Grinding Processing Techniques in Mold Manufacturing
A comprehensive guide to precision grinding methods for critical mold components, essential for producing high-quality injection molded plastic parts.
Grinding is a fundamental machining process in mold manufacturing that achieves high precision, superior surface finish, and tight tolerances. This article explores practical grinding applications for key mold components, highlighting techniques that directly impact the quality of injection molded plastic parts. The examples provided represent industry-standard practices optimized for both performance and efficiency.
From simple cylindrical surfaces to complex contours, grinding processes are tailored to each component's specific requirements. The selection of appropriate grinding methods depends on material properties, geometric complexity, dimensional accuracy needs, and surface finish specifications—all critical factors in producing molds that consistently deliver high-quality injection molded plastic parts.
(1) Grinding Process for Angle Pins
The structural shape of an angle pin is shown in Figure 1-18. One half of the end face of the fixed part is an inclined plane, and the head of the working part is hemispherical to facilitate the introduction of the angle pin into the slider. The material is 20 steel with carburizing treatment, with a hardness of over 55HRC after quenching. The working surface achieves a surface roughness of Ra 0.8μm after grinding, ensuring proper functionality in molds producing injection molded plastic parts.
The main surfaces forming the angle pin are coaxial cylindrical surfaces of different diameters. According to its size and material, hot-rolled round steel can be directly selected as the blank. In machining, it is mainly necessary to ensure the precision of its mating surfaces and the surface roughness of the sliding surfaces. Additionally, attention must be paid to the coaxiality between the cylindrical surfaces and the requirements for surface hardness, both of which influence the longevity of molds used for injection molded plastic parts.
Since the sliding surface of the angle pin has hardness requirements, a heat treatment process is generally arranged before finishing. The machining process of the angle pin can be summarized as: material preparation, rough turning, semi-finish turning, heat treatment, and grinding processes.
Figure 1-18: Angle Pin Grinding Process. Precision cylindrical grinding ensures the dimensional accuracy required for proper function in molds producing injection molded plastic parts.
After blanking, rough turning, and finish turning, except for the working surface which leaves a grinding allowance of 0.2~0.3mm on one side, all other dimensions can be processed to requirements. Then, the inclined surface of the fixed end is milled (this inclined surface can also be ground uniformly on a surface grinder after being assembled on the fixed plate). Carburizing heat treatment is then performed to achieve the required hardness of the angle pin, which is crucial for withstanding the repeated stresses encountered in producing injection molded plastic parts.
Finally, the working surface of the angle pin is ground to size requirements using a cylindrical grinder. The hemispherical surface of the guide part only has a guiding function, with lower dimensional accuracy but higher surface roughness requirements. For grinding this part, it can be polished by a fitter, or the grinding wheel can be dressed into a corresponding radius of internal R shape for grinding. For grinding similar internal and external fillets in mold rotating parts, the method of dressing the grinding wheel profile is a commonly used process in manufacturing molds for injection molded plastic parts.
In addition to the size and precision requirements of the angle pin, the existing processing equipment in the factory is also a contributing factor. For example, subsequent grinding can also be performed on a surface grinder using a rotary table. Therefore, when determining the specific process for angle pin processing, each factory may have slightly different approaches, which should be determined according to the specific conditions of the factory's equipment to optimize production of molds for injection molded plastic parts.
(2) Grinding Process for Cavity Plates
Figure 1-19: Finished Cavity Plate. The precision ground surfaces ensure proper alignment in molds for high-quality injection molded plastic parts.
Figure 1-20: Cavity Plate Before Processing. This blank will undergo precision grinding to achieve the required flatness and surface finish for injection molded plastic parts production.
The cavity plate is a component in the standard mold base. Generally, after purchasing a standard mold base, it can be disassembled for direct processing of cavity insert fixing counterbores, screw holes, runner holes, etc., on the cavity plate. As a standard mold base manufacturer producing standard mold bases, the general processing technology for cavity plates is: rough milling on the blank, quenching and tempering heat treatment, finish milling leaving a grinding allowance of 0.3~0.5mm for each plane, followed by surface grinding to achieve the required dimensions and surface roughness of the six faces of the cavity plate—critical factors in ensuring consistent quality of injection molded plastic parts.
Then, mill out the small ejection counterbores at the four corners of the back, then drill, ream, and bore the guide bush fixing holes. To ensure the coaxiality requirements between the guide bushes on the cavity plate and the guide pillars on the core plate during assembly, the cavity plate and core plate are generally placed together to machine the guide pillar holes and guide bush holes on both plates. Finally, the fitter drills the fixing screw bottom holes and taps the threads to complete the processing of the cavity plate in the standard mold base, which directly impacts the precision of injection molded plastic parts.
There are many plate-type parts in standard mold bases. Mold bases, backing plates, fixing plates, stripper plates, ejector plates, etc., all belong to this category. Different plate-type parts have different shapes, materials, sizes, precision, and performance requirements, but the profile of each plate-type part consists of planes and hole systems. They generally use 45 steel with quenching and tempering treatment, and the hardness requirement is generally between 28~32HRC, which is a hardness that can be processed by ordinary machining, making them suitable for producing molds for injection molded plastic parts.
Therefore, the processing technology of other plates is similar to that of the cavity plate mentioned above. When grinding these plate-type parts, the focus should be on ensuring the parallelism and perpendicularity between the six surfaces, as well as the surface roughness and precision grade of each surface. Generally, the processing dimensional accuracy of the template plane should reach IT7~IT8, and the surface roughness Ra should reach 0.8~3.2μm, both of which directly influence the quality of injection molded plastic parts.
If the mating surfaces of the core plate and cavity plate are directly used as the fitting surfaces of the parting surface, the processing accuracy of these two mating surfaces should reach IT6~IT7, and the surface roughness Ra should reach 0.4~1.6μm. In addition, the dimensional accuracy, perpendicularity, and hole spacing of each hole on the template must also meet the requirements. The common fitting accuracy of each hole diameter of the template is generally IT6~IT7, and Ra is 0.4~1.6μm, ensuring proper assembly and function for producing injection molded plastic parts.
If the fixing hole of the guide pillar on the moving template and the fixing hole of the guide bush on the fixed template have the same diameter, the fixed template and the moving template can be combined, positioned with process locating pins, clamped, and bored simultaneously. This not only maintains the same hole diameter and hole spacing but also ensures the coaxiality of the holes. However, with the continuous improvement of processing equipment accuracy, processing the holes of each plate separately on CNC machine tools can also ensure the installation and fitting accuracy required for high-quality injection molded plastic parts.
For templates installing sliding guide pillars, the perpendicularity requirement between the hole axis and the upper and lower template planes is grade 4 accuracy. The hole spacing between each hole on the template should be consistent, and the general error requirement is within ±0.02mm, a critical tolerance for ensuring proper alignment in molds producing injection molded plastic parts.
The processed cavity plate shown in Figure 1-19 is the cavity plate of a soap box injection mold, made of 45 steel with quenching and tempering treatment. Figure 1-20 shows the cavity plate before processing, which is disassembled from the purchased standard mold base. We will process on this plate to finally obtain the cavity plate shown in Figure 1-19, which will be used to produce high-quality injection molded plastic parts.
The processing technology of the cavity plate in this standard mold base is similar to that described earlier, so it will not be repeated here. To complete the processing of the cavity plate as shown in Figure 1-19 on this basis, the adopted processes mainly include milling, drilling, and grinding. Modern mold design generally no longer uses the surface of the cavity plate as the fitting surface of the main parting surface, but uses the fitting surface of the cavity insert and the core insert as the parting surface. Therefore, after the cavity insert is assembled, the surface of the cavity plate is generally required to be 0.1~0.3mm lower than the surface of the cavity insert to ensure proper functioning when producing injection molded plastic parts.
In mold design, the thickness dimension of the cavity plate in the standard mold base is often directly adopted. Therefore, some enterprises, to ensure that the surface of the cavity plate is lower than the surface of the cavity insert, first grind the large surface of the cavity plate on a surface grinder before other processing steps. After that, the fitter marks the water channel holes, then fixes the cavity plate on the CNC milling machine according to the datum and aligns it, and processes the side with the cavity insert fixing square hole, as shown in Figure 1-19(a). The cavity insert fixing square hole and slider square groove are processed through rough milling and finish milling in sequence, both critical for proper alignment in molds producing injection molded plastic parts.
Next, use a center drill to spot the positions of each screw hole on the CNC milling machine. The precision requirements for these holes are not very high and can be processed on the CNC milling machine or subsequently completed by a fitter. After processing this side, the workpiece is turned over to process the runner and runner hole on the back, as shown in Figure 1-19(b). It should be noted here that the runner hole is generally processed after the cavity insert is press-fitted to ensure that the runner hole on the cavity insert is aligned with the runner hole on the fixing plate, a crucial detail for proper flow in molds producing injection molded plastic parts. Some enterprises have high-precision processing equipment and can also process them separately. However, if the two runner holes processed separately are misaligned after assembly, it will make runner demolding very difficult. To avoid this situation, when processing separately, the runner hole diameter on the fixing plate is often 0.3~0.5mm larger on each side than the runner hole diameter on the cavity insert to eliminate the demolding difficulty caused by the misalignment of the two holes, ensuring efficient production of injection molded plastic parts.
(3) Grinding Process for Nameplate Punch and Die
The nameplate punching punch and die parts are shown in Figure 1-21. In the punching die, the punch working part of this punch and die component completes the blanking of the nameplate outline, and the die part completes the punching of two cylindrical holes and the "SUST"字样. It can be seen from the part drawing that the processing of this punch and die adopts the "punch and die matching method". The outer forming contour surface is a non-datum contour surface, which is matched with the actual size of the blanking die to ensure a double-sided gap of 0.06mm. The two die inner holes and the "SUST" die of the punch and die are also matched with the actual size of the punching punch to ensure proper clearance for producing high-quality injection molded plastic parts.
The external dimensions of the part are 82mm × 18mm × 25mm. The forming surfaces are the outer contour, two round holes, and the "SUST" characters. There are two M6 threaded holes on the bottom surface for fastening. The outer forming contour of the punch and die consists of straight lines and spline lines at both ends, and the side surface is a ruled surface with a relatively complex shape. The relief hole of the die part of the part is stepped, a design feature that impacts the quality of injection molded plastic parts.
The finishing of the outer forming surface can adopt CNC forming grinding or EDM wire cutting (detailed introduction will follow) methods. The bottom surface of the part also has two M6 threaded holes, which can be processed first and used as positioning references for subsequent CNC milling or CNC forming grinding operations, ensuring precision in molds used for injection molded plastic parts.
These processing methods fall into the category of special processing, which we will briefly introduce later and explain in detail in Chapter 5. Here, we will mainly provide a brief introduction to CNC milling machines, CNC lathes, and CNC engraving machines, all of which play important roles in manufacturing molds for injection molded plastic parts.
Figure 1-21: Nameplate Punch and Die Assembly. The precision ground surfaces ensure proper clearance and cut quality for nameplates that may be incorporated into injection molded plastic parts.
Grinding Tolerances Summary
Characteristics of CNC Machining in Mold Manufacturing
CNC machining offers significant advantages over traditional mechanical processing in the field of mold manufacturing, particularly for producing the high-precision components required for injection molded plastic parts. The following characteristics make CNC machining indispensable in modern mold production:
High Degree of Automation
When processing parts on CNC machine tools, the entire processing process is controlled by the CNC system according to the processing program to automatically complete the movement of the machine tool components. There is no need for manual movement of tools or worktables through wheels. The operator only needs to perform pre-processing preparations such as workpiece clamping, tool setting, and program loading, and monitor during processing, greatly improving efficiency in producing molds for injection molded plastic parts.
Strong Adaptability
The processing process of CNC machine tools is controlled by programs. When processing a certain part, it is necessary to write a processing program according to the size, shape, and technical requirements on the part drawing, and then send it to the computer of the CNC system. When the shape of the processed object changes, in addition to replacing tools and fixtures, only a new processing program needs to be written according to the processing requirements of the new object to achieve processing. Therefore, CNC machine tools have a wide processing range, can save many special fixtures, and are particularly suitable for single-piece and small-batch processing of mold parts for injection molded plastic parts.
High Processing Quality and Precision
Most CNC machine tools adopt high-performance spindles, servo drive systems, efficient and high-precision transmission components (such as ball screws, linear rolling guides, etc.) and machine tool structures with high dynamic stiffness. Measures are taken to improve the wear resistance of the machine tool and reduce thermal deformation, all of which can maintain the high geometric accuracy and positioning accuracy of the machine tool. Moreover, because CNC machine tools adopt automatic processing, human operation errors are reduced, so they have high processing accuracy, essential for producing high-quality injection molded plastic parts.
High Production Efficiency
Due to the high degree of automation of CNC machine tools, work such as marking, fixture design and manufacturing, multiple clamping and positioning, and inspection are eliminated during processing, so the production efficiency of CNC machining is higher than that of traditional machining. This efficiency gain is particularly valuable in producing molds for injection molded plastic parts, where precision and speed are both critical.
Remote Collaborative Processing
One host computer can control multiple CNC machine tools through a network, or a communication network can be established between multiple CNC machine tools. Remote collaborative processing can also be performed through a network, which is conducive to forming an integrated manufacturing system integrating computer-aided design, production management, and manufacturing. This connectivity is revolutionizing how molds for injection molded plastic parts are produced globally.
High Equipment Cost
CNC machining has obvious advantages in processing mold parts, but compared with ordinary turning and milling equipment, CNC machine tools are more expensive, more technically complex, and require higher skills for machine maintenance and programming personnel. Despite this investment, the benefits in precision and repeatability make them essential for producing high-quality molds for injection molded plastic parts.
Complex Pre-Processing Preparation
To make full use of the high performance of CNC machine tools and give play to their high efficiency advantages, it is necessary to compile part processing programs and prepare corresponding tools and fixtures before processing. Therefore, CNC machine tools are not suitable for processing parts with simple shapes, low precision requirements, and excessive blank allowances. However, for complex mold components required for high-precision injection molded plastic parts, these preparation steps are justified by the superior results.
For mold forming parts with complex generatrix surfaces of revolution, CNC turning is generally used. For complex contour shapes or spatial curved surfaces, CNC milling or rough milling followed by EDM finishing is generally used. For micro-complex shapes, special material molds, plastic insert cavities and inserts, and mold parts with special-shaped grooves, CNC EDM wire cutting can be used. For analytic geometry surfaces with high precision requirements, CNC grinding can be used, all of which contribute to producing superior molds for injection molded plastic parts.
Comparison of Grinding Processes for Injection Mold Components
Figure: Comparative analysis of surface finish capabilities and production efficiency for different grinding processes used in manufacturing molds for injection molded plastic parts.