Introduction
Mold design marks a pivotal phase in the early planning stages of injection molding, and any mistakes in moldability could severely impact the success of your injection molding endeavors. Furthermore, molds vary in shapes and sizes, crafted from different metals, with each boasting its own distinct characteristics and advantages. However, understanding common mistakes to avoid during mold design can elevate precision, quality, and performance in your mold. These can include material selection for the mold and shrinkage calculations which are critical considerations during the mold design phase. This post zeroes in on these critical aspects, aiming to furnish you with insights to bolster your moldability process, whether you’re delving into standard or custom injection molding. So, if you’re ready, let’s delve into it.
Mistake 1: Ignoring Material Selection
In any injection molding procedure, the mold acts like the negative of a photograph, transferring its geometry and surface texture directly to the injected part. Typically, this tool represents the bulk of the initial costs in injection molding. However, investing in a quality mold ensures cost-effectiveness, as it can be used multiple times in mass-manufacturing plastic parts before requiring repair or replacement.
Nonetheless, crafting a high-quality mold demands a high level of expertise in design and manufacturing to accurately produce thousands or even hundreds of thousands of parts. This is where materials selection comes in. The fact is that material selection is a crucial aspect of injection molding, as the wrong material can result in product defects, decreased performance, and increased production costs.
Again, it’s important to choose materials suited for the application, considering factors like environmental conditions and performance requirements. More importantly, fragile or soft materials, neglecting humidity effects, or using incompatible materials can have negative consequences on the quality of the plastic part.
Hence, before starting the injection mold manufacturing process, it’s essential to know the type of plastic product to be made and the material to be injected into it.
Mistake 2: Overlooking Wall Thickness
One of the essential factors to consider during the design phase of injection molded parts is wall thickness. It is an essential consideration as wall thickness can influence several crucial characteristics of a component, including its performance, aesthetics, and cost. Therefore, determining the nominal wall thickness based on functional performance requirements is necessary. Also, you must consider the allowable stress and expected life of the molded part to establish the minimum wall thickness.
The general rule is to use a uniform wall thickness on all injection molded parts, typically between 1.2mm and 4mm. However, the minimum wall thickness depends on part size, geometry, structural requirements, and resin flow behavior.
Still, choosing the proper wall thickness for your part can have drastic effects on manufacturing cost and production speed. But while there are no wall thickness restrictions, the goal is generally to choose the thinnest wall possible. Naturally, thinner walls use less material, reducing cost and cycle time and thick sections take longer to cool than thin sections.
During the cooling process, inconsistent thickness can lead to warping, twisting, or cracking, especially if thick and thin sections meet. To avoid this, design with uniform walls throughout the piece or ensure gradual thickness transitions with wall thickness variations not exceeding 10% in high shrinkage mold plastics.
Mistake 3: Neglecting Proper Draft Angles
A draft angle is essential for smoothly removing a part from the mold. Also, it prevents a puller pin from sticking to your parts, especially on cavity or display surfaces. Furthermore, draft angles and tapers reduce the risk of damaging parts during ejection, known as drag, particularly at the parting line.
Typically, a minimum draft angle of half a degree is recommended, with one and a half to two degrees preferred, especially when adding textures.
Problems arise when draft angles are absent, leading to friction and voids that can damage the part and affect the mold. This can result from plastics shrinking before cooling, causing some parts to come loose while others grab the core.
While there’s no single draft angle suitable for all parts, specific guidelines help determine the appropriate angle for each part:
- For parts up to 50mm deep, a draft angle of 1½ to 2 degrees is often sufficient, preventing damage during material shrinkage.
- Larger or deeper parts require an additional degree of draft angle for each inch of depth to accommodate the increased surface area.
- Textured parts need more draft to account for intricate textures, adding to the ideal 1½ to 2-degree angle.
- Draft angles should follow the mold’s direction, moving up or away during separation, especially towards the “top” of the mold.
- Parts with complex geometries or stress relief features like stiffeners and ribs must have draft angles.
Mistake 4: Underestimating the Importance of Rib Design
Ribs serve to reinforce molds, providing greater resistance and rigidity without increasing wall thickness.
To minimize sinking, ribs are typically designed to be about 60% of the nominal wall or the wall they are joining. Again, for glossy materials, a thinner rib, around 40% of the wall thickness, is often used. However, thin ribs can be challenging to fill, especially with added draft angles.
Proper rib design considers five key parameters:
- Thickness – which affects cooling rate and contraction, leading to potential warping.
- Rib height – which should not exceed three times the thickness of the base.
- Location – as ribs in non-critical areas may reduce impact resistance.
- Quantity – with fewer ribs being preferable as they are difficult to move once added.
- Moldability – with thin ribs posing filling challenges, often requiring mold flow and manufacturing analysis.
For a well-designed rib, important tips include considering the manufacturing process, keeping ribs short to reduce sinking, and maximizing rib root radius to avoid stress concentration.
Moreover, ribs should evenly distribute plastic flow to maintain consistent wall thickness and spacing between ribs to reduce stress concentration. Again, notches or steps in rib design should be avoided, as they can lead to stress concentration and cracking.
However, careful consideration of design angles is necessary to balance ejection ease and part insertion. Furthermore, stacking ribs toward the highest draft angle minimizes warping risk.
But that’s not all.
Using thicker wall thickness at the rib root prevents deformation during assembly.
Designing ribs requires attention to detail and adherence to various factors. Following these guidelines ensures ribs are appropriately designed for manufacturing and function.
Mistake 5: Inadequate Venting
In the injection molding process, it’s crucial to release trapped air from the mold cavity. Failure to do so can result in incomplete filling or even burn the molded part. Venting the Injection mold properly, including setting the vent and determining their number, directly impacts part quality. Insufficient venting can lead to various molding defects, such as burnt spots, weak weld lines, and poor surface finish.
Moreover, incomplete filling, irregular dimensions, and local corrosion of the mold cavity surface are common issues caused by inadequate venting. The number and location of vents must be carefully considered during mold design. Additionally, vent size depends on the material being injected, with higher viscosity materials requiring larger vent dimensions.
Vents can be placed anywhere along the parting line, typically at the end of the last areas of fill, with a theoretical spacing of every 25mm. However, in practice, this distance may be longer. In addition to vents along the parting line, vents can also be incorporated into the ejector system by utilizing the clearance between the pin and the hole. This shape can also be integrated into the mold insert itself, further aiding in effective venting. Proper venting ensures smooth production and high-quality parts.
Mistake 6: Forgetting About Shrinkage and Warpage
Shrinkage, a common occurrence in injection molding, happens during cooling when the product’s volume decreases compared to its molten resin form. Mold designers typically calculate shrinkage using methods like average shrinkage or extreme value. Yet, the primary cause of shrinkage often lies in the resin properties chosen for the project, as different materials exhibit varying rates of shrinkage.
Understanding the properties of the chosen material is crucial in managing and predicting shrinkage. Additionally, molding temperature plays a significant role, with higher mold temperatures generally leading to increased shrinkage. Striking a balance is essential to maintain quality without compromising the final piece.
Thicker walls tend to shrink more due to longer cooling times, necessitating optimized part designs to mitigate shrinkage effects. Inadequate pressure can also result in density variations, contributing to shrinkage concerns.
To address shrinkage and warpage effectively, several strategies can be employed.
Firstly, careful material selection before production begins allows for the choice of polymers with minimal shrinkage impact. Precise temperature control throughout the molding process helps maintain ideal conditions and reduces shrinkage-related variations.
Also, molding simulation software enables anticipating material behavior, allowing adjustments to parameters like temperature and pressure to minimize shrinkage risks. Collaboration between designers and engineers is vital for optimizing part design, including considerations for shrinkage factors from the outset.
Moreover, adjusting injection pressure to optimize material distribution in the mold is a valuable practice in controlling shrinkage. By implementing these strategies, manufacturers can effectively manage shrinkage and enhance the overall quality of injection molded parts.
Mistake 7: Overcomplicating the Mold Design
Plastic injection molding stands out as one of the top methods for mass-producing plastic parts. Achieving excellence in this field demands a high level of skill and experience. Mold designers and engineers leverage modern technologies like CAD/CAM/CAE to tackle intricate designs. However, these innovations should also streamline the design process by avoiding unnecessary complexities that might hinder moldability.
For instance, if the plastic parts boast simple geometry, adding extra features to the molds becomes unnecessary. A straightforward cavity mold should do the job effectively. Similarly, if basic designs meet durability and strength standards, mechanical reinforcements such as ribs become superfluous, saving time that would otherwise be spent on additional machining.
Another perk of a simple mold design is the boost in output since there are fewer parameters and components to scrutinize. Moreover, manufacturing costs, especially those related to tooling and mold maintenance, plummet due to the reduced number of mold elements. With a simple mold design, the risk involved in the design process diminishes significantly, and machining becomes much easier compared to dealing with a series of complex mold configurations.
Modifying a simple mold design is also a breeze, making it quicker and easier to meet part quality requirements.
Mistake 8: Skipping Mold Flow Analysis
Mold flow analysis is a highly advanced computer-aided simulation technique utilized in plastic manufacturing and injection molding. Hence, skipping this process during an injection molding project is never a good idea as it can lead to unimaginable difficulties. Mold flow analysis often employs specialized software to anticipate and scrutinize how molten plastic behaves as it fills the mold cavity during injection molding. Thus, it carries so many benefits, some of which are highlighted below.
For starters, through mold flow analysis, engineers and designers can glean crucial insights into various factors affecting the molding process. These factors include material properties, mold design, gate locations, and processing conditions. This analysis generates visual representations of flow patterns, pressure distribution, cooling rates, and potential issues like air traps, weld lines, and shrinkage.
Also, by conducting mold flow analysis, manufacturers can fine-tune mold design and injection parameters before physical production commences. This proactive approach circumvents plastic part defects and production hiccups, curtails trial and error during production, and ultimately enhances manufacturing efficiency and product quality. These benefits translate into cost savings, shorter development cycles, and a higher success rate in meeting desired part specifications.
As an advanced simulation tool, mold flow analysis aids in predicting potential shrinkage and warpage issues. This proactive approach facilitates mold design improvement, defect minimization, and overall enhancement of plastic manufacturing efficiency. Designers can make necessary adjustments to mold geometry, cooling systems, and gate locations to preemptively tackle these issues before mold production begins.
Conclusion
In the end, arriving at a suitable mold that meets the specific requirements of the project shouldn’t be that hard if you can avoid these 8 moldability mistakes in your injection molding projects. Selecting the right materials, considering wall thickness, and incorporating draft angles in your designs can help your moldability cause a great deal. Additionally, reinforcing your molds with ribs appropriately, providing adequate venting, and analyzing for shrinkage and warpage will take you in the right direction on your journey to a flawless mold design. Finally, taking mold flow analysis seriously as well as employing simple design where necessary will ensure that you achieve your injection molding design goals pretty quickly with less budget.
Ultimately, a mold is a tool to achieve quality plastic parts and every consideration to help enhance the productivity of this tool – including the ones highlighted here in this post, should be scrutinized critically. And that’s a wrap.
