What should be paid attention to in injection molding of thinner products?

The core difficulties in the injection processing of thin-walled injection-molded parts (usually referring to the wall thickness ≤1.5mm, which is common in household appliances/3C category) are the rapid filling, uniform pressure keeping and rapid cooling of the melt, which are prone to problems such as lack of materials, air lines, warping, low strength of weld marks, dimensional deviation, etc. The processing needs to be controlled around five core dimensions: equipment parameters, process setting, raw material treatment, mold adaptation and on-site control, and all operations are controlled by *.

1. Pretreatment of raw materials: ensure melt fluidity from the source and avoid mold filling obstacles.

Thin-walled parts require extremely high fluidity of raw materials, and impurities and moisture in raw materials will directly lead to lack of materials and bubbles, so the pretreatment needs to be more stringent:

Drying and dehumidification: hygroscopic raw materials (ABS/PC/PA/PMMA) must be fully dried, such as PC drying temperature of 120~130℃/4~6h, PA66 drying temperature of 100~110℃/3~4h, and the water content should be controlled below 0.02%; Non-hygroscopic raw materials (PP/PE) should also be stored in a dry environment to prevent condensation on the surface. After drying, they should be used as needed to avoid secondary moisture absorption.

Selection of raw materials: give priority to raw materials with high fluidity (such as MFR≥30g/10min for PP and melting finger ≥20g/10min for ABS), and avoid using recycled materials (recycled materials have poor fluidity, contain impurities, and are easy to block the gate). If masterbatch needs to be added, choose high dispersion masterbatch with the addition ratio ≤3%, which has good compatibility with raw materials and does not reduce melt fluidity.

Preheating of raw materials: during production in low temperature environment, raw materials can be preheated at low temperature (40~60℃) to avoid the sudden drop of temperature after the raw materials are fed into the barrel, which will affect the plasticization uniformity of the melt.

2. Selection and debugging of injection molding equipment: to meet the requirements of "high speed, high pressure and high precision" for thin-walled parts.

The filling time of thin-walled parts is short (usually ≤0.5s), and ordinary injection molding machines are prone to lack of pressure and speed. The equipment needs to meet the hard index, and the core components need to be debugged in place:

Requirements of equipment hard index: high-speed injection molding machine (injection speed ≥200mm/s, some precision parts ≥300mm/s), injection pressure ≥180MPa, and clamping force ≥ 350 ~ 450kg/cm (20%~30% higher than that of conventional parts) shall be selected to prevent flash caused by mold expansion during high-pressure filling; The barrel and the screw should be made of wear-resistant materials (such as nitriding treatment), and the length-diameter ratio of the screw should be ≥22:1, with high plasticizing efficiency and uniform melt mixing.

Debugging of core components: check that the nozzle and the mold gate sleeve have accurate coaxiality, no gap and no deviation, so as to prevent pressure loss during melt filling; Clean up the accumulated material and coke in the charging barrel and nozzle to avoid blocking the gate; Ensure that the clamping mechanism is tightly locked, and the template parallelism deviation is ≤0.02mm, so as to prevent the mold from slightly opening under high pressure.

Auxiliary equipment: equipped with a high-precision mold temperature machine (temperature control accuracy 1℃) and a high-speed manipulator (the picking speed matches the injection rhythm to avoid product sticking). The mold temperature machine needs to control the temperature of the cavity and the core at the same time to ensure the temperature difference ≤ 2℃.

Third, process parameter setting: the core control is "high-speed and high-pressure mold filling, low-temperature and rapid cooling, and accurate pressure keeping"

Process parameters are the core of thin-walled parts processing, and the idea of "low speed and low pressure" of conventional parts needs to be abandoned. All parameters are set around * * "rapid filling, reducing internal stress and rapid setting", and need to be refined and fine-tuned:

1. Plasticizing process: ensure that the melt is uniform and does not decompose at moderate temperature.

Barrel temperature: 10~20℃ higher than that of conventional parts (for example, PP barrel temperature 200~220℃, ABS230~250℃) to improve melt fluidity, but not exceeding the thermal decomposition temperature of raw materials; The temperature of the nozzle is 5~10℃ higher than that of the front section of the barrel to prevent the melt from condensing at the nozzle;

Screw speed: medium and high speed (300~500r/min), short plasticizing time (1~3s) and moderate back pressure (0.5~1.5MPa), which ensures uniform plasticization of melt and reduces shear overheating and degradation of raw materials.

2. Injection technology: high speed and high pressure, shortening the filling time (core! )

Injection speed: the whole process is high speed (adjusted according to the fluidity of raw materials, usually 150~300mm/s), divided into 2~3 stages (for example, the speed at the gate is slightly reduced to prevent the gate from flushing, and the inside of the cavity is high speed), and the filling time is controlled at 0.1~0.5s to prevent the melt from cooling in advance in the cavity;

Injection pressure: high pressure (120~180MPa) to ensure that the melt can overcome the resistance of the cavity and fill the thin-walled cavity quickly, and the pressure should be gradually reduced with the filling progress to prevent the product from flashing;

Injection quantity: accurately control (measurement error ≤0.5%), ensure that the cavity is full without too much molten material, and avoid excessive internal stress during pressure keeping.

3. Pressure maintaining process: low pressure for a short time to reduce internal stress and warpage.

The cavity of thin-walled parts cools rapidly after filling. If the pressure is too high, the internal stress and warpage of the product will be large, and if the pressure is insufficient, shrinkage marks and dimensional deviation will occur.

Holding pressure: 30%~50% of the injection pressure (50%~70% for conventional parts), accurately matching the cavity holding pressure requirements;

Holding time: extremely short (0.5~2s), subject to the fact that the surface of the product has no shrinkage marks and the size reaches the standard, and there is no need for continuous holding during the cooling stage;

Switching point: accurately set the switching position of holding pressure (usually switching when the cavity is 95%~98% filled), and the material shortage will be switched too early, and the internal stress will be great when switching too late.

4. Cooling process: fast and uniform cooling, accelerating setting (key! )

The melt cooling speed of thin-walled parts is fast, and uneven cooling will directly lead to warping and deformation. The cooling process should ensure uniform cavity/core temperature and short cooling time:

Mold temperature: according to the raw material setting (such as PP mold temperature 30~40℃, ABS50~60℃, PC80~90℃), the mold temperature is accurately controlled by a mold temperature machine, and the temperature difference between the mold cavity and the core is ≤ 2℃, so as to avoid warping of the product due to uneven contraction on both sides;

Cooling time: short (1~3s), based on the fact that the product does not deform or stick to the mold after demoulding, usually accounting for 30%~40% of the injection cycle (conventional parts account for more than 60%);

Cooling water: low-temperature cooling water (15~25℃) is used, and the water flow speed is ≥5m/s, so as to ensure the rapid heat exchange of the cooling waterway and no cooling dead angle.

5. Ejection process: low speed and stability to prevent product damage.

Thin-walled parts have low strength and are easy to crack and deform during ejection, and the ejection process needs to be low-speed, stable and uniform;

Ejection speed: low speed (≤50mm/s), ejecting in two stages (the first stage ejects the cavity slowly, and the second stage accelerates slightly), so as to avoid quick ejection of scratched and cracked products;

Ejection position: the ejector pins are evenly arranged, avoiding the weak parts of the product (such as weld marks and buckles), and giving priority to the overall ejection of the pallet to ensure the uniform stress of the product.

4. Mold field adaptation: Mold is the foundation, and the details need to be debugged on site.

Injection molding of thin-walled parts has high requirements for molds. In addition to the optimization of runner, cooling and exhaust in the previous design, the following adaptation and debugging should be done in field production to avoid machining defects caused by mold details:

Commissioning of runner and gate: ensure that runner is short, thick and smooth, gate is large and reasonable in position (priority is given to point gate and submerged gate, and the diameter of gate is 20%~30% larger than that of conventional parts), and the accumulated materials in runner and gate are cleaned on site to prevent the flow of melt from being blocked;

Inspection of exhaust system: the thin-walled parts are filled quickly, and air bubbles, lack of materials and weld marks are easy to appear when the exhaust is not smooth. Check whether the exhaust groove is smooth (no material accumulation or blockage) on site, adjust the depth of the exhaust groove according to the raw materials (PP/PE≤0.04mm, PC/ABS≤0.03mm), and temporarily deepen the exhaust groove (but not more than 0.05mm to prevent flash) if necessary;

Mold cleaning and protection: clean up the scraps and coke on the surface of cavity and core in time to avoid scratching the product or affecting mold filling; The surface of the cavity is kept smooth (Ra≤0.4μm) to reduce the melt flow resistance;

Mold locking inspection: after mold closing, check whether the parting surface and the insert of the mold are closely attached without gaps, so as to prevent flash of the expanding mold during high-pressure mold filling, and if necessary, appropriately increase the mold closing force (but not more than 90% of the rated mold closing force of the equipment).

5. On-site production control: the details determine the finished product rate, so as to avoid defects caused by human error.

The injection process window of thin-walled parts is narrow, and there will be defects if the parameters are slightly deviated. Fine control should be done in field production to reduce the influence of human factors:

Parameter Locking: After the process parameters are debugged up to standard, lock the operation panel. Non-professionals are forbidden to modify the core parameters such as speed, pressure and temperature at will. If adjustment is needed, it needs to be recorded and verified.

Real-time monitoring: real-time monitoring of melt temperature, injection pressure, holding pressure, mold temperature and other parameters in the production process to ensure that the parameters fluctuate within 5%; Observe the appearance of the product, find out the problems such as lack of material, air grain and warping in time, and fine-tune the parameters quickly;

Raw material control: the dried raw materials are sealed and stored to avoid secondary moisture absorption; It is forbidden to mix raw materials with different brands and fluidity to prevent uneven melt fluidity;

Equipment inspection: inspect the equipment once every 1-2 hours, and check the temperature of the barrel, the nozzle for no material leakage, the clamping mechanism for no looseness, the pressure of the hydraulic system for stability, and the cooling water channel for no water leakage or blockage. Stop the machine immediately if any problems are found.

Product storage: the demoulded products should be stored flat and use special material racks to avoid deformation caused by stacking and extrusion; Before cooling and setting of thin-walled parts, avoid contact with high-temperature and low-temperature environment to prevent secondary internal stress caused by sudden temperature change.

6. Quick solution to common defects: targeted fine-tuning to improve the yield.

The common defects in injection molding of thin-walled parts are mostly related to fluidity, mold filling, cooling and exhaust, which can be quickly and pertinently solved by the following methods without greatly adjusting the process:

Lack of material/short shot: increase the temperature of the barrel/nozzle by 10~15℃, increase the injection speed by 20~50mm/s, appropriately increase the injection pressure, clean up the blockage of the gate/runner, and check whether the exhaust slot is unobstructed;

Bubble/air mark: strengthen the drying of raw materials, increase the temperature of the barrel, accelerate the injection speed, clean the exhaust slot, and appropriately reduce the back pressure (reduce the gas entrapment in the melt);

Warp/deformation: calibrate the mold temperature (ensure the temperature difference between the cavity and the core is ≤ 2℃), reduce the holding pressure/shorten the holding time, optimize the ejection mode (ejecting the whole pallet), and store the product smoothly;

Obvious weld mark/low strength: increase the temperature of the barrel/mold, speed up the injection, design the weld mark in the non-stressed area, deepen the exhaust groove at the weld mark, and appropriately increase the injection pressure;

Flash: reduce the injection pressure/speed, reduce the holding pressure, check whether the parting surface of the mold fits, clean up the scraps of parting fabric, and appropriately improve the clamping force;

Sticking mold: raise the mold temperature by 5~10℃, speed up the cooling rate, clean up the accumulated material in the cavity, optimize the demoulding slope, and spray a small amount of demoulding agent on the surface of the cavity (a small amount for many times to avoid affecting the appearance of the product).

7. Core Taboo: These operations are absolutely forbidden to avoid batch scrapping.

It is forbidden to use low-flow grade raw materials or add a large amount of recycled materials. Insufficient melt fluidity will directly lead to material shortage and short shot;

It is forbidden to inject at low speed and low pressure. If the filling time is too long, the melt will be cooled in advance and the cavity cannot be filled.

It is forbidden to keep the pressure too high and for too long, which will lead to large internal stress, warping, cracking and even die expansion;

It is forbidden that the cavity/core temperature difference is too large, and uneven cooling will lead to unilateral shrinkage and batch warping of the product;

It is forbidden to eject quickly, the thimble is unevenly stressed, the strength of thin-walled parts is low, and it is easy to crack and deform;

Direct injection molding of raw materials that are not dried is prohibited. Wet raw materials will produce bubbles, air lines and even melt degradation.

summary

The core operation logic of thin-walled parts injection molding is * * "Fast" (high-speed filling and rapid cooling), "Accurate" (accurate temperature control, accurate pressure keeping and accurate switching), "Uniform" (uniform melt plasticization, uniform mold temperature and uniform ejection force) and "Stable" (stable parameters, stable equipment and stable field control) * *. All matters needing attention are focused on improving melt fluidity, shortening filling time, reducing internal stress and ensuring uniform cooling. As long as the four core links of raw materials, equipment, technology and mold are well controlled, the injection yield of thin-walled parts can be greatly improved and the product quality is stable.