Injection molding is developed from the principle of metal die casting. It uses the extrudability and moldability of plastics. Firstly, the material to be heated is fed into a high-temperature barrel from a hopper of an injection machine , melted and plasticized to form viscous melt.
Then, under the high pressure of a plunger or a screw , the material is injected into a closed mold with a lower temperature through a nozzle on the front end of the barrel at a large flow rate. After pressure holding, cooling and solidification, the mold is opened to eject the required injection molded product.
Injection molding process
The main process of injection molding includes three stages: plasticization and metering, injection and mold filling, and cooling and ejection.
The plastic raw materials are added into the hopper of the injection machine and brought into the barrel by the plunger or screw for heating.
After materials are heated, compacted and mixed in the barrel of the injection machine, the process of transforming loose powder or granular solid into continuous homogeneous melt is called plasticization. The melt must have uniform composition, density, viscosity and temperature distribution. Metering refers to ensuring that the injection machine can apply the plasticized melt temperature, constant pressure and quantity to the barrel through the plunger or screw.
All these actions need the injection machine to control the plunger or screw to complete in the plasticizing process.
Injection mold filling
The plunger or screw applies high pressure through the injection cylinder and piston to quickly feed the plasticized plastic melt into the closed mold cavity through the nozzle at the front end of the barrel and the gate system in the mold. The process is called mold filling: filling, pressure holding and melt back-flow.
Mold filling: the process in which the melt enters and fills the mold cavity through the nozzle and the mold gating system at a certain pressure and speed under the pushing of the plunger or screw of the injection machine.
Pressure-holding and plastic-supplementing refers to the pressure-holding and feeding stage from the moment the melt fills the cavity to the moment the plunger or screw starts to retract in the barrel.
Pressure holding refers to the process that the injection pressure continues to compact the melt in the mold cavity.
plastic-supplementing refers to filling the space caused by shrinkage of the melt gradually starting to cool in the mold cavity during the pressure maintaining process.
Back-flow refers to the reverse flow of melt in the mold cavity towards the gate and runner when the plunger or screw moves backward in the barrel.
Cooling and Solidification
Cooling and solidification refers to the process from the freezing time of the gate to the demoulding of the product. The difference between the pressure in the mold cavity and the outside atmospheric pressure during mold opening is called residual pressure:
When the residual pressure is positive, demoulding is difficult, and plastic parts are easy to scratch or crack.
When the residual pressure is negative, there are defects on the surface of the plastic parts or vacuum bubbles inside.
The mold can be opened after the plastic part is cooled and solidification, and the plastic part is pushed out under the action of the ejection mechanism to complete the injection molding process.
Annealing is a heat treatment process in which plastic parts are placed in a constant temperature heating medium (such as hot oil, hot water, hot air and liquid paraffin, etc.) for a period of time.
Humidification treatment is a post-treatment process for adjusting the water content of products. It is mainly used for polyamide and other plastic products with strong hygroscopicity and easy oxidation.It mainly eliminates residual stress and avoids dimensional deformation under the conditions of heating and heat preservation.
Automotive parts (Grille, Door, front fender, Interior and exterior parts, and others)
Home appliance ( Philip, Panasonic)
Engineer Parts/Connector parts
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The basic structure of the injection mold
The basic structure of the injection mold can be divided into seven parts: injection system, mold components, lateral moving mechanism, guide mechanism, ejection mechanism, core pulling mechanism, cooling & heating system and venting system. The details of these seven parts is as follows:
Injection system. It refers to the plastic flow path from the injection molding machine nozzle to the cavity in the mold. The injection system is composed of runner, a runner, sub-runner, gate and cold slug .
Lateral moving parting and core pulling mechanism.
In the plastic mold, the guiding mechanism mainly has the functions of positioning, guiding and enduring a certain side pressure to ensure accurate clamping of the moving and fixed molds. The clamping guide mechanism is composed of guide pin, sleeve or guide hole (directly on the template).
The ejector device mainly ejects the workpiece from the mold, and composed of ejector pins , ejection sleeves or pushing plate, an ejector pin retain plate, an ejector pin retain back plate, return in and pulling rod.
Cooling and heating system.
Plastic mold components, it refers to the components that make up the plastic mold . It mainly includes: core insert, cavity insert, inserts…
The designer’s experience of the right adoption of plastic to build a part helps alot for good performance of an injection part.The design plays a key role in this knowledge，along with information from plastic material suppliers, plastic mold tooling builders, and production companies. if you want the injection part runs properly when built in the right material, achieved in the profile designed，and displays the function required,the good design plays a role like no other.The injection part design，mold building, and manufacturing quality control principles are all bond together to achieve TQPC.
Mold Opening Direction and Parting Line
At the beginning of the design , the opening direction and parting line should be determined to ensure that the core-pulling and slider mechanism can be reduced as far as possible and the effect of parting line on appearance can be eliminated.
1. After the direction of mold opening is determined, the ribs, snap, protrusions and other structures should be designed to be consistent with the direction of mold opening as far as possible, so as to avoid core-pulling, reduce meld lines and extend the life of the mold.
2. After the mold opening direction is determined, appropriate parting line can be selected to avoid undercut of the mold opening direction, so as to better the products’ appearance and performance.
1.the appropriate draft Angle can avoid the product scratch(galling). The strip draft of smooth surface shall be ≥0.5 degree, the fine texture (sand blast surface) surface shall be greater than 1 degree, and the coarse surface shall be greater than 1.5 degree.
2.the appropriate demoulding strip draft can avoid defects, such as whiting, deformation and crack.
3. When designing products with deep pail, the draft of the outer surface should be greater than that of the inner surface as far as possible, so as to ensure that the mold core is not deviated during injection molding to obtain uniform product wall thickness, and to ensure the strength of the product opening.
1, All kinds of plastics have a certain injection wall thickness range, generally 0.5 ~ 4 mm, when the wall thickness is more than 4mm, it will cause the cooling time too long, resulting in problems such as shrinkage, at this time it should consider to change the product structure.
2. Uneven wall thickness will cause surface shrinkage.
3, Uneven wall thickness will cause voids and welding mark.
1. Reasonable application of ribs can increase product rigidity and reduce deformation.
2. The thickness of the ribs must be ≤ (0.5 ~ 0.7) T product wall thickness, otherwise the surface will shrink.
3. The single-plane draft angel of the rib shall be greater than 1.5° to avoid head injury.
1. Too small radii may cause product stress concentration and lead to product cracking.
2. Too small radii may cause stress concentration in mold cavity, resulting in mold cavity cracking.
3. Reasonable radii can also improve the processing technology of the mold. For example, the mold cavity can be directly milling with R miller, and avoid low-efficiency electrical processing.
4. Different radii may cause the shift of the parting line. Different radii or sharp corners should be selected according to the actual demand.
1.The shape of the hole should be as simple as possible, generally take round.
2. The axial direction of the hole is the same as the direction of the mold opening, which can avoid core pulling.
3. When the length-diameter ratio of the hole is greater than 2, the stripper draft shall be set. At this point, the diameter of the hole should be calculated according to the diameter of the small diameter (the maximum solid size).
4. The length-diameter ratio of blind holes is generally not more than 4 to prevent the insert pin bending.
5. The distance between the hole and the product edge is generally larger than the aperture size.
Core-pulling and slider mechanism
1, When the plastic parts , according to the direction of mold opening , can not be smoothly demoulded, the plastic mold should be designed with core-pulling or slider mechanism. The sliding or core-pulling mechanism can achieve complex product structure, but it is easy to cause defects such as weld line ,shrinkage, increase the cost of mold and shorten the life of mold.
2. When designing injection molding products, try to avoid core-pulling structure if there are no special requirements. For example, the axial direction of the hole and the direction of the rib are changed to the direction of the mold opening, and use the shut-off hole.
1. Insert in injection molding products could increase local strength, toughness, dimensional accuracy and set small threaded hole (shaft), to meet various special requirements. It will also increase the cost of the injection molded product.
2. The insert is usually copper, other metal or plastic.
3. The embedded part of the insert in the plastic part should be designed with stop turn and anti-pull structure. Such as: knurling, hole, bending, flattening, shaft shoulder, etc.
4. The plastics around the embedded parts should be appropriately thickened to prevent stress cracking of the plastic parts.
5. When designing inserts, the positioning way (hole, pin, magnetic) in the mold shall be fully considered.
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