Preform mold design principles
一. Introduction
二. Precision principles
1. Dimensional accuracy
2. Shape accuracy
三. Cooling system design principles
1. Cooling efficiency
2. Temperature uniformity
三. Cooling system design principles
1. Cooling efficiency
2. Temperature uniformity
四 .Demolding Design Principles
1. Demolding Method Selection
2. Demolding force control
五. Gate design principles
1. Gate location selection
2. Gate form selection
六．Principles of mold material selection
1. Strength and hardness requirements
2. Corrosion resistance requirements
七. Principles of runner system design
1. Runner shape and size
2. Runner balance design
八. Exhaust design principles
1. Exhaust method selection
2. Exhaust position determination
九. Cost control principles
1. Material cost control
2. Processing cost control
十. Conclusion
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一. Introduction
Preform molds are key tools for producing plastic preforms. The quality of preforms directly affects the quality of subsequent blow-molded products. Good preform mold design can ensure that the preform size is accurate, the quality is stable, the production efficiency is high and the cost is reasonable.
二. Precision principles
1. Dimensional accuracy
The dimensional accuracy of the preform is the primary consideration. When designing a mold, the size of the cavity must be accurately calculated and determined. For example, for common PET preforms, the key dimensions such as the mouth diameter, the body diameter, and the length of the preform must be designed strictly in accordance with product requirements. Usually, the dimensional tolerance should be controlled within a very small range, such as the mouth diameter tolerance may be controlled within ±0.1mm, because inaccurate mouth dimensions will lead to problems such as poor sealing of the bottle cap.
The core and cavity dimensions of the mold need to consider the shrinkage rate of the plastic material. Different plastic materials have different shrinkage rates. For example, the shrinkage rate of PET materials is generally around 1.2-2.0%. During the design, the core and cavity sizes should be enlarged or reduced in advance according to the shrinkage characteristics of the material to ensure that the size of the preform after cooling and demolding meets the requirements.
2. Shape accuracy
The shape accuracy of the preform has a great influence on the uniformity of the subsequent blowing process. The shape of the mold cavity must be highly consistent with the ideal shape of the preform, including the bottom shape, shoulder shape and side wall shape of the preform. For example, if the bottom of the preform is uneven, the thickness of the bottom of the bottle will be uneven during blowing, affecting the stability of the bottle. Therefore, when designing the mold, the cavity shape accuracy should be ensured through precise processing technology and reasonable mold structure. For example, the milling of the cavity by CNC machining center can achieve a processing accuracy of ±0.01mm.
三. Cooling system design principles
1. Cooling efficiency
The cooling system of the preform mold is a key factor in controlling the quality of preform molding and the production cycle. An efficient cooling system can quickly cool and shape the preform and shorten the molding cycle. Generally speaking, the cooling time accounts for a large proportion of the entire preform molding cycle. For example, for a conventional preform, the cooling time may account for 40-60% of the molding cycle.
The design of the cooling system should consider the flow path and speed of the cooling medium. The use of spiral cooling channels can make the cooling medium evenly distributed in the mold and improve the cooling efficiency. For example, a spiral cooling channel is designed inside the core, and the cooling medium (such as water) flows at high speed in the channel, which can quickly take away the heat inside the preform and make the internal and external temperature distribution of the preform more uniform.
2. Temperature uniformity
Ensuring uniform cooling of all parts of the preform is an important principle to avoid internal stress and deformation of the preform. If the preform is not cooled evenly, it will cause warping and dimensional instability after demolding. In order to achieve temperature uniformity, the layout of the cooling channel should be reasonable. For example, designing multiple cooling channels around the cavity and making the distance between the channels uniform can ensure that the temperature gradient on the surface of the cavity is uniform, so that the cooling speed in the wall thickness direction and the circumferential direction of the preform is basically the same.
It is also possible to adopt the method of zone cooling to adjust the diameter of the cooling channel and the flow rate of the cooling medium according to the thickness and shape characteristics of different parts of the preform. For example, for the thick-walled part of the preform, the diameter of the cooling channel can be appropriately increased or the flow rate of the cooling medium can be increased to speed up the cooling speed of the part.
四 Demolding Design Principles
1. Demolding Method Selection
The demolding methods of the preform mainly include push rod demolding, push plate demolding and pneumatic demolding. Push rod demolding is one of the most commonly used methods. It is achieved by setting a push rod inside the mold. When the mold is opened, the push rod pushes the preform out of the core. The position and number of push rods should be determined according to the shape and size of the preform. For example, for a preform with a large aspect ratio, multiple push rods need to be set at the bottom and side of the preform to ensure that the preform can be demolded smoothly and avoid deformation of the preform during demolding.
Push plate demolding is suitable for the case where the bottom area of the preform is large. The push plate can provide uniform demolding force, so that the bottom of the preform can be smoothly separated from the mold. Pneumatic demolding uses compressed air to blow the preform out of the mold. This method is suitable for some special shapes of preforms or situations where the demolding force is relatively precise, such as some high-precision pharmaceutical preforms.
2. Demolding force control
The magnitude of the demolding force directly affects the demolding quality of the preform. If the demolding force is too large, the preform will be deformed or even damaged; if the demolding force is too small, the preform cannot be demolded smoothly. When designing the mold, the magnitude of the demolding force should be calculated. The formula for calculating the demolding force is: F = A×P×f, where F is the demolding force, A is the contact area between the preform and the core, P is the unit clamping force of the plastic material on the core, and f is the demolding coefficient. By accurately calculating the demolding force, the parameters of the demolding mechanism, such as the diameter of the push rod and the thickness of the push plate, can be reasonably designed to provide a suitable demolding force.
In order to reduce the demolding force, a demolding agent coating can be applied to the surface of the core or the surface of the core can be micro-textured. The demolding agent can reduce the friction between the preform and the core, and the micro-textured treatment can allow air to enter the gap between the preform and the core to form an air cushion effect, thereby reducing the demolding force.
五. Gate design principles
1. Gate location selection
The gate location has an important impact on the quality and appearance of the bottle embryo. The gate should be selected as much as possible in the thicker part of the bottle embryo, so that the plastic melt can flow more evenly in all directions when filling the cavity. For example, for bottle embryos with thicker bottoms, the gate can be set at the center of the bottom of the bottle embryo. At the same time, the gate position should avoid affecting the appearance quality of the bottle embryo. For example, it cannot be set in a conspicuous position on the outer surface of the bottle embryo, otherwise it will leave gate marks and affect the appearance of the bottle.
The gate position should also consider the flow balance of the plastic melt. If the gate position is unreasonable, it will cause the plastic melt to stagnate and spray in the cavity, causing defects such as weld marks and bubbles in the bottle embryo. When designing a pet bottle mold with a complex shape, it may be necessary to use simulation software to analyze the flow of the melt and determine the best gate position.
2. Gate form selection
Common gate forms include pinpoint gates, side gates, and latent gates. Pin-point gate is suitable for small preforms and preforms with high requirements for appearance quality. Its advantages are small gate marks, and the plastic melt can fill the cavity at a higher speed, which is conducive to improving the surface quality of the preform. Side gate is suitable for some preforms with more regular shapes, which can allow the melt to enter the cavity evenly from the side. The latent gate is a more concealed gate form, which can automatically cut off the gate during demolding, avoid the process of manually trimming the gate, and improve production efficiency.
六．Principles of mold material selection
1. Strength and hardness requirements
The preform mold is subject to high pressure and friction during operation. Therefore, the mold material should have sufficient strength and hardness. Generally speaking, the core and cavity parts of the mold should use high-strength mold steel, such as P20 steel, whose hardness can reach about HRC30-35. This hardness can ensure that the mold will not deform during the long-term opening and closing of the mold and the preform molding process, and ensure the dimensional accuracy of the preform.
For some large or high-volume preform molds, higher strength steels such as H13 steel can also be used, which can reach a hardness of about HRC48-52 and has good thermal fatigue performance and can adapt to frequent heating and cooling cycles.
2. Corrosion resistance requirements
Since preform molds are often in contact with plastic melts, some plastic melts may be corrosive. For example, when producing some plastic preforms containing acidic additives, the mold material needs to have good corrosion resistance. Stainless steel materials such as S136 steel are a commonly used corrosion-resistant mold material. It contains a high proportion of chromium elements and can form a dense oxide film on the mold surface to prevent the intrusion of corrosive media and ensure the service life of the mold.
In addition to the corrosion resistance of the material itself, the mold surface can also be coated, such as titanium nitride coating, which can not only improve the wear resistance of the mold, but also enhance the corrosion resistance of the mold.
七. Principles of runner system design
1. Runner shape and size
The shape and size of the runner directly affect the flow properties of the plastic melt. The circular runner is the most ideal shape because it can minimize the flow resistance of the melt. When designing, the diameter of the runner should be determined according to the size of the preform and the flow rate of the plastic melt. For example, for larger preforms, the runner diameter may need to be designed to be around 6-10mm to ensure that the melt can flow into the cavity quickly and evenly.
The length of the runner should also be reasonably controlled. A runner that is too long will increase the flow resistance and heat loss of the melt. Generally speaking, the runner length should be as short as possible, and sharp bends and diameter changes in the runner should be avoided to avoid pressure loss and vortex phenomenon in the melt.
2. Runner balance design
In order to ensure that the plastic melt is evenly distributed among the cavities, a runner balance design is required. In a multi-cavity preform mold, the runner length and diameter of each cavity should be as similar as possible. If the runner is unbalanced, the quality of the preforms in different cavities will be inconsistent, such as some preforms will have defects such as short shots and flash. Computer-aided design software can be used to simulate the flow of the melt, optimize the size and shape of the runner, and achieve runner balance.
八. Exhaust design principles
1. Exhaust method selection
During the injection molding process of the preform mold, if the air in the cavity cannot be discharged in time, it will cause defects such as bubbles and burning in the preform. Common exhaust methods include exhaust groove exhaust and breathable steel exhaust. Exhaust groove exhaust is to open an exhaust groove at the appropriate position of the parting surface or cavity of the mold. The depth of the exhaust groove is generally about 0.02-0.05mm and the width is about 3-5mm. This method is simple and effective and suitable for most preform molds.
Breathable steel exhaust is to use breathable steel material in the core or cavity part of the mold. There are a large number of tiny pores inside the breathable steel, which can allow air to be discharged smoothly. This method is suitable for preform molds with high exhaust requirements, such as some high-precision, thin-walled preform molds.
2. Exhaust position determination
The exhaust position should be determined according to the shape of the preform and the flow direction of the plastic melt. Generally speaking, the exhaust position should be set at the last filling position of the melt, such as the top and shoulder of the preform. Opening exhaust grooves or using breathable steel at these locations can effectively exhaust the air in the cavity and ensure the quality of the preform.
九. Cost control principles
1. Material cost control
When selecting mold materials, try to choose low-cost materials while meeting the mold performance requirements. For example, for some small, low-production preform molds, you can choose domestic high-quality mold steel instead of imported high-end steel. At the same time, the amount of material should be reasonably calculated to avoid waste. In the mold design stage, by optimizing the mold structure, reduce unnecessary material parts, such as reasonably designing the wall thickness of the core and cavity, so that it can meet the strength requirements and reduce the amount of material.
2. Processing cost control
The processing cost of the mold is also a factor that needs to be considered during design. Choosing a suitable processing technology can reduce the processing cost. For example, for some mold parts with simple shapes, ordinary mechanical processing methods can be used, while for parts with complex shapes and high precision requirements, high-precision processing methods such as CNC machining centers or EDM can be used. At the same time, it is necessary to arrange the processing procedures reasonably to improve processing efficiency and reduce processing time.
When designing the mold, the assembly cost of the mold should also be considered. Designing mold structures that are easy to assemble, such as using standardized parts and components, can reduce the amount of adjustment and repair work during the assembly process, and can reduce assembly costs.
十. Conclusion
The design of preform molds is a complex process that takes into account multiple factors. During the design process, a series of principles such as precision principles, cooling system design principles, and demoulding design principles must be strictly followed to ensure that the designed molds can produce high-quality, high-yield, and cost-effective preforms to meet the needs of plastic bottle production.
Foshan Heyan Precision Mold Technology Co., Ltd.'s main business is preform molds, bottle blowing 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 ID.pet bottle mold
