Injection molding shrinkage simply refers to how much the polymers will contract during the cooling phase. During the injection molding process, molten plastic material is injected into a mold cavity under high pressure, and when the plastic material cools, it shrinks as it solidifies. This shrinkage can cause the molded part to be smaller than the intended size, and have an uneven shape known as war-page.
Injection molding shrinkage can vary depending on the type of plastic material used, the design of the part, and the processing conditions. It is important to take into account the expected shrinkage during the design and tooling stages to ensure that the final part meets the required specifications.
To be safe, manufacturers often compensate for shrinkage by oversizing the mold cavity or adding additional material to the part, which is then trimmed or machined after molding to achieve the desired dimensions.
Mold Shrinkage Rate
Before beginning production, the mold shrinkage rate should have been predicted to eliminate the additional cost and adjustments. All plastic materials have a minimum and maximum shrinkage percentage, which helps you evaluate the changes in the plastic from when it’s molded to when it will be completely cool. This and other factors that influence the shrinkage rate should be considered.
Types of Plastic Injection Molding Shrinkage
There are about four types of plastic injection molding, each caused under unique conditions; they include the following.
Regional Shrinkage: This is when the rate of contractions varies on the product parts that are between the ones closest to the mold gate and the regions that are nearest to the end of the fill area. This causes the part that’s near the gate to be thicker while the other end to become a little thinner. Therefore, the thinker parts pull in the thinner part, and this causes a warped shrinkage.
Thickness Shrinkage: This shrinkage takes place when the top and the bottom areas of the same mold differ in the rate of cooling. It is a situation where the top part cools faster than the lower part, and this variance causes the mold part to bow towards the cold side, shrinking more and more until that new shape is frozen into place.
Directional Shrinkage: This is a result of a variance in the molecular and fiber alignment. The shrinkage manifests itself parallel and perpendicular to the mold material’s orientation and the direction of flow. The material, therefore, shrinks more in the direction that the flow of the material takes.
In-Plane Shrinkage: It is common for polymers to shrink a little more strongly in the thickness direction than the surface plane. This is caused by a condition called mold restraint which is basically the lack of restraint in the thicker part. This difference in shrinkage causes serious warping from the corners, and it can render the product damaged and unfit for use.
How do You Calculate Mold Shrinkage?
The most convincing method to calculate the plastic injection shrinkage rate is to develop a prototype tool to mimic the cooling and gating requirements that’s used during production. Not only will this technique provide an accurate shrinkage measurement, it will also allow for the opportunity to change the tooling before the production begins in case a potential defect was predicted.
The following steps outline how to calculate the Mold Shrinkage:
First, determine the original dimensions of the mold.
Next, determine the dimensions of the molded part.
Third, subtract the dimensions of the molded part from the original dimensions of the mold.
Finally, divide the difference in dimensions by the original dimensions of the mold and multiply by 100 to get the Mold Shrinkage percentage.
After calculating the Mold Shrinkage percentage, check your answer with the given specifications.
What cause mold shrinkage
In plastic parts manufacturing, shrinkage is a normal process in injection molding that refers to how much the polymers will contract during the cooling phase. The primary cause of shrinkage is the transition from the density of melted plastic to the density of its cooled, rigid state. While it occurs during the cooling stage, minimal shrinkage can continue after the part’s ejection as the moisture and temperature stabilize. If areas of the part shrink unevenly – known as warpage – it can cause severe part defects. Beyond the shifting of the polymer’s density, other factors facilitate variable shrinkage, as well.
Other Causes of Injection Molding Shrinkage
Shrinkage is influenced by plastic composition and the material’s properties, and the processing conditions – such as flow rate and temperature – and part design can also contribute. Let’s review some of these causes:
Plastic Composition: Semi-crystalline plastics like polyethylene have a higher shrink rate than amorphous plastics, such as ABS. The molecular makeup of semi-crystalline plastics forms crystallite – a smaller, more compact structure than amorphous polymers, like polycarbonate.
Molecular Weight: Resins with a high molecular weight tend to have higher viscosity when filling an injection mold and a higher pressure drop while in the cavity. This pressure low pressure can lead to higher shrink rates in the final part.
Additives: Fillers, such as glass and ceramic fibers, have a low thermal expansion, so during the cooling process, they tend to shrink less.
Stress Level: During the molding process, the time and temperature at which the part is placed under stress can cause an excessive amount of fluid which can result in too much shrinkage and brittleness.
In addition to these causes, other variables include injection pressure, melt and mold temperatures, and the geometry of the part. Since there are a lot of factors that can influence shrinkage, it’s crucial to work with a manufacturer who can accurately calculate the part’s shrink rate to avoid defects.
How to control shrinkage during injection molding?
Every material has a shrinkage rate that is given by its manufacturer. This can be used to help predict the changes in the plastic from the time it’s molded and after it has cooled completely. Any material expands when heated and shrinks as it cools back down to room temperature. Every dimension of the plastic product will shrink a certain amount during its cooling period. Controlling this shrinkage could be the key to perfecting your final product. Let’s talk about some of the ways shrinkage can be controlled during the injection molding process.
Temperature of material
Adjusting the temperature of the plastic resin as it is heated is important for shrinkage control. The higher the material is heated before it is poured, the more the molecules expand. As it cools these molecules shrink back down. The lower the temperature of the plastic is at the time of pour, the less shrinkage will occur during the cooling process.
Temperature of mold
Controlling the mold temperature can control shrinkage. Using a cold mold allows the outer edges of a part to dry before it can fill and compress the entire space properly. Using a hot mold will create less shrinkage than a cold one. It allows the molecules of the plastic material to continue to move freely as it fills the mold and achieves the correct pressure before it begins to cool.
Pressure adjustments
The force of pressure used to inject the plastic material makes a direct difference when it comes to shrinkage rates. It is the pressure required to pack the material into place. The tighter material is packed, the less allowance there is for movement as it cools. The higher the pressure at injection, the less the plastic will shrink.
As long as pressure is applied until the plastic is solidified the shrinkage will be limited. If the pressure is released before the plastic has cooled completely, the shrinkage will worsen. Keeping the plastic restrained in place as it cools to reverse its usual shrinkage pattern controls the shrinkage but the process takes longer and costs more. Forcing air over the plastic parts also helps stabilize them.
Cold water immersion
Another way of quickly cooling a plastic part is to drop it in water that is room temperature. This cools the material below its melting point and stops the post-mold shrinkage. This helps the inner walls of the plastic solidify faster as these areas take longer to cool and solidify than the outer walls do. It is a bit risky because of the stress it causes on the product. It may cause a fracture or crack if the plastic is exposed to extreme temperatures down the line.
Determining how a plastic product will shrink and bend as it cools is important to get a perfect final product. Finding ways to control the way the material cools helps ensure that your parts come out the way they should every time. You want consistent results when it comes to injection molding projects. Formulation of materials, mold dimensions, and processing details will all affect shrinkage. Let we find the best way to control shrinkage and help you achieve the best results possible for your next injection molding project.
Conclusion
Injection molding shrinkage is a common issue in plastic part manufacturing. But it can be avoided by following some steps, which include the assessment of the resin shrinkage rate to ascertain the reliability of the polymer, the composition of the polymer, the injection molding process, and the plastic part shape. All these elements play a critical role in preventing the occurrence of shrinkage.
Overall, understanding the causes and how to solve shrinkage in plastic injection molding is a great determinant to the success of every injection molding process.
