Figure 6.1 Assembly drawing

This paper analyzes structure of injection molding mold for a plastic handle. It first conducts 3D modeling of plastic handle's exterior, analyzes plastic product's application scenarios, and selects product's materials. 3D software is then used to analyze product's volume and estimate the total injection volume per shot. An appropriate injection molding machine model is selected. After selecting parting surface and arranging mold cavities, design begins with injection molding system, demolding mechanism, side core pulls, mold opening and closing guides. Design utilizes a side gate, uses a CI LKM standard mold base, and employs a push-rod ejection method. Finally, a cooling system is designed for mold to ensure product's molding quality.

Injection Molded Part Structure and Processability Analysis:
1. Receive part design documents and technical requirements
Obtain part's 3D model and 2D engineering drawings, and clarify part's material properties (considering shrinkage, fluidity, and heat resistance), dimensional tolerances, surface quality, and assembly relationships (clearance/interference with other parts).
2. Confirm part's production batch size (small batch size ≤ 10,000 pieces, medium batch size 10,000-100,000 pieces, large batch size ≥ 100,000 pieces).
3. Part Manufacturability Analysis
Wall Thickness Uniformity: Avoid wall thickness variations greater than 30% (to prevent sink marks and bubbles). For example, if wall thickness of a part is 2mm, thickness in adjacent areas should not exceed 2.6mm.
Mold Draft Angle: Set based on material (for example, for ABS parts, outer surface should have an angle of ≥1°, and inner surface should have an angle of ≥0.5° to avoid damage).
Rice and Sharp Corners: Eliminate sharp corners, with a minimum radius of ≥0.3mm (to reduce mold stress concentration and extend mold life).
Hole and Rib: Hole diameter ≥1mm (to prevent needle breakage), rib thickness ≤ 1/2 the wall thickness (to prevent shrinkage).

 

Figure 1.1 3D drawing of plastic part

 

Figure 1.2 2D drawing of product part
3. Determining Position of Parting Surface
Parting surface structures typically include flat, vertical, and inclined parting surfaces.
1) Position designed parting surface at part with the largest contour.
2) Parting surface should be chosen to minimize difficulty of machining molded part itself.
3) Mold should retain finished plastic part after movable mold is opened.
4) Ensure that lateral core pulling molding process operates normally without any interference or obstruction.
5) Parting surface should be positioned to facilitate mold venting.
Analyzing handle structure, parting surface was ultimately located at the largest cross-section of handle base.

 

Figure 2.2 Parting surface position
Cavity Layout: When determining specific number of cavities for an injection mold, primary considerations are production efficiency and machining accuracy, as well as difficulty of mold design. Increasing number of cavities complicates the overall mold design, increasing both mold manufacturing costs and production costs, but reducing production waste. After comprehensively considering mold design, design of each component, design and processing requirements, a one-mold, two-cavity layout was ultimately adopted.

 

Figure 2.3 Cavity layout
Gating System Design: Gating system is "plastic channel" and includes main runners, branch runners, gates, and cold wells. Selection should be based on part structure:

 

Figure 3.3 Runner and side gate
Gating Types: Direct Gate (suitable for large parts, eliminates waste but leaves a mark), Side Gate (suitable for small and medium-sized parts, easy to remove), and Point Gate (suitable for parts with high surface requirements, such as cosmetic cases, requiring a three-plate mold).
Runner Dimensions: Calculate based on material flow (e.g., ABS branch runner diameter is 6-8mm, PP can be reduced to 4-6mm). Avoid excessive runner lengths that cause pressure loss.
Molded Part Design: Molded parts are categorized into various types based on their structure: integral embedded, modular, and monolithic.
Integral type involves machining molded part onto mold plate. This structure offers the best strength and rigidity, but is difficult to machine and is therefore less commonly used. Integral embedded type offers relatively good strength and rigidity, making it particularly suitable for multi-cavity molds and small and medium-sized molds. Furthermore, this type of molded part leaves no joint marks, which does not affect product's appearance. It is also easier to machine and is the most commonly used.
Combined molded parts are suitable for products with deep cavities or smaller volumes. Since these two types of molds are subject to significant wear on core, a modular design facilitates later maintenance and replacement.
Since internal and external structures of handle are not overly complex, I chose to use an integrally embedded cavity and core.

 

Figure 4.1 3D diagram of die

 

Figure 4.2 3D drawing of punch
Guide Mechanism Design: Guide mechanisms are located at four corners of mold base. Guide sleeves have an interference fit with fixed platen, and guide pins have an interference fit with movable platen.

 

Figure 5.3 Guide mechanism layout
Design of Lateral Parting and Core Pulling Mechanism: After mold is molded and cooled, mold must be opened. However, parts often have holes or slots on the side. Direct mold opening can result in defects or even defective parts. Therefore, side holes require lateral core pulling for demolding.
In this project design, side holes on side wall of product have a small core pulling distance, so commonly used inclined guide pin + slider core pulling mechanism can meet molding requirements.

 

Figure 5.5 Core pulling mechanism
Mold Demoulding Mechanism Design: After molding, plastic parts may remain on mold core due to thermal expansion and contraction. Manual demolding is only suitable for small-scale production. Large-scale production requires a mold release mechanism to complete mold release process.
Mold Release Mechanism Selection: Based on production design experience, we know that box-like objects can be demolded using a push-rod mechanism because it is simple to manufacture, maintain, replace, and offers high precision. Therefore, this design also uses a push-rod mechanism for mold release.

 

Figure 5.4 Push rod distribution
Cooling System Design:
1. Cooling circuit should be distributed as evenly as possible to ensure uniform cooling across mold, prevent product deformation and residual stress caused by uneven cooling.
2. Cooling water channel should be as close to mold cavity surface as possible, but should not affect mold strength.
3. Water channel diameter should be appropriate to ensure cooling water flow rate and volume, avoiding excessively fast or slow flow that may affect cooling efficiency.

 

Figure 5.7 Cooling water channel

Assembly Drawings: Mark mold's overall dimensions, closing height, mold opening stroke, and key mating relationships. Include a parts list (including part name, material, quantity, and standard number).
Parts Drawings: Each non-standard part (such as cavity, core, and slide) must be labeled with dimensions, tolerances, surface roughness (e.g., cavity surface Ra ≤ 0.8μm), and heat treatment requirements (e.g., P20 must be tempered to HRC 28-32).

 

How Injection Molds Work: First, raw material pellets are heated. Once fully melted, they are injected into mold, cooled, and held under pressure. Injection molding machine's pull forces movable and fixed platens apart. Slide, driven by inclined guide pins, then moves laterally, pulling core. After reaching a certain distance, injection molding machine's ejector pin passes through a hole in movable mold base plate. Ejector pin presses against push plate, and push rod on push plate ejects molded part, thus completing cycle. Ejector mechanism is reset by reset pin, and slide is reset by inclined guide pins when mold is closed.

 

Figure 6.1 Assembly drawing