As a professional supplier in the field of commodity moulds, I’ve witnessed numerous challenges and design errors that can significantly impact the quality and efficiency of the moulding process. In this blog, I’ll share some of the most common design errors in commodity moulds and provide practical solutions on how to correct them. Commodity Mould
1. Inadequate Draft Angles
Error Description
Draft angles are crucial in mould design as they allow the finished part to be easily ejected from the mould. One of the most prevalent mistakes is the use of inadequate draft angles. When the draft angles are too small or non – existent, it becomes difficult to remove the part from the mould. This can lead to part damage, such as scratches, deformation, or even breakage during the ejection process. Additionally, it may increase the cycle time as more force is required to eject the part, which can reduce the overall productivity of the moulding operation.
Causes
The root cause of this error often lies in a lack of understanding of the material properties and the ejection requirements of the part. Designers may not fully consider the shrinkage characteristics of the plastic material during cooling. Different plastics have different shrinkage rates, and if these are not accounted for, the part may stick to the mould surface. Another cause could be the desire to achieve a specific aesthetic or functional feature on the part, which may lead to designers sacrificing the necessary draft angles.
Correction Methods
The first step in correcting this error is to analyze the material being used and determine its shrinkage rate. Based on this information, appropriate draft angles can be calculated. As a general rule, for most thermoplastics, a draft angle of at least 1 – 2 degrees per side is recommended. However, for more complex parts or materials with high shrinkage rates, larger draft angles may be required. If the mould has already been manufactured with inadequate draft angles, it may be possible to modify the mould surface by adding a thin layer of material and then machining the correct draft angle. In some cases, using a mould release agent can also help to ease the ejection process temporarily, but this is only a short – term solution.
2. Incorrect Gate Design
Error Description
The gate is the point where the molten plastic enters the mould cavity. Incorrect gate design can cause a variety of problems. If the gate is too small, it can restrict the flow of plastic into the cavity, leading to incomplete filling of the part, short shots, or weld lines. Weld lines occur when two or more flow fronts of molten plastic meet and do not fully fuse together, resulting in a weak area in the part. On the other hand, if the gate is too large, it can cause excessive pressure on the part during filling, leading to flash (excess plastic that seeps out of the mould cavity) and difficulty in removing the gate vestige from the finished part.
Causes
This error can be attributed to inaccurate flow analysis during the design stage. Designers may underestimate the viscosity of the plastic material or the complexity of the part geometry, resulting in an inappropriate gate size. Additionally, a lack of experience in gate design can also lead to mistakes. For example, choosing the wrong type of gate for a particular part, such as using a direct gate when a submarine gate would be more suitable.
Correction Methods
To correct an incorrect gate design, a detailed flow analysis should be conducted using computer – aided engineering (CAE) software. This software can simulate the flow of plastic through the mould cavity and help determine the optimal gate size and location. If the mould has already been made, it may be possible to modify the gate by machining it to the correct size or changing its location. In some cases, adding a secondary gate or using a hot runner system can improve the filling of the part. For example, a hot runner system can maintain the plastic at a consistent temperature, reducing the viscosity and improving the flow, which can be beneficial for parts with complex geometries.
3. Poor Venting
Error Description
During the injection moulding process, air is trapped inside the mould cavity as the molten plastic fills it. If there is insufficient venting, this trapped air can cause a variety of problems. It can lead to burn marks on the part surface, as the compressed air heats up due to the high pressure of the incoming plastic. Burn marks not only affect the aesthetic appearance of the part but can also weaken the material. In addition, poor venting can result in incomplete filling of the part, as the trapped air creates a back – pressure that resists the flow of plastic.
Causes
The main cause of poor venting is often a lack of attention to detail during the mould design process. Designers may not consider the volume of air that needs to be displaced during filling or may underestimate the importance of proper venting. Another cause could be the use of improper venting techniques or the placement of vents in ineffective locations. For example, vents that are too small or too far from the areas where air is likely to be trapped will not function effectively.
Correction Methods
To improve venting, the first step is to identify the areas where air is most likely to be trapped, such as the corners or thin – walled sections of the part. Vents can then be added in these areas. The size and number of vents should be determined based on the size and complexity of the part, as well as the type of plastic material being used. In general, vents should be small enough to prevent plastic from leaking out but large enough to allow the air to escape easily. For existing moulds with poor venting, it may be possible to add additional vents by machining small grooves or holes in the mould surface. Another solution is to use a porous metal insert in the mould, which can act as a natural vent.
4. Insufficient Cooling
Error Description
Proper cooling is essential for the quality and productivity of the injection moulding process. Insufficient cooling can cause the part to take longer to solidify, which increases the cycle time and reduces the overall production efficiency. It can also lead to uneven shrinkage of the part, resulting in warping, dimensional inaccuracies, and internal stresses. Warped parts may not fit properly in the assembly or may have a reduced mechanical strength.
Causes
This error can be due to an improper design of the cooling channels in the mould. The cooling channels may be too small, too far apart, or have an inefficient layout, which restricts the flow of coolant and reduces the heat transfer efficiency. Another cause could be the use of an inappropriate coolant or a coolant system with insufficient capacity.
Correction Methods
To correct insufficient cooling, a detailed thermal analysis should be performed to determine the optimal layout and size of the cooling channels. The cooling channels should be designed to ensure a uniform flow of coolant around the mould cavity. In some cases, it may be necessary to add more cooling channels or modify their shape to improve the heat transfer. For existing moulds, it may be possible to retrofit the cooling system by adding additional channels or upgrading the coolant pump to increase the flow rate. Using a more efficient coolant, such as a glycol – water mixture, can also improve the cooling performance.
5. Mismatched Material and Mould Design
Error Description
Using the wrong material for a particular mould design or vice versa can lead to a range of problems. For example, if a mould is designed for a high – viscosity plastic but a low – viscosity plastic is used, the plastic may flow too easily and cause flash. Conversely, if a low – viscosity plastic is expected but a high – viscosity plastic is used, the part may not fill completely. Additionally, different plastics have different chemical and mechanical properties, and using an incompatible material can cause corrosion of the mould or degradation of the part.
Causes
This error often occurs due to a lack of communication between the material suppliers, designers, and manufacturers. Designers may not have a complete understanding of the material properties or may make assumptions based on previous experience without considering the specific requirements of the current project.
Correction Methods
To avoid this error, it is crucial to have a clear communication channel between all parties involved in the project. Designers should work closely with material suppliers to select the most suitable plastic material for the part and the mould. They should consider factors such as the part’s function, the required mechanical properties, and the processing conditions. If a mismatch has already occurred, it may be necessary to modify the mould design to accommodate the material or to change the material to match the existing mould design.
In conclusion, as a commodity mould supplier, I understand the importance of addressing these common design errors to ensure the high – quality production of moulded parts. By being aware of these issues and implementing the appropriate correction methods, we can improve the efficiency of the moulding process, reduce production costs, and enhance the overall quality of the products.
Industrial Equipment Mold If you are in need of high – quality commodity moulds or facing challenges with your existing moulds, I invite you to contact me for a detailed discussion. Our team of experts is ready to provide you with customized solutions to meet your specific requirements.
References
- "Injection Moulding Handbook" – By Osswald, Turng, and Gramann
- "Mould Design for Plastics" – By Rosato and Rosato
- Various industry – specific research papers on plastic injection moulding design and optimization.
Taizhou Tangwei Mould Co., Ltd.
Address: Mould City, Xicheng, Huangyan, Taizhou, Zhejiang, China
E-mail: cjc@twmould.com
WebSite: https://www.tangweimould.com/
