Injection Molding Design: Proven Guidelines & Expert Tips

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.

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!

Importance of Design for Injection Molding

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.

Determines Complexities of Manufacturing

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.

Ensures Manufacturing Feasibility

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.

Prevents Parts Failure

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.

Design Guidelines for Injection Molding

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

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.

Parting Line

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 Angle

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 and Bosses

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.

Gate Location and Type

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.

Ejector Pins

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

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.

Round Corners

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).

Surface Finishing

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.

Material Selection

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.

Mold Base and Cavity Layout

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.

Cooling System

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.

Runner and Gate

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.

Ejection System

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.

Mold Materials and Surface Finishing

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.

Common Injection Molding Design Issues and Solutions

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.

Sink Marks and Warping

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

• Extremely high melt or mold temperature
• Incredibly low holding or injection pressure
• Defective mold structure design
• Insufficient holding or cooling time and pressure

Solutions 

• Ensure a gradual and longer cooling process to prevent internal stresses
• Maintain even wall thickness to facilitate the flow of molten plastic in a single direction through the mold cavity
• Use adequate holding pressure and time to allow the cooling of material near the part’s surface
• Reduce the mold or material temperature

Flash and Part Sticking

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

• Inappropriate exhaust system design and control
• Inadequate clamping force
• Deficient mold design and degraded molding condition
• Excessive injection pressure or high mold temperature
• Inadequate mold release agents
• Insufficient cooling time

Solution

• Ensure the exhaust channel has the right size
• Apply high clamping force for the plate to avoid space in between
• Redesign the mold to allow smooth flow of molten material and proper ventilation
• Coat the mold properly with the right release agents
• Optimize the injection pressure, mold temperature, and cooling time for the specific material used

Short Shots and Burn Marks

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

• Insufficient injection pressure
• Trapped air pockets obstruct the free flow of molten plastic
• Using material with extremely high viscosity
• Inappropriate design of the gate and runner system
• Extremely high melting temperature

Solutions

• Widen the available vent or add more air vents to ensure better ventilation
• Use sufficient mold temperature to avoid rapid and inconsistent material cooling
• Reduce the injection speed to mitigate the risk of trapped air
• Increase speed and pressure or use a thinner base material for better flow

Gas Traps and Voids

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 

• Poor ventilation in the mold
• Uneven filling of the mold cavity
• Trapped air compression and ignition 
• Insufficient molding pressure
• Material’s vulnerability to voids due to significant changes in its density

Solutions

• Increase the mold temperature
• Redesign or retool the runner system and gate positioning
• Use materials with lower viscosity to prevent air bubbles from forming
• Limit the cycle time to prevent trapped air from compressing and igniting
• Increase the injection pressure to expel trapped air from the cavity effectively

Parting Line Mismatch and Deflection

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

• Uneven clamping force
• Dimensional variations in mold components
• Too high injection pressure and temperature
• Thermal expansion of the mold
• Insufficient cooling time

Solutions

• Ensure proper clamping and alignment of the mold
• Keep mold temperature consistent throughout the molding process
• Optimize parameters for the material used
• Post-molding heat treatment can reduce residual stress

Injection Molding Process Control for Quality Plastic Parts

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 Process Overview

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:

Machine Selection and Setup

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:

• Clamping Force: The machine should provide sufficient clamping force to hold the mold securely during the process.
• Injection Unit Size: The injection unit should be large enough to provide sufficient melt volume to fill the mold cavity without overpacking or under-filling the part.
• Screw Type and Size: The screw should provide consistent melt quality and flow rate. The screw diameter should also provide the right shot size and melt density.
• Temperature Control: The machine should have a high-quality temperature control system to maintain uniform heating and cooling throughout molding.
• Material Handling: The machine should also have an efficient material handling system that can transport the material from the storage area without contamination.

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.

Process Parameters and Optimization

Injection molding process control involves monitoring and adjusting several parameters for optimal results. Here are some critical parameters to consider:

• Injection Pressure and Speed: These parameters determine how quickly the molten plastic material fills the mold cavity. The injection pressure should be high enough to fill the mold cavity. However, it should not be so high that it causes flash or part distortion. It should ensure that the material fills the cavity in the shortest possible time without degrading.
• Injection Temperature: Injection temperature affects the flow and viscosity of the plastic material. The plastic material should be heated to its melting point and kept at a stable temperature throughout the process. Machinists can monitor and control the temperature with thermocouples at different points in the mold cavity.
• Holding Pressure and Time: The holding pressure should be such that it prevents the material from flowing back into the injection unit. The holding time should allow the plastic material to cool and solidify completely. The time will depend on the part’s wall thickness and complexity.
• Cooling Time: The choice of cooling time should depend on the material’s thermal properties and the part’s wall thickness. Thermocouples can also help monitor the cooling time. Machinists can adjust the time by changing the cooling channel layout or increasing the size.
• Ejection: The ejection system should ensure smooth and consistent ejection, avoiding damage to the part and the mold. Ejection force should also depend on the part’s size and complexity.

Quality Control and Inspection

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.

Get Plastic Parts with High Quality and Visual Appealing

Need quality plastic parts? RapidDirect’s injection molding services have you covered. With an excellent combination of skilled technicians and state-of-the-art equipment, we offer parts with exceptional precision and visual appeal.

RapidDirect offers a wide range of materials and finishing options to elevate the quality of your injection molding projects. Our experienced technicians are here to assist in optimizing your mold design and selecting the most suitable materials and finishes for your project.

We recognize the challenges involved in mold-making and injection molded part production, so we’ve created a streamlined process to guide you through each step with ease.

1. Start Your Design: Begin by creating your design to meet the specific requirements of your project.
2. Upload Your Design to Get a Quote: Easily upload your design to our platform and receive an instant quote tailored to your needs.
3. Get Your Parts Delivered: Once everything is finalized, we handle the production and ensure your parts are delivered promptly.

Conclusion

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.