What should be paid attention to in injection molding of pneumatic and shock-absorbing plastic parts?

Due to the core use requirements of pneumatic and shock-absorbing plastic parts (pneumatic parts require air tightness, pressure resistance and accurate size, and shock-absorbing parts require elastic stability, fatigue resistance and hardness adaptation), injection molding needs to focus on three cores: material property matching, molding accuracy control and mechanical property guarantee, so as to avoid problems such as lack of materials, bubbles, deformation and insufficient elasticity. The following are the core points for attention in different dimensions, which are in line with the actual mass production scene and can be landed simply:

First, determine the core material and match the processing technology (pneumatic/shock absorber materials are quite different, so avoid common technology).

The mainstream materials of the two types of plastic parts are different, so it is necessary to define the material characteristics first, and then determine the injection parameters, core materials and processing basic requirements:

Pneumatic plastic parts: priority is given to PA6/PA66 (nylon), POM (polyoxymethylene) and PC+ABS, which require high rigidity, good air tightness and no air holes, and are suitable for pneumatic joints/valve bodies/sealing rings, etc.

Key points: PA6/PA66 should be fully dried in advance (moisture content ≤0.2%) to avoid air bubbles and cracking after molding, which will affect air tightness; POM should avoid high temperature degradation and strictly control the temperature of the barrel.

Shock-absorbing plastic parts: TPR/TPE/TPU (elastomer), EPDM (ethylene propylene diene monomer) and silica gel are preferred, which require elastic stability, accurate Shore hardness (mostly 30~80A), fatigue resistance and suitable shock-absorbing pad/bumper/sealing sleeve, etc.

Key points: the melting uniformity of elastomer should be strictly controlled to avoid cold material and lack of material, and ensure the elasticity of plastic parts is consistent; Silicone needs to be matched with a special silicone injection molding machine to control mold temperature and vulcanization.

Second, mold design and debugging: adapt to the performance of plastic parts and ensure the quality from the source.

Mold is the core to determine the precision, air tightness and elasticity of plastic parts, and it needs targeted design to avoid the defects in later molding:

Pneumatic parts mold:

Runner/gate: balanced runner is adopted, and the gate position is away from the sealing surface/mating surface to prevent the residual gate from affecting the air tightness; Small size pneumatic parts use point gate, which is convenient for glue cutting and has no residue;

Exhaust system: exhaust grooves with a depth of 0.01~0.02mm should be made at the end and dead corner of the cavity to avoid air being trapped when the molten material is filled to form internal bubbles (bubbles are the biggest hidden danger of air tightness failure of pneumatic components);

Accuracy control: the tolerance of cavity/core shall be reduced by 50% according to the requirements of plastic parts, and the sealing surface and mating surface shall be mirror polished (Ra≤0.4μm) without scratches and steps, so as to ensure the close fitting of plastic parts.

Shock absorber mold:

Runner/gate: large-diameter runner+fan gate is used to ensure the rapid and uniform filling of elastomer melt and avoid cold spots (cold material will lead to local insufficient elasticity and easy cracking);

Cavity demoulding: large demoulding slope+soft demoulding ejector pin to avoid the elastic parts being pulled and deformed when demoulding, which will affect the appearance and elasticity;

Hardness adaptation: shrinkage allowance is reserved for the cavity size (the shrinkage rate of elastomer is large, such as TPU shrinkage rate of 1.5%~3%), so as to avoid the deviation of hardness and elasticity caused by the dimensional deviation of plastic parts.

General mold requirements: the mold cooling system is uniform to avoid deformation caused by uneven local cooling of plastic parts; The mold cavity should be clean and free of oil stains and impurities to prevent the surface of plastic parts from being short of materials and scratches.

Third, the control of injection molding process parameters: accurately adjust parameters in parts to avoid core defects.

Process parameters directly determine the molding quality of plastic parts. Pneumatic parts are strictly controlled to be "bubble-free and stable in size" and damping parts are strictly controlled to be "even in elasticity and free of materials". Key points for adjusting core parameters are as follows:

(1) Pneumatic parts (mainly PA6/PA66/POM)

Drying treatment: PA6/PA66 is dried at 80~90℃ for 4~6h, POM is dried at 70~80℃ for 2~3h, and it is put on the machine immediately after drying to avoid moisture regain;

Temperature control: the barrel temperature is calibrated according to the material (PA66: 230 ~ 260℃, POM: 180 ~ 200℃), and the mold temperature is 60~80℃ (to improve melt fluidity, reduce internal stress and avoid cracking of plastic parts);

Injection parameters: low-speed and high-pressure filling is adopted, gas entrapment is avoided at low speed in the early stage, and molten material is ensured to fill the cavity at high pressure in the later stage, and the holding pressure is 60%~80% of the injection pressure, and the holding time is appropriately extended (filling the cavity to reduce shrinkage cavity and bubbles);

Cooling time: sufficient cooling (for example, the cooling time of PA66 plastic parts is ≥ the product wall thickness ×1.5s), so as to avoid deformation caused by incomplete cooling and demoulding of plastic parts, which will affect the fitting accuracy and air tightness.

(2) Shock absorbers (mainly TPR/TPE/TPU)

Temperature control: the barrel temperature is 160~200℃ (adjusted by hardness, the temperature of soft parts is low), and the mold temperature is 30~50℃ (to avoid internal stress caused by too fast cooling of elastomer and ensure elastic recovery);

Injection parameters: high-speed and low-pressure filling is used to fill the cavity quickly, so as to reduce the cooling stratification of molten material and avoid local material shortage and cold joint marks (cold joint marks will greatly reduce the fatigue resistance of shock absorber and make it easy to break);

Holding pressure and cooling: the holding pressure is low (40%~50% of the injection pressure) to prevent the plastic parts from over-holding pressure from causing high hardness and decreased elasticity; The cooling time is moderate to ensure that the plastic parts are elastic and stable without warping after demoulding.

4. Post-molding treatment: targeted treatment to improve the final performance of plastic parts.

Some plastic parts need to be treated after molding to eliminate internal stress, improve air tightness/elasticity and avoid later use failure:

Post-processing of pneumatic parts:

PA6/PA66 plastic parts: Humidity control treatment (put in water or humidity environment for 24~48h) to eliminate internal stress, improve the toughness and dimensional stability of plastic parts, and avoid cracking in use;

Sealed pneumatic parts: do air tightness pre-test (simple air pressure test) to eliminate unqualified products with bubbles and shrinkage cavities to avoid air leakage after installation.

Post-treatment of shock absorber:

Elastomer plastic parts: tempering (heat preservation at 60~80℃ for 1~2h) to eliminate internal stress of demoulding, restore the elasticity of plastic parts to a stable state, and ensure the hardness and fatigue resistance;

Remove flash: use low-temperature freezing edge removal/manual soft scraper edge removal to avoid the edge damage of plastic parts and the elastic layer falling off due to hard scraping.

V. Control of mass production process: ensure consistency and avoid batch defects.

Pneumatic/shock absorber parts are mostly industrial accessories, which require high batch consistency, so it is necessary to control the whole process in mass production:

Raw material control: the same batch of plastic parts uses the same brand and batch of raw materials, and the raw materials need to have material reports, and the elastomer needs to be tested for hardness consistency to avoid performance differences between batches;

First piece inspection: after each batch is turned on, make the first piece, check the size, air tightness and fitting accuracy of pneumatic parts, and check the hardness, elasticity and overall size of shock absorber parts, and mass production can only be carried out after passing the test;

Process patrol: patrol every 1~2 hours, focusing on the appearance (no material, bubbles, scratches) and key dimensions of plastic parts, and randomly sampling the fatigue resistance of elastomers (simple press rebound test);

Equipment maintenance: clean the barrel and nozzle regularly to prevent the material in the barrel from degradation; Check the mold seal and exhaust slot to avoid air bubbles and material shortage caused by the blockage of exhaust slot.

6. Core Pit Avoidance Point: The most common processing error of two types of plastic parts.

Pneumatic parts: air bubbles due to insufficient drying of raw materials → air tightness failure; When the injection speed is too fast, it will be wrapped in air → internal shrinkage cavity; Insufficient pressure keeping → shrinkage marks of plastic parts, which affect the fit;

Shock absorber: too low melt temperature → cold joint mark, lack of material, poor fatigue resistance; The holding pressure is too high → the hardness of plastic parts is too high and the elasticity is insufficient; Improper demoulding method → plastic parts are strained and deformed;

Common error: poor exhaust of mold → air bubble and lack of material; Uneven cooling → plastic deformation; Improper handling of flash → affecting sealing/elasticity.

Core summary

The core of pneumatic plastic parts injection molding is "no moisture, no bubbles, accurate size and good sealing", focusing on three links: drying, exhaust and pressure keeping; The core of shock-absorbing plastic parts is "uniform elasticity, accurate hardness, no cold joint and fatigue resistance" * *, focusing on controlling melt uniformity, injection speed and demoulding protection. Both types of plastic parts need to customize the process according to the material characteristics, and avoid defects from the whole process of mold, injection molding and post-treatment, so as to ensure that plastic parts match the requirements of working conditions.