Introduction to Hole Machining: Critical for Molds for Plastic Injection

Hole machining is a fundamental process in manufacturing, serving as the backbone for creating precision components across industries. Among its most vital applications is in the production of molds for plastic injection—a sector where dimensional accuracy, surface finish, and positional consistency directly determine the quality of final plastic parts. From guide pin holes to bolt clearance holes, every hole in molds for plastic injection must meet strict specifications to ensure mold alignment, part ejection, and long-term durability. This page explores the four most成熟 (mature) and widely adopted hole machining methods—drilling, hole expanding, reaming, and boring—along with their associated equipment, tools, and best practices tailored to the unique demands of molds for plastic injection.

Each method addresses specific stages of hole production, from roughing to finishing, and is selected based on factors like hole size, precision requirements, and the complexity of the mold design. For molds for plastic injection, where even微小 (microscopic) deviations can lead to part defects or mold failure, choosing the right machining approach is paramount. Below is a detailed breakdown of each process, highlighting how they integrate into the manufacturing workflow of molds for plastic injection.

1. Drilling: The Foundation of Hole Machining for Molds for Plastic Injection

Drilling is the most common and foundational hole machining method, primarily used for roughing holes in metal and plastic components—including the mold bases, cavities, and cores that form molds for plastic injection. Its primary purpose is to create initial holes that serve as the base for subsequent精加工 (finishing) processes, such as expanding, reaming, or boring. In the context of molds for plastic injection, drilling is indispensable for producing bolt (screw) clearance holes, thread bottom holes, and preliminary positioning pin holes—all of which are critical for assembling and aligning mold components.

1.1 Core Drilling Principles for Molds for Plastic Injection

Drilling operates on the principle of rotating a cutting tool (typically a twist drill) to remove material and create a cylindrical hole. For molds for plastic injection, two primary drilling approaches are employed, each selected based on the mold part’s design and the required coaxiality (alignment between the hole and external features):

• Stationary Workpiece, Rotating Drill: Used on drilling machines, milling machines, or boring machines. Here, the drill spins at high speed while the mold part (e.g., a mold base or cavity insert) remains fixed. This method is ideal for most molds for plastic injection components, as it accommodates irregularly shaped parts and allows for flexible hole placement.
• Rotating Workpiece, Stationary Drill: Employed on lathes, where the mold part rotates while the drill is held stationary in the lathe tailstock. This method is preferred when the hole requires strict coaxiality with an external cylindrical surface—for example, in the production of mold cores or pins for molds for plastic injection, where misalignment could cause uneven plastic flow.

While drilling is efficient for roughing, it has limitations: drilled holes typically have low precision (IT12–IT14 tolerance class) and a rough surface finish (Ra 25–12.5 μm). For molds for plastic injection, this means drilling is almost always followed by secondary processes to meet the mold’s precision needs.

1.2 Drilling Machines for Molds for Plastic Injection

The choice of drilling machine depends on the size of the mold part, the number of holes, and the production volume of molds for plastic injection. Three primary types of drilling machines are used in mold manufacturing:

① Bench Drilling Machine

Bench drilling machines (or "bench drills") are compact, lightweight, and designed for small-scale operations. They feature a simple structure with a motor-driven spindle mounted on a bench, making them easy to operate and maintain. However, their capacity is limited: they can only drill holes with diameters less than 12 mm. In the production of molds for plastic injection, bench drills are commonly used in repair shops or low-volume production lines for drilling small holes in mold inserts, such as ejector pin clearance holes or sensor mounting holes. Their portability and low cost make them a staple for small-scale mold workshops.

② Vertical Drilling Machine

Vertical drilling machines (or "upright drills") are more robust than bench drills, with a vertical spindle that delivers both main (rotational) and feed (axial) movements. The spindle’s feed motion can be manual or motorized, and the worktable can be adjusted vertically along the machine’s column to accommodate mold parts of varying heights. Vertical drills are ideal for single-piece or small-batch production of molds for plastic injection, as they can handle medium-sized components (e.g., mold cavities up to 500 mm in height) and drill holes up to 50 mm in diameter. Their rigidity ensures consistent hole depth and position, which is critical for aligning mold halves in molds for plastic injection.

③ Radial Drilling Machine

Radial drilling machines (or "radial drills") are the most versatile option for large-scale mold manufacturing. They feature a horizontal arm that can move vertically along the column, slide radially (left/right) along the arm, and rotate 360° around the column. This flexibility allows the drill to be positioned over large or heavy mold parts—such as mold bases for automotive molds for plastic injection—without repositioning the workpiece. Radial drills can drill holes up to 100 mm in diameter and are widely used in medium to large mold shops for their efficiency and precision. They are particularly valuable for drilling multiple holes in large mold structures, where accuracy in hole spacing directly impacts mold assembly.

1.3 Key Tool: Twist Drill for Molds for Plastic Injection

The twist drill is the most common cutting tool used in drilling for molds for plastic injection. Typically made from high-speed steel (HSS) or carbide (for harder mold materials like pre-hardened steel), twist drills are designed to efficiently remove material while maintaining structural integrity. The structure of a twist drill—critical for its performance in molds for plastic injection machining—is divided into three main components: shank, neck, and body (Figure 3-20).

Figure 3-20: Typical structure of a twist drill used in drilling holes for molds for plastic injection. The tool’s design ensures efficient chip evacuation and precise hole formation.

• Shank: The cylindrical or tapered end of the drill that connects to the machine’s spindle. It transmits torque from the machine to the drill and ensures secure clamping. For molds for plastic injection machining, tapered shanks are preferred for larger drills (over 12 mm) as they provide better torque transfer and centering.
• Neck: The narrow section between the shank and the body. It serves as a transition zone and provides space for chip evacuation. In twist drills used for molds for plastic injection, the neck is often marked with tool specifications (e.g., diameter, material) for easy identification.
• Body: The longest section of the drill, consisting of two main parts:
  • Cutting Part: The tip of the drill, responsible for removing material. It features two cutting edges (lips) and a chisel edge that centers the drill in the workpiece. For molds for plastic injection materials (e.g., P20, H13 steel), the cutting edges are ground to a specific angle (usually 118°) to optimize cutting efficiency and tool life.
  • Guiding Part: The portion of the body behind the cutting part, featuring two symmetric helical flutes and lands (narrow ridges between the flutes). The lands act as guides, ensuring the drill stays on course and smoothing the hole wall—critical for preventing hole deviation in molds for plastic injection components. The helical flutes facilitate chip evacuation and the delivery of cutting fluid, which cools the tool and reduces friction during drilling.

Drilling lays the groundwork for all subsequent hole machining processes in molds for plastic injection. By selecting the right machine, tool, and parameters, manufacturers can ensure that the initial holes meet the necessary roughing standards, setting the stage for high-precision finishing.

2. Hole Expanding: Refining Rough Holes for Molds for Plastic Injection

Hole expanding (or "countersinking" in some contexts, though technically distinct) is a secondary machining process that follows drilling to refine the hole’s dimensions and quality. It uses a specialized tool called an expanding drill to enlarge a pre-drilled hole, improve its roundness, and correct minor axial misalignment. For molds for plastic injection, hole expanding is a critical step between rough drilling and precision finishing (e.g., reaming or boring), as it creates a more uniform hole that simplifies subsequent processes and enhances final precision. Common applications in molds for plastic injection include expanding bolt holes to accommodate larger fasteners, refining guide pin holes, and preparing holes for thread tapping.

2.1 Principles and Advantages of Hole Expanding for Molds for Plastic Injection

Hole expanding works by removing a thin layer of material (typically 1–3 mm per side) from the inner wall of a pre-drilled hole. Unlike drilling, which relies on a chisel edge to start the hole, expanding drills have multiple cutting edges (usually 3–4) that distribute cutting forces evenly, reducing vibration and improving hole quality. This even force distribution is particularly beneficial for molds for plastic injection components made from hardened steels, where uneven forces can cause tool deflection or hole distortion.

The key advantages of hole expanding for molds for plastic injection include:

• Higher Precision: Expanded holes achieve tolerances of IT10–IT11, a significant improvement over the IT12–IT14 of drilled holes. This precision is essential for molds for plastic injection components like guide bushings, where even small dimensional deviations can cause mold misalignment.
• Improved Surface Finish: Expanding produces a surface roughness of Ra 6.3–3.2 μm, which is smoother than the Ra 25–12.5 μm of drilled holes. A smoother surface reduces friction between moving mold parts (e.g., ejector pins) in molds for plastic injection, extending mold life.
• Axial Correction: Expanding can correct minor axial misalignment (up to 0.1 mm) in pre-drilled holes. This is critical for molds for plastic injection hole systems (e.g., multiple guide pin holes), where positional accuracy ensures mold halves align correctly during closure.
• High Efficiency: Expanding uses larger feed rates (0.2–0.5 mm/rev) than drilling, reducing cycle time. For high-volume production of molds for plastic injection, this efficiency translates to lower manufacturing costs.

2.2 Expanding Drills for Molds for Plastic Injection

Expanding drills (or "core drills") are specialized tools designed for the expanding process. They differ from twist drills in several key ways to optimize performance for molds for plastic injection machining:

• Multiple Cutting Edges: Most expanding drills have 3–4 cutting edges (instead of 2 in twist drills), which distribute cutting forces and reduce tool wear. For molds for plastic injection materials like H13 steel, this design extends tool life by minimizing heat buildup.
• Guiding Lands: Expanding drills feature wider guiding lands than twist drills, which improve stability and ensure the tool stays centered in the pre-drilled hole. This is critical for maintaining hole roundness in molds for plastic injection components.
• Reduced Chisel Edge: Unlike twist drills, expanding drills have no chisel edge—instead, the cutting edges meet at a sharp point that follows the pre-drilled hole. This eliminates the "push" force associated with drilling, reducing tool deflection and improving hole straightness.

Figure 3-21: Hole expanding process using a specialized expanding drill to refine a pre-drilled hole in a mold component for molds for plastic injection. The process improves precision and surface finish.

2.3 Expanding Applications in Molds for Plastic Injection

Hole expanding is a versatile process that supports various stages of molds for plastic injection manufacturing. Key applications include:

• Pre-Finishing for Reaming/Boring: Expanding is often used as a pre-processing step before reaming or boring. By creating a uniform, low-tolerance hole, expanding reduces the material removal required in subsequent processes, extending tool life and improving efficiency. For example, in the production of precision定位 pin holes (positioning pin holes) in molds for plastic injection, expanding prepares the hole for reaming, ensuring the final hole meets IT7 tolerance.
• Final Machining for Low-Precision Holes: For holes that do not require ultra-high precision (e.g., some bolt clearance holes or vent holes in molds for plastic injection), expanding can serve as the final machining step. Its IT10–IT11 tolerance and Ra 6.3–3.2 μm surface finish are sufficient for these non-critical applications.
• Thread Preparation: When tapping threads in molds for plastic injection components, expanding is used to refine the pre-drilled thread bottom hole. A uniform hole ensures consistent thread depth and pitch, preventing thread stripping or misalignment.

In summary, hole expanding bridges the gap between rough drilling and precision finishing in molds for plastic injection manufacturing. Its ability to improve hole quality, correct misalignment, and enhance efficiency makes it an indispensable step in producing high-quality mold components.

3. Reaming: Precision Finishing for Small-to-Medium Holes in Molds for Plastic Injection

Reaming is a semi-finishing and finishing process designed for small-to-medium diameter (typically 1–50 mm) unhardened holes, making it ideal for critical components in molds for plastic injection. Unlike drilling or expanding, which focus on material removal, reaming emphasizes dimensional accuracy and surface finish by removing a very thin layer of material (0.05–0.2 mm per side) from the inner hole wall. For molds for plastic injection, reaming is used to produce high-precision holes such as guide pin holes, ejector pin holes, and valve gate holes—where even micron-level deviations can affect mold performance or part quality.

3.1 The Reaming Process: A Combination of Cutting and Forming

Reaming is a complex process that combines cutting, scraping,挤压 (extrusion), and smoothing—making it distinct from other hole machining methods. The reamer (the primary tool) has multiple cutting edges (usually 4–12) that remove small chips from the hole wall, while its cylindrical calibration section applies gentle pressure to smooth the surface. This dual action is critical for molds for plastic injection, as it produces holes with both high dimensional accuracy and a polished finish.

Key characteristics of the reaming process for molds for plastic injection include:

• Low Material Removal: Reaming removes only a small amount of material, which minimizes tool deflection and ensures the hole retains its original position. This is essential for molds for plastic injection hole systems, where positional accuracy (e.g., between guide pin holes) is critical.
• Cutting-Scraping-Extrusion Integration: The reamer’s cutting edges remove material, while its calibration section scrapes burrs and extrudes the hole wall to improve surface finish. This integrated action produces holes with surface roughness as low as Ra 1.6–0.8 μm—far smoother than expanding or drilling.
• Fixed-Size Tooling: Reamers are "fixed-size" tools, meaning their diameter is precision-ground to match the final hole size. For molds for plastic injection, this ensures consistency across multiple mold components (e.g., matching guide pin holes in upper and lower mold halves).

3.2 Reamer Design and Types for Molds for Plastic Injection

Reamers are engineered to meet the strict precision demands of molds for plastic injection. Their structure consists of three main components: shank, neck, and working section—each tailored to optimize performance:

• Shank: Transmits torque from the machine to the reamer. For molds for plastic injection machining, shanks are available in cylindrical (for small reamers, <12 mm) or tapered (for larger reamers, >12 mm) designs. Tapered shanks provide better centering and torque transfer, which is critical for high-precision reaming.
• Neck: A narrow transition zone between the shank and working section. It provides space for chip evacuation and prevents the reamer from binding in the hole—important for molds for plastic injection materials that produce long chips (e.g., aluminum).
• Working Section: The most critical part of the reamer, divided into three subsections:
  • Guide Cone: A tapered section at the tip that guides the reamer into the pre-drilled/expanded hole. For molds for plastic injection, the guide cone angle is typically 15°–30° to ensure smooth entry and prevent hole damage.
  • Cutting Section: Features multiple cutting edges (4–12) ground to a specific rake angle (usually 5°–10°) to optimize material removal. For mold steels (e.g., P20), a higher rake angle reduces cutting forces, while for harder materials (e.g., H13), a lower rake angle improves tool durability.
  • Calibration Section: A cylindrical section with a slight taper (0.005–0.01 mm per 100 mm) that ensures the reamer fits the hole tightly. It has two key functions: (1) scraping and extruding the hole wall to improve surface finish, and (2) maintaining the final hole diameter. For molds for plastic injection, the calibration section is precision-ground to within ±0.002 mm of the target diameter.

Reamers for molds for plastic injection are classified into two main types based on their operation method:

① Hand Reamers

Hand reamers are designed for manual operation, typically used for small-batch production or repair of molds for plastic injection. They have a longer calibration section (2–3 times the hole diameter) to enhance guiding, as manual operation lacks the stability of machine reaming. However, the longer calibration section increases friction and makes chip evacuation more difficult—so hand reamers are best suited for soft materials (e.g., aluminum) or small holes (<10 mm) in molds for plastic injection. They are often used to finish holes in mold inserts that cannot be clamped in a machine.

② Machine Reamers

Machine reamers are used with drilling machines, milling machines, or CNC machines—making them ideal for high-volume production of molds for plastic injection. Unlike hand reamers, their calibration section is shorter (1–1.5 times the hole diameter) because machine stability provides sufficient guiding. To improve centering, the cutting section’s cone angle is usually 8°–30°, which helps the reamer follow the pre-drilled hole path. Machine reamers are available in HSS or carbide designs: HSS reamers are cost-effective for soft mold materials, while carbide reamers are preferred for hardened steels (e.g., H13) used in high-wear molds for plastic injection.

Figure 3-22: Left: Hand reamer with a long calibration section for manual reaming of small holes in molds for plastic injection. Right: Machine reamer in action, finishing a guide pin hole in a mold component.

3.3 Reaming Applications and Precision in Molds for Plastic Injection

Reaming is the go-to process for producing high-precision small-to-medium holes in molds for plastic injection. Its key applications include:

• Guide Pin Holes: Guide pins ensure mold halves align correctly during closure—critical for preventing part flash in molds for plastic injection. Reaming produces guide pin holes with IT7–IT8 tolerance and Ra 1.6–0.8 μm surface finish, ensuring a precise fit between the pin and hole.
• Ejector Pin Holes: Ejector pins push finished plastic parts out of the mold cavity. Reamed ejector pin holes have tight tolerances (IT8–IT9) and smooth surfaces, reducing friction between the pin and hole and preventing pin seizing—an issue that can damage molds for plastic injection or cause part defects.
• Valve Gate Holes: In hot-runner molds for plastic injection, valve gates control plastic flow into the cavity. Reaming produces valve gate holes with ultra-high precision (IT6–IT7) and a polished surface (Ra 0.8–0.4 μm), ensuring consistent flow and preventing plastic degradation.

Reaming is the gold standard for small-to-medium precision holes in molds for plastic injection. Its ability to produce tight tolerances and smooth surfaces makes it indispensable for critical mold components, ensuring the final mold performs reliably and produces high-quality plastic parts.

4. Boring: Precision Machining for Large and Complex Holes in Molds for Plastic Injection

Boring is a versatile finishing process used to machine large, deep, or complex holes—including those in large molds for plastic injection components like mold bases, cavity blocks, and hydraulic cylinder housings. Unlike drilling, expanding, or reaming (which use fixed-size tools), boring uses a single-point cutting tool that can be adjusted to achieve the exact hole diameter. This flexibility makes boring ideal for molds for plastic injection applications where hole sizes exceed 50 mm, or where strict positional accuracy is required for multi-hole systems (e.g., engine block molds or large automotive part molds).

4.1 Boring Principles and Advantages for Molds for Plastic Injection

Boring operates by rotating a single-point cutting tool (or the workpiece) to remove material from the inner wall of a pre-drilled or cast hole. For molds for plastic injection, the process is typically performed on boring machines, lathes, milling machines, or CNC machines—each offering unique benefits for different mold designs. The key advantage of boring is its ability to achieve both high dimensional accuracy and precise positional control, making it the preferred method for large or complex holes in molds for plastic injection.

Key benefits of boring for molds for plastic injection include:

• Large Hole Capacity: Boring can machine holes with diameters exceeding 100 mm—far larger than the maximum capacity of reamers or expanding drills. This is critical for molds for plastic injection components like large cavity blocks, where holes for cooling lines or hydraulic cylinders may be 100–300 mm in diameter.
• High Precision: Bored holes achieve tolerances of IT6–IT8, with surface roughness as low as Ra 1.6–0.4 μm. This precision is essential for molds for plastic injection hole systems (e.g., multiple cooling holes in a mold base), where consistent hole size and spacing ensure uniform cooling of plastic parts.
• Positional Accuracy: Boring allows for precise control of hole location and orientation. For molds for plastic injection with multi-hole systems (e.g., guide pin holes in a large mold base), boring ensures hole spacing is accurate to within ±0.01 mm—preventing mold misalignment and part defects.
• Versatility: Boring can machine not only cylindrical holes but also tapered holes, stepped holes, and counterbores—all of which are common in molds for plastic injection. For example, counterbored holes are used to recess bolt heads in mold bases, while stepped holes accommodate multi-diameter components like valve stems.

4.2 Boring Equipment and Tools for Molds for Plastic Injection

The choice of boring equipment depends on the size and complexity of the molds for plastic injection component. Four primary types of machines are used for boring in mold manufacturing:

① Boring Machines

Boring machines (or "horizontal boring mills") are specialized for machining large, heavy workpieces—ideal for large molds for plastic injection bases and cavity blocks. They feature a horizontal spindle that holds the cutting tool, and a movable worktable that positions the workpiece. Boring machines offer three linear axes (X, Y, Z) and a rotational axis (W), allowing for precise positioning of holes in 3D space. For molds for plastic injection with complex hole systems, boring machines can achieve hole spacing accuracy of ±0.005 mm—critical for high-precision mold designs.

② Lathes

Lathes are used for boring cylindrical workpieces, such as mold cores or hydraulic cylinders for molds for plastic injection. On a lathe, the workpiece rotates while the boring tool is fed axially along the spindle axis. This method is ideal for producing deep, straight holes with high coaxiality—e.g., the inner hole of a mold core used to form a plastic tube. Lathes can bore holes up to 500 mm in diameter and 2000 mm in depth, making them suitable for long molds for plastic injection components.

③ Milling Machines

Milling machines (vertical or horizontal) are versatile for boring small-to-medium holes in molds for plastic injection components. They use a rotating spindle to hold the boring tool, and the workpiece is clamped to a movable table. Milling machines are ideal for boring holes in irregularly shaped mold parts (e.g., cavity inserts) and can be equipped with CNC controls for automated hole positioning. For prototype molds for plastic injection, milling machines offer fast setup times and flexibility for design changes.

④ CNC Machining Centers

CNC machining centers (vertical or horizontal) are the most advanced option for boring in molds for plastic injection manufacturing. They combine the capabilities of boring machines, lathes, and milling machines into a single system, with automated tool changers and multi-axis control. CNC machining centers can machine complex hole systems in molds for plastic injection with high repeatability (±0.002 mm) and efficiency, making them ideal for high-volume mold production. They are particularly valuable for molds with 5-axis hole systems (e.g., aerospace molds for plastic injection), where holes are oriented at angles to the workpiece surface.

The primary tool used in boring for molds for plastic injection is the single-point boring tool. These tools consist of a tool holder and a replaceable cutting insert (made from carbide or ceramic). The cutting insert is ground to a specific shape (e.g., square, round, or diamond) to optimize material removal and surface finish. For molds for plastic injection materials like H13 steel, carbide inserts with a TiAlN coating are preferred for their wear resistance and heat tolerance.

Figure 3-24: Boring process on a horizontal boring machine, machining a large cooling hole in a mold base for molds for plastic injection. The machine’s precision controls ensure the hole meets strict dimensional and positional requirements.

3.3 Boring Applications in Molds for Plastic Injection

Boring is a cornerstone process in the manufacturing of large or complex molds for plastic injection. Its key applications include:

• Large Cavity Holes: For molds for plastic injection used to produce large parts (e.g., automotive bumpers or appliance housings), boring machines are used to machine the large cavity holes in the mold base. These holes can be 500–1000 mm in diameter and require precision to ensure the mold halves align correctly.
• Cooling Line Holes: Cooling lines are critical for regulating mold temperature and reducing cycle time in molds for plastic injection. Boring is used to machine large cooling line holes (20–50 mm in diameter) in mold bases, ensuring uniform hole size and spacing for consistent cooling. Tapered or stepped cooling holes—machined via boring—improve coolant flow and heat transfer.
• Hydraulic and Pneumatic Holes: Large molds for plastic injection use hydraulic or pneumatic systems to actuate ejectors, slides, or cores. Boring machines produce the large-diameter holes (30–100 mm) required for hydraulic cylinders and pneumatic lines, ensuring precise fit with the hydraulic components.
• Multi-Hole Systems: For molds for plastic injection with complex hole systems (e.g., engine block molds or medical device molds), CNC machining centers use boring to produce multiple holes with precise spacing and orientation. This ensures that all mold components (e.g., guide pins, ejectors, and sensors) align correctly during assembly.

Boring is the most versatile and precise process for large or complex holes in molds for plastic injection. Its ability to handle large diameters, complex geometries, and strict positional requirements makes it indispensable for manufacturing high-quality, high-performance molds.