What should be paid attention to when processing injection molded products?

Injection molding product processing is a systematic project involving material characteristics, equipment parameters, mold design, process control and other links, and the deviation of any link may lead to product defects (such as shrinkage, bubbles, flash, deformation, etc.). Combined with the practical experience of industrial production, injection molding should focus on the following six core links and 28 key points for attention, covering the whole process from prenatal preparation to postpartum treatment:

First, prenatal preparation: lay the foundation for processing (avoid batch defects)

1. Preparation and inspection of raw materials

Drying treatment: hygroscopic plastics (such as PA, PC, ABS, PET) need to be dried in advance, otherwise it will produce defects such as bubbles, crazing, brittle cracks, etc.

Drying temperature: PA 80-120℃, PC 100-120℃, ABS 70-90℃, PET 120-140℃;

Drying time: 2-4 hours (adjusted according to humidity), and the water content after drying should be ≤0.05%(PA should be ≤ 0.02%);

Avoid excessive drying: if the drying temperature of PC is too high, it will lead to degradation, yellowing and embrittlement.

Purity of raw materials: check whether plastic particles are mixed with impurities (such as dust, metal chips and other plastics), which will affect the surface quality and mechanical properties of products and need to be filtered through a screen (usually 80-120 mesh screen).

Masterbatch/additive matching: Masterbatch should be compatible with the base material (for example, PP masterbatch can't be used in ABS), and the addition ratio should be even (usually 2%-5%) to avoid color flower and color difference; The dosage of additives (such as antioxidants and flame retardants) should be controlled. Excessive dosage may lead to precipitation or performance degradation.

2. Mold inspection and maintenance

Mold cleaning: the mold cavity, core, gate and exhaust slot should be free of oil stain, glue stain and residual plastic, otherwise, the product surface will be scratched, stuck and short of materials; Avoid using hard tools (such as wire brushes) when cleaning to avoid scratching the mold cavity.

Mold status:

Check the surface smoothness of the cavity (especially the external parts mold) for no rust and wear;

Guide posts, guide sleeves and thimbles shall be adequately lubricated (using high-temperature grease) without sticking;

The cooling water channel is smooth and free from blockage (the water passing test can be passed to ensure that the temperature difference between inlet and outlet water is ≤ 5℃);

The exhaust slot is unobstructed (0.01-0.03mm in depth and 5-10mm in width) to avoid air bubbles and burning caused by trapped air.

Mold installation: mold installation is accurate in positioning, and the pressure plate is fastened evenly to avoid mold deformation caused by eccentric load; The clearance between the thimble and the cavity is moderate (0.01-0.02mm) to prevent flash or top injury to the product.

3. Equipment debugging preparation

Parameter calibration of injection molding machine: confirm the accuracy of injection pressure, holding pressure, injection speed, temperature control and other parameters. Pressure gauges and thermometers need to be calibrated regularly (error ≤ 2%).

Equipment status check:

The barrel and screw are free from abrasion and corrosion, and the screw speed is uniform without jamming;

The pressure of the hydraulic system is stable (fluctuation ≤±5bar) and there is no leakage in the oil circuit;

The heating ring and thermocouple work normally, and the temperature of each section of the barrel is uniform (the temperature difference is ≤ 3℃).

Second, process parameter setting: the core control link (determining product quality)

1. Temperature control (affecting plastic fluidity and molding quality)

Barrel temperature: set according to the melting point and fluidity of plastics, and follow the principle of "low front section and high rear section" (to avoid the retention and degradation caused by premature melting of materials);

Crystalline plastics (such as PP, PE, PA): barrel temperature = melting point+20-50℃(PP 160-220℃, PA6 220-260℃);

Amorphous plastics (such as ABS, PC, PMMA): barrel temperature = glass transition temperature+100-150℃(ABS 180-240℃, PC 240-300℃);

Heat-sensitive plastics (such as PVC and POM): The barrel temperature should be strictly controlled to avoid exceeding the degradation temperature (PVC 160-190℃, POM 180-220℃), and heat stabilizers should be added if necessary.

Nozzle temperature: 5-10℃ higher than the highest temperature of the barrel, so as to avoid material cooling and solidification at the nozzle, resulting in blockage or material shortage; But too high temperature may lead to casting.

Mold temperature:

Crystalline plastic: high mold temperature (PP 40-80℃, PA6 60-100℃) can promote crystallization, improve product strength and toughness, and reduce shrinkage deformation;

Amorphous plastic: the mold temperature is low (ABS 40-60℃, PC 60-90℃), which shortens the cooling time, improves the production efficiency and avoids product warping;

The mold temperature of exterior parts should be uniform (temperature difference ≤3℃), otherwise it will lead to product color difference and shrinkage marks.

2. Pressure and speed control (affecting the mold filling and holding effect)

Injection pressure: set according to the product structure, wall thickness and fluidity to ensure that the material fills the mold cavity;

Thin-walled parts and complex parts: high injection pressure (80-120 MPa);

Thick-walled parts and simple parts: low injection pressure (40-80 MPa);

Excessive pressure can easily lead to flash and increase of internal stress in the product; Too little pressure can easily lead to material shortage and shrinkage marks.

Holding pressure and time:

Holding pressure = 50%-80% of injection pressure (to avoid product warping and mold sticking caused by excessive pressure);

Holding time: to ensure the complete feeding of the melt, which is generally 1/3-1/2 of the cooling time (thick-walled parts need to be extended, such as 5-10 seconds; Thin-walled parts 1-3 seconds); Insufficient pressure holding will lead to shrinkage and depression, and too long will increase the cycle cost.

Injection speed: follow the principle of "slow-fast-slow" to avoid impacting the mold cavity or getting trapped;

Initial stage of mold filling: low speed (to prevent melt splashing and flow marks at the gate);

Mid-filling stage: high speed (filling the cavity quickly to reduce cooling shrinkage);

At the end of mold filling: low speed (avoiding melt overflow and reducing flash);

Heat-sensitive plastics (such as PVC) need to be injected at low speed to prevent shear overheating and degradation.

3. Cooling time control (affecting production efficiency and product stability)

Cooling time = product wall thickness ×(5-10) (for example, the cooling time is 10-20 seconds for a 2mm thick ABS product), so as to ensure that the temperature of the product during demoulding is ≤ thermal deformation temperature (to avoid deformation after demoulding);

If the cooling time is too long, the production efficiency will be reduced, and if it is too short, the product will be warped, die-stuck and sticky on the surface.

The cooling effect can be optimized by adjusting the flow rate of cooling water channel and the temperature of mold, and the temperature of thick-walled parts can be controlled by circulating cooling water or oil temperature machine.

4. Back pressure and screw speed (affecting the plasticizing quality of materials)

Back pressure: generally set at 5-15MPa, the function is to compact materials, exhaust gas and improve plasticization uniformity;

Excessive back pressure: the material is seriously heated by shear, easy to degrade and discolor, and the energy consumption increases;

Low back pressure: uneven plasticization of materials, bubbles and delamination inside the product.

Screw speed: set according to plastic characteristics, generally 30-100r/min;

High viscosity plastics (such as PC, PMMA): low rotation speed (30-60r/min) to avoid shearing overheating;

Low viscosity plastics (such as PE and PP): high rotational speed (60-100r/min) to improve plasticizing efficiency;

Too fast rotation speed will easily lead to material degradation, and too slow rotation speed will lead to insufficient plasticization.

Third, molding process control: real-time monitoring and adjustment (to avoid batch defects)

1. Mold filling stage monitoring

Observe the filling state of the melt: whether it is stable and impact-free, and there are no flow marks and scorch marks at the gate;

If there is material shortage, adjust the injection pressure and speed first, and then increase the temperature of the barrel (to avoid the flash caused by blindly increasing the injection volume);

If flash occurs, first check the mold gap, and then reduce the injection pressure and speed, or appropriately reduce the temperature of the barrel.

2. Monitoring during pressure maintaining and cooling stage

Monitor product shrinkage: if shrinkage marks appear, appropriately increase the holding pressure and extend the holding time;

Observe the demoulding state of the product: there is no warping, deformation and top injury during demoulding. If the mold sticks, the mold temperature can be appropriately increased, the demoulding agent can be sprayed (a small amount for many times to avoid affecting the surface quality), or the ejector pin layout can be optimized.

3. Real-time adjustment of common defects

Causes of defect types and adjustment measures

Bubble/craze materials contain water, have poor exhaust, and are degraded due to too high material temperature. Strengthen drying, clean the exhaust tank, and reduce the material temperature/back pressure.

Shrinkage/depression has insufficient pressure keeping, short cooling time and high material temperature, so that the pressure keeping/time is increased, the cooling time is prolonged and the material temperature is lowered.

Excessive flash injection pressure, large mold gap and high material temperature reduce injection pressure/speed, adjust mold gap and reduce material temperature.

The warpage deformation die has uneven temperature, insufficient cooling time, large internal stress, uniform die temperature, prolonged cooling time, optimized gate position and annealing treatment.

Surface scratches/pits have impurities in the mold cavity, the material temperature is too high to degrade, the mold cavity is cleaned by too much release agent, the material temperature is lowered, and the amount of release agent is reduced.

Lack of material/short shot injection pressure/speed is insufficient, material temperature is too low, gate is blocked, injection pressure/speed is increased, material temperature is increased, and gate is cleaned.

Iv. production process operation: standard implementation (ensuring stability and safety)

1. Feeding operation

The feeding should be uniform and continuous to avoid the interruption or accumulation of materials in the barrel;

Avoid mixing foreign materials into the hopper (such as tools and sundries) and prevent damage to the screw and the charging barrel;

Moisture-absorbing plastics should be used as they are added to avoid moisture absorption again after drying.

2. Product picking, placing and inspection

Handle the product gently to avoid collision and scratch (especially appearance parts), and use special fixture or sucker to prevent fingers from touching the product surface (sweat stains will lead to oxidation and discoloration);

Every 10-20 molds are sampled, and the product size (key dimensions are detected by three-coordinate measuring instrument), surface quality and assembly performance are checked. If any abnormality is found, the machine shall be stopped immediately for adjustment.

3. Equipment maintenance and safety

Monitor the running status of the equipment in the production process: whether the temperature, pressure and screw speed of the barrel are stable, whether there is abnormal noise or leakage;

It is forbidden to open the safety door and touch moving parts (such as screws and thimbles) when the equipment is running;

In case of mold sticking, blockage and other faults, it is necessary to stop the machine to relieve pressure before handling, and it is forbidden to start the machine forcibly.

Five, postpartum treatment: improve product performance and appearance (key finishing link)

1. Deburring and trimming

Burrs on product gates, runners and parting surfaces need to be removed. Manual grinding (1000-2000 mesh sandpaper), vibration grinding, laser deburring and other methods can be used to ensure that the burr height is ≤0.03mm (appearance parts are ≤ 0.01 mm);

Avoid damaging the surface of the product, especially the transparent parts and high-gloss parts.

2. Heat treatment (eliminating internal stress)

For products with easy deformation and large internal stress (such as PC, ABS, PA), annealing treatment is required:

Temperature: 10-20℃ lower than the plastic thermal deformation temperature (PC 100-120℃, ABS 60-80℃);

Time: 2-4 hours, slowly cool to room temperature (cooling rate ≤5℃/h) to avoid secondary stress.

3. Surface treatment (according to product requirements)

Exterior parts can be treated by oil spraying, electroplating, screen printing, bronzing, etc. Before treatment, it is necessary to ensure that the surface of the product is free of oil and dust (cleaned by ultrasonic);

For functional products such as flame retardant and antistatic, relevant properties (such as flame retardant grade and surface resistance) should be tested to ensure that they meet the requirements.

4. Quality inspection and packaging

Final inspection: check the dimensional tolerance (in line with the requirements of drawings), surface quality (without defects) and mechanical properties (such as tensile strength, impact strength and sampling inspection);

Packaging: anti-static bags, pearl cotton, blister boxes and other packaging materials are used to avoid scratches and deformation during product transportation; Measures to prevent moisture and dust (especially hygroscopic plastic products) should be taken in batch packaging.

Vi. Special precautions for different plastic materials

Matters needing attention in processing plastic material characteristics

PP (polypropylene) is crystalline, with good fluidity and easy shrinkage. The mold temperature is 40-80℃, and the holding time is sufficient (to avoid shrinkage). Avoid long-term detention at high temperature (easy to degrade)

ABS (Acrylonitrile-Butadiene-Styrene) is amorphous, with good toughness and easy to generate internal stress. The drying temperature is 70-90℃(2-3 hours). The mold temperature is even (40-60℃); Postpartum annealing is required.

PC (polycarbonate) has high transparency, high temperature resistance and poor fluidity. The drying temperature is 100-120℃ (more than 4 hours). The barrel temperature is 240-300℃ (to avoid degradation over 320℃); High injection pressure (80-120 MPa); Mold temperature 60-90℃

PA (nylon) is hygroscopic, crystalline and wear-resistant, and must be fully dried (80-120℃ for 4-6 hours); Mold temperature 60-100℃ (to promote crystallization); Glass fiber reinforcement can be added (pay attention to screw wear)

PVC (polyvinyl chloride) is heat-sensitive, degradable and toxic. The barrel temperature is 160-190℃ (strictly controlled). Adding heat stabilizer and lubricant; Avoid contact with copper (accelerated degradation); draughty

PMMA (plexiglass) has high transparency, poor fluidity and easy to scratch. The barrel temperature is 200-250℃. The mold temperature is 60-80 DEG C; Slow injection speed (to avoid silver streaks); Mold cavity needs high finish.

Summary: the core principle of injection molding

The key of injection molding process lies in "precise control, whole-process monitoring, and adjustment as needed": strict pre-production preparation ensures that basic conditions reach the standard, process parameters are optimized to control molding quality, batch defects are avoided through standardized operation and real-time adjustment, and product performance is improved through post-production treatment. At the same time, it is necessary to adjust the processing scheme according to the plastic material characteristics, product structure requirements (such as wall thickness and complexity) and mold design level, so as to stably produce injection products that meet the quality requirements.

In actual production, it is suggested to establish a "process parameter file" to record the optimal processing parameters of different products and materials, so as to facilitate rapid debugging in subsequent batch production; At the same time, equipment maintenance, mold maintenance and operator training are carried out regularly to improve production stability and product qualification rate.