is heated and shrinks when it is cooled. Plastic material is no exception. Each plastic material has a distinct value for how much it will shrink after it is heated and then allowed to cool. This value is referred to as the shrinkage rates and is listed as so many "inches per inch." That means that for each inch of dimension on the plastic product, the material will shrink a certain amount of inches. Usually, these materials shrink somewhere between 0.000 inch/inch up to approximately 0.050 inch/inch. Shrinkage can also be thought of in terms of percentage. A part that has a 0.010 inch-per-inch shrinkage will shrink a total of I %. One with a shrinkage rate of 0.020 inch-per-inch will shrink 2%. One with a 0.005 rate will shrink 1/2 of l %.
All plastics are generally categorized as having either low, medium, or high shrinkage. Low shrinkage is within a range of 0.000 to 0.005 inch/inch. Medium shrinkage is within a range of 0.006 to 0.010. High shrinkage is anything over 0.010.
It is important to understand the relation between shrinkage and amorphous versus crystalline materials. Amorphous materials tend to have low shrinkage rates and the shrinkage occurs equally in all directions. This is called "isotropic" shrinkage. Crystalline materials tend to have high shrinkage rates and the shrinkage is greater in the direction of flow than across the direction of flow. This is called "anisotropic" shrinkage. An exception to this anisotropic rule exists when using reinforced materials. These will shrink less in the direction of flow and more across the direction of flow. This is due to the orientation of the reinforcement fibers.
Because of the inherent differences between amorphous material shrinkage and crystalline material shrinkage, there is a greater range of shrinkage control for amorphous materials. Crystalline materials have a tendency towards higher shrinkage rates in general, but have much less response to processing parameter changes toward shrinkage control. The following information, while general, applies more towards amorphous materials than crystalline.
The Effects Of Temperature Adjustments
One way of altering the amount of shrinkage for a specific product or material is to adjust the temperature of the plastic while it resides in the barrel. In general, the higher the plastic temperature, the greater the amount of shrinkage. This is because of the activity of the individual plastic molecules; as the temperature rises, these molecules expand more and take up more space. The higher the temperature, the greater the expansion. The reverse of this is also true; the lower the temperature, the lower the degree of expansion, therefore the lower the amount of shrinkage as the plastic cools.
A general rule-of-thumb is that shrinkage rates can change 10% by changing barrel temperatures 10%. Thus, if a material exhibits a shrinkage rate of 0.005 in/in. at a barrel temperature of 500 degrees, it can be lowered to 0.0045 or raised to 0.0055 by altering the barrel temperatures to 450 or 550 degrees respectively. These are extreme changes and may not be practical for other reasons, but they represent the 10% rule-of-thumb.
Shrinkage can be adjusted by altering temperatures of the mold also. A hot mold will create less shrinkage than a cold mold. This is because the cold mold solidifies the plastic "skin" sooner than a hot mold, resulting in a shrinking of plastic before full injection pressure is applied. On the other hand, a hot mold allows the molecules to continue to move and be compressed by injection pressure before solidifying. This results in less shrinkage because the molecules are not allowed to move as much after solidifying. A rule-of thumb here is that a 10% change in mold temperature can result in a 5% change in original shrinkage.
The Effects Of Pressure Adjustments
Injection pressure has a direct effect on shrinkage rates. The higher the injection pressure, the lower the shrinkage rate. This is because the injection pressure packs the plastic molecules together. The higher the pressure, the tighter the molecules are packed. The more they are packed, the less movement they are allowed as they are cooled. This lower movement results in lower shrinkage. The pressure rule-of-thumb states that a 10% change in pressure can cause a 10% change in shrinkage rate. Of course, the shrinkage is controlled only for as long as the pressure is applied. As long as the pressure is applied until the plastic has cooled to its point of solidification, the shrinkage will be controlled. If the pressure is relaxed before that point, the shrinkage will increase because the molecules have been allowed to move again.
Post-Mold Shrinkage
There is a constant battle between maintaining the quality of a molded product and reducing the cost of molding that product. Controlling the shrinkage is only a part of that battle, but it should be understood that the lower the desired amount of shrinkage, the longer the cycle, and the higher the cost. Of course the opposite of this is also true. In fact, under certain molding conditions, once the part is out of the mold it may continue to cool and shrink for up to 30 days! Admittedly, the first 95% of the cooling and shrinking takes place within the first few hours after removal from the mold, but that last 5% can take up to a month to stabilize and finalize. Even if the shrinkage is controlled to achieve that first 95% through molding parameter adjustments, the theoretical cycle time could evolve into 10 minutes for a part that we know should normally run at a 30 second cycle. One way of minimizing the cycle while controlling the shrinkage is to control the shrinkage after the product is ejected from the mold instead of while it is still in the mold. The cycle time can be reduced, thus the cost of molding can be reduced. This is what post-mold cooling and shrinking is all about.
Post-mold shrinkage is normally controlled by restraining the molded product in a fixture that holds it in place while it cools. The product is purposefully bent and bowed in directions opposite the normal shrinking and cooling patterns that develop when a part cools. This is to over-compensate for shrinkage so the part will spring back after cooling to a shape that is desired. This must be done through trial-and-error by measuring cooled parts to determine how to adjust the fixture to give the desired results.
When using post-mold cooling/shrink fixtures, it is necessary to leave the cooling product in the fixture for the equivalent time of approximately 6 full cycles. Therefore, it is necessary to have at least 6 fixtures, or 'stations" in place at all times. Forcing air over the parts helps stabilize them.
Another method of post-mold cooling is to simply drop the molded parts in a container of cold water. The temperature of the water must be maintained at below room temperature (approximately 60 degrees) but there is no advantage in having it lower than that because once the entire plastic mass drops to a temperature below its melting point or glass transition point, it will not continue to shrink. The post-mold cooling is only being done in an effort to effect that cooling for the center portions of the walls, which take longer to reach that point of solidification than the external skin of the walls.
There is a danger in using any method of post-mold shrinkage control because the practice does induce varying degrees of mechanical stress to the molded product. This stress is caused by the forcing of molecules into positions that they are not seeking on their own. When this is done, stress is concentrated on the molecules that are being stretched and compressed. This stress is maintained as the part cools and is locked-in after the part has fully cooled and shrunk. Then, if it is ever exposed to extreme temperatures or mechanical abuse, the stress is relieved and the product may fracture, crack, or shatter depending on how much stress was induced during the post-mold cooling operation.
Douglas M. Bryce
Chief Consultant
IPLAS
dbryce@iplas.com
http://www.iplas.com
012003A © 2003 IPLAS/Douglas M. Bryce