Injection Molding Design Guide For High-Precision Production

custom plastic parts, injection molding solutions

The injection molding design guide collects methods to help optimize the injection molding process and is an important part of the manufacturing. 55 million tons of plastic are processed every year to make almost every type of part. Injection molding is the primary process for manufacturing these plastic parts. The process is a fast and cost-effective way to mass-produce components ranging from high-precision engineering parts to cosmetic casings. If we need to better improve efficiency and quality, we have to pay attention to injection molding design guide. This injection molding design guide provides some main thinking directions to help and optimize to achieve better results and complete the project perfectly.
Injection molding design guide-Draft-Angle

Injection Molding Process Design

Acquiring a fundamental grasp of the injection molding process is essential for comprehending the key considerations when designing parts for injection molding and effectively forecasting production schedules. Here are the detailed steps involved:

Design the CAD Model

Begin by creating a CAD model and make decisions regarding color, texture, and resin selection.

Design Verification

Utilize Sungplastic’s services to verify your design through 3D printed, CNC machined, and urethane cast prototypes. This step assesses whether the plastic part meets function, strength, aesthetics, and durability requirements.

DFM Analysis (Design for Manufacturability)

Sungplastic offers a complimentary DFM analysis along with your quote, providing critical insights into potential issues or risks for tool manufacturing and injection molding. Address any identified issues by updating the CAD design to avoid costly tooling modifications. This stage is crucial because adjustments made during the design phase are significantly more efficient and economical than modifications made post-tooling.

Additionally, Sungplastic will inform you about how tooling and injection molding will impact your plastic parts, considering factors such as:
Gate location (ensuring it doesn’t affect the cosmetic surface)
Parting line and split line location (evaluating their aesthetic impact)
Ejection mark placement (inside and non-disruptive to functionality)
Knit line appearance (where flow fronts meet)
Other risks like tolerances and deformation

Tooling Design

Review the tooling drawing for approval and communicate any concerns until you are satisfied with the design.

Steel Cutting

Typically, CNC machining, electrical discharge machining (EDM), and wire cutting are required to create core, cavity, inserts, and lifters. Sliders, tooling base, and ejection pins are usually purchased as off-the-shelf components. Skilled bench workers assemble the tooling, ensuring proper fit and functionality, including the operation of side actions and cooling systems.

T1 Samples (First Trial)

Once assembly is complete, the tooling factory produces one or two trial molds to address as many issues as possible before providing the first samples to the customer. These T1 samples are then sent for function testing and dimensional measurements. T1 samples are not typically perfect, so the tooling factory may require a week or two for adjustments, such as gate location and injection machine parameters. Polishing and texturing of the tooling are typically performed after the customer approves the T1 sample and there are no major tooling modifications needed.

T2 and T3 Samples (Second and Third Trial)

For less complex tooling and well-documented design requirements, samples with minimal quality issues should be received by the third trial (T3). In cases where design-related quality issues persist (e.g., holes in the plastic part), knit lines may be inevitable.

Kickoff Production

Once the samples are approved, production commences. For new product launches, starting with a single-cavity tool is recommended to expedite time-to-market. Subsequently, production velocity can be increased by implementing multi-cavity or family tools.

Understanding these steps in the injection molding process is pivotal for successful product design and production planning. Optimizing the process is the key procedure of injection molding design guide.

Maintaining Uniform Wall Thickness

In the realm of injection molding design guide, wall thickness stands as a paramount factor necessitating meticulous consideration during the part design phase. Wall thickness significantly impacts various critical aspects of the part, including its mechanical properties, visual appeal, and overall cost. A thoughtful approach to wall thickness not only enhances the quality of the end product but also shields you from costly disruptions in your production schedule stemming from molding complications and mold adjustments.

While manufacturability remains a vital concern, it’s essential to derive the nominal wall thickness primarily from functional performance prerequisites. Delve into an evaluation of permissible stress levels and the anticipated lifespan of the part to determine the appropriate nominal or minimum wall thickness. Overly thin walls may demand excessive plastic pressure or result in air entrapment issues, where the plastic fails to fill completely. Conversely, excessively thick walls incur higher expenses, owing to increased material consumption and extended machine cycle durations.

In our injection molding design guide, recommended wall thickness for common resins available:
ABS: 0.045 – 0.140 inches
Acetal: 0.030 – 0.120 inches
Acrylic: 0.025 – 0.150 inches
Liquid crystal polymer: 0.030 – 0.120 inches
Long-fiber reinforced plastics: 0.075 – 1.000 inches
Nylon: 0.030 – 0.115 inches
Polycarbonate: 0.040 – 0.150 inches
Polyester: 0.025 – 0.125 inches
Polyethylene: 0.030 – 0.200 inches
Polyethylene sulfide: 0.020 -0.180 inches
Polypropylene: 0.025 – 0.150 inches
Polystyrene: 0.035 – 0.150 inches

Establishing a consistent and uniform wall thickness within the part is imperative for two significant reasons:

  • Flow Path Consistency
    When the molten polymer courses through the mold, it naturally follows the path of least resistance. Thicker walls tend to attract preferential flow, which can result in the formation of air traps and weld lines within the part.
  • Uniform Cooling
    Plastics exhibit poor heat conductivity. Maintaining uniform wall thickness ensures even cooling of the material, leading to uniform shrinkage as it solidifies. In contrast, variations in wall thickness result in disparate rates of shrinkage, leading to potential warping issues in the finished parts.

In cases where achieving a uniform thickness in your design proves challenging, you have several options to consider:

Option 1: Rounded Corners and Smooth Transitions
Incorporating rounded corners and smooth transitions serves to mitigate stresses on the walls and minimizes discrepancies in shrinkage rates during material cooling. Sharp corners, in contrast, not only introduce stress points in the part but also drive up manufacturing costs due to the need for expensive Electrical Discharge Machining (EDM) to create the mold. Rounded corners facilitate smooth material flow through the mold. When designing these corners, ensure the maintenance of uniform thickness throughout the feature. The interior radius should measure at least half of the wall thickness, while the exterior radius should equal the inside radius plus the wall thickness.

Option 2: Draft Angles
To prevent the part from locking onto the mold due to shrinkage during cooling, draft angles are crucial. Draft angles ease the removal of the part from the mold. Insufficient draft can lead to cosmetic imperfections known as drag marks, resulting from the part adhering to the mold surface. Most CAD systems offer tools to add draft angles, typically best addressed in the final stages of the design to maintain simplicity. Reference tables with recommended draft angles are available for common mold texturing standards like VDI and Moldtech textures. Generally, 1-2 degrees of draft angle per side suffice for standard applications, while 3-5 degrees per side are suitable for increased texture, and 5 degrees or more per side are recommended for heavy textures.


Bosses serve as localized raised areas utilized for the purpose of fastening and connecting various parts. When it comes to injection molding, bosses offer a practical means to reinforce your component without compromising manufacturability. In the realm of injection molding design, thin-walled structures are favored to enhance mold longevity, part quality, production speed, and more. However, this preference for thin walls can result in a lack of strength and structural support in the produced parts. Here’s where bosses play a crucial role. They are strategically positioned to infuse additional structural integrity into specific areas, such as screw holes and slots, where it is essential.

Furthermore, bosses find utility in achieving precise alignment during assembly. By designing bosses that can interlock with each other, much like dowel pins, rapid and accurate assembly becomes feasible. The design of bosses carries substantial significance, with one key factor being shrinkage, a frequent consideration in injection molding design guide. When designing a boss for a screw hole, for instance, it’s imperative to compensate for shrinkage by creating a slightly smaller diameter. Equally important is the thickness of the boss, which should generally not exceed 60% of the overall wall thickness to prevent unsightly sink marks caused by shrinkage.


Ribs are employed to bolster the structural integrity of a part and enhance its load-bearing capacity. Although introducing ribs to a part is a straightforward concept, it can become more complex when working with plastics. Plastic injection-molded parts typically maintain a uniform thickness, but ribs are the exception to this rule. Typically, the wall thickness of ribs falls within the range of 50% to 75% of the nominal wall thickness. Additionally, the bottom of the rib should feature a fillet, with the fillet radius typically ranging from 0.25 times to 0.5 times the nominal wall thickness. This radius should not be less than 0.010 inches.

It is essential to establish a height limit for ribs. Ideally, rib height should not exceed 2.5 times the nominal wall thickness. Conventional draft angles, typically ranging from 0.5 degrees to 1 degree per side, should be applied to ribs to facilitate molding.

However, it’s crucial to note that ribs can lead to sink marks, resulting from significant shrinkage during cooling. These sink marks manifest as cosmetic defects on the part’s surface. Consequently, to mitigate this issue, the wall thickness of the ribs must be significantly reduced compared to the nominal wall thickness of the part.

Parting Lines

The parting line, which corresponds to where the mold separates during the opening and closing process, dictates the necessary draft direction for your design. While the common perception is that the parting line often runs down the center of the part, it’s not always the most ideal location. Consider, for instance, a widely produced product like a LEGO brick; its parting line isn’t in the middle but at the very bottom. This principle applies to various other products, such as plastic cups. Generally, it’s advisable to avoid placing the parting line on filleted surfaces. Doing so would require a high-precision mold, raising production costs. Any misalignment can result in flash and potential cosmetic defects. The most favorable placement for your parting line is typically on sharp edges.


Gate design is a crucial aspect of injection molding design guide, and there are four primary gate types to consider: edge gates, sub-gates, hot tip gates, and sprue gates.

  • Edge Gates
    Ideal for flat parts, edge gates are positioned at the part’s edge and result in a scar along the parting line.
  • Sub-Gates
    Commonly used, sub-gates require ejector pins for automatic trimming. They come in various variations, such as banana gates, smiley gates, and tunnel gates, which allow you to move the gate away from the parting line when necessary to enhance filling.
  • Hot Tip Gates
    Located at the top of the mold, hot tip gates are primarily used in round or conical geometries and exclusively with hot runner injection molds.
  • Sprue Gates
    Direct or sprue gates are utilized in single-cavity molds, typically for large cylindrical parts. They are the simplest to manufacture and maintain, offering cost-effectiveness. However, they leave a noticeable scar at the point of contact.

Ejector Pins

Ejector pins play a vital role in the injection molding design guide by pushing the part out of the mold once it has cooled. However, they often leave marks on the finished part, which may be seen on certain household products. Typically, these pin marks appear on surfaces that are not visible to the end user, such as the interior of a housing. To ensure effective ejection, the ejector pin pads must be positioned on a surface perpendicular to the direction in which the ejection pins will push.

Sungplastic’s Injection Molding Design Guide

Our injection molding design guide refers to a wide range of aspects. We will guide how to select the appropriate material for specific applications, provide processing of maintaining uniform wall thickness and avoiding issues like warping, sink marks, or voids, the placement of parting lines, details on gate design, runner systems, and considerations for minimizing material waste and optimizing flow, guide on designing structural features like ribs and bosses for added strength and functionality, dealing with undercuts and complex part geometries, adding draft angles to facilitate part ejection from the mold, achieving desired surface finishes and textures, tolerances for various dimensions and features, mold design including cavity layout, cooling channels, and gating location and ejector systems design.

We ensure consistent part quality and minimizing defects, are committed to provide high quality and loyal services at cost-effective approaches.

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