Injection molding is among the most widely used manufacturing processes in the world. You may have used it to produce plastic parts that must be assembled before their actual application.
Their correct assembly depends on how different components join together, which is affected by the injection molding tolerances you have set.
Not ensuring proper tolerances can increase your production costs; therefore, you should optimize them. This article will discuss several ways to control plastic injection molding tolerances using design for manufacturing, tool design, selection of materials, and process control.
What Are the Steps of the Plastic Injection Molding Process?
Injection molding starts with injecting plastic resins from a hopper into the barrel. There, a reciprocating screw moves it along heating devices so that the resins melt as they move forward toward the mold cavity.
Once the molten plastic fills the cavity, it is allowed to cool and solidify. The mold then opens, and ejector pins are used to remove the molded part. The mold closes after that, and the process is repeated to produce the next part.
Ideal for mass production, you can also use it for prototyping and other low-volume applications.
Why Are Tolerances Important for Injection-molded Parts?
As the molten plastic cools inside the mold, it shrinks naturally. While this is to be expected, you may face problems if the molded part’s dimensions vary significantly from your original design.
In that case, your individual parts may not align, and joining them together to obtain a final product becomes difficult. This leads to a loss of functionality, forcing you to invest further to ensure better results.
Therefore, you should incorporate a precise range of allowable variations, called injection molding tolerances, beforehand to ensure high-quality results.
By setting maximum and minimum values for your part’s dimensions, you set standards for your worst-case scenario.
How to Optimize Injection Molding Tolerances
There are four main ways you can optimize your plastic injection molding process to optimize your tolerances:
Use Design for Manufacturing for Part Design
Design for manufacturing (DfM) refers to optimizing your part’s design early in the product design stage to enable cheaper and more efficient results. It is easier to identify and eliminate design concerns at that stage of product development.
Experienced manufacturing services like Kemal have professional designers onboard who can help you apply these principles to your design. Suggested design features include:
Overall Part Size
The importance of part tolerance becomes more pronounced the larger your part gets. Larger parts are more susceptible to experiencing warping, shrinkage, and hence more distortion. This is why part size is an important consideration in DfM.
Wall Thickness
Factors that affect the rate of shrinkage include wall thickness, temperature, material flow, and the rate of cooling. Therefore, to achieve a steady shrink rate in your parts, you should ensure that your part design has uniform wall thickness.
This eventually leads to a lower rate of defects such as warping, cracking, twisting, and sink marks.
There are several ways to achieve uniform wall thickness, some of which are listed below:
- Incorporate ribs in your part design.
- Select the right materials.
- Unless you really need them, avoid including features such as sharp internal corners, bosses with poor design, and long spans without support.
- Including a radius on sharp corners.
- Avoid thick walls.
Draft Angle
Draft angles are included to enable easy removal of a part from the injection mold after cooling. As the mold fully surrounds the part, this removal process can damage the part’s structure due to friction.
If you don’t include draft angles in your injection molding design, your parts may experience shrinkage, causing them to get stuck inside the mold.
Draft angles are expressed in terms of degrees or millimeters.
There are no set standards regarding how you should include them in your design, but experts suggest the following:
- For most parts, you should keep the draft angle between 1° and 2°.
- A 1° draft angle should be added for a 1-inch depth.
- The surface texture also affects the size of the angle. A 3° angle is recommended for light textures, while heavy textures require draft angles of greater than 5°.
- It is recommended to have a 0.5° angle for all vertical surfaces.
Bosses
Bosses refer to cylindrical projections located on the walls of a part. They help with the alignment of different parts during assembly, serve as a bearing for other components, and act as points of fixation by enabling the insertion of screws in them.
You should ensure that the walls of the bosses are not too thick.
This is done due to the following reasons:
- To prevent sink marks and voids
- To increase cycle time
- To avoid the fragmentation of the plastic while fastening it
Moreover, bosses should be added to the closest sidewall to ensure the distribution of extra loads, enhancing the frigidity and material flow of the part.
Choose the Right Material
Another way of optimizing your plastic injection molding tolerances is to choose an appropriate material for your process.
Since the nature of the material used affects how much it undergoes shrinkage, you should keep the following points in mind:
- Material composition: Since amorphous plastics have a relatively less compact structure than semi-crystalline plastics, they undergo less shrinkage.
- Molecular weight: Plastic resins with a greater molecular weight exhibit more viscosity and experience a larger pressure drop. This causes a greater degree of shrinkage.
- Use of additives: The shrink rate of your plastics decreases if you add fillers with a low thermal expansion coefficient, which is a measure of how much a material’s size changes in response to a change in temperature.
Consider Mold Tools
In addition to ensuring you use the right part design and material for your injection molding process, you also have to pay attention to the mold you use.
To account for material shrinkage, the mold tool is usually made larger than is required. Parts that have a combination of thin and thick walls undergo different rates of cooling, which causes defects such as warping and sink marks. This impacts injection molding tolerances.
Therefore, mold makers consider the following aspects when designing the mold:
Tool Cooling
As we have mentioned before, how well your material cools down before ejection affects the quality, functionality, and appearance of your product.
Uniform cooling in the mold is achieved by incorporating cooling channels at specific locations in the mold. Experts also track the time it takes to fill the mold, the viscosity of the plastic resin used, and the injection pressure.
Tool Tolerance
Variations in the dimensions of your injection-molded part are not only caused by shrinkage. In fact, these variations are added to those already present in the process as a result of mold tolerances.
Although molds are usually manufactured using precisely controlled CNC machining, their tolerances should also be factored in.
Toolmakers also ensure that the mold is “metal safe” to enable design modifications and to save time and money.
Any design tweaks are included in the original mold without having to make another one from scratch. Out-of-tolerance dimensions are machined away to ensure dimensional accuracy.
Location of the Ejector Pins
Ejection follows the cooling stage in injection molding. The mold tool opens, and the ejector pins remove the part from the cavity. You must ensure that the part is ejected as quickly as possible, which necessitates positioning ejector pins at the right points.
Not doing so can damage the part since improper locations of the pins can cause undesired indentations on the part. In addition to this, some materials do not achieve full rigidity when they are ejected. An uneven ejection process can cause warping.
Location of the Gate
The gate refers to the opening through which molten plastic resin flows into an injection mold. Its location should be carefully designed since not doing so affects the cosmetic appearance of the molded part.
More importantly, it may also cause warping and shrinkage due to the uneven filling of the mold.
Mold designers consider the following things when designing the gate location:
- Gates should not be situated adjacent to features such as pins and cores, which can block the flow of material around them.
- Gates should be located near thick walls to guarantee complete filling.
- Gates should be positioned where the cross-section is the deepest to enhance the material flow and prevent sink marks and voids.
Implement Repeatable Process Controls
Once all of the aforementioned optimizing methods are applied to control your plastic injection molding tolerances, you can actually start the manufacturing process.
However, there are still other factors that you have to keep an eye on. These are related to the molding conditions themselves and include the injection temperature, pressure, and the time the mold holds the molten material (holding time).
These have to be tweaked to optimize the process so that you can ensure dimensionally accurate parts.
You may consider adding temperature and pressure sensors in the mold, especially for complex parts, to constantly track, get feedback on, and correct these variables. Ensuring a steady temperature and pressure usually results in higher-quality results.
Achievable Plastic Injection Molding Tolerances
The tables below list suggested tolerances for common plastics used in injection molding. Tolerances are commonly written in terms of a value with a +/- sign, denoting the maximum acceptable variations.
As you can see in the tables below, tolerances depend on the material used and the range of dimensions. Commercial tolerances for each dimension range are greater than the corresponding precision tolerances.
Moreover, adding fillers such as glass fibers (GF) reduces the range of tolerances needed as the shrinkage rate decreases.
Dimensional Tolerances
The dimensional tolerances of the commonly used plastics are listed below. For precise dimensions above 100 mm, you should conduct a project review for all materials.
Straightness or Flatness Tolerances
These tolerances are added to account for warping, which happens as a result of variations in the rates of shrinkage along the direction of the material flow.
Hole Diameter Tolerances
As you can see in the table below, the larger the diameter of the hole, the larger the tolerance that should be set.
Blind Hole Depths Tolerances
A blind hole is a hole that does not fully pass through a component. In injection molding, they are secured at one end, which makes them more susceptible to deformation in response to a strong force of material flow.
Concentricity or Ovality Tolerances
This refers to setting the wall thickness, which is the difference between the inside and outside diameters of a part.
Kemal: Your Plastic Prototype Manufacturing Service Provider
Kemal realizes how important injection molding can be for your manufacturing needs. That is why we follow strict international tolerance standards and quality control inspections to ensure we achieve excellence in production and meet your provided design specifications.
Our experience serving numerous clients belonging to various industries has enabled us to learn and grow so that we can cater to all your prototyping and mass production demands.
Our team of professionals has the necessary expertise to offer feedback and suggestions regarding your part tolerances before starting production.
FAQs
Conclusion
To sum up our discussion, injection molding is a useful technique to produce a large variety of durable parts. In spite of our efforts, some dimensional variance is always present in the molded product.
By using the abovementioned optimization methods, we can ensure that these variations remain within the allowable limits and that we achieve a high-quality, consistent product.
