What are the post-processing technologies of injection molded parts after processing?

After the processing of injection molded parts is completed, it is necessary to optimize the appearance, performance and assembly adaptability through a series of post-processing technologies. The core objectives of different technologies are quite different, which can be divided into four major directions: appearance optimization, performance improvement, structure modification and surface function enhancement. The following are detailed classifications:

First, the process of appearance optimization: solving surface defects and improving visual texture.

The core of this kind of process is to eliminate the appearance problems (such as flash, bubbles and shrinkage marks) caused by injection molding, or to make the surface more in line with the product design requirements through modification. Common processes include:

1. Deburring/Deflashing

Function: Remove excess plastic (flash, burr) caused by mold clamping gap or injection pressure at the edge, parting surface and gate of injection molded parts to avoid affecting assembly accuracy and feel.

Common ways:

Manual deburring: It is suitable for small batch and complex structural parts (such as corner buckles of intelligent sweeper), and it is finely trimmed with art knife, sandpaper and file, with low cost, low efficiency and poor consistency.

Mechanical deburring: it is commonly used in batch production, such as vibration grinding (putting the workpiece, grinding stone and abrasive into a vibrating bucket to deburr through friction) and sandblasting deburring (striking the surface with high-pressure sand particles and achieving deburring+matte effect at the same time).

Freezing and deburring: For elastic and soft rubber parts (such as silicone sealing ring of sweeper), the workpiece is frozen at low temperature to embrittlement, and then the high-speed shot is used to impact the burr to avoid soft rubber deformation.

2. Polishing

Function: Reduce the surface roughness (Ra value) of injection molded parts, and achieve high-gloss, sub-gloss or mirror effect, which is commonly used in exposed parts such as the upper cover and control panel of sweeper.

Classification and application:

Rough polishing: Use grinding wheel and sandpaper (80#-400#) to polish the obvious shrinkage marks and weld marks on the surface, which lays the foundation for subsequent fine polishing.

Fine polishing: Manual or mechanical polishing with fine sandpaper (600#-2000#) and polishing paste (such as chrome oxide and alumina paste) can achieve sub-gloss effect with Ra≤0.2μm m..

Mirror polishing: For high-gloss parts such as ABS, PC, etc. (such as the base of the display panel of the sweeper), the wool wheel and diamond polishing paste are used for high-speed polishing, and finally a mirror-like reflective effect (Ra≤0.02μm) is achieved.

3. Surface Cleaning

Function: Remove the residual release agent, oil stain, dust or abrasive on the surface of injection molded parts to avoid affecting the adhesion of subsequent painting and printing processes.

Common ways:

Water washing: batch parts are washed with high-pressure spray and neutral cleaning agent, and then dried by hot air (suitable for plastic parts, such as dust box shell of sweeper).

Ultrasonic cleaning: for complex structural parts (such as the gearbox shell of sweeper, with deep holes and grooves), the tiny bubbles generated by ultrasonic vibration are used to peel off dirt, which makes the cleaning more thorough.

Plasma cleaning: for plastics with low surface tension (such as PP, PE), the oil stain and weak boundary layer are removed by plasma bombardment, and the surface adhesion is improved at the same time (often used for pretreatment before subsequent screen printing and bonding).

Second, the performance improvement process: enhance the structural strength or environmental adaptability.

This kind of process focuses on improving the mechanical properties (such as impact resistance and aging resistance) or environmental resistance (such as temperature resistance and chemical corrosion resistance) of injection molded parts, which are commonly used in the stress parts of sweepers (such as roller brush brackets and driving wheel housings).

1. Annealing

Principle: eliminate the internal stress caused by uneven cooling during injection molding (internal stress may cause cracking and deformation of parts during storage or use, especially PC, PA and other materials).

Process: Put the injection-molded parts into an oven, keep the temperature for 1-4 hours at a temperature 50-100℃ lower than the melting point of the materials (such as about 120-130℃ for PC), and then slowly cool to room temperature (cooling rate ≤5℃/h) to avoid generating new internal stress.

Application scenario: the transparent PC window of the sweeper (it is necessary to avoid cracking due to internal stress after long-term use), and the driving wheel sleeve made of PA66 (it needs to bear a certain torque, and the internal stress is too large to cause fracture).

2. Aging treatment (aging)

Principle: accelerate the stability of materials (especially thermoplastic elastomers and reinforced plastics) through natural or artificial environment to avoid dimensional change or performance attenuation in subsequent use.

Classification:

Natural aging: place the parts in a normal temperature and humidity environment for 1-2 weeks, so that the molecules of the materials are slowly rearranged and the size is gradually stabilized (suitable for small batches or parts with low aging requirements).

Artificial aging: keep the temperature in the oven at 60-80℃ for 24-72 hours to speed up the aging process (suitable for mass production, such as TPU soft rubber roller brush sheath of sweeper, which needs to avoid untidy fitting due to aging shrinkage).

3. Coating

Function: form a protective film on the surface to enhance wear resistance, chemical corrosion resistance or ultraviolet aging resistance.

Common types:

Wear-resistant coating: such as polytetrafluoroethylene (PTFE) coating, sprayed on the surface of the roller brush shell of the sweeper to reduce the wear caused by friction with the ground.

Anti-ultraviolet coating: For outdoor sweeper parts (such as the fuselage shell of commercial sweeper), spray coating containing ultraviolet absorber to avoid fading and embrittlement after long-term exposure.

Chemical-resistant coating: such as epoxy resin coating, used for the inner wall of dust box of sweeper to prevent acidic/alkaline substances in dust from corroding plastics.

Third, the process of structural modification: adapting to assembly requirements

This kind of technology mainly aims at the dimensional deviation or assembly interface problem of injection molded parts, and realizes accurate adaptation through mechanical processing, which is common in the precision assembly parts of sweepers (such as motor mounting seats and sensor brackets).

1. Machining

The core purpose: to correct the key dimensions of injection molded parts (such as aperture, shaft diameter and flatness), or to process structures that cannot be molded at one time by injection molding (such as deep holes and threaded holes).

Common processing methods:

Drilling/tapping: when the size of the hole reserved for injection molding parts deviates, use a drilling machine to enlarge the hole; Or direct tapping (such as M3 threaded hole of motor bracket of sweeper, which cannot be molded once by injection, and needs to be tapped later).

Milling: correct flatness or process special-shaped grooves (for example, the clearance between the sensor and the installation groove of the sweeper should be ≤0.1mm, and finish milling with a milling machine when the injection accuracy is insufficient).

Cutting: trimming the edge size of large pieces by laser cutting or wire cutting (for example, the fuselage side plate of the sweeper needs to be cut and corrected if the overall size is too large after injection molding).

2. Bonding/Welding

Function: Splicing multiple injection-molded parts into complex components (used when a single injection cannot form a large size or complex structure).

Common ways:

Glue bonding: it is bonded with special plastic glue (such as chloroform glue for ABS and epoxy glue for PC), which is suitable for non-stressed parts (such as decorative strip of sweeper and fuselage shell). Its advantages are no obvious joints and good appearance.

Ultrasonic welding: using high-frequency ultrasonic (15-40kHz) to make the contact surface of two plastic parts generate heat by friction, and then cooling and bonding, which is suitable for stressed parts (such as the upper cover and lower cover of dust box of sweeper, which need to be sealed and bear certain pressure), and has the advantages of high welding strength and high efficiency (only 0.5-2 seconds for a single solder joint).

Hot plate welding: For large-area or special-shaped parts (such as the water tank shell of the sweeper, which needs to be sealed all around), the contact surface of the two parts is melted with a heating plate, and then pressed and cooled, which has good sealing performance (water leakage prevention).

Fourth, the surface function strengthening process: giving special functions or logos.

This kind of process mainly adds functionality (such as conductivity and anti-skid) or LOGO (such as logo and parameters) to injection-molded parts, which are common in the operation panel and functional parts of sweepers.

1. Screen Printing/Pad Printing.

Function: Print LOGO, operation icon or parameters on the surface (such as "switch" and "mode" icon on the control panel of the sweeper).

Difference:

Screen printing: Suitable for flat or simple curved surface (such as the flat cover of sweeper). Ink is transferred to the surface through screen printing, with high pattern accuracy (0.1mm line width) and strong adhesion.

Pad printing: Suitable for complex curved surface or concave-convex surface (such as the circular button of sweeper). The ink is transferred from the steel plate to the surface of the workpiece through the silicone pad printing head to adapt to the irregular shape.

2. Laser Marking

Principle: High-energy laser (such as fiber laser and CO laser) is used to etch the plastic surface to form a permanent mark (no ink, no falling off).

Advantages: high accuracy (two-dimensional code and tiny characters can be etched), high efficiency (only 0.1-0.5 seconds for a single logo) and environmental protection (no ink pollution).

Application scenario: The product serial number and safety certification mark (such as "CE" and "RoHS") of the sweeper need to be kept for a long time and are not easy to wear.

3. Conductive Treatment

Function: endow the plastic surface with electrical conductivity and avoid static electricity accumulation (static electricity may absorb dust or interfere with electronic components, especially the internal circuit board bracket of the sweeper).

Common ways:

Spraying conductive paint: paint containing metal powder (such as copper powder and silver powder) is sprayed on the surface to form a conductive layer (surface resistance ≤ 10Ω).

Vacuum coating: In vacuum environment, metals (such as aluminum and nickel) are evaporated and deposited on the plastic surface to form a uniform conductive film (with more stable conductivity, suitable for high-precision electronic components).

Summary: Selection principle of post-treatment process

On-demand matching: Select the process according to the function (stressed/exposed/assembled), material (PC/ABS/PA) and product requirements (appearance/performance/cost) of the parts, such as polishing+screen printing for exposed high-gloss parts and annealing+ultrasonic welding for stressed parts.

Cost balance: manual process (such as manual deburring) is low in cost but low in efficiency, suitable for small batch; Automation technology (such as vibration grinding and laser marking) has high initial investment, but it is suitable for large quantities and needs to be combined with capacity selection.

Compatibility: it is necessary to consider the connection between processes, such as "plasma cleaning → screen printing" (improving ink adhesion after cleaning) and "annealing → mechanical processing" (eliminating internal stress first, then processing to avoid cracking).

Take the "roller brush bracket" of intelligent sweeper (PA66+30% glass fiber material) as an example, its post-processing flow is usually: deburring (vibration grinding) → annealing (eliminating internal stress) → drilling and tapping (machining) → spraying wear-resistant coating (enhancing wear resistance), and finally meeting the use requirements of "bearing roller brush torque and resisting long-term friction".