What Are Necessary for Injection Mold Design?
1. Design Basis
(1) Dimensional accuracy and correctness of relevant dimensions
The determination of the external quality and specific dimensions of plastic products is based on the specific requirements and functions of the entire product:
- Plastic products with high demands on external quality but lower requirements on dimensional accuracy, such as toys.
- Functional plastic products with strict dimensional requirements.
- Plastic products with stringent requirements on both appearance and dimensions, such as cameras.
(2) Whether the Demolding Angle Reasonable
The demolding angle directly affects the demolding process and quality of plastic products, determining whether the injection process can proceed smoothly:
- There should be sufficient demolding angle.
- The angle should correspond to the parting line or parting surface of the plastic product during molding.
- Whether it will affect the accuracy of appearance and wall thickness dimensions.
(3) Whether it will affect the strength of a certain part of the plastic product
2. Design Procedures
(1) Analysis of Plastic Product Drawings and Samples
- Geometric shape of the product
- Dimensions, tolerances, and design benchmarks
- Technical requirements
- Plastic name, grade
- Surface requirements
(2) Number and Arrangement of Cavities
- Product weight and injection volume of the injection molding machine
- Product projected area and clamping force of the injection machine
- Mold dimensions and effective area for mold installation on the injection machine (or distance between injection machine tie bars)
- Product precision and color
- Presence of side cores in the product and their treatment methods
- Production batch of the product
- Economic benefits (production value per mold)
Once the number of cavities is determined, the arrangement of these cavities is carried out, which involves the positioning of the cavities within the mold.
The arrangement of cavities is connected to mold dimensions, design of the gating system, balancing of the gating system, design of core-pulling (slider) mechanisms, design of inserts and cores, and design of the heat exchange system.
All these considerations are interconnected with the selection of parting lines and gate positions.
Hence, during the specific design process, necessary adjustments need to be made to achieve a more refined design.
3. Determination of Parting Lines
- Should not affect the appearance.
- Should be conducive to ensuring product accuracy and mold processing, especially cavity machining.
- Should facilitate the design of the gating system, venting system, and cooling system.
- Should aid in mold opening (parting or demolding), ensuring that the product remains on one side of the moving mold during demolding.
- Should allow for convenient arrangement of metal inserts.
4. Gating System Design
The design of the gating system involves selecting the main runner, determining the shape and size of the sub-runners, choosing the gate locations, and specifying the gate type and gate cross-sectional dimensions. When using a point gate, to ensure the removal of the sub-runner, attention should also be paid to the design of the sub-gate device, sub-gate mechanism, and gate mechanism.
In the process of designing the gating system, the first step is to choose the location of the gate.
The selection of gate locations directly impacts product molding quality and the smooth progress of the injection process.
Therefore, the choice of gate locations should adhere to the following principles:
- Gate locations should preferably be chosen on parting surfaces to facilitate mold processing and easy cleaning of the gate during usage.
- The distances between gate locations and various parts of the cavity should be as consistent as possible, while maintaining the possible shortest flow path.
- Gate locations should ensure that plastic flows smoothly into wide and thick-walled sections of the cavity, facilitating smooth plastic flow.
- Gate locations should be placed at the thickest section of the plastic part.
- Avoid direct impact of plastic against cavity walls, cores, or inserts when flowing into the mold cavity, allowing plastic to flow into different parts of the cavity promptly and preventing deformation of cores or inserts.
- Strive to prevent weld lines from forming on the product or position weld lines in less critical areas.
- Gate locations and the direction of plastic flow should ensure even parallel flow into the mold cavity, aiding the expulsion of internal gases.
- Gates should be situated in areas that are easy to clean while minimizing any impact on the product’s appearance.
5. Exhaust System Design
The design of the exhaust system plays a crucial role in ensuring the quality of product molding.
Using exhaust slots, these slots are generally positioned at the last areas of the cavity to be filled. The depth of the exhaust slots varies with different plastics, and it is primarily determined by the maximum allowable gap where plastic does not produce flash.
For instance, for ABS, it’s about 0.04mm, while for polypropylene, it’s below 0.02mm.
Clearances provided by core inserts, ejector pins, and specialized exhaust plugs are used for exhaust.
Sometimes, ejector pins are necessary to prevent vacuum deformation during ejection. Additionally, anti-vacuum elements are designed to prevent products from being vacuum-sealed against the mold surface.
6. Cooling System Design
The design of the cooling system is a relatively intricate task, as it involves considering both cooling effectiveness and uniformity, as well as the impact of the cooling system on the overall mold structure. This includes:
- The arrangement of the cooling system and the specific form it takes.
- Determining the specific positions and dimensions of the cooling system.
- Focusing on cooling for critical parts, such as the moving mold or inserts.
- Cooling for side slides and side cores.
- Designing cooling components and selecting standard cooling elements.
- Designing sealing structures.