Injection molding is one of the most popular methods for rapid manufacturing. The injection molding draft angle is a critical parameter in this process that has a significant impact on product quality.
In this article we will focus on draft angle injection molding. We explain what injection molding is, why it's important, and what you should consider when designing molds for injection molding.
What is the draft angle in injection molding?
The injection molding tilt angle is an essential geometric characteristic of injection molds. This is the taper or degree of inclination of the mold walls along the demolding direction.
As it is a property of the shape, the draft angle is also visible on the component. Its unit of measurement is degrees.
Typically, engineers use an angle of about 1.5 degrees. However, for certain special applications the angle may be between 0.5 and 10 degrees.
Why is draft angle important in injection molding?
The concept of tilt angle is easy to understand. However, its benefits are not that intuitive to understand and require some explanation. It is the most important of all training angles. Small miscalculations can ruin the entire product line and even damage injection molding equipment.
Without further ado, let’s dive into the various reasons that make it an important parameter in injection molding.
Ejection of smooth parts
Part ejection is a crucial step in the molding process and an important reason for the tilt angle of injection molding. Just before the part is ejected, it touches the walls of the mold. Contact stresses are high enough to deform the part through friction during ejection.
Therefore, it is desirable to minimize the effects of this friction force. Engineers do this by incorporating a sketch. Once the part is pushed out by the ejector pins, the entire part bounces and loses contact with the mold walls. This lack of contact means there is no friction.
Without the release angle, the part would rub against the mold surface during the entire release process, as shown in the figure. This has an extremely detrimental effect on surface quality and manufacturing tolerance.
Furthermore, the lack of friction also reduces the required ejection force, which has a positive effect on the power requirement and speeds up the ejection process.
To forgive
Warping is one of the most serious defects in the injection molding process. To explain what it is, see the illustration in the previous section, which shows parts with and without drafts.
If no inclination angle is foreseen in the design, a vacuum may form in the cavity created when the part is ejected. This vacuum causes the part to fold in on itself. This undesirable deformation is warping.
As you can see, this problem does not exist with a draft angle for injection molding. As soon as the ejection process begins, the part separates from the mold, creating space for air to fill the voids, preventing the formation of a vacuum. Therefore there is no deformation.
Surface Finish
We briefly discussed this advantage of draft injection molding in the previous section. The chamfer means the part does not rub against the shape, which means the surface quality is maintained.
An important consideration here is the variation of the draft angle with surface structure requirements. In addition to the approximate geometric characteristics of the component, the inclination angle must also take into account the characteristics of the surface structure.
This means that any craters/projections that make up the surface structure must also have enough space to separate from the mold wall. We'll talk more about this soon.
Reduces depth draft
The mold halves separate along the design line. This drawing line is strategically positioned to avoid critical defects such as deep drawing and burr formation.
Deep drawing solves the problem of the part getting stuck in the mold. The draft angle of injection molding helps reduce the negative effects of deep drawing, with a larger draft angle reducing scratches.
Production costs
Mold angles have a huge impact on the cost of injection molds. From the above discussion, we can derive numerous ways in which injection molding tilt angle helps to reduce costs.
First, the elaborate molds produce parts with better surface, less distortion and fewer deep drawing problems. This means manufacturers incur less costs on finishing parts and defective parts.
Furthermore, maintenance costs are also much lower due to air currents. The friction mentioned above also damages the mold, which needs to be periodically polished and even replaced due to wear.
Finally, the demolding angle also saves costs because it speeds up the injection molding process. This results in faster cooling cycles and part ejection.
Design Guidelines for Draft Angle in Injection Molding
The draft angle in injection molding is a sensitive parameter that must be precisely coordinated by mold designers. In this section we present a list of recommendations for selecting the correct tilt angle for specific applications.
Mechanical adjustments and thermal shrinkage
Injection molding changes the thermal contraction of the resin/molten metal during the cooling cycle. This is because shrinkage depends on the geometry of the part.
Retraction always occurs in the direction of the geometry's center of gravity. This means that the outer surfaces of the part shrink away from the mold and separate from the walls, while the inner surfaces shrink inward toward the mold and hold it tighter. As a result, the geometric dimensions of the part can change and affect its ability to form proper mechanical fits during assembly.
Therefore, designers must ensure that draft angles included in their designs do not violate dimensional tolerances after shrinkage. This especially applies to all components and surfaces that are part of a mechanical adjustment.
Greater draft for rough textures
We have already discussed the relationship between textures and injection molding design. The release slope must create enough space so that the texture is not scraped off during ejection.
The rule is: the coarser the texture, the greater the departure angle should be. This makes it easier to eject the pieces and preserves the quality of the texture.
For a reflective surface, a tilt angle of 0.5 degrees is generally appropriate. However, for every 0.1 mm increase in surface roughness, the draft angle must be increased by 0.4 degrees. For particularly coarse samples, engineers can also choose extreme tilt angles of up to 10 degrees.
Superior design for deeper functions
The deeper the vertical features of the component become, the more susceptible it becomes to deformation. The deeper parts have more empty spaces to create a vacuum during ejection. Therefore, they deform more and more quickly.
The solution is simple: increase the draft angle during injection molding to obtain deeper details. The general rule is to add another degree of slope for every inch of depth of the piece. This rule may change slightly depending on the size and material of the part, but generally applies to most injection molding jobs.
This is an extremely efficient method for machining components with features such as deep pockets and cavities.
Central cavity approach for functionality and aesthetics
The center cavity approach is a technique used on parts where you want the outside to be smooth while the inside can remain somewhat rough. This requirement is usually for aesthetic purposes. For protective housings, for example, the outer surface is smooth and shiny for visual reasons. It may also be a functional requirement when the outside needs to slide or the inside needs to be assembled with other components.
In any case, with the core cavity process, the injection molding draft angle for the core is a little larger, so the part shrinks as it shrinks. In other words, the internal part adheres to the mold while the external part completely separates from the walls.
As a result, the quality of the external surface remains intact, but the internal surface has greater roughness.
Quick Overview: Tips and Tricks for Determining Lean Angle
It can be a little difficult to absorb so much information about shape angles at once. To help our readers, here is a comprehensive list of tips and tricks:
- For general injection molding work, an injection molding draft angle of 1.5 degrees is recommended.
- Incorporate draft angles into the cavity and core, although the core's draft angle should be slightly larger.
- For every inch the depth of the piece increases, add 1 degree of slope. By depth, of course, we mean the dimension along the drawing direction.
- Smooth surface structures require a small draft and rough structures require a large draft. Increase draft by 0.4 degrees for every 0.1 mm increase in surface roughness.
- Add sketches to all vertical elements such as walls, ribs, fins, recesses, etc.
- Use the center cavity approach when the outer surface of the component needs to be smooth.
- If the component has features that require lateral drawing (additional drawing direction), also include drafting slopes in those features.
- Hard materials with abrasive properties require larger rake angles than soft, elastic materials.
University Degree
This concludes our discussion of what tilt angle is, why it is beneficial, and what you should know about it. We hope the reading was interesting and expanded your knowledge as an injection molding designer. Do you have an injection molding project?
We also offer our customers comprehensive support from the start. Our free Design for Manufacturing (DFM) analyzes help you optimize your tooling designs before starting production. We invite you to contact us today with your injection molding projects and tooling!
Common questions
When should a draft angle be included in the design process?
Although the inclination angle is a mandatory requirement for perfect production, it affects the functionality of the parts. Therefore, it is recommended to include the draft angle at the very beginning of the design process, that is, during preliminary design and prototype construction.
Can I avoid tilt angles?
Due to the serious consequences, in most cases it is not advisable to neglect departure angles. They are an essential part of the manufacturing phase and affect the quality of production. Under certain conditions, e.g. For example, if the part is very small or the tolerances are very tight, the designer may choose to avoid stretching, but this is still not recommended.
What are positive and negative drafts?
A positive draft angle occurs when the draft elements “taper” into the mold. This means that the elements are wider at the base and narrower in shape. A negative angle is the opposite of this. It “taperes outward into the shape.”
