Milling Processing Examples in Plastic Mold Manufacturing
A comprehensive guide to precision milling techniques for critical components in plastic mold systems, focusing on the manufacturing processes that ensure high-quality plastic molded parts.
Milling is a fundamental machining process in the production of plastic mold components, offering precise control over complex geometries and surface finishes essential for high-performance plastic molded parts. This detailed guide explores three critical components in injection mold lateral core-pulling mechanisms: the chute pressure block, slide block, and wedge block. Each component requires specific milling techniques to achieve the dimensional accuracy, surface quality, and mechanical properties necessary for reliable operation in producing plastic molded parts.
The manufacturing processes described here represent industry-standard practices optimized for efficiency, precision, and repeatability—key factors in maintaining consistent quality across large production runs of plastic molded parts. By understanding these milling examples, manufacturers can better appreciate the complexity involved in creating the precision tools that shape modern plastic components.
1. Milling of Chute Pressure Blocks
For lateral core-pulling mechanisms in injection molds, slide blocks require precise lateral movement within chutes, which are typically composed of planar surfaces. These surfaces demand high wear resistance and low surface roughness to ensure smooth operation and long service life in producing plastic molded parts. To balance wear resistance with manufacturing and assembly simplicity, chutes are rarely machined directly into mold plates. Instead, rectangular grooves are milled into the plates, and chute pressure blocks are secured within these grooves using screws, forming the guide channels for slide movement as shown in Figure 1-9.
Both the chute pressure blocks and slide blocks are critical components in lateral parting and core-pulling mechanisms. Figure 1-10 illustrates a simple chute pressure block design— a 40mm×15mm×10mm hexahedron featuring two countersunk through-holes for mounting screws. This seemingly straightforward component plays a vital role in maintaining the precision required for consistent production of plastic molded parts.
While the chute pressure block's structure is simple, consisting primarily of flat surfaces and holes, its mating surfaces with the slide block require high precision and hardness. The typical manufacturing process begins with rough milling and semi-finish milling to create the hexagonal shape, followed by drilling the countersunk screw holes. After heat treatment to achieve the required hardness, final dimensional accuracy is achieved through surface grinding. This process ensures the component can withstand the repeated stresses encountered in high-volume production of plastic molded parts.
Detailed Milling Process for Chute Pressure Blocks
Rough Milling Operations
The initial rough milling stage establishes the basic hexahedral shape from the raw material. This process removes large volumes of material efficiently while leaving sufficient stock for subsequent operations. For plastic molded parts production, maintaining consistent blank dimensions at this stage is crucial for ensuring uniform results in later processing steps.
Cutting parameters are optimized for material removal rate rather than surface finish, typically using carbide end mills with appropriate helix angles for the workpiece material. Coolant is applied continuously to manage heat buildup and extend tool life during this demanding phase.
Semi-Finish Milling
Semi-finish milling brings the component closer to its final dimensions, establishing the critical surfaces that will later be ground to achieve the required precision. This stage focuses on creating flat, parallel surfaces with sufficient accuracy to minimize grinding time while ensuring adequate stock remains for the finishing process.
For plastic molded parts molds, maintaining precise squareness between adjacent surfaces is particularly important at this stage, as it directly impacts the functionality of the chute mechanism during operation.
Hole Machining
The countersunk through-holes for mounting screws are typically machined after the basic shape is established. This sequence prevents distortion of the critical mating surfaces that could occur if holes were drilled earlier in the process. The drilling operation is followed by countersinking to recess the screw heads below the surface.
Proper hole positioning is essential for correct alignment when mounting the pressure block in the mold, which directly affects the movement accuracy of the slide mechanism and ultimately the quality of plastic molded parts produced.
Heat Treatment & Final Grinding
After machining the basic geometry, the component undergoes heat treatment to achieve the required hardness for wear resistance in service. This is particularly important for plastic molded parts production where thousands or millions of cycles may be required.
Following heat treatment, precision surface grinding brings all critical surfaces to their final dimensions with tight tolerances, typically ±0.005mm, and achieves the required surface finish (Ra 0.8μm or better) for proper sliding performance with the mating slide block.
Surface Finish Requirements for Plastic Mold Components
Figure 1-11: Comparison of surface finish requirements for different mold components used in producing plastic molded parts
2. Milling of Slide Blocks
Slide blocks are typically solid components composed of planar and cylindrical surfaces, featuring precision mating斜面 and sliding guide surfaces. When lateral core-pulling mechanisms are integrated into the slide block design, they also include lateral molding surfaces that directly form features on plastic molded parts. In addition to dimensional accuracy, machining must ensure proper positional relationships between surfaces and achieve low surface roughness for reliable operation.
Figure 1-11 illustrates a slide block with an angled guide pin hole. Key machining requirements include precise planar surfaces with appropriate surface finishes, accurate positioning and dimensions for the rounded square hole that secures the lateral core, and proper alignment of all functional features. These elements are critical for ensuring consistent quality in the plastic molded parts produced.
While the dimensional tolerance for the angled guide pin hole is not extremely tight—since it maintains a relatively large clearance with the guide pin during operation—its positional accuracy is crucial. This positioning ensures the core-pulling movement lags appropriately behind the mold opening motion.
The inner surface of the angled guide pin hole makes sliding contact with the outer surface of the guide pin, requiring both high surface quality and hardness. Consequently, slide blocks undergo heat treatment followed by internal hole lapping to correct any heat treatment distortion and reduce surface roughness. Alternatively, wire EDM can be used to machine the angled guide pin hole, which requires pre-drilling a wire entry hole before heat treatment, with the final EDM operation performed afterward.
Slide Block Manufacturing Process Flow
Forging and Initial Preparation
The process begins with a forged blank, selected for its improved mechanical properties compared to cast or rolled materials. This initial material preparation ensures the slide block can withstand the stresses encountered in high-volume production of plastic molded parts. The forging is inspected for defects before proceeding to machining operations.
Rough and Semi-Finish Milling
The forged blank undergoes rough milling to remove excess material and establish the basic external geometry. This is followed by semi-finish milling to bring dimensions closer to their final values while maintaining appropriate stock for subsequent operations. Critical surfaces that will form the sliding interfaces in the completed mold for plastic molded parts receive special attention to ensure proper flatness and parallelism.
Square Hole Machining
The rounded square hole for securing the lateral core is machined using specialized milling operations or EDM. This feature requires tight positional accuracy relative to other functional surfaces to ensure proper alignment of the core that shapes features on plastic molded parts. The corners are typically rounded to reduce stress concentration and facilitate machining.
Hole Machining Operations
Multiple hole machining operations are performed, including drilling and boring or milling the angled guide pin hole, drilling return spring holes, and machining two screw holes to final dimensions. Each hole's position and orientation are critical to proper functioning of the slide mechanism and ultimately the quality of plastic molded parts produced.
Heat Treatment
The component undergoes heat treatment to achieve the required hardness, typically in the range of 50-55 HRC, balancing wear resistance with toughness. This heat treatment ensures the slide block can withstand the repeated stresses of millions of molding cycles while maintaining dimensional stability for consistent production of plastic molded parts.
Final Grinding and Lapping
After heat treatment, precision grinding brings the top and bottom surfaces, sliding guide surfaces, side surfaces, end surfaces, and angled surfaces to their final dimensions and tolerances. The final step involves lapping the angled guide pin hole to achieve the required surface finish, ensuring smooth operation and long service life in producing high-quality plastic molded parts.
3. Milling of Wedge Blocks
The wedge block's primary function is to lock the lateral core-pulling slide during the mold clamping phase, preventing retraction of the slide during the injection and packing stages when pressure is applied to the plastic molded parts. The angle of its inclined surface is typically 2-3 degrees greater than that of the angled guide pin. Figure 1-12 shows a typical wedge block design, which locates via a square boss with two chamfers that fits into corresponding slots and recesses in the mold plate, and is secured with two screws as illustrated in Figure 1-13.
This wedge block design, except for the angled guide pin hole and screw holes, consists entirely of planar surfaces. From a geometric perspective, it could theoretically be completely machined using milling processes. However, despite its seemingly simple structure, the milling process involves multiple setups that complicate production. Additionally, the wedge block requires high hardness, further increasing manufacturing complexity.
In practice, wire EDM is often used for more efficient production of these components. For our purposes, however, this part serves to illustrate the types of surfaces that can be processed using milling techniques in the production of plastic molded parts and their associated tooling.
Comparison of Milling Processes for Plastic Mold Components
| Component | Key Machining Features | Surface Finish Requirements | Hardness Requirements | Critical Tolerances |
|---|---|---|---|---|
| Chute Pressure Block | Hexahedral surfaces, countersunk screw holes | Ra 0.8μm on sliding surfaces | 50-55 HRC | ±0.005mm on critical dimensions |
| Slide Block | Multiple planar surfaces, angled guide pin hole, rounded square hole | Ra 0.4μm on sliding surfaces, Ra 0.2μm on lapped hole | 50-55 HRC | ±0.003mm on mating surfaces, positional tolerance ±0.01mm |
| Wedge Block | Angled surfaces, locating boss, mounting holes | Ra 0.8μm on contact surfaces | 52-58 HRC | Angular tolerance ±5', positional tolerance ±0.01mm |
The manufacturing process for wedge blocks is similar to that for slide blocks. It begins with rough milling and semi-finish milling of the forged blank to establish the basic shape. The locating boss top surface, front and rear vertical surfaces, angled surfaces, and side surfaces are left with grinding allowances, while other surfaces are milled to final dimensions. Next, the angled guide pin hole is drilled and bored or milled, followed by drilling the two screw holes to their final dimensions.
After heat treatment to achieve the required hardness, precision grinding is performed on the boss top surface, both side surfaces, front and rear vertical surfaces, and angled surfaces to meet final dimensional requirements. The final step involves lapping the angled guide pin hole to achieve the surface finish and dimensional accuracy specified for reliable performance in plastic molded parts production.
Each of these components—chute pressure blocks, slide blocks, and wedge blocks—plays a critical role in the proper functioning of lateral core-pulling mechanisms in injection molds. Their precise manufacturing ensures the consistent production of high-quality plastic molded parts with complex geometries that would be impossible to achieve with simpler mold designs.
The milling processes described here represent the industry standard for producing these components, balancing precision requirements with manufacturing efficiency. By understanding and implementing these processes correctly, mold manufacturers can produce tooling that delivers reliable performance and consistent quality in plastic molded parts production, even over millions of molding cycles.
The precision milling techniques outlined for these critical mold components—chute pressure blocks, slide blocks, and wedge blocks—demonstrate the sophisticated manufacturing processes required to produce high-quality plastic molded parts. Each component's design and production process is optimized to ensure durability, precision, and performance in demanding injection molding environments. By mastering these milling processes, manufacturers can create molds capable of producing consistent, high-quality plastic molded parts with complex geometries, meeting the stringent requirements of modern manufacturing applications.
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