1、 Formation mechanism of burrs and burrs
Flying edges and burrs are common defects in the injection molding process in Dalian, mainly manifested as excess thin or filamentous plastic at the edges or parting surfaces of plastic products. The formation mechanism mainly involves the following aspects:
Poor mold closure: When the fitting accuracy of the mold parting surface is insufficient or the locking force is not enough, the molten plastic will squeeze into the mold gap under high pressure, forming burrs.
Excessive injection pressure: Excessive injection pressure can exceed the capacity of the mold, forcing plastic into the tiny gaps of the mold.
Improper melt temperature: If the temperature is too high, the flowability of the plastic will be too strong, which can easily penetrate into the gaps of the mold; If the temperature is too low, higher injection pressure is required, which indirectly leads to flash.
Influence of material properties: Some materials with good fluidity, such as PP, PA, etc., are more prone to produce burrs.
Mold wear: After long-term use, the mold parting surface wears and creates gaps, creating conditions for the formation of burrs.
2、 Optimization strategy for process parameters
1. Reasonably set the locking force
Locking force is the first line of defense against flying edges. The required locking force should be calculated based on the projected area of the product and material properties. The general empirical formula is: locking force (ton)=projected area of the product (cm ²) x material coefficient (ton/cm ²). For high liquidity materials, the coefficient is taken as 0.6-0.8; Ordinary materials are taken as 0.4-0.6. In actual production, a safety margin 10-15% higher than the calculated value should be set.
2. Accurately control injection pressure
Adopting multi-stage injection pressure control, using higher pressure during the filling stage to ensure complete filling, and gradually reducing pressure during the holding stage. Recommended settings:
The first injection pressure is 60-80% of the machine's maximum pressure
Reduce the second paragraph to 50-60%
Set the holding pressure to 30-40%
At the same time, in conjunction with V-P (speed pressure) conversion point control, switch to holding pressure when the mold cavity is 95-98% full.
3. Optimize temperature parameters
The melt temperature should be controlled around the middle value of the recommended material range. For materials that are prone to flash, try lowering the temperature by 5-10 ℃. The temperature of the mold affects the cooling rate and material flowability, and is generally controlled at 20-30 ℃ below the glass transition temperature of the material. For products with high smoothness requirements, higher mold temperatures (such as 80-100 ℃) can be used in combination with longer cooling times.
4. Scientifically set injection speed
Adopting a "slow fast slow" speed curve:
In the initial stage, slow injection (20-30% to high speed) is used to stabilize the front end of the melt
Rapid filling in the middle stage (70-90% speed) to avoid cold materials
Reduce the speed at the end to 30-40% to minimize impact
For thin-walled parts, the speed can be appropriately increased, while for thick walled parts, the speed should be reduced.
3、 Key points of mold design and maintenance
1. Optimization of parting surface design
The parting surface should meet the following requirements:
Control the clearance within 0.02mm
Adopting a stepped or inclined structure to increase sealing performance
Set overflow chutes (depth 0.05-0.1mm) at key locations to guide flash generation in controllable positions
Add auxiliary locking mechanism to large molds
2. Top out system design
Improper ejection system can cause mold deformation and flash:
The layout of the top pin should be balanced to avoid local stress concentration
The setting of the ejection stroke is reasonable, generally ranging from 2-3mm to the maximum thickness of the product
Adopting delayed ejection function to ensure sufficient cooling of the product
3. Selection of mold materials
The cavity and core materials should have:
High hardness (HRC52 or above) to resist plastic erosion
Good polishing performance (up to mirror level)
Appropriate matching of thermal expansion coefficient
Recommend using pre hardened steel (P20), quenched steel (718H), or hard alloy
4. Regular mold maintenance
Establish a mold maintenance system:
Check the wear of the parting surface every 5000-10000 mold cycles
Regularly clean the exhaust duct and cooling water channel
Timely replace worn guide posts and guide sleeves
Surface rust prevention treatment
4、 Material selection and processing
1. Material flowability control
Select materials with appropriate fluidity based on product structure:
Complex thin-walled components can be made of high flow materials (such as MFR>30)
Simple thick walled components are made of ordinary flowable materials (MFR10-20)
Caution should be exercised when adding flow modifiers as it may increase the risk of flash edges
2. Material drying treatment
Adequate drying can prevent gas decomposition from causing burrs:
Moisture absorbing materials (such as PC, PA) need to be dried at 120 ℃ for more than 4 hours
Non hygroscopic materials (such as PS, PP) should also be dried at 80 ℃ for 2 hours
Use a dehumidification dryer to maintain a dew point below -40 ℃
3. Proportion of recycled materials used
Strictly control the proportion of returned materials:
Products with high appearance requirements shall not exceed 10%
Ordinary products can be relaxed to 20-30%
Each batch of recycled materials needs to be filtered and re pelletized
5、 Monitoring and adjustment during the production process
1. First piece inspection process
Establish a strict first item inspection system:
Check if there are any flying edges at the parting line
Measure whether the key dimensions are within the tolerance range
Conduct destructive testing to check internal quality
Record all process parameters as a benchmark
2. Process monitoring methods
Adopting modern monitoring methods:
Install mold pressure sensor to monitor the force on the parting surface
Use an infrared thermometer to monitor the temperature in various areas of the mold
Using a visual inspection system to automatically identify flash defects
Establish SPC control system to analyze process fluctuations
3. Exception handling program
The processing steps for discovering flying edges:
Immediately stop the machine and check the closed status of the mold
Check if the locking force has decreased
Verify the recent fluctuations in process parameters of 20 molds
Clean the parting surface of the mold
Gradually adjust the process parameters (first lower the temperature, then adjust the pressure)
6、 Handling skills for special situations
1. Multi cavity mold flash control
Special attention should be paid to multi cavity molds:
Ensure balanced filling of each cavity (difference<5%)
Check the temperature consistency of each nozzle when using a hot runner system
For cavities that are prone to burrs, the gate size can be set separately
2. Flyedge treatment for transparent products
Transparent parts are more sensitive to burrs:
Using high-precision molds (with a clearance of less than 0.01mm)
Using low-temperature and low-pressure processes
Choose specialized transparent materials
The surface of the mold is chrome plated or polished to Ra<0.05 μ m
3. Micro component flash control
Special requirements for micro injection molding:
Adopting a high rigidity small tonnage dedicated injection molding machine
Mold temperature control accuracy ± 1 ℃
Injection speed response time<10ms
Use vacuum assisted exhaust
By systematically optimizing process parameters, improving mold design, strictly controlling materials, and establishing a scientific production management system, the problems of burrs and burrs in the injection molding process can be reduced, and product quality and production efficiency can be improved. In practical applications, targeted adjustments should be made based on specific product and material characteristics, and a continuous improvement attitude should be maintained in order to achieve stable high-quality production.
