Creating injection molded parts requires careful consideration of numerous variables that can impact the functionality and quality of the final product. Common issues, such as sink marks, flow lines, and warping, underscore the need to thoroughly understand effective design principles.
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This article presents the key injection molding design guide to help you create the best plastic parts. You&#;ll also learn about process control, mold creation strategies, and tips for avoiding common pitfalls. Keep reading!
Injection molding is a manufacturing method where molten plastic is injected into a mold cavity to form a specific shape. The mold&#;s structure and the part being produced significantly influence the process&#;s success. For part designers, understanding these elements is essential to achieving optimal results. Here&#;s why careful design consideration is so critical in the injection molding process.
After reviewing the design, product designers and engineers can anticipate potential complications during manufacturing. This detailed analysis helps reduce uncertainties before production begins. Additionally, understanding these complexities clarifies the mold&#;s shape and structure, ensuring the creation of the right tooling for the desired products.
At the initial stage of plastic part production, it can be uncertain whether a part is suitable for manufacturing. However, proper design helps determine the feasibility of the process from the outset. This allows manufacturers to identify potential challenges, such as parts getting stuck in molds, and ultimately saves time and costs, ensuring the product is affordable and produced more efficiently.
An inadequate design process can compromise the functionality and appearance of injection molded parts. Such parts may fail to perform as intended due to molding defects or other mechanical issues. Following a comprehensive guide will help in selecting the appropriate molding parameters and preventing critical problems that could lead to part failures.
Injection molding is a complex process that demands precise design considerations to ensure successful production. Mistakes in design can lead to significant delays and increased costs once the process is underway. To avoid these issues, it&#;s crucial to adhere to proper guidelines. Here are some key factors to consider when parts are being designed for injection molding.
Wall thickness can influence several key features of a component, including its performance, aesthetics, and cost. Therefore, you should determine the nominal wall thickness based on the functional performance requirements. You should consider the allowable stress and expected lifetime of the molded part to establish the minimum wall thickness.
The rule of thumb is to use a uniform wall thickness throughout the injection molded parts. Generally, it is ideal to keep the wall thickness between 1.2mm and 3mm. Excessively thin walls will require high plastic pressure and cause air traps. On the other hand, overly thick walls will incur more expenses because of longer cycle times and greater material usage.
Whenever a component requires variation in wall thickness, you must ensure a gradual transition between the sections. You can achieve this by incorporating chamfers on sloped edges or corners. Likewise, using fillets for rounded edges or corners will ensure that the molten plastic fills the mold and cools evenly.
The parting line is where the two halves of the mold meet to produce the final product. Any mismatch or misalignment can lead to flash defects on the molded part. To minimize these defects, it is crucial to create a parting line that is simple and straight. A straightforward parting line is easier to manufacture, requires less maintenance, and can result in a better overall finish.
When designing the parting line, it&#;s generally better to place it on sharp edges rather than filleted surfaces. This reduces the need for a mold with tight tolerances, which can help control production costs. It&#;s also important to consider the visual impact of the parting line on the final product. The line should be positioned to minimize visibility and avoid crossing critical surfaces or features like text or logos, ensuring the final product meets aesthetic standards and enhances the overall quality of the process.
Draft angles on surfaces of injection-molded parts allow for easy removal from the mold without damage. The required draft angle depends on factors like wall thickness, material shrinkage, post-production finishing needs, etc.
The average draft should increase by 1 degree per inch of depth, but a minimum of 1.5 to 2 degrees is typically safe for most components. Heavy textures may require up to 5 degrees per inch of depth. An inadequate draft can cause aesthetic flaws like drag marks. You can add draft angles using CAD systems. However, it would be best to do this in the final stages of design to minimize complexity.
Ribs help to strengthen part walls where two walls meet at a 90-degree angle. They help increase the structural integrity and increase the load-bearing capacity of the part. On the other hand, bosses have raised areas used for fastening and aligning parts. They also strengthen parts in areas like screw holes and slots.
The base thickness of support ribs should be a maximum of two-thirds the thickness of the adjoining wall. Rib height should not exceed 2.5 times the nominal wall thickness (2.5T). It is important to consider shrinkage. To avoid sink marks, the thickness of the boss should not exceed 60% of the overall wall thickness.
The gate in injection molding is an essential component that directly connects to the plastic part and controls the flow of melted plastic resin into the cavity. The size, shape, and location of the gate have a significant impact on the finished product. It affects its structural integrity and exterior appearance.
There are four common types of gate designs for different types of injection molds: edge, sub, hot tip, and sprue. As the name suggests, edge gates are located at the edge of flat parts and leave a scar on the parting line. Sub-gates are common and have different variations, such as banana, smiley, and tunnel gates. They require ejector pins to trim automatically and are helpful when moving the gate location away from the parting line for better filling.
Hot tip gates are only used with hot runner molds. They are often located at the top of the mold for round or conical geometries. On the other hand, sprue gates are ideal for single-cavity molds that are large and cylindrical. They often leave a large scar at the point of contact but are easy to manufacture and maintain.
The gate selection depends on the part structure, material choice, dimensional requirements, and aesthetic needs of the end product. A key rule is to locate gates away from high-stress or impact areas to minimize the risk of defects. It is also essential to eliminate secondary de-gating operations and place them in the thickest area to achieve the best fill. In some cases, multiple gates may be necessary depending on the part&#;s size, geometry, and plastic polymer type.
This is a crucial part of the injection molding setup that helps push parts out of the mold after they are sufficiently cool. They often leave marks on the parts. Therefore, Therefore, the part designer needs to ensure they are positioned on flat surfaces perpendicular to the movement direction of the ejector pin.
Part shape, draft angles, wall depth, and wall texture determine the number and placement of pins. These factors will influence how the part adheres to mold walls. Material choice will also affect the size and placement of these pins. For instance, stickier resins will require more ejection force. Likewise, softer plastic polymers will require wider or more pins to help distribute the ejection force to avoid molding defects.
Undercuts and threads are recessed or overhanging features that make it difficult to eject a plastic part from the mold with a single pull. Ensuring the part can be ejected with a single, unidirectional pull is essential for keeping injection molding costs low. Doing this will help keep the cost low. Therefore, it is important to avoid threads and undercuts in plastic parts.
To avoid undercuts, you can orient features parallel to the draw line, and use lifters and sliders. Lifters help release internal undercuts without draft. Once the part cools, the lifter can push up at an angle to remove the undercut from the mold. In contrast, sliders use angled pins attached to the core mold to release external undercuts.
To improve the efficiency and quality of plastic molded parts production, designers and engineers should aim for rounded features rather than sharp corners and edges. Sharp edges require more pressure to fill, increasing the risk of part damage and defects during ejection. Rounded internal and external corners help plastic flow more smoothly and reduce residual stress and cracking.
The radius of internal corners should be at least 50% of the adjacent wall thickness. On the other hand, external corners should be 150% of the adjacent wall thickness. For vertical features like bosses and snap fits, the base should be rounded. The boss radius should be 25% of the adjacent wall and a minimum radius of 0.381mm (0.015 inches).
Plastic parts can have different surface finishes that affect their texture, look, and feel. Choosing the right finish is crucial as it determines the tooling and material needed. Rough finishes require higher draft angles and impact the material selection. The mold surface may also need preparation to achieve the desired finish. The slightest imperfection in the mold surface can transfer to the molded part. The more post-production finishing needed, the higher the cost and longer it takes to complete the mold.
Injection molding involves using a variety of plastic resins, each with its specific physical and mechanical properties. Material selection impacts the part&#;s functionality in its intended environment. Key considerations when selecting injection molding materials include material shrinkage rate, assembly, and cost.
Material shrinkage rate varies based on the plastic type and processing conditions, which can affect part performance and geometry. You should also consider the material&#;s ability to handle assembly processes such as mechanical fastening and welding. While the desirable attributes of the plastic material are essential, you must also consider the cost of purchasing, machining, and finishing the plastic to minimize production costs.
The tooling defines the shape of the intended plastic part, so all components must be in optimal condition for a smooth process. Here are some tips to consider when working on the mold tooling design.
The mold tooling includes the mold base, cavity, core inserts, and other components. The mold base provides the foundation for the mold, while the cavity and core inserts create the shape of the part. The design of the mold tooling affects the accuracy and consistency of the molding process.
The mold must be durable, easy to maintain, and easy to disassemble and assemble for repairs and maintenance. The mold tooling should be built with precision to ensure proper alignment of the cavity and core. The cavity layout of the mold base must also give access to the hollow and core inserts, permitting simple maintenance and repair. This reduces the risk of defects and improves part quality.
The cooling system is a crucial part of the mold, as it controls the temperature of the mold cavity and the plastic material. Effective cooling is vital for solidifying the plastic and controlling shrinkage.
The system should be designed to ensure uniform cooling throughout the mold cavity. Cooling channels should be positioned near areas that take longer to cool, preventing interference with the gating and runner systems. Machinists should also optimize the setup to achieve the shortest possible cycle time.
The runner and gate system controls the flow of molten plastic into the mold cavity. The gate is the entry point for the plastic to enter the cavity, and the runner system channels the plastic to the gate. The gate and runner system affects the efficiency of the molding process and the quality of finished products.
The gate size, location, and shape should optimize material flow, minimize part stress, and avoid defects in the part. The runner system should minimize pressure drop, ensure the even distribution of material, and avoid dead spots where plastic can accumulate and cause defects.
The ejection system removes the finished part from the mold cavity. Its design should take into account the part&#;s geometry, the number of undercuts, and its stiffness. To prevent damage during ejection, designers can incorporate ejector pins, sleeves, or hydraulic systems. Additionally, the ejection system must be robust enough to withstand the forces needed to remove the part. Proper placement of the ejection system relative to the gating and runner systems is also crucial to avoid interference.
The material used for the mold affects its lifespan and the quality of the finished product. To ensure optimal performance, the mold material should have a high melting temperature, good thermal conductivity, and excellent wear resistance. Choosing a suitable material can help reduce cycle time, extend the mold&#;s lifespan, and reduce the risk of part defects.
Each mold is unique and requires careful consideration during the machining process. The materials used must be machined with precision to avoid surface defects that can transfer to the molded part. It is important to remove visible marks on the mold surface left by end mills through additional finishing, like bead blasting or polishing. The degree of finishing required can impact the cost and timeline of the mold tooling process.
RapidDirect offers outstanding injection mold tooling services to improve the molding process and the quality of molded parts. We provide comprehensive DFM analysis for your injection molding projects to improve mold and part design. As a result, you can save enough time and money, while getting superior-quality products.
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Injection molding defects can arise during manufacturing, affecting the product&#;s functionality. These issues often stem from factors like molding parameters or material selection. While many defects can be mitigated by fine-tuning the molding process, some may necessitate redesigning the mold tooling or upgrading production equipment.
Let&#;s explore some of the typical issues and how you can resolve them.
A sink mark occurs as tiny depressions on flat surfaces of molded parts. Sink marks typically happen due to the shrinking of a molded part&#;s inner component, causing the material to sink inward from the outside.
Warpings are unexpected bends and twists on injection molded components due to the irregular internal shrinkage in the cooling process. It puts unintended stress on various areas of the molded component. This stress forces the molded parts to bend and twist while cooling. You can notice this in parts that are flat but have gaps when placed on a flat surface.
Causes
Solutions
Flash, spew, or burrs refer to a situation where excess molding material appears as a thin line at the edge of the component. It usually occurs due to the flow of some material out of the intended channels. Although a flash counts as a subtle defect, it may become a severe product defect if it affects its functionality.
On the other hand, part sticking involves the molded part adhering to the mold surface, making it difficult or impossible to eject.
Causes
Solution
A short shot is a defect on molded parts when the molten material fails to fill the entire mold cavity. As a result, the molded component is incomplete after cooling and ejection. Short shots are considered severe defects because they affect the molded part&#;s appearance and function.
Burn marks as black rust-colored marks on the surface or edges of the molded component. Although these defects do not usually impact the integrity of parts, they become a severe problem when they burn the molded component such that it causes degradation.
Causes
Solutions
These air trap defects are among the most critical flaws. They appear as trapped air or air bubbles in the molded components. These trapped bubbles can cause structural and aesthetic faults. Likewise, if the air originally within the mold gets hot and compressed tight enough, it can explode, destroying both the molded component and the mold.
Vacuum voids are trapped air bubbles found in injection molded parts. Manufacturers sometimes refer to these defects as air pockets. Although quality control experts categorize voids as minor defects, more substantial voids can weaken the molded component.
Causes
Solutions
Parting line mismatch is a defect where the two halves of the mold do not line up correctly. It results in a visible seam or gap along the parting line of the molded part. Deflection occurs when the molded part warps or bends out of its intended shape during cooling. Both defects can result in parts not meeting the required specifications, leading to increased scrap rates and reduced productivity.
Causes
Solutions
To ensure high-quality plastic products, it is essential to have strict process control throughout the manufacturing process. Before we go into the key steps for achieving process control in injection molding, let&#;s have a brief overview of the process.
Injection molding involves melting plastic polymers and solidifying them under pressure in molds that give the components their shapes. This continuous cycle includes many steps. After heating the plastic resins, the gate opens upon applying the appropriate pressure to the mold tooling. The melted plastic is then injected into the mold.
Once the molten resin reaches the end of the barrel, the gate is closed. The two parts of the mold then close simultaneously and are held together by the clamp pressure. After the holding phase, the screw retracts, and the part cools in the mold. Once the part cools, the mold opens, and ejector pins or plates push the part out. The finished part is then ready for finishing processes.
With this in mind, let&#;s check out the various aspects of the process control:
Selecting the right injection molding machine and setting it up correctly will help achieve process control and produce high-quality plastic parts consistently.
Consider the following factors:
Overall, there should be room for tracking critical process parameters such as temperature, pressure, and cycle time. Machinists should be able to easily detect any variations in the process parameters and adjust them in real-time to prevent defects in the finished product.
Injection molding process control involves monitoring and adjusting several parameters for optimal results. Here are some critical parameters to consider:
Quality control and inspection aim to guarantee that the molded parts meet the quality and performance requirements. There are different aspects, including process capability studies, visual and dimensional inspection, and functional testing. They help identify sources of variability and suggest improvements to the process.
Efficient quality control ensures that molded parts are free of defects and surface blemishes and that they meet the specified tolerances and functional requirements. Quality control and inspection processes must be done regularly to ensure that the parts meet the specified quality, safety, and performance standards.
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Injection molding is a versatile and efficient technique for producing high-quality custom plastic components across various industries. However, to achieve optimal results, it&#;s essential to follow a well-defined guide, offering a clear understanding of the process.
The design principles discussed in this article will help you streamline the process, ensuring cost-effective production and shorter cycle times. Design errors can be costly. Get in touch with RapidDirect today for expert guidance on your injection molding projects. We&#;re here to deliver top-quality results.
Experienced product designers deeply understand injection molding processes and consider numerous factors in plastic part design. This article focuses on essential elements such as wall thickness, draft angles, ribs, holes, pillars, snaps, interference fits, and tolerances in plastic molded part design.
Determining the appropriate wall thickness is crucial. Other features like ribs and fillets reference the wall thickness. The wall thickness of a plastic product depends on various requirements, including the external forces it must withstand, support for other parts, properties of the plastic material, weight, electrical performance, dimensional accuracy, stability, and assembly requirements.
Typically, the wall thickness for thermoplastic materials ranges from 1 to 6mm, with 2 to 3mm being most common. For larger parts, thicknesses can exceed 6mm. Table 1 shows recommended values for the wall thickness of various thermoplastics.
MaterialsMinimum Wall ThicknessRecommended Values For Small WorkpiecesRecommended Values For Medium WorkpiecesRecommended Values For Large WorkpiecesNylon0.450.761.52.4~3.2PE0.61.251.62.4~3.2PS0.751.251.63.2~5.4PMMA0.81.52.24~6.5PVC1.21.61.83.2~5.8PP0.851.541.752.4~3.2PC0.951.82.33~4.5POM0.81.41.63.2~5.4ABS0.812.33.2~6Table 1Uniform wall thickness is a key principle in plastic part design. Uneven thickness can cause inconsistent melt flow and cooling shrinkage, leading to defects like sink marks, voids, warping, or even cracking. It can also result in shrinkage marks, internal stresses, distortion, color variations, or differences in transparency. Thinner walls may compromise strength and rigidity during use and assembly. Economically, overly thick parts increase material costs and production time. Areas with thicker plastic cool slower, leading to sink marks. Figure 1 illustrates uniform wall thickness design.
Figure 1If a transition from thicker to thinner sections is unavoidable, it should be gradual, maintaining a maximum ratio of 3:1 in wall thickness, as shown in Figure 2.
Figure 2In many cases, designers can use ribs to modify the overall wall thickness, which not only saves material and reduces production costs but also shortens cooling time. Cooling time is approximately proportional to wall thickness.
Additionally, designers must consider the flow path, the distance the molten material travels from the gate to all parts of the cavity. Generally, there&#;s a proportional relationship between flow path and wall thickness. A larger wall thickness means a longer flow path. If the ratio of flow path to wall thickness is too high, material shortage or incomplete filling can occur far from the gate. Therefore, increasing wall thickness might be necessary in some cases.
Sharp angles often lead to defects and stress concentration in parts. These areas are prone to unwanted material accumulation during post-processing treatments like electroplating or painting. Stress concentration can cause fractures under load or impact. Therefore, it&#;s advisable to avoid sharp angles in design. Figure 3 provides an example of sharp angle design.
Figure 3At the onset of designing an injection-molded product, it&#;s crucial to establish the ejection direction and parting line. This ensures minimal core-pulling mechanisms and reduces the impact of parting lines on the appearance. Once the ejection direction is set, structures like ribs, snaps, and protrusions should align with it to avoid core pulling, reduce seam lines, and extend mold life. The appropriate parting line can then be chosen to enhance appearance and performance.
During ejection from the mold, the part must overcome ejection and opening forces. Opening refers to the part&#;s detachment from the cavity. As the part cools inside the mold, it shrinks, causing the hole walls to grip the core tightly. Friction between the part and core, vacuum adhesion at the hole bottom, and other factors make ejection forces significantly greater than opening forces. Excessive ejection forces can deform the part, cause whitening, wrinkling, and surface abrasions.
Draft angles are crucial in determining the magnitude of ejection forces. Since injection-molded parts often adhere to the convex mold due to cooling shrinkage, equal draft angles on both concave and convex molds ensure uniform wall thickness and prevent the part from sticking to the hotter concave mold after ejection. In special cases where the part is required to stick to the concave mold post-ejection, the draft angle on the adjoining concave part can be reduced, or an undercut can be deliberately added to the concave mold.
There&#;s no fixed value for draft angles; they are usually determined based on experience. Highly polished outer walls can have draft angles as small as 1/8 or 1/4 degree. For deeper or textured parts, the draft angle should increase correspondingly. Conventionally, an additional 1 degree of draft angle is required for every 0.025mm depth of texture.
Moreover, while larger draft angles generally facilitate easier ejection, it&#;s vital to maintain dimensional accuracy. The dimensional errors caused by draft angles must stay within the precision range. For parts with significant shrinkage or complex shapes, larger draft angles should be considered.
The strength of plastic parts does not solely depend on increased wall thickness. In fact, increased thickness can lead to internal stresses due to shrinkage, thereby reducing strength. The key to enhancing the strength of plastic parts lies in their stiffness. This is often achieved through a combination of thin-wall styles and strategically placed ribs to increase the section modulus.
However, adding ribs results in increased thickness at the junction with the main wall. This thickness typically depends on the largest inscribed circle, determined by the rib thickness and the radius of the root fillet. With a base material thickness of 4mm, changing the rib thickness and root fillet radius alters the diameter of the largest inscribed circle. Figure 4 illustrates how local increases in wall thickness can lead to shrinkage deformation on the back, affecting the appearance. Proper design can reduce the likelihood of surface indentations, thus improving part quality.
Figure 4From the analysis, it&#;s evident that the thickness of the rib should be minimized within limits. If the rib is too thin, its height must be increased to maintain stiffness. However, excessively thin ribs can lead to deformation under pressure, difficulties in filling during molding, and sticking to the mold. The radius of the rib&#;s base should not be too small to avoid stress concentration.
Generally, the radius of the rib&#;s root should be at least 40% of the rib thickness. The rib thickness should be between 50% and 75% of the base material&#;s wall thickness, with the higher ratio limited to materials with low shrinkage rates. The height of the rib should be less than five times the thickness of the base material. Ribs must have draft angles and be oriented in the direction of ejection or use movable mold components. The spacing between ribs should be more than twice the thickness of the base material.
To achieve uniform stiffness in all directions, the simplest method is to add ribs both longitudinally and transversely, intersecting at right angles. However, this can increase wall thickness at intersections, leading to greater shrinkage. A common solution is to add a round hole at the intersection to create uniform wall thickness, as shown in Figure 5.
Figure 5Incorporating holes in plastic parts for assembly or functionality is common. The size and placement of these holes should ideally not compromise the product&#;s strength or add complexity to the manufacturing process. Key factors to consider:
There are various types of holes, such as through holes, blind holes, and stepped holes. From an assembly perspective, through holes are more common and easier to produce than blind holes. In terms of mold design, through holes are structurally more straightforward. They can be formed with cores fixed in both the movable and fixed parts of the mold, or with a single core in either part. The former creates two cantilever beams under the action of the molten plastic, but with short arms, resulting in minimal deformation.
The latter, generally forming a simply supported beam, also has minimal deformation. When using two cores, their diameters should differ slightly to prevent misalignment and ensure smooth mating surfaces. Blind holes, formed with a cantilever beam core, are more prone to bending under the impact of the molten plastic, leading to irregularly shaped holes. Generally, the depth of a blind hole should not exceed twice its diameter. For blind holes with diameters of 1.5mm or less, the depth should not exceed the diameter. The wall thickness at the bottom of a blind hole should be at least one-sixth of the hole diameter to avoid shrinkage.
Side holes are typically formed using side cores, which can increase mold costs and maintenance, especially if the side cores are long and prone to breaking. If feasible, the design can be improved as shown in Figure 6, to mitigate these issues.
Figure 6Bosses, typically protruding from the wall thickness, are used for assembling products, separating objects, and supporting other parts. Hollow bosses can accommodate inserts or tighten screws. These applications require sufficient strength to withstand pressure without cracking. Bosses are generally cylindrical, as this shape is easier to mold and offers better mechanical properties.
Ideally, bosses should not be designed as isolated cylinders. They should be connected to outer walls or used in conjunction with ribs. This approach enhances the strength of the boss and facilitates smoother flow of the plastic material. The connection to the outer wall should be a thin-wall connection to avoid shrinkage.
The base of the boss where it meets the base material should have a fillet radius of 0.4 to 0.6 times the thickness of the base material. The wall thickness of the boss should be between 0.5 and 0.75 times the thickness of the base material. The top of the boss should have a chamfer for ease of screw installation. Draft angles are also necessary on bosses. These design requirements are similar to those for ribs, making bosses a variant of ribs. Refer to Figures 7 and 8 for these relationships.
Figure 7Figure 8Many bosses are used to connect self-tapping screws. The internal threads on these bosses are formed through cold flow processing, which deforms the plastic without cutting it. The size of the threaded boss must be sufficient to withstand the screw&#;s insertion force and the load it carries. The hole diameter in the boss should ensure that the screw remains secure under specific torque and vibration conditions.
The outer diameter of the boss must withstand the circumferential force generated during screw tightening without breaking. To facilitate screw insertion, a recess is often created at the top of the boss, slightly larger than the nominal diameter of the thread. Calculating the dimensions of a boss can be complex.
A simplified estimation method from a foreign website is recommended, based on the nominal diameter of the screw. First, identify the material used, then apply the corresponding coefficient from the table to the screw&#;s nominal diameter to determine the appropriate size.
Snap-fit assembly is a convenient, cost-effective, and environmentally friendly method of connection. The snap-fit components are molded simultaneously with the product, eliminating the need for additional fasteners like screws. Assembly simply involves snapping the corresponding parts together.
The principle of a snap-fit involves pushing a protruding part of one component past an obstacle on another component. This process involves elastic deformation, and once the obstacle is cleared, the parts snap back into their original shape and lock together, as shown in Figure 9. Snap-fit connections can be either permanent or releasable.
Figure 9Structurally, snap-fits can be categorized into cantilever, annular, and ball shape, as detailed in Figure 10.
Figure 10Two critical angles in snap-fit design are the retraction side and the entrance side. Generally, a larger retraction side is preferred for a more secure fit. When the retraction side approaches 90 degrees, the snap-fit becomes permanent, as shown in Figure 11.
Figure 11The maximum allowable deflection for a uniform section snap-fit can be calculated using: Y = el² / (1.5t). This formula assumes deformation only in the snap hook. In practice, some deformation also occurs near the snap-fit, which can be considered a safety factor.
The force required to produce a deflection Y in the snap-fit: P = wt²Ee / (6l).
The assembly force can be estimated with: W = P(μ + tga) / (1 &#; tga).
For releasable snap-fits, the release force can be calculated using the same formulas, substituting angle a with angle b.
Table 2 provides some coefficients needed for these calculations.
Materials(e)(%)GPaCoefficient(s) of FrictionPS23.00.3ABS22.10.2SAN23.60.3PMMA22.90.4LDPE50.20.3HDPE41.20.3PP41.30.3PA31.20.1POM42.60.4PC22.80.4Annular snap-fits use a ring&#;s internal protrusions to engage with a shaft&#;s groove. Based on the release angle, they can be either releasable or non-releasable. The ring expands elastically during insertion and removal, typically made from materials with good elasticity.
Figure 12 illustrates an annular snap-fit.
Figure 12The maximum size of the annular snap-fit&#;s protrusion can be calculated using:
y = Sd((K + v) / E + (1 &#; v) / E) / K
Where S is the design stress, v is Poisson&#;s ratio, E is the elastic modulus, and K is a geometric coefficient calculated as: K = (1 + (d/D)²) / (1 &#; (d/D)²).
The expansion force on the sleeve can be calculated with:
P = (tan a + μ) / Sydlπ / K
Where μ is the coefficient of friction.
Table 2 provides Poisson&#;s ratios for various unfilled materials, with friction coefficients shown in Figure 17.
Interference fits, used to connect holes and shafts, are effective for transmitting torque and other forces. This type of connection is convenient and straightforward. The primary consideration in designing interference fits is the amount of interference: too little interference leads to unreliable connections, while too much makes assembly difficult and increases the risk of cracking.
When designing interference fits, it&#;s important to consider the tolerances of the hole and shaft, as well as the operating temperature, since temperature variations can significantly affect the interference amount. Most shafts are metallic, and to ensure a reliable connection, it&#;s common to add knurling or grooves on the mating shaft. The general formula for calculating interference is:
Y = Sd((K + v) / E) / K
Where S is the design stress, v is Poisson&#;s ratio, E is the elastic modulus, and K is a geometric coefficient calculated as:
K = (1 + (d/D)²) / (1 &#; (d/D)²)
The assembly force can be calculated with:
W = Sdlπμ / K
Where μ is the coefficient of friction, and l is the length of engagement. The Poisson&#;s ratio can be found in Table 3.
MaterialsPoisson&#;s RatioPS0.38PMMA0.4LDPE0.49HDPE0.47PP0.43PA0.45PC0.42PVC0.42PPO0.41PPS0.42Steel0.38Figure 13In addition to interference fits, other methods for joining plastic parts include heat staking, welding, and ultrasonic welding. Each of these methods has its own set of advantages and is suitable for different applications based on the material properties and the requirements of the assembly.
Most plastic products can achieve high precision in dimensional tolerances. However, materials with high shrinkage rates or softer materials can be more challenging to control. Product design must consider the use environment, plastic material, and product shape to set appropriate tolerances. As customer demands increase, the concept of fit and finish must evolve. The goal is to achieve a balance between fit, precision, and aesthetics.
Injection molding is generally categorized into three quality levels: general-purpose, medium precision, and high precision.
There are no inherently bad materials, only inappropriate choices for specific applications. Designers must thoroughly understand the properties of available materials and carefully test them to study their impact on the performance of molded products.
The most commonly used materials in injection molding are thermoplastics, which are divided into amorphous and semi-crystalline plastics. These two categories differ significantly in molecular structure and performance affected by crystallization. Semi-crystalline thermoplastics are typically used for parts requiring high mechanical strength, while amorphous thermoplastics, less prone to bending, are often used for casings.
Figure 14Thermoplastics are available in unreinforced, glass fiber reinforced, and mineral or glass bead filled varieties. Glass fibers mainly enhance strength, rigidity, and temperature resistance; minerals and glass fibers reduce warping but offer lower reinforcement. Specific changes in properties due to reinforcements should be confirmed with material suppliers or through experimentation.
Some thermoplastics, especially PA6 and PA66, are highly hygroscopic, which can significantly affect their mechanical properties and dimensional stability.
Considerations related to processing and assembly are crucial. Integrating multiple functions into a single component can save on costly assembly expenses. This principle is beneficial for calculating production costs. High-performance materials (rigidity, toughness) can allow for thinner walls, shortening production cycles. Therefore, listing all standards and systematically evaluating them is essential.
Sharp corners often lead to defects and stress concentration in plastic parts, which can cause fractures under load or impact. Larger rounded corners (fillets) offer a solution to this problem. They not only reduce stress concentration but also facilitate smoother flow of the plastic during molding and easier ejection of the finished product.
Figure 15If the internal corner is rounded and the external corner is sharp, the area at the turn will still be thicker than other parts, leading to shrinkage. A solution is to round both internal and external corners to achieve uniform wall thickness. In this case, the external radius is the sum of the internal radius and the base wall thickness.
The design principles for corner radii also apply to cantilever snap-fits. In these snap-fits, the cantilever arm needs to bend and fit into place. If the radius of the corner (R) is too small, it can lead to excessive stress concentration, making the product prone to breaking when bent. Conversely, if R is too large, it can result in shrinkage marks and voids. Therefore, there is a specific ratio between the corner radius and wall thickness, typically ranging from 0.2 to 0.6, with an ideal value around 0.5.
In summary, this article has covered various critical aspects of structural design for injection molded parts, including wall thickness, draft angles, ribs, holes, bosses, snap-fits, interference fits, tolerances, and rounded corners. Each of these elements plays a vital role in the overall functionality, durability, and quality of the final product.
However, it&#;s important to remember that structural design is also influenced by environmental factors, specific conditions, and unique requirements of each project. These factors necessitate a tailored approach to each design challenge.
The goal of this comprehensive overview is to equip aspiring and practicing structural design engineers with the knowledge and insights needed to excel in their field. By understanding and applying these principles, designers can create more effective, reliable, and high-quality injection molded parts.
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