Due to the greatly varied market demands for molds and molded products, there are many kinds of changes and thus high requirements for us in such aspects as shape, size, material, structure and more, causing us many problems and difficulties in the production process of molds and products. How to effectively control the geometric dimensions of the mold and the product is just one of them.
For different types of molds and products, there are different control techniques and methods for dimensions control. Personally, I usually start from the following aspects.
Mold Design Control
Firstly, we have to fully understand the technical requirements for the mold structure, material, hardness, and precision, etc., including whether the shrinkage rate of the plastic material is proper, and whether the 3D model of the product is complete, in a bid to perform reasonable analysis.
2. Thorough consideration should be given to various factors that may influence the product appearance, including shrinkage, flow marks, draft angle, weld lines and cracks of the injection molded product.
3. Try to simplify the processing method of the mold without impeding the function, shape and texture of the injection molded product.
4. Whether the selection of the parting surface is appropriate; the mold machining, product appearance and deburring of the molded part should be carefully considered.
5. Is the ejection method appropriate; use the ejector pin, the stripper plate, the ejector sleeve or other methods; whether the ejector pin and stripper plate are appropriately located.
6. What temperature control method is suitable for the plastic part; which circulation system structure is suitable for temperature control oil, temperature control water, or coolant, etc.; whether the size, quantity and location of the coolant holes are appropriate.
8. Whether the type of the gate, the size of the runner and the gate, as well as the location of the gate are appropriate.
7. Whether the injection volume, the injection pressure and the clamping force of the injection molding machine are sufficient; whether the nozzle R, and the nozzle sleeve diameter, etc. match properly.
Perform comprehensive analysis and make preparation from all of these aspects, to strictly control the product from the very beginning.
Manufacturing Process Control
Although comprehensive consideration and arrangement have been carried out in the design stage, there are still many problems and difficulties during the actual production process. We should try our best to meet the original intention of the design and find the most effective and most economical processing method in actual production.
1. Choose the economical machine tools for 2D and 3D processing.
2. We should also consider a string of production process requirements and solutions, e.g., the appropriate fixtures to facilitate preparation for production, and the rational application of cutters, so as to prevent product deformation, avoid fluctuations in product shrinkage, and eliminate product warpage caused during ejection, thus improving the precision of mold manufacturing, minimizing errors and preventing mold accuracy changes, etc.
3. Here, I’d like to mention a summary of the reasons for the dimensional error and the corresponding proportion, drafted by the British Plastics Federation (BPF):
A: The mold manufacturing error is about 1/3 for dimensions control; B: The error caused by mold tear and wear is 1/6; C: The error caused by the uneven shrinkage of the molded part is about 1/3 for dimensions control; D: The error caused by the difference between the preset shrinkage and the actual shrinkage is 1/6.
Total error = A + B + C + D, so we can see that the mold manufacturing tolerance should be kept within 1/3 of the dimensional tolerance of the molded part, otherwise it is difficult for the mold to guarantee the geometric dimensions control of the molded part.
General Production Control
It is a common problem and phenomenon that geometric fluctuations occur after the plastic part is molded:
1. Control of material temperature and mold temperature. Different plastic materials have different requirements for temperature. Different situations will occur if the plastic features a poor fluidity or two materials are mixed. The plastic should be controlled within the optimal fluidity range, which is easy to achieve. However, the control of the mold temperature is more complicated. Different geometry, size and wall thickness of the molded product have certain requirements for the cooling system. The length of the cooling time is largely dependent on the mold temperature.
Therefore, the mold should be kept at an allowable as low as possible temperature, so as to shorten the injection cycle, and improve production efficiency. If the mold temperature changes, the shrinkage rate also changes; if the mold temperature remains stable, then the dimensional accuracy is stabilized, thereby preventing such defects of the molded part as deformation, poor glossiness, etc., and ensuring the optimal physical properties of the plastic. Of course, there is a commissioning process. In particular, the injection molding of multi-cavity molds is more complicated.
2. Pressure & Venting Control:
The proper injection pressure and clamping force should be determined when the mold is being commissioned. The air in the mold cavity and the core, and the gas generated by the plastic must be vented through the mold vents. If the gas is not smoothly vented, there will be the three molding defects of underfilling, weld marks or burns, which sometimes appear on the same part. However, as long as flashing doesn’t occur, the depth of the vents should be as deep as possible. Behind the shoulder, larger venting grooves are usually designed for the gas to vent through the shoulder quickly. If needed, a special venting groove can be designed on the ejector pin, for which the reason is the same, i.e., 1. No flashing occurs; 2. The venting is fast and effective.
3. Plastic Part Dimensional Compensation Control
Due to the difference in the shape and size of some plastic parts, deformation and warpage may occur in different situations with the change of temperature and loss of pressure after product ejection. At this time, some assistive fixtures can be used to make remedies for dimensions control in a timely manner after the molded part is released, so that when the product is naturally cooled and shaped, better adjustment and remedy results can be obtained. If strict management is ensured throughout the injection molding process, the dimensions of the injection molded part can be very well controlled.