Glossary for Injection Molding Design Guidelines

Whether your injection-molded part is simple or complex, understanding how certain elements can impact the manufacturability or the finished part quality is critical. The injection molding design guidelines provided in this glossary can provide you with general rules for proper design. Material selection and part complexity may change these values as each resin behaves differently when processed. QuickParts offers in-depth design for manufacturability (DFM) evaluations to ensure your product can be molded to your specifications.

A-side/B-side

In injection molding, the mold is divided into two halves, known as the A-side and B-side. The A-side, also referred to as the cavity side, typically contains the primary cavity where the molten plastic is injected. It is responsible for forming the external surface of the final product. The B-side, also known as the core side, usually contains the core elements that shape the internal features of the product. Proper design of both A-side and B-side is crucial for ensuring the precision and quality of the molded product.

Barrel

The barrel is a heated cylindrical component in an injection molding machine where plastic pellets are melted. The barrel houses the injection screw, which rotates and moves axially to melt and convey the plastic toward the mold. The temperature in the barrel must be precisely controlled to ensure the plastic melts uniformly.

Bosses

Bosses are features used for mating parts with screws or other fasteners, mounting the part to something else, providing alignment during assembly, or providing structural support. Typically, when used with screws or fasteners, they are round. They may also be used as a point that receives a heat-staked threaded insert during a secondary operation.   

Proper design considerations, such as wall thickness, draft angles, and ribs or gussets, are essential to ensure that bosses perform their intended functions effectively without causing defects or manufacturing issues. Generally, ribs are used to tie the outside walls to the bosses. A rule of thumb is that the thickness of the boss should be 60% or less of the part’s nominal wall thickness. For easy removal from the mold, it generally should have a 1° draft angle.

Cavity

The cavity is the hollow space within the mold that shapes the final product. It is precisely machined to the desired dimensions and surface finish of the product. The cavity design must consider factors like parting line placement, venting, and cooling to ensure that the molded part meets specifications. Multiple cavities can be incorporated in a single mold to produce several parts simultaneously, increasing production efficiency.

Cooling Channels

Cooling channels are passages within the mold through which coolant, typically water, is circulated to solidify the molten plastic. The design and placement of cooling channels are critical as they influence the cooling rate, which affects the cycle time and the quality of the molded part. The mold maker will ensure proper cooling channel design so the part cools uniformly, minimizing defects such as warpage or shrinkage and improving the overall efficiency of the injection molding process.

Core

The core is the internal part of the mold that forms the hollow or recessed areas of the product. It is often part of the B-side of the mold and is designed to create internal features like holes or undercuts. Core design must consider ease of removal, cooling efficiency, and structural integrity. It may require additional mechanisms, such as core pulls or lifters, to release complex parts without damaging them.

Cycle Time

Cycle time is the total time required to produce one complete part in the injection molding process. It includes the phases of injection, cooling, and ejection. Optimizing cycle time is essential for maximizing production efficiency and reducing costs. Each phase must be carefully controlled; for instance, too short a cooling time can lead to defects, while too long a cycle time reduces productivity.

Draft Angles

A draft angle is a design feature applied to the vertical sections of the part. It is a slight angle that tapers the sides of the part so that it can be easily removed from the mold. Without draft angles, the friction created when removing the part could damage its surface or leave injector pin marks. Draft angles are typically measured in degrees from the vertical axis of the part. Common draft angles are from 1° to 2°, but the exact angle depends on the material, part design, and complexity of the mold.

Features that require draft angles are any deep surface that could become stuck, including walls, ribs, posts, bosses, and gussets. Textures and polished surfaces also require draft angles. For textures, the angle will depend on how heavy the grain is. Polished surfaces require draft to avoid drag marks. In the case of internal features like holes or cavities, the draft angle is applied in the opposite direction to ensure easy removal from the mold core.

The plastic being used and the part complexity can also influence the required draft angle. Different materials have different shrinkage rates and flow characteristics, which can influence the required draft angle. For example, materials that shrink more during cooling might require larger draft angles. More complex parts with intricate designs may need larger draft angles to ensure easy ejection from the mold.

Ejection Pins

Ejection pins are rods that push the finished part out of the mold once it has solidified. They are typically located in the B-side of the mold and must be precisely timed and positioned to avoid damaging the part. Ejection pins must exert enough force to remove the part without leaving marks or causing deformation. Proper design and placement of ejection pins are vital for maintaining the quality of the finished product.

Family Mold

A family mold is a type of mold designed to produce multiple different parts in a single cycle. This is achieved by incorporating multiple cavities, each shaping a different component. Family molds are efficient for manufacturing complex assemblies with various parts, as they reduce the number of molds needed and streamline the production process. However, designing family molds requires careful balancing to ensure consistent quality across all parts produced.

Fillet and Radii

Sharp angles and corners should be avoided when possible. Plastic doesn’t flow well into sharp corners, which can result in mold stress and a functionally weak part. Fillets typically refer to rounded internal or external corners, often with a specific function like stress reduction or improved material flow. Radii refers to the curvature of any edge or feature, not limited to corners. As a rule of thumb, the outside radii should be one wall thickness larger than the inside radii.

Flash

Flash is the excess plastic that leaks out of the mold cavity during the injection process, forming thin, unwanted protrusions along the parting line. Flash can occur due to improper clamping pressure, mold wear, or design flaws. It often requires additional post-processing to remove. Minimizing flash involves precise mold design, maintenance, and operation to ensure tight seals and proper alignment during molding.

Gates

A gate is a small opening that enables plastic to enter the mold and fill the cavity. The size of the gate is determined by the material used. For example, shear-sensitive materials require larger gates to reduce sheer heating, which is the temperature increase that happens when the material passes through the gate. Undersized gates cause a buildup of material pressure. This causes the material to be squeezed into the cavity too quickly in a motion called jetting, resulting in waved imperfections around the gate.

Gates can be located at various points around the mold cavity, depending on the part and material. The gate leaves behind a blemish, so typically, placement will be in less visible areas. Gates can take different forms: wide, narrow, tapered, or maintain a consistent diameter. The gate height is generally 40-60 percent of the wall thickness, which is also a good starting point for the width (the side-to-side dimension of the gate at the part).

Gate placement can influence your design choices and your part’s quality. Your mold designers will choose the optimal gate location based on requirements for mechanical loading, fill pattern, and aesthetics. Ideally, it is away from pins and cores in a thick area of the part that won’t affect its functionality or aesthetic.

Hopper

The hopper is a container that feeds plastic pellets into the barrel of the injection molding machine. It is typically located above the barrel and uses gravity to ensure a continuous supply of material. The hopper may include features like a drying system to remove moisture from the pellets, which is critical for maintaining material quality and preventing defects in the molded parts.

Injection Screw

The injection screw is a rotating and reciprocating component within the barrel that melts and conveys the plastic through the barrel toward the mold. Its design includes flights and mixing sections that facilitate the melting, homogenization, and pressurization of the plastic. The injection screw’s efficiency directly impacts the quality and consistency of the final product.

Knit Lines

As material flows into that cavitation, the place where two flow fronts meet is called a knit line. This typically occurs when there is a core or area that it flows around, such as a hole. These lines can vary from practically invisible to highly visible, depending on the material choice and geometry of the part. The primary issue is that knit lines have reduced mechanical properties.

Using nominal wall thickness can help help that walls are so thick that the resins begin to cool too quickly. Gate placement should be so that the knit lines are minimized or moved to an area that won’t have stress applied.

Parting Line

The parting line is the line where the two halves of the mold meet to form the final product. It may also be referred to as the part seam. The placement of the parting line can affect the part’s aesthetic. It will add width or length to the part; although the amount is small (about 0.005 in ), it could create issues for mating parts that fit one inside the other, for example.

Plasticate

Plasticating is the process of melting plastic pellets in preparation for injection molding. This involves heating the pellets in the barrel and mixing them using the injection screw. The goal is to achieve a uniform melt with the right viscosity for injection. The efficiency of the plasticating process depends on factors like barrel temperature, screw speed, and back pressure, all of which must be carefully controlled to ensure high-quality molded parts.

Ribs and Gussets

Ribs are thin, elongated features that extend from the surface or wall of a molded part to provide reinforcement and increase stiffness, enhancing the part’s strength and rigidity, especially in thin-walled sections.  They allow for the use of less material while maintaining structural integrity, reducing both weight and cost. Ribs should be designed with a thickness that is typically not more than 60% of the adjacent wall thickness to avoid sink marks and ensure proper cooling, but this can vary with the material. The height and spacing of ribs should be optimized to provide the required reinforcement without causing molding defects. Ribs that are too tall or too closely spaced can lead to warping or cooling issues. Like other features in molded parts, ribs should have draft angles (around 1°) to facilitate easy ejection from the mold. Adding fillets at the base of ribs can help reduce stress concentrations and improve the flow of the molten plastic during molding.

Gussets in injection molding are reinforcing features that provide additional support to corners, walls, or junctions in a molded part, increasing the part’s strength and stability of the part, particularly in areas where loads are applied. They help distribute stresses more evenly, reducing the likelihood of warping, bending, or cracking. Gussets should be placed in areas where additional support is needed, such as at the base of a vertical wall or around mounting points. The size and shape of gussets should be optimized to provide the necessary support without adding excessive material. Triangular or trapezoidal shapes are common. Gussets should have draft angles to facilitate easy ejection from the mold, typically around 1°.

Runners

Runners are channels that distribute molten plastic from the sprue to the gates of the mold cavities. They play a crucial role in ensuring that the plastic flows evenly and fills each cavity properly. Different types of runners, such as cold runners and hot runners, have their advantages and disadvantages. Cold runners are simpler and less expensive, but they require more material and create more waste. Hot runners are more complex and costly but reduce material waste and cycle time, improving overall efficiency.

Shrinkage

Shrinkage is the contraction of plastic as it cools and solidifies within the mold. It can cause dimensional inaccuracies and affect the final product’s fit and function. All materials will shrink to some degree. Designing for shrinkage involves understanding the material’s shrinkage rate and incorporating appropriate allowances in the mold design. Controlling cooling rates and ensuring uniform cooling can also help minimize abnormal shrinkage and its effects on the molded part.

Sprue

The sprue is the main channel through which molten plastic flows from the nozzle of the injection molding machine into the runners. It is the first point of entry for the plastic and must be designed to minimize pressure loss and ensure smooth flow. The sprue design, including its length and diameter, affects the overall flow characteristics and efficiency of the injection molding process. Proper sprue design is essential for achieving high-quality molded parts with minimal defects.

Undercuts

An undercut is an indentation or protrusion that prohibits the ejection of a part from a one-piece mold. Snaps, holes, and vents may need to be modified to be used without the use of lifters or slide actions. Sometimes, moving the parting line will allow the undercut feature to be more easily ejected with a straight pull. Doing so can affect draft angles and how parts mate during assembly, so this should be evaluated as well. Shutoffs can also be used for some undercuts.  A shutoff, a piece of metal sticking out from the core side of the mold to produce an overhang without a side action, can be used to produce the underside or a hook or cantilever. When it is not possible to design around an undercut, slides or lifters can be used. However, these add cost to the mold. External undercuts, recesses, or projections on a part’s outer surface, which prevent the part’s direct removal from the mold cavity, can be released with a slide or slider. Lifters are used to release the part when the undercut is on the inside surfaces of the part.

Vent

Vents are tiny openings in the mold that allow air and gases to escape during the injection process. Proper venting is crucial for preventing defects such as burn marks, voids, and incomplete filling. Vents must be strategically placed to ensure effective evacuation of gases while preventing the escape of molten plastic. Designing effective vents involves balancing their size and location to maintain optimal mold performance.

Wall Thickness

Wall thickness is the distance between one surface of your part and its opposite sheer surface. The wall thickness should be designed to meet the strength, stiffness, and dimensional stability needed for the part without being too thick or too thin.

When possible, uniform thickness is preferred because it makes cycle times repeatable and consistent, results in fewer cosmetic defects, and can lower the part cost. Nominal thickness improves moldability because the melted plastic can move through the cavitation of the mold more easily. If it is moving from a thick wall section to a thin wall section, the pressure increases, and there may be flow hesitation, which could result in surface defects or short shots. Conversely, flow from a thin wall section to a thick wall may result in warping, sink marks and voids, or other defects. If maintaining a nominal thickness isn’t possible, there are some design options that can help.

Ribs, stiffening features, curves, and corrugations can help reduce wall thickness while providing rigidity. Coring out the section removes excess material, making the part easier to mold. Below are some recommended wall thicknesses for commonly used materials.

Recommended Wall Thicknesses For Commonly Used Thermoplastics

Warpage

Warpage is the distortion or bending of a molded part due to uneven cooling or internal stresses. It can significantly affect the part’s dimensional accuracy and functionality. Controlling warpage involves optimizing mold design, cooling rates, and processing conditions to ensure uniform cooling and stress distribution. Addressing warpage in the design phase helps produce high-quality parts that meet stringent specifications and performance requirements.

Moving Beyond Injection Molding Design Guidelines

When you consider that hundreds of plastic choices are available and infinite levels of part complexity, it becomes clear that general injection molding design guidelines may not be sufficient for your injection molded part; this is why every quote from Quickparts incorporates a comprehensive design for manufacturability (DFM) analysis to optimize your design for efficient, cost-effective production. Before we begin your injection molded project, we work closely with you to ensure the finished product will meet your specifications.

Ready to get started? Contact us to learn more or to start a project.