What factors should be considered in the customized design of preform molds?
一. Factors related to the characteristics of preform products
1. Preform size and precision requirements
2. Shape and structural characteristics of bottle embryos
3. Material properties of bottle embryos
二. Factors related to mold production efficiency
1. Selection of the number of cavities
2. Mold opening and closing speed and degree of automation
三. Mold quality and reliability factors
1. Selection of mold materials
2. Cooling system and demoulding system design
四. Cost control and economic factors
1. Mold manufacturing cost budget
2. Mold service life and maintenance cost
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一. Factors related to the characteristics of preform products
1. Preform size and precision requirements
Dimension specifications: Customized preform molds must first consider the size of the preform, including diameter, height, wall thickness, etc. Preforms of different sizes require matching mold cavities and core sizes. For example, for preforms with smaller diameters, the core diameter of the mold must be precisely designed to ensure the accuracy of the inner diameter of the preform; for preforms with higher heights, the mold opening stroke and ejection mechanism must be able to adapt to the demolding requirements of the preform.
Precision standards: The precision requirements of preforms vary depending on the purpose. If it is a preform used for pharmaceutical packaging, the dimensional accuracy requirements are extremely high. For example, the tolerance of the bottle mouth diameter may be controlled within ±0.05mm to ensure a good seal of the bottle cap. This requires that the mold adopts high-precision processing equipment and processes during the design and manufacturing process, such as using CNC machining centers to process cavities and cores, and performing precise dimensional inspection and adjustment after assembly.
2. Shape and structural characteristics of bottle embryos
Shape complexity: The shapes of bottle embryos are diverse, and some bottle embryos may have special shapes, such as special-shaped shoulders and reinforcing ribs at the bottom. For bottle embryos with complex shapes, the mold design should consider the filling method of the plastic melt. For example, for bottle embryos with internal compartments or complex textures, the gate position and runner system need to be carefully designed to ensure that the plastic melt can be evenly filled into each part to avoid defects such as short shots and weld marks.
Special structural requirements: If the bottle embryo has a special structure, such as nesting or assembly with other parts, the mold design should consider the molding of these structures. For example, if there is an external thread structure on the bottle embryo, the mold should be designed with a corresponding demolding mechanism, such as rotary demolding or lateral demolding, to prevent damage to the threaded part of the bottle embryo during demolding.
3. Material properties of bottle embryos
Plastic fluidity: Different plastic materials have different fluidity. For example, PP (polypropylene) has good fluidity, while PC (polycarbonate) has relatively poor fluidity. When designing a preform mold, for plastics with good fluidity, a relatively small gate size and a simpler runner system can be designed; for plastics with poor fluidity, a larger gate and a more optimized runner are required to ensure that the plastic melt can smoothly fill the cavity.
Shrinkage rate difference: The shrinkage rates of various plastics are different. For example, the shrinkage rate of PET is generally around 1.2-2.0%, and the shrinkage rate of PP is about 1.0-2.5%. Considering the shrinkage rate of the preform material, the core and cavity sizes of the mold should be designed with corresponding compensation. For example, when designing a mold, the size of the core and cavity should be pre-enlarged or reduced according to the shrinkage rate of the plastic to ensure that the size of the preform after cooling and demolding meets the requirements.
二. Factors related to mold production efficiency
1. Selection of the number of cavities
Determine the number of cavities of the mold according to the production scale of the preform. If it is a small batch production, a single-cavity or double-cavity mold may be sufficient, so that the mold structure is relatively simple and the design and manufacturing cycle is short. For example, for the trial production of new products or customized small batch orders, single-cavity molds are convenient for adjusting and optimizing process parameters.
For large-scale production, multi-cavity molds can improve production efficiency. However, multi-cavity molds are difficult to design and manufacture, and issues such as runner balance and cooling uniformity need to be considered. For example, for an 8-cavity preform mold, to ensure that the plastic melt filling speed and pressure of each cavity are the same and the cooling effect is consistent, this requires fine optimization in the design of the runner system and cooling system.
2. Mold opening and closing speed and degree of automation
Mold opening and closing speed design: Reasonable mold opening and closing speed is important for improving production efficiency. Too fast mold opening speed may generate greater impact force and damage the mold; too slow mold opening speed will increase the molding cycle. When custom designing, the appropriate mold opening and closing speed should be determined according to the size, weight and demolding requirements of the mold. For example, for large preform molds, the mold opening speed may be relatively slow to ensure smooth opening of the mold.
Automation function integration: Consider whether automation functions need to be integrated into the mold. If the preform production process needs to be automated, such as automatic demoulding, automatic shearing gate, etc., the mold should be designed with a corresponding structure. For example, a pneumatic demoulding device or an automatic shearing device can be installed on the mold, and the compatibility of these automated devices with the injection molding machine control system should be considered.
三. Mold quality and reliability factors
1. Selection of mold materials
Select appropriate mold materials according to the production requirements of the preform and the working conditions of the mold. For the core and cavity parts that are subject to high pressure and friction, high-strength and high-hardness mold steel should be selected. For example, H13 steel has good thermal fatigue performance and wear resistance, and is suitable for long-term and high-frequency production of preform molds.
If the preform mold needs to contact corrosive plastic materials, such as plastics containing acidic additives, materials with good corrosion resistance, such as S136 stainless steel, should be selected. At the same time, the cost and processability of the material should also be considered. On the premise of meeting the mold quality requirements, try to choose cost-effective materials.
2. Cooling system and demoulding system design
Cooling system optimization: The design of the cooling system directly affects the quality of the preform and the service life of the mold. It is necessary to ensure that all parts of the preform are cooled evenly to avoid deformation or internal stress of the preform caused by uneven cooling. For example, spiral cooling channels or partitioned cooling can be used to design the layout of the cooling channels and the flow rate of the cooling medium according to the wall thickness and shape characteristics of the preform.
Demolding system reliability: The design of the demolding system must ensure that the preform can be demolded smoothly while avoiding damage to the preform. According to the shape and structure of the preform, choose a suitable demolding method, such as push rod demolding, push plate demolding or pneumatic demolding. For example, for a preform with a large bottom area, push plate demolding may be more suitable because it can provide uniform demolding force. When designing the demolding mechanism, the magnitude of the demolding force should be calculated, and the number, position and size of the push rods should be reasonably determined.
四. Cost control and economic factors
1. Mold manufacturing cost budget
When customizing preform molds, the manufacturing cost of the mold should be considered. The cost includes material cost, processing cost, assembly cost, etc. Cost estimation is carried out according to the requirements of the preform and the complexity of the mold. For example, for preform molds with complex shapes, special processing techniques such as EDM may be required, which will increase processing costs; while for molds with simple shapes, the cost can be reduced by optimizing the design and reducing the processing steps.
Reasonably control the size and weight of the mold to avoid unnecessary material waste. For example, under the premise of meeting the requirements of preform size accuracy and strength, try to reduce the wall thickness and template size of the mold to reduce material costs. At the same time, the production batch of the mold should be considered. For molds produced in small batches, some low-cost manufacturing methods and materials can be used.
2. Mold service life and maintenance cost
Service life considerations: The designed preform mold should have a reasonable service life. The service life of the mold can be extended by selecting suitable materials and optimizing the design structure. For example, surface treatment technology, such as nitriding, can improve the wear resistance and corrosion resistance of the mold, thereby extending the service life of the mold.
Maintenance cost analysis: Consider the maintenance cost of the mold during use. Design a mold structure that is easy to maintain, such as modular design, to facilitate the replacement and repair of parts. For example, the gate part of the mold is designed as an independent module. When the gate is damaged, it can be easily replaced without repairing the entire mold, thereby reducing maintenance costs.
Foshan Heyan Precision Mold Technology Co., Ltd.'s main business is preform molds, blow molds, bottle cap molds and other products. It has a strong engineering, R&D, technology, and sales team, advanced processing equipment and rich project experience. If you want to know more or have special needs, please call 13318345050, WeChat at the same number.pet bottle mold
