Injection molding is the main molding method for producing various shapes of plastic products from thermoplastic or thermosetting plastics using plastic molds. During the production process, defects such as shrinkage marks, burrs, bubbles, and warping may occur. The following introduces methods to reduce injection molding defects from aspects such as raw materials, molds, equipment, process parameters, and operations.

In terms of raw materials

Raw material drying

Reason: Many plastic materials have hygroscopicity, such as nylon (PA), polycarbonate (PC), etc. If the raw materials are not fully dried, moisture will vaporize and form bubbles during the injection molding process, affecting the appearance and performance of the product.

Measure: Select appropriate drying equipment and conditions based on the characteristics of the raw materials. For example, for nylon raw materials, a dehumidification dryer is usually used, with a drying temperature controlled at 80-100 ℃ and a drying time of 4-6 hours, to reduce the moisture content of the raw materials to below 0.1%.

Raw material selection and proportioning

Reason: Different grades and types of plastic raw materials have significant differences in performance. If the raw material selection is improper or the ratio is not reasonable, it may lead to defects in the product. For example, using materials with poor fluidity can easily lead to defects such as insufficient filling during the injection molding process.

Measure: Select appropriate raw material grades and types according to the requirements of the product. For thin-walled products, materials with good fluidity should be selected; For products that require high strength, composite raw materials with added reinforcement materials can be chosen. At the same time, strictly follow the formula for raw material ratio to ensure the stability of raw material performance.

In terms of molds

mould design

Gate Design

Reason: The position, shape, and size of the sprue have a significant impact on the filling and flow of plastic. Unreasonable gate design may lead to defects such as weld lines and insufficient filling in the product.

Measure: Select the appropriate gate type based on the shape and structure of the product, such as point gate, side gate, hidden gate, etc. The gate position should be selected at the thicker part of the product wall to avoid direct impact on the core or insert, in order to reduce the occurrence of weld marks. At the same time, design the gate size reasonably to ensure that the plastic can smoothly fill the mold cavity.

Exhaust system design

Reason: During the injection molding process, if the air inside the mold cavity and the gas generated by the decomposition of the plastic cannot be discharged in a timely manner, defects such as pores and burning will form on the surface of the product.

Measures: Reasonable exhaust grooves should be set on the mold, and the depth and width of the exhaust grooves should be determined according to the characteristics of the plastic and the size of the product. Generally speaking, the depth of the exhaust groove is 0.01-0.03mm and the width is 3-5mm. For large or complex shaped products, a mosaic mold structure can also be used to exhaust using the fitting gap of the inlay.

Cooling System Design

Reason: Uneven cooling can lead to inconsistent shrinkage of the product, resulting in defects such as warping and deformation.

Measure: Design a reasonable cooling water channel to ensure that the cooling water can flow evenly through all parts of the mold. The diameter, spacing, and layout of cooling water channels should be determined based on the shape and size of the product. Generally speaking, the diameter of the cooling water channel is 8-12mm, with a spacing of 25-35mm. At the same time, the temperature and flow rate of the cooling water are controlled to ensure uniform and stable mold temperature.

Mold maintenance

Reason: The mold may experience wear and blockage during use, which affects the quality of the product.

Measures: Regularly clean and maintain the mold, inspect the mold cavity, core, gate, exhaust groove and other parts for wear, blockage or damage. Timely replace worn parts, clean blocked exhaust ducts and gates, and ensure the normal operation of the mold.

Equipment aspect

Injection molding machine selection

Reason: The specifications and performance of the injection molding machine should match the requirements of the product. If the injection volume, locking force and other parameters of the injection molding machine are too small, it may lead to insufficient filling of the product or defects such as burrs.

Measure: Choose a suitable injection molding machine based on the weight, size, and shape of the product. Generally speaking, the injection volume of an injection molding machine should be 1.3-1.5 times the weight of the product, and the locking force should be calculated and determined based on the projected area of the product and the injection pressure of the plastic.

equipment maintenance

Reason: Various components of the injection molding machine may experience wear and aging during long-term use, which affects the stability of the injection molding process and the quality of the products.

Measures: Regularly maintain and upkeep the injection molding machine, check whether the hydraulic system, electrical system, heating system, etc. of the injection molding machine are working properly. Timely replace worn parts, adjust equipment parameters, and ensure stable performance of the injection molding machine.

In terms of process parameters

temperature control

Barrel Temperature

Reason: Excessive temperature in the material barrel can cause plastic to decompose, resulting in defects such as bubbles and discoloration; If the temperature of the material cylinder is too low, it will cause the flowability of the plastic to deteriorate, resulting in insufficient filling.

Measure: Reasonably set the temperature of each section of the material barrel according to the characteristics of the plastic and the requirements of the product. Generally speaking, the temperature of the material barrel gradually increases from the feeding section to the nozzle section, but should be controlled below the decomposition temperature of the plastic. For example, for polypropylene (PP) raw materials, the barrel temperature can be set to 180-230 ℃.

Temperature

Reason: Mold temperature has a significant impact on the crystallinity, shrinkage rate, and surface quality of the product. Excessive mold temperature can prolong the cooling time of the product and reduce production efficiency; If the mold temperature is too low, it will cause the surface of the product to become rough and the internal stress to increase.

Measure: Reasonably control the mold temperature based on the characteristics of the plastic and the requirements of the product. For crystalline plastics such as polyethylene (PE), polypropylene (PP), etc., the mold temperature can be appropriately increased to promote the crystallization of the plastic and improve the strength and rigidity of the product; For non crystalline plastics such as polystyrene (PS), acrylonitrile butadiene styrene copolymer (ABS), etc., the mold temperature can be appropriately lowered to reduce the cooling time of the product.

pressure control

Pressure

Reason: Insufficient injection pressure can lead to insufficient plastic filling, resulting in defects such as material shortage and weld marks; Excessive injection pressure can cause problems such as burrs and increased internal stress in the product

Measure: Reasonably set the injection pressure according to the shape, size, and flowability of the plastic of the product. Generally speaking, the injection pressure should be adjusted according to the actual situation, and the injection pressure should be minimized as much as possible while ensuring the smooth filling of the mold cavity with plastic.

Hold Pressure

Reason: The function of the holding pressure is to compact the plastic inside the cavity after filling it with plastic, preventing shrinkage of the product. Insufficient holding pressure can cause severe shrinkage of the product, resulting in shrinkage marks; Excessive holding pressure can cause internal stress in the product and even lead to mold damage.

Measure: Reasonably set the holding pressure based on the shrinkage rate of the product and the characteristics of the plastic. The holding pressure is generally 60% -80% of the injection pressure, and the holding time should be determined according to the thickness of the product and the cooling rate of the plastic.

SPEED CONTROL

injection speed

Reason: Excessive injection speed can cause turbulence in the plastic cavity, resulting in defects such as air streaks and burning on the surface of the product; If the injection speed is too slow, it will prolong the cooling time of the plastic and produce weld marks.

Measure: Reasonably select the injection speed based on the shape of the product and the characteristics of the plastic. For thin-walled or complex shaped products, high-speed injection can be used to reduce the cooling time of the plastic and improve the filling effect; For thick walled products or products with high surface quality requirements, low-speed injection can be used to avoid defects such as air streaks and burning.

screw speed

Reason: Excessive rotation speed of the screw can cause excessive shear force on the plastic inside the barrel, leading to plastic decomposition; If the screw speed is too low, it will cause uneven plasticization of the plastic, affecting the quality of the product.

Measure: Reasonably set the screw speed based on the characteristics of the plastic and the performance of the injection molding machine. Generally speaking, the screw speed should be adjusted according to the actual situation, and the screw speed should be minimized as much as possible while ensuring sufficient plasticization of the plastic.

Operational

Operating specifications

Reason: Improper operation by operators, such as uneven feeding and too fast mold opening and closing speed, may lead to defects in the product.

Measure: Provide professional training to operators to familiarize them with the operating procedures and process requirements of injection molding machines. During the operation, strictly follow the operating procedures to ensure uniform feeding, moderate mold opening and closing speed, and stable injection molding process parameters.

Product retrieval and post-processing

Reason: Improper removal or delayed post-processing of the product may result in deformation, damage, and other issues.

Measure: When removing the product, appropriate tools should be used to avoid damage to the product. For some easily deformed products, a slow ejection mechanism can be used to eject them, and timely shaping treatment can be carried out after removal. At the same time, according to the requirements of the product, necessary post-processing such as annealing, humidity control, etc. should be carried out to eliminate the internal stress of the product and improve its performance.