Most plastic products are made by injection molding. This is mainly due to the high productivity and extremely low unit cost of the process. As with any manufactured component, tolerance is critical. If not specified or controlled correctly, the final parts will not fit together during assembly. This kind of error needs to be avoided especially because the upfront cost of the mold is very high. This paper will describe how to control injection molding tolerance and ensure high quality through DFM (Design for manufacturing) principles, material selection, tool design and process control.
Why is tolerance so important?
For example, if two flat parts need to be bolted together, the positional tolerance of the holes on each part must consider all possible cases. Even if one part is at its minimum tolerance and the other part is at its maximum tolerance, they must still fit during assembly. In this case, it seems simple, but when multiple parts need to be assembled, one part may cause the whole assembly to not work properly. Tolerance analysis, such as worst case method, tolerance stack and statistical analysis, can be used to optimize the injection molding tolerance of multi part components.
Factors affecting injection molding tolerance:
1. Part design
One of the most important ways to limit warpage, excessive shrinkage, and part misalignment is to use DFM principles when designing parts. This is best achieved by working with injection molding services early in the design process to prevent costly redesign later in the design phase.
wall thickness - parts with variable wall thickness may have uneven shrinkage. When thick areas cannot be avoided, coring must be used to maintain uniform wall thickness. Uneven wall thickness will lead to part deformation, which will affect tolerance and assembly. Thicker walls are not always the best choice for increasing strength; Where possible, it is best to use stiffeners and gussets to improve the strength of the parts.
draft angle - the draft angle is crucial to ensure easy ejection from the tool. If the best condition is not reached, the parts may get stuck during ejection, scraping and warping of the finished product. The draft angle can vary from 0.5 ° to 3 °, depending on the part design and surface finish.
boss features - when assembling multiple plastic parts, bosses are usually used to accommodate fasteners. If the boss is too thick, a dent may be left on the part. If they are not connected to the side walls by ribs, they may be significantly deformed. This will make the assembly of these parts almost impossible.
2. Material selection
Injection molding plastics can be made of a variety of resins. The choice of these materials depends mainly on the application of the final product. Each resin has a different shrinkage. This shrinkage needs to be considered when designing the mold, and the mold size is usually adjusted by the material shrinkage percentage. If multiple material components are required, different shrinkage rates need to be designed. If the design tolerance is not appropriate, the parts may not be assembled together, which is a costly error in injection molding.
Injection molding tolerance is mainly determined by material shrinkage and part geometry. Material selection needs to be finalized before tools are designed and manufactured. The tool design is highly dependent on the selected material.
3. Tool design
Once a material is selected, the tool is usually oversized to account for the shrinkage of the relevant material. However, the shrinkage is not uniform in all dimensions. For example, thicker parts have a different cooling rate than thinner parts. Therefore, a complex part with a mixture of thin and thick walls will have a variable cooling rate. The resulting warpage or subsidence can seriously affect the injection tolerance and assembly. To limit these effects, tool manufacturers consider the following factors when designing mold features.
tool cooling - controlled cooling is essential to maintain uniform shrinkage. Poor tool cooling will lead to uncontrolled shrinkage, which will lead to serious deviation of parts from their tolerance requirements. The intelligent placement of cooling channels can significantly improve the consistency of parts.
tool tolerance - tools that exceed the tolerance will lead to all subsequent injection molding parts, and the error will be added in addition to any error caused by shrinkage. However, in the CNC machining process, the tool tolerance is usually strictly controlled and monitored, so the tool out of tolerance is rarely the reason for the part out of tolerance. In addition, these tools are usually "steel safe". This means that key dimensions or features can be adjusted by additional milling when manufacturing tools. If the finished dimension of some parts is not within the tolerance range, the additional material allows fine adjustment of the tool by machining. For example, a tight tolerance hole feature on a part may have a tool designed with a core pin on the wider side of the tolerance. If the hole needs to be adjusted, it will be processed thinner to make the hole thinner.
thimble position - the thimble pushes it out of the mold when the mold is opened; This needs to be done as quickly as possible to minimize cycle time. If the ejector pin is placed in an undesirable position, the parts may be damaged. Some materials are not completely rigid when leaving the tool, and uneven ejection may lead to serious warpage and dimensional inconsistency.
gate position - the gate is a part of the resin inflow tool. If placed in an undesirable position, this will result in a poor appearance. In addition, uneven filling rate can also lead to warpage and irregular shrinkage. Complex parts often require multiple gates to achieve uniform filling and mitigate these challenges.
4. Process control
Despite all the previous design work and material considerations to optimize the injection tolerance of the parts, the parts may still exceed the tolerance when the first batch of samples are delivered. Once all the above methods are combined, the next step to improve tolerance compliance is to adjust the process. Controlling temperature, pressure and holding time are some of the most common methods to improve the quality of parts. Once the ideal condition set is determined, the mold can create consistent parts with very small dimensional changes between parts.
In complex multi feature parts, it may be beneficial to embed pressure and temperature sensors in tools to measure these parameters in the manufacturing process to achieve real-time feedback and process control. Maintaining the pressure and temperature in the tool at all times helps to ensure consistent tolerances.
In complex multi feature parts, it may be beneficial to embed pressure and temperature sensors in tools to measure these parameters in the manufacturing process, so as to realize real-time feedback and process control. Maintaining the pressure and temperature in the tool at all times can largely ensure consistent tolerances.
