What are the core processes of injection molding
During the Injection Molding production and processing process, plastic material characteristics, mold structure and process parameters will directly affect the molding quality, dimensional accuracy and appearance yield of plastic parts. If you want to accurately control injection molding production and reduce defects and defective products, you must have a thorough understanding of the core physical characteristics and process rules ofplastic molding. The following is a systematic breakdown of the seven core molding skills necessary for the injection Molding Industry, covering key knowledge points such as shrinkage, fluidity, and crystallinity, and adapting to all scenarios of daily machine adjustment, mold design, and process optimization.
Molding Shrinkage Control Techniques
Molding shrinkage of plastic parts is a common characteristic of thermoplastic plastic processing. Compared with thermoset plastics, thermoplastic plastics shrink to a larger extent, in a wider range, and with stronger directional differences. It is also the core reason for dimensional deviation and warpage deformation of plastic parts. The influencing factors are mainly divided into four categories:
High-precision mold shrinkage design guidelines
Plastic fluidity control skills
The fluidity of plastic determines how smoothly the melt fills the mold cavity, which directly affects the occurrence of molding defects such as lack of material, shrinkage and flash. It is the core reference for machine adjustment and mold design.
Plastic flow grade classification
Commonly used plastics can be divided into three major categories based on molding suitability:
Three major influencing factors on liquidity
Key points of plastic crystalline molding
Thermoplastic plastics can be divided into two categories: crystalline plastics and amorphous (amorphous) plastics based on their condensation forming characteristics. Crystallinity is the process in which randomly arranged molecules gradually arrange themselves into regular arrangements and solidify when the plastic melt is cooled. It is also a key indicator to distinguish the plastic molding characteristics. Simple industry differentiation standards: Under normal conditions, crystalline plastics are mostly translucent or opaque, while amorphous plastics are mostly transparent (special cases: polymethylpentene is a crystalline plastic but highly transparent and some amorphous plastics are opaque).
Core precautions for crystalline plastic molding
Key points in processing heat-sensitive and hydrolyzable plastics
Some plastics have poor high temperature resistance and are heat-sensitive materials. When it stays in a high temperature environment for too long, the gate cross-section is too small, and the melt shear is too large, it is easy to cause yellowing, degradation, decomposition and deterioration. Common materials: rigid polyvinyl chloride, polyvinylidene chloride, vinyl acetate copolymer, polychlorotrifluoroethylene, etc. After this kind of plastic decomposes, it will produce toxic and corrosive gases and impurities, which can damage the human body, equipment and molds. Processing requirements: Prioritize the use of screw injection molding machines, enlarge the cross-section of the pouring system, chrome-plating the mold and barrel to prevent accumulation of material in dead corners; strictly control the molding temperature, and add special stabilizers to weaken the heat sensitivity of the material.
Some plastics are prone to absorbing water. Even if a trace amount of moisture remains, hydrolysis reactions will occur under high-temperature and high-pressure injection molding environments, resulting in resin decomposition, melt bubbling, and reduced fluidity, ultimately causing surface defects and damaged mechanical properties of plastic parts. Therefore, before this type of plastic is put into production, it must be fully heated and dried according to standard parameters to prevent the raw materials from absorbing moisture again.
Solving techniques for stress cracking and melt fracture
Some plastics are highly sensitive to stress, and internal stress is easily generated during injection molding, causing the plastic parts to become brittle. After subsequent extrusion by external forces and contact with solvents, cracking problems will easily occur. Optimization plan: The raw materials are fully dried in advance, and anti-cracking additives are added; the structure of the plastic part is optimized to reduce sharp corners, uneven wall thickness, and redundant inserts to avoid stress concentration; the mold is increased in the demoulding slope, and the gate and ejection structure are optimized; the material temperature, mold temperature, injection pressure and cooling time are reasonably adjusted during production to avoid low-temperature brittle demolding; the plastic part is annealed after molding to eliminate internal residual stress and avoid the plastic part from being exposed to various solvents.
When the flow rate of the polymer melt is too fast and exceeds the critical value, transverse cracks will appear on the surface when the melt passes through the nozzle and gate. This phenomenon is called melt rupture, which will seriously damage the appearance and structural performance of the plastic part. Improvement method: Select raw materials suitable for the melt flow rate, enlarge the nozzle, runner, and gate cross-sections, appropriately reduce the injection speed, and increase the material temperature to ensure smooth filling of the melt.
Plastic thermal properties and mold cooling process
The specific heat capacity, thermal conductivity and thermal deformation temperature of different plastics vary greatly, which directly determines the plasticizing requirements, molding cycle and cooling plan:
The mold needs to be equipped with an independent heating and cooling system based on the characteristics of the plastic and the structure of the plastic part: timely cooling after high-temperature molding will shorten the molding cycle and reduce the crystallinity; when the residual heat of the raw material is insufficient, the mold heating system will be activated to stabilize the mold temperature, improve melt fluidity, and avoid uneven cooling of the inside and outside of thick-walled plastic parts. For large-area plastic parts with unstable fluidity, local alternating hot and cold temperature control can be used to ensure molding quality.
Techniques for controlling hygroscopicity of plastics
Affected by additives and molecular structure, plastics are divided into two categories: hygroscopic and adhesive types and non-hygroscopic types. The moisture content of raw materials must be strictly controlled during production. If the moisture content exceeds the standard, the moisture will vaporize under high temperature and pressure, causing plastic hydrolysis, resulting in blistering, flow marks, and poor gloss of the plastic parts. At the same time, the mechanical properties such as tensile and impact of the plastic parts will be greatly reduced. Therefore, all hygroscopic plastics must be preheated and dried in strict accordance with industry standards, and sealed and protected during the production process to avoid secondary adsorption of moisture in the air by raw materials, and to avoid molding defects from the source.
- The material temperature and mold temperature are low, and the melt fluidity is poor. Increase the barrel temperature and mold temperature appropriately.
- Insufficient injection pressure/speed, increase injection pressure and injection speed; increase injection section stroke
- The gate/runner is too small, the gate position is unreasonable, and the melt reaches the end and the pressure loss is large.
- Poor exhaust, trapped air blocks the material, thicken the exhaust groove 0.015-0.03mm
- If the raw material has poor fluidity, it can be replaced with a high-flow grade; check whether the material is dry enough.
- The material temperature and mold temperature are low, and the melt fluidity is poor. Increase the barrel temperature and mold temperature appropriately.
- Insufficient injection pressure/speed, increase injection pressure and injection speed; increase injection section stroke
- The gate/runner is too small, the gate position is unreasonable, and the melt reaches the end and the pressure loss is large.
- Poor exhaust, trapped air blocks the material, thicken the exhaust groove 0.015-0.03mm
- If the raw material has poor fluidity, it can be replaced with a high-flow grade; check whether the material is dry enough.