When designing any injection molded part, you need to consider the shrinkage and shrinkage of the material and the shape of the part. Plastic shrinkage is the dimensional change that occurs when a molded part cools after injection molding. Most part shrinkage occurs during the cooling phase while the part is still in the molding tool, but a small amount of shrinkage occurs after demolding as the molded part continues to cool, especially for Delrin and POM. Generally speaking, after the part is demolded, most of the excess heat has dissipated and most shrinkage problems have occurred. However, the part may continue to shrink slightly for hours or even days until the temperature and moisture content stabilize and match the surrounding environment. Therefore, for consistency and proper part stability, dimensional inspections should wait at least one day after the part is demolded.
Common Typical Shrinkage Rates for Injection Molding
Plastic injection molded part shrinkage is expressed in thousandths of an inch per linear inch (0.00X /in/in). Typical shrinkage rates vary between 0.001/in/in and 0.020/in/in, depending on the material, wall thickness, cooling rate, and other variables. The average is about 0.006/in/in.
Calculation method of injection molding shrinkage rate
The calculation method of injection molding shrinkage rate is mainly based on the difference between mold size and plastic part size.
- Basic formula: S=(D-M)/D×100%, where S is the shrinkage rate, D is the mold size, and M is the plastic part size.
- Simplified calculation: D=M+MS, or use D=M+MS+MS² when accurate calculation is required.
- Standard conditions: Usually measured after 24 hours at 23℃±0.1℃ and relative humidity of 50±5%.
How Model Compensation vs. Mold Compensation is Calculated
When calculating shrinkage, mold engineers simply scale the mold tooling by 1.00X. Before CAD, engineers would scale up the part to compensate for shrinkage by multiplying every number on the drawing by 1.00. At Elimold, shrinkage compensation occurs primarily during the moldmaking phase, so the stable part dimensions should be consistent with the CAD model specifications and part drawings. In other words, we compensate for shrinkage so that the part specifications match the model.
Plastic injection molding shrinkage also varies based on wall thickness. Material suppliers typically provide a material data sheet that specifies the shrinkage range for the material. For example, for a 0.100-inch wall thickness, the shrinkage is 0.005-0.007/in/in. Conversely, if the wall thickness is 0.100 inches during verification and inspection, the average shrinkage of the part along those walls is expected to be 0.006 inches. When developing parts with tight tolerances, it is absolutely critical to consider shrinkage during the mold design phase to ensure that the final part meets the model specifications.
Fine-Tuning Shrinkage Compensation
Moldmakers can fine-tune the shrinkage of a part by adjusting the density of the material (i.e., how strong the fill material is) and the length of the cooling time in the mold. If the part is large, the tolerances are critical, or a new or exotic material is being used, then we always recommend a mold test before starting production. Generally, the injection molding shop will help you improve your product design, which includes calculating the exact shrinkage of your part. The shop that has this capability and can provide this solution can reduce costs and time. Communicating product design for manufacturability with the injection molding shop is definitely worth the extra time and investment to perform. The cost is orders of magnitude lower than reworking or scrapping a tool due to a part being out of tolerance.
Asymmetric Shrinkage
Different plastic materials behave differently from one another due to the complex chemical composition of polymers. Therefore, it is critical to review the datasheet of the material before developing a tool. Materials with asymmetric shrinkage properties add another layer of complexity. These plastics shrink differently in one direction than in another.
For example, a polymer filled with long glass fibers will shrink more in the transverse (cross) direction than in the longitudinal (flow) direction. This presents a dilemma for the mold designer. The material supplier’s documentation will state that the X and Y axes shrink differently. This is not a problem for long straight parts like popsicle sticks or rulers, but it can be a challenge for parts with complex geometries.
If the part is complex, such as a hole and the flow front meeting at different angles and running in different directions at different locations on the part, it cannot be accurately calculated and modeled. Even if the desired level of confidence and reliability can be achieved, the time and cost of modeling the results will be an expensive and unnecessary study.
For example, the complexity even goes down to simple geometries like round holes. They will become oval in the tool due to the molten plastic flow and subsequent shrinkage relative to the fiber direction in the material. Standard components like core pins cannot be used.
An approximation of mold shrinkage is applied by averaging the shrinkage to balance the longitudinal and lateral shrinkage across the part. This allows for a good first round approximation and requires sign-off on the methodology by all parties as there can be variance in results. Critical features are then changed or added after the first shot based on analysis of material flow and shrinkage. The bottom line is, if tight tolerances are critical, try to avoid asymmetrical shrinkage resins.
The impact of mold shrinkage on product precision
Mold shrinkage has a great impact on the precision of plastic products. If the mold shrinkage is too large, the product size deviation will be too large, and problems such as flash and shrinkage will occur, affecting the quality of the product. If the mold shrinkage is too small, the product size will be too small, which will also affect the quality.
Therefore, when designing and manufacturing plastic molds, the impact of mold shrinkage needs to be fully considered. By accurately controlling the mold shrinkage, the accuracy and quality of the product size can be ensured.
Causes of Plastic Injection Molding Shrinkage Marks
Shrinkage marks may form in the plastic injection molding process for one or more reasons, including processing methods, component geometry, material selection, and plastic mold design. Geometry and material selection are usually determined by the raw material supplier and are not easily changed. However, mold manufacturers have many factors in mold design that may affect shrinkage in plastic injection molding processing. Cooling runner design Gate type and gate size may have various effects. For example, a small gate (such as a tube gate) cools much faster than a tapered gate. Cooling the gate too early will reduce the filling time in the mold cavity and increase the chance of shrinkage marks. For molders, adjusting processing conditions is one way to solve the problem of plastic processing shrinkage. Filling pressure and time have a significant impact on shrinkage. After the part is filled, excess material continues to fill the mold cavity to compensate for material shrinkage. If the filling phase is too short, shrinkage will be aggravated, resulting in more or larger shrinkage marks.
How to control the shrinkage rate of plastic products molded by injection molding?
Controlling mold shrinkage rate can start from the following aspects:
- Select the appropriate plastic material: Different plastic materials have different shrinkage rates. Selecting the appropriate material can reduce the impact of mold shrinkage rate on product precision.
- Design a reasonable mold structure: Reasonable mold structure design can reduce the impact of mold shrinkage rate on product precision.
- Accurate mold manufacturing: Accurate mold manufacturing can ensure the accuracy of mold size, thereby reducing the impact of mold shrinkage rate on product precision.
- Select the appropriate molding process: The appropriate molding process can reduce the impact of mold shrinkage rate on product precision. For example, adjusting molding temperature, pressure and other parameters can improve the shrinkage of the product.
- Use mold flow analysis software: Mold flow analysis software can simulate and analyze plastic molds, predict mold shrinkage rate and optimize it, thereby reducing the impact on product precision.