Formed part design
Mold parts can be divided into formed parts and structural parts according to their functions. Formed parts refer to structural parts that directly participate in forming the cavity space, such as concave mold (cavity), male mold (core), inserts, rows, etc.; Structural parts refer to parts used to complete various actions during installation, positioning, guidance, ejection and forming, such as positioning rings, nozzles, screws, pull rods, ejector pins, sealing rings, distance pull plates, pull hooks, etc. wait. See the next section for commonly used structural parts. When designing formed parts, full consideration should be given to the molding shrinkage rate of the rubber material, the demoulding slope, the craftsmanship of manufacturing and maintenance, etc.
5.4.1 Forming shrinkage of rubber compound
The molding shrinkage of rubber materials is affected by many aspects, such as rubber material type, rubber part geometry and size, mold temperature, injection pressure, mold filling time, holding time, etc. Among them, the most significant influence is rubber material type, rubber Part geometry and wall thickness. Different rubber materials have different shrinkage ranges (see Chapter 2, Commonly Used Plastics). The specific shrinkage rate is based on the recommended value. If there is any change, it needs to be determined by the person in charge.
It is worth noting that when increasing the shrinkage value for the same plastic part, the reference points selected for 3D design and 2D design should be the same, otherwise the 3D and 2D designs will be inconsistent.
5.4.2 Draft slope
Reasonable demoulding slope is a necessary condition to facilitate demoulding and obtain high-quality surface requirements. When designing plastic parts, a more reasonable draft is generally given. However, due to sometimes poor consideration, the rubber parts are selected or have an unreasonable draft angle, which will inevitably affect the surface quality of the plastic parts. Therefore, the draft angle of the plastic parts should be checked during mold design, and related The person in charge negotiates to resolve unreasonable areas. The following are general requirements for draft angle:
(1) Commonly used rubber materials such as ABS, HIPS, PC, PVC, etc., the demoulding slope of the outer surface of the plastic parts should be selected as follows:
For small plastic parts with a smooth outer surface, the draft angle is ≥ 1˚; for large plastic parts, the draft angle is ≥ 3˚
External surface etched surface Ra < 6.3, draft angle ≥ 3˚; Ra ≥ 6.3, draft angle ≥ 4˚
The spark pattern surface of the outer surface Ra < 3.2, the draft angle ≥ 3˚; Ra ≥ 3.2, the draft angle ≥ 4˚
(2) Regardless of whether the bone position and column position on the inner surface of the plastic part are designed with a draft angle, when designing the mold, the draft angle should be increased or modified according to the following requirements.
The thickness of the root of the bone is less than 0.5t ("t" is the wall thickness of the plastic part); the thickness of the top of the bone should be greater than or equal to 0.8mm. The specific demoulding slope is based on the determined thickness difference and the height of the bone. Depends. If the draft angle is required on both sides of the length of the bone, a larger draft angle should be selected without affecting the internal structure of the plastic part.
The requirements for column positions shall be modified according to the contents of Chapter 3, Section 3.
(3) When increasing or modifying the demoulding slope of the rubbing or punching position, select it according to the requirements for the stepped parting surface in Section 2 of Chapter 5. If the structure of the plastic part is affected, the solution should be negotiated with the relevant person in charge. .
5.4.3 Processability of formed parts
When designing the mold, the formed parts should have good assembly, processing and maintenance performance. In order to improve the processability of formed parts, the following points should be considered:
(1) Cannot produce sharp steel or thin steel
As shown in Figure 5.4.1a; 5.4.1b; 5.4.1c
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(2)Easy to process
Ease of processing is the basic requirement for the design of molded parts. When designing molds, the processing performance of each part should be fully considered, and the processing technology requirements should be met through reasonable inlay combinations. For example, in order to make the stop of the plastic part easy to process, the inlay shown in Figure 5.4.2a and 5.4.2b is generally used.
spelling structure. Other combination methods or no inlaying are unreasonable design structures.
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(3) Easy to resize and repair
For molded parts, the combined structure should be considered for parts that may change in size, as shown in Figure 5.4.3; for the bump and scratch locations that are prone to wear, an inlaid structure should be used for strength and convenience in maintenance.
(4) Ensure the strength of molded parts
(5)Easy to assemble
For molded parts with inlaid structures, ease of assembly is a basic requirement in mold design, and errors during installation should be avoided. For regular-shaped inserts or multiple inserts with the same dimensions in the mold, the design should consider avoiding misaligned installation of the inserts and turning installation of the same insert. A commonly used method is asymmetric fastening or positioning of inserts. As shown in Figure 5.4.4b.
In Figure 5.4.4a, the fastening position is symmetrical, which can easily lead to misaligned installation of insert 1 and insert 2, and the same insert can also be easily rotated and installed. In Figure 5.4.4b, the fastening positions of each insert are asymmetrically arranged, and the fastening arrangements of insert 1 and insert 2 are also different, thereby avoiding misaligned installation and turning installation of the same insert. In addition, in order to avoid misaligned installation, asymmetric arrangement of positioning pins can also be used.
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(6) Cannot affect the appearance
When designing molded parts, not only the process requirements must be considered, but also the appearance requirements of the plastic parts must be ensured. Whether the clamping wire is allowed to exist in the plastic part is the prerequisite for determining whether the insert can be made. If the clamping wire is allowed to exist, the inlay structure should be considered. Otherwise, other structural forms can only be used. In Figure 5.4.5, if clamping lines are allowed on the surface of the plastic part, the inlay structure can be used to facilitate processing; in Figure 5.4.6, clamping lines are not allowed on the front surface of the plastic part. To facilitate processing or other purposes, the clamping The line position is moved to the side wall, thereby adopting a mosaic structure. In Figure 5.4.7, when the arc is not allowed to clamp the wire, the insert structure is changed and the wire clamping position is moved to the inner wall.
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(7) Comprehensive consideration of mold cooling.
After the molded parts adopt an inlay structure, if local cooling is difficult, other cooling methods or the overall structure should be considered.
