How to choose injection molding machine nozzle?

Time:2021-04-02 18:36:19 / Popularity: / Source:

How to choose injection molding machine nozzle 
Basic form of nozzles: nozzles can be divided into straight-through nozzles, locked nozzles, hot-runner nozzles and multi-runner nozzles.
Straight-through nozzle is a commonly used nozzle. Its characteristic is that spherical surface of nozzle directly contacts spherical surface of main casting sleeve of mold. Arc radius and runner of nozzle are smaller than that of mold. During injection, high-pressure melt is directly filled into cavity through runner system of mold, with fast speed, low pressure loss, convenient manufacturing and installation.
Locking nozzles mainly solve problem of salivation of straight-through nozzles, are suitable for processing low-viscosity polymers (such as PA). Nozzle flow channel can be closed during pre-molding to prevent melt drooling, it can be opened under action of injection pressure during injection, so that melt can be injected into cavity. Working principle of injection cylinder is: when injection cylinder enters oil, piston drives piston rod and bearing placed in thrust seat to push screw forward or backward.
Through nut on piston rod head, axial position of two parallel piston rods and axial position of the injection screw can be adjusted synchronously. During injection, thrust seat pushes screw through thrust shaft for injection; while pre-plasticizing, oil motor drives thrust shaft to drive screw to rotate to achieve pre-plastic. When block-shifting cylinder is filled with oil, it realizes forward or backward motion of injection block, ensures that injection nozzle is in close contact with arc surface of main casting sleeve of mold to generate injection block pressure that can seal melt.

Precision requirements for injection parts

After assembly, integral injection part must be placed on frame, nozzle must be tightly connected with main casting sleeve of mold to prevent overflow. It is required to make center line of injection part concentric with center line of clamping part; In order to ensure matching accuracy of injection screw and inner hole of barrel, parallelism of two injection cylinder holes, positioning center hole of barrel and symmetry of center line must be ensured;
For horizontal machines, parallelism of two guide holes of seat shift cylinder and symmetry of its center must also be ensured, while for vertical machines, it is necessary to ensure parallelism between two seat shift cylinder holes and cylinder positioning center hole, symmetry of center line.
Factors affecting above-mentioned position accuracy are dimensional accuracy, geometric accuracy, manufacturing accuracy, assembly accuracy of associated component hole and shaft.
Clamping part is one of important parts of injection molding machine. Its function is to realize opening and closing movement, so that mold is closed and system is elastically deformed to reach clamping force and lock mold.

Requirements for mold parts:

① Opening and closing movement of moving mold plate should be high-speed, stable and silent;
② Mold clamping mechanism must meet requirements of rated clamping force to reliably lock mold;
③ Clamping parts have enough mold space and mold plate stroke;
④ Moving mold plate movement should be reliable and safe, protect safety of person and mold, set up double insurance;
⑤ Clamping part and its mold have sufficient strength and rigidity.
Mold clamping part is composed of a mold clamping frame, a mold clamping mechanism, a mold adjustment device, a protection device, and an ejection device.

Mold grinding technology

As an important tool for workpiece manufacturing, molds will inevitably have some defects in processing, which will result in workpieces that cannot be used well after they are manufactured. In order to better manufacture related workpieces, seven measures are introduced in detail.

1. Reasonable selection and dressing of grinding wheels

Grinding wheel of white corundum is better. Its performance is hard and brittle, it is easy to produce new cutting edges. Therefore, cutting force is small and grinding heat is small. Medium particle size is used, such as 46-60 mesh. Hardness of grinding wheel is medium soft and soft (ZR1, ZR2 and R1, R2), that is, coarse-grained, low-hardness grinding wheels, which have good self-excitation and can reduce cutting heat.
When fine grinding, it is very important to choose an appropriate grinding wheel. In view of high vanadium and high molybdenum conditions of mold steel, GD single crystal corundum grinding wheel is more suitable. When processing cemented carbide and materials with high quenching hardness, diamond grinding wheels with organic binders are preferred. organic Binder grinding wheel has good self-grindability and roughness of ground workpiece can reach Ra0.2μm.
In recent years, with application of new materials, CBN (cubic boron nitride) grinding wheels have shown very good processing results. Precision processing on CNC forming grinders, coordinate grinders, and CNC internal and external cylindrical grinders is better than other types of grinding wheels.
In grinding process, attention should be paid to dressing grinding wheel in time to keep grinding wheel sharp. When grinding wheel is passivated, it will rub and squeeze on the surface of workpiece, causing burns on the surface of workpiece and reducing its strength.

2. Reasonable use of cooling lubricant

Play three functions of cooling, washing and lubrication to keep cooling and lubrication clean, so as to control grinding heat within allowable range to prevent thermal deformation of workpiece.
Improve cooling conditions during grinding, such as using oil-immersed grinding wheels or internal cooling grinding wheels.
Introduce cutting fluid into center of grinding wheel, cutting fluid can directly enter grinding area, play an effective cooling role, prevent burns on the surface of workpiece.

3. Reduce quenching stress after heat treatment to a minimum

Because of quenching stress and net-like carbonized structure under action of grinding force, structure undergoes a phase change, which can easily cause cracks in workpiece. For high-precision molds, in order to eliminate residual stress of grinding, low temperature aging treatment should be carried out after grinding to improve toughness.

4. Eliminate grinding stress

To eliminate grinding stress, mold can be immersed in a salt bath at 260℃-315℃ for 1.5min, then cooled in 30℃ oil. In this way, hardness can be reduced by 1HRC and residual stress can be reduced by 40%-65%.

5. Constant temperature grinding

For precision grinding of precision molds with dimensional tolerances within 0.01mm, attention should be paid to influence of ambient temperature and constant temperature grinding is required. From calculation, it can be seen that for a 300mm long steel, when temperature difference is 3℃, material will have a change of about 10.8μm, (10.8=1.2*3*3, deformation per 100mm is 1.2μm/℃), each finishing process needs to fully consider influence of this factor.

6. Using electrolytic grinding processing

Electrolytic grinding can improve mold manufacturing accuracy and surface quality.
During electrolytic grinding, grinding wheel scrapes oxide film instead of grinding metal, so grinding force is small, grinding heat is also small, and there will be no grinding burrs, cracks, burns, etc. General surface roughness can be better than Ra0 .16μm; In addition, wear of grinding wheel is small, such as grinding of cemented carbide, wear of silicon carbide grinding wheel is about 400%-600% of weight of cemented carbide; when electrolytic grinding is used, wear amount of grinding wheel is only 50%-100% of carbide grinding amount.

7. Reasonably choose amount of grinding

Adopt fine grinding method with small radial feed, even fine grinding. Such as: appropriately reduce radial feed and speed of grinding wheel, increase axial feed, so that contact area between grinding wheel and workpiece is reduced, heat dissipation conditions are improved, and surface temperature is effectively controlled.

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