Left and right armrests are important parts of a treatment chair. Product diagram is shown in Figure 1. Maximum size of product is 201.34 mm * 81.78 mm * 60.61 mm, average thickness of plastic part is 12.00 mm, material of plastic part is PA6+50% GF, shrinkage rate is 1.004, and weight of plastic part is 124.20 gram. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling of injection molding, flow lines, pores, warpage deformation, silver streaks, cold materials, jet lines, etc.

 

Figure 1 Product Picture of Left and Right Handrails
It can be seen from Figure 1 that structure of plastic part is L-shaped, one end of cylinder has an inner hole, which needs to be inserted into hardware, and its circumference has a uniform tooth shape, which can play a stable role after being embedded in plastic.
Purpose of inserts in plastic parts is to increase local strength, hardness, wear resistance, conductivity or magnetic permeability of plastic parts; or to increase stability of shape and size of plastic parts, reduce deformation, and improve accuracy; or to reduce size of plastic and meet other requirements. Materials of inserts include various non-ferrous or ferrous metals, as well as glass, wood, and molded plastic parts, among which metal inserts are most commonly used.
Use of inserts generally increases cost of plastic parts and complicates mold structure. In addition, installation of inserts in mold during molding of plastic parts will reduce productivity of plastic parts, and it is difficult to realize automation. Molded plastic part is used as an insert mold, which is generally called an insert mold. Overmolding mold can achieve same effect as two-color mold, it is also a good choice when two-color molding machine is lacking.
After metal insert is inserted into plastic, there will be various problems, common precautions are as follows;
1) After metal insert is inserted into plastic, in order to ensure that it is firm and reliable, does not loosen, does not slip off, it is necessary to make steps, undercuts, patterns or knurling in embedded part of metal insert to prevent it from falling off.
2) Shape and size design of metal inserts need to pay attention to craftsmanship, inserts are easy to process, can be reliably positioned during injection molding, can be easily ejected and demolded after injection.
3) Due to different properties of two materials, especially different coefficients of thermal expansion, metal insert may cause plastic parts around insert to crack. If a copper nut is inserted into PC material during injection molding, it is prone to cracking. It is necessary to preheat insert during injection molding to avoid cracking.

 

4) It is not suitable to design a multi-cavity mold with a mold for placing insert, because it takes time to place insert and delays injection cycle. Therefore, a comprehensive evaluation is required during mold design.
Material of plastic part is PA6+50%GF, shrinkage rate is 1.004. Nylon and glass fiber can play a reinforcing role. After glass fiber is added to plastic, shrinkage rate is significantly reduced, decline is getting bigger and bigger with increase of glass fiber content. When designing a mold containing a high proportion of glass fiber plastic, it is necessary to pay attention to small shrinkage rate of large or complex products, while shrinkage rate is anisotropic.
Plastic part is symmetrical, mold design cavity ranks 1+1, mold base is CI3540 standard mold base. Gate is designed for submersible thimble feeding. Side of elbow of plastic part with insert is designed on ground side of mold, and slider core needs to be designed. Inclined positioning blocks are added to sky side, operating side and opposite operating side of mold base to lock mold after it is closed.

 

Fixed mold and movable mold bases also have positioning pins to increase rigidity.
In order to facilitate fixing of inserts, slider is driven by an oil cylinder. Bottom of slider is designed with a wear-resistant plate 17, made of 9CrWMn, heat-treated HRC60-62, and an oil groove is opened on the surface. When oil cylinder is core-pulled, it is necessary to design inclined wedge 19 in order to lock oil cylinder. For molds with movable core-pulling, cylinder can be easily locked with a wedge. In fixed mold core pulling or some hot runner molds, sometimes it is difficult to lock oil cylinder, and it is necessary to design oil cylinder to lock oil cylinder or other mechanisms to lock it. Before closing mold, slider needs to be reset to facilitate insertion of insert. A guide post is designed at the bottom of cylinder mounting plate to provide guidance for movement of cylinder piston rod and increase rigidity.
When designing mold of cylinder core pulling, it is necessary to pay attention to design of a connecting block on the head of cylinder piston rod. One side of connecting block is made with a screw hole, and thread of cylinder piston rod is screwed into this screw hole. The other end of connecting block is designed with a T-shaped block, and a T-shaped groove is designed on corresponding part of slider seat to connect with connecting block. Do not screw thread of piston rod directly into slider, so as not to cause slider to move unsmoothly.

 

Figure 2 Mould diagram of left and right armrests

 

Figure 3 3D mold diagram
Mold temperature of nylon products is very high during injection molding. Therefore, cooling system must be adequately designed. An insulation board is designed on fixed mold panel. Both movable and fixed molds are designed with enough straight-through water circuits to ensure normal progress of injection molding.
Molding machine of mold is NB300, structure of mold is a European mold, hanging mold beam 8 is designed on sky side. Plastic parts are ejected by a ejector pin. European mold is characterized by strong tension and strong ejection. No spring is designed on return pin. Thimble plate must be designed with a travel switch and a center bracket to facilitate detection of effective reset of thimble plate. Make threaded holes at K.0 hole at the bottom of thimble plate, usually M12, M16 and M20, which are convenient for connecting return rod.