Injection molding is the most common manufacturing process for making plastic products composed of multiple parts that must be assembled at the final stage of production. Part assembling involves the proper alignment and joining of different parts. Here, tolerance is very important, and if not correctly specified and controlled, assembling will fail.
Errors relating to tolerance are always problematic due to the cost of an injection mold. Therefore, there is a need to know how to control injection molding tolerances. This article will introduce how you can control plastic molding tolerances using design for manufacturing (DfM) materials selection, tool design, and process control.
Why are Tolerances Important for Injection Molded Parts?
The degree of variations in any rapid prototyping process depends on its accuracy, and although injection molding is fairly precise, a little variation still exists. This variation makes it important to determine the permissible range of deviation for the effective functioning of parts after assembly.
Plastic molding tolerances are critical in assembling products having multiple injection molded parts. For example, if you want to join two plastic molded parts using a bolt, you need to drill a hole in both parts. Any error in the location and size of the holes can result in errors during assembling and loss of function. Therefore, there is a need for locational tolerances on both parts for maximum function.
In simple terms, controlling and optimizing injection molding tolerances is a way of assuming the “worst-case scenario.” It involves determining the permissible range of deviation that aids the maximum functioning of products.
How to Optimize Injection Molding Tolerances
To optimize injection molding tolerances, you can optimize the product design using Design for Manufacturing, using the right injection materials, tweaking the injection mold design, and process controls. This section will introduce each category so that you can achieve a realistic plastic injection molding tolerance.
During the Design Phase
Manufacturers that use injection molding encounter problems such as warping, excessive shrinking of parts, and part misalignment during the process, which affect the tolerance of injection molded parts. To counter this, designers ensure that every product design sticks to Design for Manufacturing (DfM) as it can limit the occurrence of such issues.
You can get access to a good DfM by engaging in a good rapid prototyping service with wide experience in injection molding (like RapidDirect) early in the design process. Below are four factors you should consider in terms of part design.
· Overall Size
The larger the overall size of the products or the parts you want to make, the higher the importance of tolerance. In plastic injection molding, an increase in the size of a product will likely distort the product, can lead to warping, or shrinking. Therefore, to reduce this, considering the size is important.
· Wall Thickness
Shrinkage is the contraction of the plastic part during the cooling phase. It is an integral injection molding process controlled by factors such as part’s wall thickness, temperature, etc.
Having a uniform wall thickness will lead to a stable shrink rate which reduces cosmetic defects such as warping, sinking, cracking, and twisting. You can have a uniform wall thickness by doing the following:
- Avoid parts geometries such as sharp internal corners, long unsupported spans, and poorly designed bosses
- Use rib to strengthen walls if essential
- Place radius on inside corners alleviates warping
- Use the right materials with wall thickness in mind
Also, you should avoid using thick walls as this reduces the cooling rate. Consequently, it will increase the shrinking rate and consequently warping,
· Draft Angles
Draft angles are an important part of injection molding design as they facilitate the easy removal of a part from an injection mold. The ease of removal can reduce the damage because of friction, minimizes wear and tear, and ensures a smooth finish.
Not incorporating draft angle in your injection molding can result in shrinkage and parts getting stuck during ejection (plastic materials such as nylon will still produce a perfect job at 00). Draft angle is measured in degrees/inch/millimeter, but there is no standard injection molding tolerances rule when incorporating one into product design. However, we have some rules of thumb you could follow.
- A draft angle of 10 to 20 is suitable for most parts.
- Add 10 for 1-inch depth.
- Use 30 for light texture and >50 for heavy texture
- Use 0.50 on all vertical surfaces.
Bosses are a critical part of product design used for fastening components during part assembling. Designing a boss comes with a few yet important considerations. One is that the wall of the boss must not be thick. Incorporating a thick boss in your design will lead to the following:
- Creation of voids and sinks mark
- Expanding the cycle time
- Splitting of the plastic while fastening
Also, you should core bosses (i.e., attach them to the nearest sidewall). Doing this will lead to additional load distribution for the part, and improvement of the part frigidity and material flow.
Select the Right Material for Tight Injection Tolerance
Your choice of material plays a huge role in injection molding tolerance based on having uniform shrinkage (i.e., the contraction of an injection molding material during the cooling phase due to the change in density). Shrinkage depends on factors such as wall thickness, temperature, parts, and the type of material. In terms of material selection, you should use the following factor to aid your choice:
- Plastic Composition: Amorphous plastics, e.g., ABS, have lower shrinkage than semi-crystalline plastic e.g., polyethylene, due to their less-compact structure.
- Molecular Weight: High molecular weight resins will have high viscosity and a high-pressure drop which increases the shrink rate.
- Additives: The addition of fillers with low thermal expansion will reduce the shrink rate.
Different resins have different shrinkage rates. Therefore, you must factor this into material selection and injection mold design to reduce cosmetic defects such as warping, sinking, cracking, and twisting, which affects the tolerance of injection molded parts
Keep Mold Tools in Mind
On selecting the ideal materials, mold designers offer to oversize the mold to account for material shrinkage. Different materials have different shrinkage rates due to uneven thicknesses. To reduce this, you should take note of the following when designing the mold.
· Tool Cooling
Cooling is a crucial step in injection molding, which determines the quality of the finished product. It involves the uniform cooling of the heated plastic polymer before its ejection.
Cooling must be uniform, as non-uniform cooling leads to shrinkage, sink marks, jetting, warping, etc., which affects the final product’s appearance, tolerance, and functioning.
In order to achieve uniform cooling, injection mold designers should place cooling channels in the mold at strategic and effective points. Also, there might be a need to monitor the following parameters:
- Injection pressure
- Resin viscosity
- Fill time
· Tool Tolerance
An injection mold is commonly made using CNC machining, which makes it possible to achieve a tight tolerance that maintains accuracy throughout the cycle of heating and cooling of the process.
Tolerance will ensure that parts cool properly without reducing accuracy. While it is not common due to the use of CNC machining, not controlling tolerance when making a mold can result in severe defects such as warping, shrinking, sinking, etc.
· Ejector Pin Location
An ejector pin is a feature in an injection mold that pushes the final product from the mold. The pin comes in different shapes (flat shapes are the best), through which it applies some amount of force to push the product out. Consequently, when it is in the wrong place, it can cause unwanted indentations on the finished product. Also, in non-rigid materials or materials with non-uniform cooling, an ejector pin can rupture the unfinished product leading to several cosmetic defects and physical aberration.
· Gate Location
The gate is the part of the mold through which the injection molding materials enter the mold. When deciding the location of the gate, you should consider the following
- Place the gate in the deepest cross-section: This will minimize sinking and void. It will also improve plastic flow.
- Place the gate in thick-walled areas for complete packing.
- Do not place the gate beside obstructions such as pins and cores.
- Do not allow the gate location to affect the runner’s distortion and the user’s aesthetics.
The gate must be located in the right place as placing it in the wrong place can lead to the following:
- Uneven fill rate: This will lead to warping and shrinkage
- Poor cosmetic finishes.
Execute Repeatable Process Controls
Process controls are ways by which manufacturers calibrate variables that can affect part quality. These variables are an integral part of every manufacturing process, and their calibration helps to reduce the deviation. Common variables are temperature, pressure, and holding time. A few ways of achieving this includes:
- You can embed temperature and pressure sensors in the mold to measure the mold environment and provide real-time feedback and repeatable process controls.
- Resins have a high thermal expansion coefficient which can lead to alteration in size due to a change in temperature. Therefore, when working with parts at a consistent temperature.
Achievable Plastic Injection Molding Tolerances
To achieve real injection molding tolerances, there are some achievable plastic injection molding tolerances you can introduce to your plastic mold design. Below are the common ones for major plastics used in plastic injection molding:
· Dimensional Tolerances +/- mm
Maintaining the degree of accuracy can be very challenging. Therefore, designers make use of the (+/-) sign to show a range in measurement. Each material has a different tolerance range as the dimensions increase. The table above shows the dimensional tolerance of major plastic used in injection molding.
· Straightness / Flatness Tolerances
Warping occurs due to different mold shrinkage rates in the direction of mold flow and across the flow. It can occur due to different wall thickness, which has different shrink rates. Tweaking the mold design, better gate position, and process control can minimize warping. However, you might need to have a practical tolerance in terms of plastics as warping is hard to reach 100%.
· Hole Diameter Tolerances +/- mm
The larger the hole size, the more the need to consider tolerance. The chart above explicitly shows the tolerance for different sizes of hole diameter.
· Blind Hole Depths Tolerances +/- mm
Blind holes are holes drilled into a workpiece using an insert core without breaking through it. They are fixed and held at one end, which increases their tendency to undergo deformation under a strong meld flow force. The chart above shows the different tolerance you can use.
· Concentricity/Ovality Tolerances +/- mm
This involves determining the wall thickness (the difference between the outside diameter and inside diameter). The chart above shows the different tolerance and change in cost as regards achieving this tolerance.
There is always a degree of variations in injection molding that makes it important to have a permissible range of deviation so that for effective functioning of parts after assembly. As a result, injection molding tolerances are critical in assembling products having multiple injection molded parts.
On controlling and optimizing injection molding tolerances, it is possible to determine the permissible range of deviation that aids the maximum functioning of products. Common ways of achieving this depend highly on DfM, material selection, and process control, and this article helps you to simplify the common ones that will be very useful in your project.
Get Started with RapidDirect
Producing high-performing and consistent products from multiple injection-molded parts comes with targeting and reducing variations in injection molding. Through part optimization, it is possible to make parts that are reliable and of high quality.
Partnering with us at RapidDirect can be your best decision in making parts of high quality. We are an expert rapid prototyping company well experienced in injection molding service with the necessary machine and experience to actualize that goal. Our team comprises qualified individuals committed to delivering a high-quality product at the right speed.
Injection molds are made using CNC machining, which has high accuracy and precision. Typically, it is possible to achieve a tolerance of +/- 0.005 inches. It is also possible to achieve more restrictive tolerances using the method. However, this depends on the machine and team experience.
Using a tight tolerance in injection molding will lead to an increase in the cost of manufacturing. Also, it makes it more difficult to make and assemble multiple parts to the final product. Therefore, there is also an increase in the labor cost and high wastage of materials
Injection molding is very accurate, making it suitable for fabricating many types of materials. Although it has some design restrictions, the mold is precise and is typically within 0.005 inches. Consequently, it is a very reliable method of production used by many rapid prototyping services.