What Is Mold Insert Design?
Insert Overview
In order to save material, make processing convenient and simple, and ensure the mold has sufficient life, a set of molds should have inserts. Inserts refer to the parts except for loading/unloading and other functional components. Inserts are generally embedded in the template plate.
Inserts are generally made of cold-work steel, hot-work steel, high-speed steel or plastic mold steel and other alloy materials. Others such as alloy steel, alloy aluminum, cobalt-nickel-tungsten new materials can be used.
For a large number of molds, in order to accurately meet the mold specifications required by injection molding OEM projects, the cavity is generally custom-made and directly machined on the fixed cavity plate by professional injection molding mold manufacturers.
The insert design requirements are as follows:
① Have sufficient strength and rigidity to withstand the lateral pressure of the plastic melt.
② Have sufficient hardness and wear resistance to withstand the friction of the material flow; usually the insert material hardness should be above HRC35, and in special high-volume injection molding production it should be HRC50～52 or above.
③ Good polishing performance, the surface should have high smoothness, and the surface roughness Ra should be below 0.4μm.
④ Good cutting performance and process performance; important precision parts should be polished as much as possible, and general parts should be milled as much as possible.
⑤ Easy to repair, and the processing of wearing parts must consider the operating structure.
⑥ Have sufficient precision; generally, the fit precision of hole parts reaches H6～H7, and shaft parts reach h4～h6.
Selection of Insert Materials and Their Heat Treatment
① Forming parts refer to mold parts that are in direct contact with plastic to form products, such as cavities, cores, sliders, inserts, lifters, side cores, etc.
② The material of forming parts is directly related to the quality and life of the mold and determines the appearance and internal quality of the molded plastic products. It must be chosen very carefully. Generally, it should be selected based on the contract requirements and according to the requirements and characteristics of the product and mold by experienced injection molding tooling suppliers.
③ The application principle of forming part materials is: according to the type of plastic to be formed, the shape of the product, dimensional accuracy, appearance quality and use requirements of the product, production batch size, etc., taking into account the cutting, polishing, welding, texturing, deformation, wear resistance and other properties of the material, while considering economy and mold manufacturing conditions and processing methods used by injection molding factory partners, to select different types of steel.
④ For molds for forming transparent plastic products, both the cavity and core need to be made of steel with high mirror polishing performance, such as 718 (P20+Ni type), NAK80 (P21 type), S136 (420 type), H13 type steel, etc. Among them, 718 and NAK80 are pre-hardened and do not require further heat treatment. S136 and H13 type steels are in annealed state with hardness generally HB160～200. After rough machining, vacuum quenching and tempering are required. The hardness of S136 is generally HRC40～50, and the hardness of H13 type steel is generally HRC45～55 (can be determined according to the specific grade).
⑤ For molds with high appearance quality requirements, long life, and large batch production, the selection of forming part materials is as follows:
a. The cavity needs to be made of steel with high mirror polishing performance, such as 718 (P20+Ni type), NAK80 (P21 type), etc., all in pre-hardened state, no further heat treatment required; the surface is nitrided, nitriding depth 0.15～0.2mm, hardness HV700～900.
b. The core can be made of medium and low-grade P20 or P20+Ni type steel, such as 618, 738, 2738, 638, 318, etc., all in pre-hardened state; for molds with small production batches, domestic plastic mold steel or imported high-quality carbon steel S50C, S55C can also be used.
⑥ For molds with general appearance quality requirements, the selection of forming part materials is as follows:
a. For small and precision molds, both cavity and core should preferably use medium-grade imported P20 or P20+Ni type steel.
b. For medium and large molds, where the molded plastic has no special requirements for steel, the cavity can use medium and low-grade imported P20 or P20+Ni type steel; the core can use low-grade imported P20 type steel or imported high-quality carbon steel S50C, S55C, etc., or domestic plastic mold steel.
c. For cavities with internal shrinkage texture, try to avoid using P20+Ni type 2738 (738) grade.
⑦ For steel structural parts with low appearance quality requirements and special requirements for wear and corrosion of the steel by the molding material, the selection of forming part materials is as follows:
a. For medium and large molds, the cavity can use low-grade imported P20 or P20+Ni type steel, or imported high-quality carbon steel S55C, S50C or domestic P20 or P20+Ni type plastic mold steel; the core can use domestic high-quality carbon steel.
b. For small molds with high output and complex structure, the cavity can use low-grade imported P20 or P20+Ni type steel, or domestic P20 or P20+Ni type plastic mold steel; the core can use domestic plastic mold steel.
⑧ For test molds, polystyrene with corrosive plastics and plastics containing flame retardants, if the product requirements are high, imported corrosion-resistant steel can be selected, requiring S136 quenched to HRC50～52; for general requirements, domestic corrosion-resistant steel can be used.
⑨ For molds where the plastic has strong friction and impact on the steel, such as molds used to inject nylon + glass fiber material, imported or domestic H13 type steel with high wear resistance, high thermal stress fatigue resistance and excellent anti-seizing and anti-thermal cracking properties should be selected by custom injection molding solutions providers, such as SKD11, SKH9, HRC56～58.
⑩ Forming parts generally use the same material as the parts of the person being manufactured. For parts of the mold that are difficult to cool or require high cooling effect, the insert material should be copper alloy or aluminum alloy.
⑪ The material selection principles for relatively hard moving parts in the mold are as follows:
a. Transparent parts should use high-grade imported steel with good polishing performance, such as 718, NAK80, etc.
b. Non-transparent parts should generally use medium-grade imported steel with relatively high hardness, such as 618, 738, 2738, 638, 318, etc., with surface nitriding treatment, nitriding depth 0.15～0.2mm, hardness HV700～900.
c. If the mold requirements are low, low-grade imported steel or domestic steel can also be used, with nitriding hardness generally HV600～800.
⑫ The selection principles for higher hardness are as follows:
a. For corrosion resistance requirement, select S136 quenched to HRC50～52.
b. For molds with fiber material and wear resistance requirements, SKD11 can be selected, quenched to HRC56～58.
c. For moving parts with wear resistance requirements, such as lifters, push blocks, etc., 8407 can be selected, quenched to HRC43～45 and then nitrided.
d. For high hardness hard materials, W302 can be selected, quenched to HRC50～52.
e. For long-life molds, M2 steel can be considered.
Segmentation of Main Parting Surface
Shape and Type of Parting Surface
The separable contact surface on the mold used to take out the plastic product and the gate is called the parting surface, also known as the clamping surface.
The parting surface can be flat, curved or stepped, but should be as simple as possible to facilitate plastic product forming and mold manufacturing. In general, a simple plane is used as the parting surface, and complex forms are only used in special cases.
The first step in mold design is to select the position of the parting surface. The selection of the parting surface is affected by factors such as plastic part shape, wall thickness, molding method, post-processing process, appearance of the plastic part, dimensional accuracy of the plastic part, demolding method of the plastic part, mold type, number of molds, mold exhaust, inserts, gate placement and the structure of the injection molding machine. For mold designers and injection molding manufacturing service providers, the correct selection of the parting surface is crucial to mold manufacturing and operation.
The forms of parting surfaces usually include the following types, see Figure 7-10.
Figure 7-10 Parting Surface
Selection Principles of Parting Surface
① The parting surface should preferably be opened at the position with the largest cross-sectional contour of the product to facilitate smooth demolding of the product.
② The parting surface should be set in a position that ensures no impact on the appearance quality of the plastic part, such as corners or edges. Prevent excessively thick flash caused by the parting surface, and the flash generated by the parting surface should be trimmed and eliminated as much as possible.
③ During injection molding, try to accumulate plastic on one side, so the parting surface should be selected as much as possible at the place where the product remains on the moving mold side, with the core set on the moving template, relying on the ejection force of the operation, the plastic part remains on the moving mold side.
④ The parting surface should not adversely affect the plastic part to a large extent. For plastic parts with high precision requirements, if the dimensions divided by the parting surface are inaccurate, dimensional errors will be caused.
⑤ When generally dividing the product, the part with high precision requirements should be placed in the opening direction of the fixed template for the longer parting distance or core-pulling part, and the shorter side should be side-cored.
⑥ Because the side core-pulling force is small, for large products with large projected area, the parting surface with large projected area should be placed on the main plane of the moving and fixed mold clamping, the front, and the parting surface with small projected area should be side parting surface.
⑦ The parting surface should be as simple as possible and avoid using complex shapes.
⑧ When the parting surface is used as the main exhaust surface, the parting surface should be designed at the end of the material flow to facilitate exhaust.
⑨ When selecting a semi-planar parting surface, it should be beneficial to cavity processing and product demolding convenience.