Machining process planning is a critical aspect of modern manufacturing, serving as the blueprint for transforming raw materials into finished products with precision and efficiency. This document outlines the fundamental concepts that form the basis of effective process planning, from basic definitions to complex production considerations. Understanding these principles is essential for anyone involved in manufacturing engineering, production management, or related fields, particularly when working with diverse manufacturing methods such as the injection molding process.
The following sections explore the core components of machining process planning, providing detailed explanations of each concept and their practical applications in various manufacturing scenarios. Whether dealing with simple one-off parts or complex mass-produced components, these fundamental principles remain consistent, guiding engineers and technicians in developing optimal manufacturing strategies that may include the injection molding process depending on product requirements.
1. Process
The process refers to the sequence of operations that directly alter the shape, dimensions, relative positions, and properties of the production object (raw materials or blanks) to transform them into finished products or semi-finished products. In the context of mold manufacturing, the process constitutes the primary part of the production cycle, encompassing operations such as casting and forging of blanks, heat treatment to modify material properties, mechanical machining of parts, and surface treatment processes.
Other labor processes, such as technical preparation for production, inspection, transportation, and storage, are considered auxiliary processes in mold manufacturing and do not fall under the category of part machining processes. This distinction is crucial in all manufacturing domains, including the injection molding process, where efficient separation of primary and auxiliary processes contributes to overall production optimization.
A complete process consists of one or several sequentially arranged operations through which the blank is transformed into a finished part. These operations can be further divided into setups, stations, steps, and passes. Each of these subdivisions plays a specific role in ensuring the efficient and accurate transformation of materials, whether in traditional machining or specialized processes like the injection molding process.
The definition and organization of processes vary somewhat across different manufacturing methodologies. For instance, in the injection molding process, the primary process focuses on material preparation, mold setup, injection parameters, and cooling cycles, while auxiliary processes include mold cleaning, part inspection, and packaging.
Figure 1: Illustration of a typical manufacturing process flow, showing the transformation from raw material to finished component through various stages, similar to how the injection molding process transforms plastic pellets into finished parts.
2. Operation
An operation refers to that part of the process which is completed continuously by one or a group of workers, at a single workplace, on the same workpiece or on a group of workpieces simultaneously. The operation is the basic unit that constitutes the process, and its division is primarily based on whether the workplace (equipment), processing object (workpiece) changes, and whether the processing is completed continuously.
Specifically, an operation maintains consistency in four aspects: the operating worker(s), the processing location, the part being processed, and the continuity of processing. This definition holds true across various manufacturing techniques, including the injection molding process, where each operation within the production cycle maintains these consistent elements.
In the context of the injection molding process, a typical operation might involve setting up a specific mold in an injection machine, configuring the machine parameters, and running a production batch of identical parts. This operation remains consistent until the mold is changed, the machine is reconfigured for a different part, or the production run is completed.
The clear definition of operations is essential for production planning, scheduling, and cost estimation. By breaking down the manufacturing process into distinct operations, managers can more effectively allocate resources, track progress, and identify bottlenecks. This is equally true in discrete manufacturing and in processes like the injection molding process, where operational efficiency directly impacts production costs and lead times.
Figure 2: A manufacturing operation showing consistent worker, workplace, and workpiece - key elements in defining an operation, whether in traditional machining or the injection molding process.
3. Step
Within a single operation, it is often necessary to use different cutting tools and cutting parameters to machine different surfaces. To facilitate analysis and description of the operation content, operations can be further divided into steps. A step is defined as that part of an operation which is completed when the machined surface, cutting tool, and cutting parameters (speed and feed rate) remain unchanged.
An operation can consist of multiple steps or just one step, depending on the complexity of the part and the machining requirements. The two determining factors for a step are the machined surface and the cutting tool. If either changes, or if the processing is not continuous despite both remaining the same, it generally constitutes a new step.
In the injection molding process, similar principles apply to the concept of steps, though the terminology differs. For example, different phases of the injection cycle—such as filling, packing, cooling, and ejection—can be thought of as analogous to steps in machining, each with distinct parameters and purposes.
When a workpiece undergoes several identical steps consecutively after secondary clamping, these are often documented as a single step in process documentation to simplify the operation content. For instance, as shown in Figure 2-2, when drilling four 10mm holes consecutively in a part, the operation can be written as "Drill 4×φ10mm holes" as a single step.
This approach to defining steps streamlines process documentation while maintaining clarity. In the injection molding process, similar simplifications are made when documenting repeated cycles or identical operations, ensuring that process sheets remain comprehensive yet concise.
Figure 3: Example of multiple identical drilling operations documented as a single step, a concept that also applies to repeated cycles in the injection molding process.
4. Pass
Within a single step, if the metal layer to be removed from the machined surface is sufficiently thick that it requires multiple cuts, each individual cut is referred to as a pass. A pass is thus a part of a step, and a step can include one or several passes depending on the material thickness and machining requirements.
The number of passes required is determined by several factors, including the material properties, the desired surface finish, the power of the machine tool, and the capabilities of the cutting tool. For hard or tough materials, more passes may be necessary to achieve the desired result without damaging tools or equipment.
In the injection molding process, a similar concept exists in the form of multiple injection stages or multiple cycles to achieve the desired part quality. While not exactly analogous to machining passes, these stages serve the same fundamental purpose—breaking down a complex transformation into manageable segments to ensure quality and process control.
Each pass in machining typically removes a specific amount of material, with the final pass often being a finishing pass that removes a minimal amount to achieve the precise dimensions and surface quality required. This incremental approach allows for greater control over the machining process and helps prevent tool wear and workpiece damage.
Understanding when and how to implement multiple passes is crucial for efficient machining. Excessive passes can increase production time and costs, while too few passes may compromise part quality. This balance is equally important in the injection molding process, where the number of cycles and their parameters must be optimized for both quality and efficiency.
Figure 4: Illustration of multiple passes in a turning operation, where each pass removes a layer of material, similar to how multiple stages might be used in the injection molding process.
5. Positioning and Setup
To machine a surface on a workpiece that meets specified technical requirements, it is necessary to locate the workpiece in a precise position on the machine tool or fixture before machining. This process is known as workpiece positioning. After positioning, due to the forces encountered during machining—such as cutting forces and gravitational forces—the workpiece's position may shift. Therefore, a clamping mechanism must be used to secure the workpiece and maintain its established position.
The entire process from positioning to clamping is collectively referred to as setup. Proper setup is critical for ensuring dimensional accuracy and repeatability in manufacturing, whether in traditional machining operations or in specialized processes like the injection molding process, where mold positioning and clamping are essential for part quality.
In a single operation, a workpiece may require only one setup or multiple setups. However, it is generally desirable to minimize the number of setups in any manufacturing process, including the injection molding process. Each additional setup introduces potential sources of error and increases the auxiliary time required for production.
In the injection molding process, proper setup involves precise positioning of the mold halves, ensuring proper alignment, and applying the correct clamping force. This setup directly impacts part quality, with misalignment leading to flash, dimensional inaccuracies, or even mold damage.
Advanced fixturing systems and modular setups have been developed to reduce setup times and improve accuracy in modern manufacturing. These systems allow for quick changeovers between different workpieces while maintaining positioning precision, a capability that is particularly valuable in flexible manufacturing environments where production runs may be short and varied, similar to how quick mold change systems benefit the injection molding process.
Figure 5: Workpiece positioning and clamping in a machining fixture, a process analogous to mold setup in the injection molding process.
6. Station
A station refers to each position occupied by the workpiece, along with the fixture or machine's movable parts, relative to the cutting tool or machine's fixed parts, after a single workpiece setup. The concept of stations is particularly important in multi-station manufacturing processes where multiple operations can be performed without repositioning the workpiece.
To reduce the number of workpiece setups, various rotary tables, index fixtures, or transfer fixtures are commonly used. These devices allow the workpiece to undergo multiple machining operations at different stations after a single setup. Each machining position occupied by the workpiece on the machine tool is therefore referred to as a station.
In the injection molding process, while the concept isn't identical, multi-cavity molds or family molds can be thought of as incorporating station-like principles, where multiple identical or different parts are produced in a single molding cycle, each in its own "station" within the mold.
Figure 2-3 illustrates the use of a multi-station fixture for drilling, reaming, and boring holes in a part. This fixture enables the workpiece to be sequentially positioned at four stations: loading/unloading (station 1), drilling (station 2), reaming (station 3), and boring (station 4). After completing one step, the rotary part of the machine fixture indexes the workpiece 90 degrees relative to the fixture's fixed part for the next step.
The adoption of multi-station fixtures reduces workpiece setup times, shortens process cycles, and improves productivity—advantages that are also realized in the injection molding process through the use of automated systems and multi-cavity molds. This approach is particularly beneficial in high-volume production environments where efficiency gains translate directly to cost savings.
Figure 6: Multi-station machining setup demonstrating how a workpiece can be processed at multiple stations without re-clamping, a concept that enhances efficiency similarly to multi-cavity molds in the injection molding process.
7. Production Program
The production program refers to the quantity of products (including waste and spare parts) that an enterprise should produce within a planned period, along with the schedule. The planning period is typically one year, so the production program is often referred to as the annual output or production volume. When formulating a process plan, the production type is generally determined based on the product's (or part's) production program.
Most mold components are produced as single pieces, while standard frames or standard parts are usually produced in larger quantities. In the injection molding process, production programs can vary widely, from small batches of custom parts to high-volume production runs of millions of identical components.
For standard mold base components or standard parts, the production program is calculated using the following formula:
Nz = N × n (1 + α) (1 + β)
Where:
- Nz — Production program of the part, pieces/year
- N — Production program of the product, units/year
- n — Number of this part in each product, pieces/unit
- α — Spare part rate, %
- β — Scrap rate, %
This formula accounts for the basic product demand, the number of each part required per product, and additional factors for spare parts and potential scrap. In the injection molding process, similar calculations are used to determine material requirements, considering factors like runner systems, scrap rates, and recycling possibilities that are unique to plastic processing.
Figure 7: Comparison of production volumes across different manufacturing types, including the injection molding process which can span the entire range from low to high volume production.
8. Production Type
Production type refers to the classification of an enterprise's (or workshop's, section's, team's, or workstation's) degree of production specialization. It is generally determined based on the size of the product's (or part's) production program and the variety of products manufactured. Once the production program for a part is determined, production is typically scheduled in batches within a specific timeframe. The quantity of parts produced in each batch is referred to as the batch size.
Production types can be categorized into three main types: single-piece production, batch production, and mass production. Mass production is rarely seen in general mold manufacturing enterprises, but is common in high-volume manufacturing processes like the injection molding process for consumer goods.
Single-piece production involves manufacturing a variety of products, with only one or a few of each product made. A single workstation may perform multiple operations on various products, with little repetition. General mold manufacturing typically falls into the single-piece production category.
Batch production refers to the periodic production of products in batches. The variety of products is not excessive, but each product is produced in a certain quantity. A workstation periodically processes certain operations on different workpieces in batches. For example, standard mold plates, mold bases, guide pillars, and guide bushes commonly used in molds are produced in batches.
In the injection molding process, batch production is extremely common, with production runs varying from small batches of custom components to large batches of standardized parts. The flexibility of the injection molding process allows manufacturers to efficiently produce both small and large batches, with quick changeovers between different part types.
Based on product characteristics and batch size, batch production can be further divided into small-batch, medium-batch, and large-batch production. Different production types have distinct considerations regarding process equipment, processing methods, technical requirements for operators, production costs, and part interchangeability. Therefore, it is essential to clarify the production type when formulating a process route.
The injection molding process is particularly versatile in adapting to different production types, with equipment and tooling available for everything from rapid prototyping (single-piece production) to high-volume manufacturing with automated systems. This flexibility makes it a preferred manufacturing method for many industries, allowing for efficient scaling from initial prototypes to full production runs.
| Item | Single-piece Production | Batch Production | Mass Production |
|---|---|---|---|
| Workpiece | Frequently changing | Periodically changing | Long-term unchanged |
| Blank Manufacturing & Machining Allowance | Low precision, large allowance | Medium precision, moderate allowance | High precision, small allowance |
| Machine Equipment | General-purpose | General & special-purpose | Specialized & automatic |
| Fixtures | Standard fixtures | Special fixtures | High-efficiency special fixtures |
| Worker Skill Level | Highly skilled | Moderately skilled | Lower skill, higher setup skill |
| Part Interchangeability | Fitted, no interchangeability | Generally interchangeable | Fully interchangeable |
| Process Documentation | Simple process cards | Detailed process documents | Very detailed documents |
| Application in Injection Molding | Prototypes, custom molds | Standard components, medium runs | Consumer products, high-volume parts |
Table comparing characteristics across production types, including their application in the injection molding process.
Summary
The fundamental concepts of machining process planning form the backbone of efficient and effective manufacturing operations. From understanding the overall process flow to optimizing individual steps, each concept plays a crucial role in transforming raw materials into finished products with precision and cost-effectiveness. These principles apply across various manufacturing methodologies, including the injection molding process, where they inform decisions about tooling, production scheduling, and quality control.
By mastering these concepts, manufacturing professionals can develop optimal process plans tailored to specific production requirements, whether for single-piece mold components or high-volume parts produced through the injection molding process. The ability to correctly identify operations, steps, passes, stations, and setups, combined with an understanding of production programs and types, enables organizations to balance quality, efficiency, and cost in their manufacturing operations.