Introduction
Think mold shrinkage does not affect part quality? Think again. In injection molding various factors affect the part quality of a molded part. However, some factors like mold shrinkage have a more significant impact on the quality of these parts.
Mold shrinkage essentially depicts a reduction in part volume or size resulting from the contraction of molten plastic materials as it cools and solidify. But the importance of mold shrinkage is far reaching especially when it comes to mold design and part quality.
By the end of this post, you will have gotten all the information you need on the types and impacts of mold shrinkage and the strategies to control it. So, without much ado, let’s dive in.
Causes of Mold Shrinkage
To begin with, Several factors are responsible for causing mold shrinkage. Nevertheless, they can be categorized into three major groups including material properties, processing conditions, and part design.
- Material Properties
When selecting materials for injection mold, it is essential to be familiar with the properties of these materials to understand their interaction with injection molding parameters like heat and pressure.
This will help predict a material’s shrinkage rate and choose a suitable material that conforms to the objective and design specifications of the project. Incidentally, injection mold uses many types of plastics including thermosetting and thermoplastics which have different shrinkage rates.
For instance, Polypropylene, Polyethylene Terephthalate, and HDPE which are semi-crystalline plastics have lower shrinkage rates than ABS, PMMA, and PC which are amorphous plastics. Furthermore, reinforcing polymers with fillings like fiberglass can help lower the shrinkage rate with better dimensional stability for plastic parts.
- Processing Conditions
Another cause of mold shrinkage comes from injection molding processing. With the presence of heating and injection pressure constantly applied to molds and polymers, the tendency for significant contraction during cooling becomes high.
For a better understanding, consider polymers under very high melting temperatures which signifies a lower viscosity. Consequently, polymers will have more fluidity in a molten state and a higher shrinkage as they harden.
Also, increasing injection pressure leads to increased filling speed which is a leading cause of shrinkage. Hence, optimizing injection pressure and temperature often gives the best outcome of minimal shrinkage.
Additionally, reducing the cooling rate also helps to mitigate against excessive shrinkage leading to better dimensional stability and part quality.
- Part Design
One consequence of varying a mold’s wall thickness is shrinkage. Think of it as the spark that lights a bonfire. Naturally, uniform wall thickness allows the free flow of molten polymers with hindrances. However, the introduction of differences in the wall thickness leads to disparities in things like flow speed and pressure which also affects the cooling rate of the molten material. The result is the rapid cooling of thin areas while thicker areas take longer to cool, introducing shrinkage to the whole process. Hence, you’ll need to account for the expected shrinkage based on the disparities in wall thickness for your plastic part design. Using software to simulate the flow through those constrained areas can come in handy in measuring shrinkage.
Effectively managing these factors is essential to ensure the final product meets the desired dimensional accuracy and performance standards.
Impact of Injection Mold Design on Shrinkage
Shrinkage rate is an important factor to put into consideration when designing injection molds considering its significant impact on the quality of the part. However, it is important to identify the necessary considerations designers need to carefully examine to deliver the most suitable mold design that eliminates excessive and unequal shrinkage and its negative consequences.
Firstly, mold designers need to critically consider the compatibility of the part material as well as the mold material as this combination can either break or bake a flawless plastic part. This is because mold cooling needs to support the plastic part material to ensure that one of the materials does not excessively cool before the other. This balance in cooling ensures that the plastic material in the mold has adequate time to cool and solidify without resulting in excessive shrinkage which comes with dimensional inaccuracies and defects like warpage.
Also, the injection pressure and melting temperature are crucial to ensure that mold shrinkage does not occur. Thus, optimal values should be specified by designers to ensure effective injection mold design that delivers superior part quality.
Furthermore, ensuring uniform wall thickness will help the molten material flow within the mold with ease allowing for uniform shrinkage when the part is demolded. Incidentally, knowing the shrinkage value is essential in compensating for expected shrinkage during the design of the mold.
Types of Shrinkage
Identifying the type of shrinkage encountered during an injection molding process is important in deciding on the best approach to tackling this challenge.
- Linear Shrinkage: First there is the linear shrinkage which often occurs as a result of extensional and shear forces acting on the fluids as they flow inside the mold. These forces often cause the plastic molecules to shrink along the flow direction of the polymer resulting in a linear shrinkage.
- Volumetric Shrinkage: In the case of volumetric shrinkage contraction is the causative agent for this type of shrinkage as it solidifies reducing the overall volume of the plastic part. Frankly, plastics can shrink up to three-quarters of their original size causing massive dimensional inaccuracies and sometimes excessive reduction in part density.
- Directional Shrinkage: This is a variation of the linear shrinkage. However, in this case, rather than shrinking parallel to the direction of polymer flow, plastic molecules will shrink either in radial orientation or in extensional orientation. This also determines the orientation of the warpage.
Methods to Measure Shrinkage
There are essentially two methods which technicians and designers use in measuring shrinkage including direct measurement technique and the use of shrinkage prediction software.
- Direct measurement techniques: This method often requires arranging a prototype of the gating and cooling scenarios for the actual mold and measuring the shrink rate from this setup. This method is very accurate as a prototype often depicts the actual parameters including the flow rate, pressure, and temperature specified for the injection molding.
- Use of shrinkage prediction software: Alternatively, using advanced CAD software with capabilities for simulation can also be very effective. This software can simulate the actual parameters specified for the injection molding to determine the shrinkage rate.
Strategies to Control Mold Shrinkage
Several strategies are available to help you control mold shrinkage with many of them easily applied. However, the core of the strategies centre around material selection, processing parameters, and mold design adjustment as highlighted below.
· Material Selection: Material selection is usually the first point of call when there is a need to control shrinkage. Selecting a suitable material with a known shrinkage rate significantly helps to minimize shrinkage as compensation can be made during part design for the known shrinkage rate of the material. Additionally, ensuring you use material with a low shrinkage rate such as semi-crystalline plastics such as PET, PP, and HDPE can immensely help in significantly reducing mold shrinkage.
· Processing Parameters: The need to regulate processing parameters including pressure, temperature, and cooling time is both critical and appropriate considering the significant impact it has on the shrinkage level. First, slightly increasing the melting point can help with polymer flow within the mold. However, when the temperature becomes excessively high there is the tendency of replacing easy and faster fluid flow with reduced part quality. Defects such as sink marks and warpage which are results of shrinkage can begin to show up.
· Mold Design Adjustments: Arresting mold shrinkage from its foundation is often the best way to prevent its occurrence. Thankfully, advancement in technology makes room for easy and accurate mold adjustment during design.
The use of applications like Computer Aided Design (CAD) often goes a long way to help adjust mold design parameters. Some applications will even afford designers the ability to simulate fluid flow to accurately estimate the level of shrinkage to expect.
Especially with complex designs, the need to add features like draft angles, ribs, and uniform wall thickness are effective ways to stop mold shrinkage in its tracks.
But that’s not all.
The use of manufacturing constants like shrink factors as well as employing expansion cores are other effective ways to reduce mold shrinkage. All of these strategies are very efficient proactive steps in avoiding shrinkage and producing a flawless and defect-free, plastic part.
Role of Injection Molding Manufacturers
To achieve a perfect injection molded part, various personnel involved in the process have to take their responsibilities seriously. Still, the bulk of the responsibility for avoiding excessive shrinkage and its challenges lies with mold manufacturers.
Firstly, collaboration is a fundamental role of any injection molding manufacturer. It’s not just enough to be one of the best, the collective knowledge of a community of experienced injection molding manufacturers can make a huge difference, especially in a world of rapid technological advancements. This connection will help manufacturers stay ahead with technological information, techniques, and strategies for tackling mold shrinkage and its negative effects. This can also positively impact the level of part quality manufactured.
Additionally, having a team of experienced engineers and technicians is key to maintaining a high level of quality manufacturing as their training and extensive experience can come in handy for innovative ways to solve manufacturing challenges.
As an example of an experienced and reliable injection molding manufacturer, TDL has been able to proffer so many innovative solutions to mold shrinkage issues in their many productions. One such is the automotive project involving the production of door handle covers where precision and dimensional accuracy were at the forefront of the project objective. The use of a careful material selection process coupled with the use of computer software for mold flow simulation and data collection for real-time shrinkage analysis took center stage. This helped the experienced engineers and designers to optimize production parameters including melting temperature, injection pressure, and mold cooling time.
In the end, a flawless door handle cover that met all design specifications of functionality, dimension accuracy, and aesthetics.
Common Challenges and Solutions
Once shrinkage occurs in an injection molding process, several common challenges accompany it. These anomalies fall under warping and distortions or dimensional inaccuracies.
· Warping and Distortion: It can be worrisome when demolding a plastic part only to find that it has been distorted. Shrinkage due to non-uniform wall thickness is often the cause of this type of challenge. Consequently, allowing for more consistent wall thickness during design and uniform cooling across the mold is often the best way to mitigate against warpage and distortions.
· Dimensional Inaccuracy: Dimensional inaccuracies are another common challenge of shrinkage. This anomaly comes from excessive shrinkage resulting from inadequate time for cooling as stress builds up in the mold during the cooling stage causing a sharp contraction. Hence, providing adequate cooling time for plastic will exempt these parts from dimensional errors resulting in undersized parts.
Conclusion
At the end of the day, understanding mold shrinkage helps to design a suitable mold using optimal parameters to control excessive mold shrinkage as well as its negative consequences. Essentially, this knowledge is key to controlling shrinkage and helping to deliver part quality that meets design specifications and end-user satisfaction. More importantly, understanding that selecting a suitable material is pivotal to part production quality. Additionally, it combines a vigorous mold and plastic design with parameter optimization to get a flawless plastic part. Nevertheless, the ability to take advantage of advanced simulation software can be instrumental in accurately predicting shrinkage rate which is then used to optimize plastic part design for an excellent output.
