In the extrusion blow molding industry, the mold is not only a core forming tool but also a key factor that directly determines product quality, production efficiency, and long-term operating costs. A properly selected mold can perfectly match the performance of the extrusion blow molding machine, produce high-quality products with stable dimensions and excellent appearance, and significantly reduce the unit production cost through high-efficiency operation. On the contrary, an inappropriate mold may lead to a series of problems such as high defective rate, frequent equipment failures, low production efficiency, and even fail to meet basic product requirements, resulting in unnecessary investment losses.
For manufacturers using extrusion blow molding machines, selecting the right mold is a systematic project that requires comprehensive consideration of product design, material characteristics, equipment parameters, production scale, cost budget and other factors. Many enterprises only focus on the price of molds when purchasing, ignoring the matching with equipment and the actual production demand, which eventually leads to the situation that the mold cannot be used normally or the production benefit is far below expectations. As a professional manufacturer of extrusion blow molding machines with years of experience, Apollo has in-depth technical accumulation in mold matching and process optimization. Its full series of extrusion blow molding equipment can adapt to various types of blow molds, and its professional engineering team can provide targeted mold selection guidance for customers to ensure the best cooperation between equipment and molds.
This article will systematically explain the core functions and classification of extrusion blow molds, sort out the key factors that need to be considered in mold selection, analyze the characteristics and applicable scenarios of different mold materials and structural designs, explain the matching principles between molds and Apollo extrusion blow molding machines in detail, provide detailed mold price reference and investment return analysis, and summarize common mistakes and maintenance methods in mold procurement. Through comprehensive and in-depth interpretation, this paper aims to help extrusion blow molding enterprises master scientific mold selection methods, avoid procurement risks, and maximize production efficiency and investment returns.
1. The Critical Role of Molds in Extrusion Blow Molding Production
1.1 Core Functions of Extrusion Blow Molds
Extrusion blow molding is a process that extrudes molten plastic into a tubular parison through an extruder, then puts the parison into a mold cavity, and blows compressed air into the parison to make it expand and fit the inner wall of the mold cavity, finally obtaining a hollow plastic product after cooling and shaping. In this whole process, the mold undertakes multiple core functions and is the direct carrier of product forming.
First of all, the mold defines the shape and size of the product. The shape of the mold cavity completely determines the external contour of the final product, and the precision of the cavity processing directly affects the dimensional tolerance of the product. For products with assembly requirements such as plastic bottles and barrels, the dimensional accuracy of the mouth and thread is particularly important, which depends entirely on the machining accuracy of the corresponding parts of the mold.
Secondly, the mold affects the wall thickness distribution of the product. Through the design of the cavity shape, the cooperation with the die head and the blowing process, the mold can control the stretching degree of the parison in different positions, so as to realize the reasonable distribution of the wall thickness. For products that need to bear pressure or impact, the wall thickness can be increased at the stress position through mold design to improve the structural strength of the product.
Third, the mold determines the surface quality of the product. The surface finish, texture, logo, pattern and other effects of the product are all directly formed by the surface of the mold cavity. High-precision polishing or texture etching on the surface of the mold can transfer to the surface of the product to obtain the desired appearance effect.
Fourth, the mold undertakes the cooling and shaping function. Most of the cooling process of the product after blow molding is completed through the cooling water channel inside the mold. The reasonable design of the cooling system can shorten the molding cycle, improve production efficiency, and reduce the product deformation caused by uneven cooling.
1.2 Main Categories of Blow Molding Molds
Extrusion blow molds can be divided into different categories according to different classification standards, and different types of molds are suitable for different production scenarios.
According to the product application category, molds can be divided into packaging bottle molds, industrial barrel molds, automotive part molds, daily necessities molds, and large industrial component molds. Packaging bottle molds are mostly used for producing beverage bottles, detergent bottles, pharmaceutical bottles and other small and medium-sized hollow products, usually with small volume and high requirements for dimensional accuracy of the bottle mouth. Industrial barrel molds are used for producing 20L, 50L, 200L and other specifications of plastic barrels, with large volume and high requirements for structural strength. Automotive part molds are used for producing automotive fuel tanks, air ducts, bumpers and other hollow parts, with complex shapes and often with special structures such as inserts and core pulling.
According to the number of cavities, molds can be divided into single-cavity molds and multi-cavity molds. A single-cavity mold has only one forming cavity in one mold, which is mostly used for large-size products or small-batch multi-variety production. A multi-cavity mold has two or more cavities at the same time, which can form multiple products in one molding cycle, greatly improving production efficiency, and is mostly used for mass production of small and medium-sized products.
According to the structural form, molds can be divided into integral molds, combined molds and molds with special mechanisms. Integral molds have simple structure, good rigidity and low cost, and are suitable for products with simple shapes. Combined molds are assembled by multiple parts, which is convenient for processing and replacing local vulnerable parts, and is suitable for medium and large molds. Molds with special mechanisms such as side core pulling and thread demoulding are used for products with side concave, thread and other structures that cannot be demoulded directly.
1.3 Key Differences from Injection Molding Molds
Many people easily confuse blow molding molds with injection molding molds, but there are essential differences between the two due to different forming principles. First of all, the cavity pressure during blow molding is much lower than that during injection molding, usually only a few tenths to a few megapascals, so the rigidity requirement of the blow mold is relatively lower than that of the injection mold, and the wall thickness of the template is thinner under the same product size.
Secondly, blow molding molds only need to form the outer surface of the product, and the inner surface of the product is formed by air blowing, so there is no core structure similar to injection molds, and the structure is relatively simpler. Injection molds need to design both cavity and core to form the inner and outer surfaces of the product at the same time, with more complex structure.
Thirdly, the cooling system layout is different. Blow molding molds are mainly cooled on both sides of the cavity, while injection molds need to arrange cooling water channels in both the cavity and the core. In addition, the exhaust system of blow molding molds is more important, because the air in the cavity needs to be discharged quickly during the blowing process, otherwise it will cause defects such as incomplete product forming and surface scorching.
Understanding these differences helps to avoid applying the experience of injection mold selection to blow mold procurement, and make more targeted selection decisions.
2. Core Factors to Consider When Selecting a Blow Mold
2.1 Product Design and Application Requirements
The primary basis for mold selection is the specific design and application requirements of the product. All mold designs and configurations must serve the final product performance. First of all, the overall size and structural complexity of the product directly determine the basic size and structural form of the mold. Large-size products need large molds, and products with complex structures such as side depressions, handles, and threads need to be equipped with corresponding core pulling, insert and other mechanisms.
Secondly, the dimensional tolerance requirements of the product affect the machining accuracy level and material selection of the mold. Products with high precision requirements need to use high-stability mold materials and high-precision processing technology, and the mold cost will increase accordingly. For example, pharmaceutical packaging bottles have extremely high requirements for the dimensional accuracy of the bottle mouth to ensure the sealing performance with the cap, so the mold processing accuracy requirements are much higher than those of ordinary daily chemical bottles.
Thirdly, the surface quality requirements of the product determine the surface treatment method of the mold cavity. Products that require high transparency and high gloss need mirror-polished mold cavities; products with matte or frosted effect need sandblasting or etching treatment on the mold surface; products with trademark logos and scale lines need to carve corresponding patterns on the mold surface. Different surface treatment processes have different costs and processing cycles.
Fourthly, the mechanical property requirements of the product affect the structural design of the mold. Products that need to bear internal pressure or fall impact need to optimize the wall thickness distribution through mold design, and improve the structural strength through reasonable rounded transition and wall thickness gradient. These requirements will be fed back to the mold design scheme, affecting the cavity shape and wall thickness distribution design.
2.2 Material Compatibility and Shrinkage Characteristics
Different plastic raw materials have different physical and chemical properties, which have an important impact on mold material selection and cavity size design. First of all, the shrinkage rate of different materials varies greatly. After the plastic product is cooled from the high-temperature molding temperature to room temperature, the size will shrink to a certain extent. When designing the mold, it is necessary to reserve a corresponding shrinkage margin on the cavity size according to the shrinkage rate of the material, so as to ensure that the product size after cooling meets the design requirements.
For example, the shrinkage rate of HDPE is usually between 1.5% and 3%, the shrinkage rate of PP is about 1.2% to 2.5%, and the shrinkage rate of PETG is only about 0.2% to 0.8%. If the shrinkage rate is not accurately calculated in the mold design, the final product size will be out of tolerance and cannot be used normally. For materials with large shrinkage rate and large fluctuation range, it is also necessary to consider leaving a certain adjustment space in the mold design to facilitate subsequent process debugging.
Secondly, the corrosion and abrasion of different materials to the mold are different. For example, PVC materials will release corrosive gases during processing, which will corrode the mold cavity for a long time. It is necessary to choose corrosion-resistant mold steel or do anti-corrosion surface treatment. For materials filled with glass fiber or mineral fillers, they have strong abrasion to the mold, so it is necessary to choose mold materials with high hardness and wear resistance, or do surface strengthening treatment.
In addition, the processing temperature and viscosity of different materials are different, and the requirements for mold structure and exhaust are also different. Materials with high viscosity need larger exhaust grooves, and materials with high processing temperature have higher requirements for mold thermal stability.
2.3 Matching with Extrusion Blow Molding Machine Parameters
The mold must be perfectly matched with the extrusion blow molding machine to give full play to the performance of both. If the mold does not match the equipment, it may not be installed at all, or it may not achieve the expected production effect after installation. The core parameters that need to be matched include clamping force, platen size, tie bar clearance, extrusion output, blow system and so on.
Clamping force is the maximum clamping force that the equipment can provide. During blow molding, the internal air pressure will generate a force to push the two mold halves open. The clamping force provided by the equipment must be greater than this force, otherwise the mold will not be tightly closed, resulting in flash and other defects. The required clamping force is related to the projected area of the product on the mold parting surface and the blowing pressure. The larger the product size and the higher the blowing pressure, the greater the required clamping force. When selecting a mold, it is necessary to calculate the required clamping force and ensure that it is within the rated clamping force range of the equipment.
The size of the platen and the position of the mounting hole determine whether the mold can be firmly installed on the equipment. The bottom area of the mold should be adapted to the size of the platen. It should not be too small to cause unstable installation, nor too large to exceed the range of the platen. The mounting screw holes on the mold should correspond to the T-slot or screw hole positions on the equipment platen to facilitate fixed installation.
The tie bar clearance refers to the distance between the four tie bars of the clamping unit, which determines the maximum external dimension of the mold that can be installed. The width and height of the mold must be smaller than the tie bar clearance, otherwise the mold cannot be put between the tie bars for installation. Many enterprises ignore this parameter when purchasing molds, resulting in the mold being too large to be installed on the existing equipment, causing economic losses.
In addition, the extrusion output of the equipment must match the total weight of the products produced by each mold cycle. If the total weight of products per cycle is too large and the extrusion output of the equipment is insufficient, the parison extrusion time will be too long, prolonging the molding cycle and reducing production efficiency. If the total weight of products is too small, the extrusion speed will be too fast, which is not easy to control the parison quality. The blowing system of the equipment, including blowing pressure, blowing flow rate and blowing mode, should also be matched with the mold design to ensure the blowing effect.
2.4 Production Volume and Batch Flexibility
The scale of production and the characteristics of product batches are also important factors to consider when selecting molds. For mass production of a single product, priority should be given to multi-cavity molds with high production efficiency and long-life mold materials, so as to maximize production efficiency and reduce unit product cost. Although the one-time investment of such molds is relatively high, the cost can be quickly recovered through large-scale production, and the long-term benefit is higher.
For the production mode of small batches and multiple varieties, it is more appropriate to choose single-cavity molds with simple structure and low cost. The single-cavity mold has low manufacturing cost, short development cycle, and is convenient for product replacement, which can better adapt to the flexible production needs of multiple varieties. In terms of material selection, you can also choose a slightly lower grade of material to reduce the initial investment. If the product is updated quickly and the mold elimination speed is fast, there is no need to choose a mold material with an excessively long service life, which will cause unnecessary waste.
In addition, the future production plan should also be considered. If the product demand is expected to grow steadily in the next few years, you can appropriately leave a certain margin when selecting the mold, or directly choose a multi-cavity mold to meet the future capacity demand, avoiding the repeated investment caused by secondary mold purchase. If the product is still in the market verification stage, you can first make a simple test mold for small-batch trial production, and then open a formal mass production mold after the product scheme is stable, so as to reduce the trial and error cost.
2.5 Budget and Total Cost of Ownership
Budget is one of the important constraints in mold selection. Many enterprises tend to only pay attention to the one-time purchase cost when purchasing molds, and choose the cheapest scheme, but ignore the long-term use cost, resulting in higher total cost of ownership in the whole life cycle of the mold.
The total cost of ownership of the mold includes not only the initial purchase cost, but also the later maintenance cost, repair cost, replacement cost, and the impact on production efficiency and product yield. A cheap mold with poor quality may have problems such as short service life, high failure rate and high defective rate in the later use process. The maintenance cost and the loss caused by defective products in a few years may far exceed the price difference when purchasing. On the contrary, a high-quality mold with a slightly higher price has a long service life, low failure rate, stable product quality and high production efficiency, and its long-term comprehensive benefit is much higher.
Therefore, when selecting molds, we should take the total cost of ownership in the entire service life as the evaluation standard, not just the initial purchase price. Weigh the relationship between initial investment and long-term benefits, and choose the most cost-effective scheme within the budget range. For long-term mass production projects, we should appropriately increase the initial investment and choose high-quality molds, which can bring more丰厚的 returns in the later stage.
3. Key Structural Design Elements of High-Quality Blow Molds
3.1 Cavity and Parting Line Design
The cavity is the core forming part of the mold, and its design quality directly determines the shape and quality of the product. When designing the cavity, the first step is to reasonably select the position of the parting line. The parting line is the joint line of the two halves of the mold, and its position will affect the appearance of the product, the difficulty of demolding, and the processing cost of the mold.
The principle of selecting the parting line is to place it at the position with the largest contour of the product to ensure smooth demolding. At the same time, try to place the parting line in an inconspicuous position of the product to avoid affecting the appearance quality. For products with high appearance requirements, the parting line can be designed at the edge or bottom of the product to reduce the visual impact. In addition, the parting line should be designed as simple as possible to avoid too complex curved shape, which will increase the difficulty of mold processing and matching, and increase the cost.
Draft angle is another important parameter of cavity design. Due to cooling shrinkage, the product will tightly wrap the mold cavity after forming. A proper draft angle can ensure that the product can be smoothly demolded from the cavity without scratching the surface or even being stuck. The size of the draft angle should be determined according to the material characteristics, product height and surface roughness. Generally speaking, the draft angle of blow molding products is between 0.5 degree and 2 degrees. For materials with large shrinkage rate and deep cavity, the draft angle should be appropriately increased.
All corners of the cavity should adopt smooth transition fillets as far as possible, avoiding sharp corners. Sharp corners are not only easy to cause stress concentration of products, reducing the mechanical strength, but also easy to cause stress concentration of the mold itself, affecting the service life of the mold. Reasonable fillet design can also improve the flow state of the parison during blowing, make the wall thickness distribution more uniform, and reduce the phenomenon of local excessive thinning.
The dimensional accuracy of the cavity must be strictly controlled according to the product tolerance requirements. The processing accuracy level should be reasonably selected according to the product size specification and tolerance requirements. For the matching parts such as the bottle mouth and thread, higher processing accuracy is required to ensure the interchangeability and sealing performance of the product. At the same time, the surface treatment of the cavity shall be determined according to the product appearance requirements. Common surface treatment methods include mechanical polishing, mirror polishing, sandblasting, texture etching, laser engraving, etc. Different treatment processes have different costs and service lives, and should be selected in combination with product requirements and budget.
3.2 Cooling System Design
The cooling system is one of the most important parts of the blow mold, which directly determines the length of the molding cycle and the dimensional stability of the product. The cooling process accounts for more than 60% of the entire molding cycle. Optimizing the cooling system can significantly shorten the cycle time and improve production efficiency.
The design principle of the cooling water channel is to make the cooling water evenly flow through all parts of the cavity, ensure uniform cooling of all parts of the product, avoid local overheating leading to product deformation and inconsistent shrinkage. The diameter, spacing and distance from the cavity surface of the water channel are key parameters. The larger the diameter of the water channel, the better the cooling effect, but it is limited by the mold structure. The spacing between water channels should be uniform to avoid uneven cooling. The distance between the water channel and the cavity surface should be appropriate. Too close will lead to uneven temperature on the cavity surface, and too far will reduce the cooling efficiency.
For large and complex molds, a partition cooling design can be adopted to divide the mold into multiple independent cooling zones, each with independent water inlet and outlet. By adjusting the water flow and temperature of different zones, precise temperature control of each part can be realized, and the cooling uniformity can be further improved. For parts with thick wall and slow heat dissipation, the density of water channels can be appropriately increased to strengthen cooling and balance the cooling speed of each part.
The design of the cooling system should also consider the water flow state. Turbulent flow has better heat exchange effect than laminar flow. Reasonable water channel diameter and flow rate design should ensure that the cooling water is in turbulent flow state in the pipeline to improve heat exchange efficiency. The water channel interface shall be designed in a unified standard to facilitate connection with the external cooling water system. At the same time, the sealing performance of the water channel shall be ensured to avoid water leakage.
High-quality molds are usually designed with cooling simulation through professional thermal analysis software in the design stage to predict the temperature field distribution and cooling effect, optimize the layout of water channels, and avoid problems found only after trial production, saving the time and cost of mold modification.
3.3 Exhaust System Design
Exhaust system is a very important but easily ignored part of blow molding molds. During the blowing process, the air originally existing in the mold cavity must be discharged quickly with the expansion of the parison. If the exhaust is not smooth, the air will be trapped in the cavity, resulting in incomplete product forming, surface dents, scorching marks and other defects, seriously affecting the appearance and quality of the product.
The most common exhaust structure is the exhaust groove arranged along the parting surface. The exhaust groove is usually processed on one side of the parting surface, with a depth of usually 0.05mm to 0.2mm and a width of 5mm to 20mm. The specific size depends on the viscosity of the plastic material. Materials with low viscosity need thinner exhaust grooves to avoid flash. In addition to the parting surface, exhaust plugs or exhaust inserts can also be set at the highest point of the cavity and the position where air is easy to be trapped to strengthen the exhaust effect.
For large molds or products with complex shapes, porous exhaust plates can be used at appropriate positions, or breathable steel can be used as local inserts to achieve better exhaust effect. The breathable steel has countless tiny pores inside, which can allow air to pass through but not molten plastic, and has very good exhaust effect. However, the cost of breathable steel is high and it is easy to be blocked after long-term use, so it needs regular maintenance.
When designing the exhaust system, the position and size of the exhaust should be reasonably determined according to the product shape and material characteristics to ensure that all air in the cavity can be discharged smoothly during the blowing process. At the same time, it should be ensured that the exhaust groove will not leave obvious traces on the product surface, affecting the appearance quality.
3.4 Flash Land and Trimming Structure
In the extrusion blow molding process, a certain amount of excess material will be formed at the parting line, called flash, which needs to be removed in the subsequent process. The flash land structure on the mold plays a role in clamping the parison and controlling the flash thickness, which directly affects the trimming quality and material utilization rate.
The flash land refers to the pressing surface at the edge of the cavity on the parting surface, with a certain width. When the mold is closed, the excess parison material is squeezed between the flash lands of the two halves of the mold to form a thin flash. The width and clearance of the flash land should be designed reasonably. Too wide will increase material waste, and too narrow will lead to insufficient clamping force and serious burrs on the product.
For products that require high appearance, the flash position can be designed with a shear edge structure to cut off the flash during mold closing, so that the product has almost no residual flash after demolding, reducing the workload of subsequent trimming. However, the shear edge structure has high requirements for mold processing accuracy and mold closing accuracy, and the cost is relatively high.
In addition, the bottom of the mold is usually designed with a tail material cutting structure to cut off the excess material at the bottom of the product. The cutting edge should have sufficient hardness and sharpness to ensure flat and burr-free incision. The design of these trimming structures directly affects the subsequent processing efficiency and finished product quality of the product, and is an important part of the mold design that cannot be ignored.
4. Mold Material Selection and Performance Comparison
4.1 Common Mold Steel Grades and Characteristics
The selection of mold material directly affects the service life, processing accuracy, maintenance cost and price of the mold. The main materials used for manufacturing blow molding molds include carbon structural steel, alloy structural steel, stainless steel, aluminum alloy, etc. Different materials have great differences in hardness, wear resistance, corrosion resistance, thermal conductivity and price.
Ordinary carbon steel, represented by 45# steel, is the most economical mold material. It has moderate strength, good machinability and low price, but its hardness and wear resistance are general, and it is easy to rust and deform. It is mostly used for low-demand trial molds, small-batch production molds or large non-stress-bearing parts of molds. The service life of 45# steel molds is usually between 100,000 and 300,000 shots, which is suitable for products with short life cycle and low quality requirements.
P20 pre-hardened plastic mold steel is the most widely used material for blow molding molds. It has good comprehensive performance, uniform hardness after pre-hardening treatment, good machinability and polishing performance, and certain wear resistance and corrosion resistance. The service life of P20 steel molds can reach 300,000 to 800,000 shots, which can meet the needs of most conventional mass production. The price is moderate, and the cost performance is very high. It is the preferred material for most packaging bottle molds and industrial barrel molds.
For higher requirements, S136 and other corrosion-resistant mold steels can be selected. This kind of steel has high chromium content, excellent corrosion resistance and polishing performance, can reach a very high mirror effect, and is suitable for producing high-gloss transparent products and products with high appearance requirements. At the same time, it can resist the corrosion of corrosive gases released by materials such as PVC, and the mold has a longer service life, which can reach more than 1 million shots. However, the price of S136 steel is much higher than that of P20, which is suitable for high-end product production with high quality requirements and long service life.
In addition, there are die steels such as Cr12MoV with higher hardness and better wear resistance, which are used for mold parts that are easy to wear, such as cutting edges, to extend the service life of vulnerable parts.
3.2 Aluminum Alloy Molds: Advantages and Limitations
Aluminum alloy is another common blow mold material, which is famous for its light weight and high thermal conductivity. The thermal conductivity of aluminum alloy is several times that of steel, so the cooling effect is very good, which can significantly shorten the molding cycle and improve production efficiency. At the same time, aluminum alloy molds are light in weight, easy to install and debug, and have low requirements for the bearing capacity of the machine template.
Aluminum alloy molds are easy to process and have short manufacturing cycles, which can shorten the product development cycle. They are very suitable for new product trial production and small-batch multi-variety production modes. The price of aluminum alloy molds is usually lower than that of alloy steel molds of the same specification, and the initial investment is small.
However, aluminum alloy molds also have obvious limitations. First of all, the hardness and wear resistance are low, and the surface is easy to scratch and wear. The service life is usually only tens of thousands to 200,000 shots, which is not suitable for long-term mass production. Secondly, the aluminum alloy has low strength, and is prone to collapse and deformation under large clamping force, so it is not suitable for large high-pressure blow molding molds. In addition, the surface of aluminum alloy mold is not easy to do texture etching and other treatments, and the surface treatment methods are limited.
Therefore, aluminum alloy molds are mostly used for product trial production, small-batch production, or products with low requirements for surface quality and service life. For long-term mass production, steel molds are still the more reliable choice.
4.3 Surface Treatment Process to Improve Mold Performance
On the basis of selecting the appropriate base material, the performance of the mold can be further improved through surface treatment, including improving surface hardness, wear resistance, corrosion resistance and demolding performance. Common surface treatment processes include nitriding, chrome plating, nickel plating, PVD coating and so on.
Nitriding treatment forms a high-hardness nitriding layer on the surface of the mold, which can significantly improve the surface hardness and wear resistance of the mold, and also improve the fatigue resistance. The nitriding layer has good bonding force with the base material and is not easy to fall off. It is a very cost-effective surface strengthening method. After nitriding treatment, the service life of P20 steel mold can be increased by more than 50%.
Chrome plating treatment forms a hard chromium layer on the mold surface, which improves hardness, wear resistance and corrosion resistance at the same time. The chrome-plated surface is smooth and bright, which can improve the surface gloss of the product and has a certain demolding effect. However, the chrome plating layer may peel off after long-term use, and the environmental protection problem of the chrome plating process also needs attention.
PVD physical vapor deposition coating is a more advanced surface treatment technology, which can form a layer of superhard coating such as TiN and TiCN on the mold surface. The coating has extremely high hardness and excellent wear resistance, and the friction coefficient is low, which has a good demolding effect. The PVD coating is thin and will not affect the dimensional accuracy of the mold. It is suitable for high-precision molds, but the treatment cost is relatively high.
When selecting surface treatment methods, comprehensive consideration should be given according to product material, quality requirements and production batch to achieve the best balance between performance and cost.
5. Matching Guide Between Apollo Blow Molding Machines and Molds
5.1 Matching Principles for Different Machine Series
Apollo produces a full range of extrusion blow molding machines, from small single-station machines to large multi-station high-speed machines, covering a wide range of applications. Different series of machines have different parameters and have different requirements for mold matching.
For small and medium-sized blow molding machines, which are mainly used for producing 10ml to 20L packaging bottles and daily necessities, the clamping force is usually between 5 tons and 50 tons. This type of machine is suitable for single-cavity or multi-cavity molds. When selecting multi-cavity molds, it is necessary to ensure that the total projected area of all cavities does not exceed the allowable range of clamping force, and the overall size of the mold meets the requirements of tie bar clearance. For high-speed double-station machines, the mold opening and closing speed is fast, so the weight of the mold should be controlled within a reasonable range to avoid excessive inertia affecting the operation stability.
For large blow molding machines, which are used for producing 50L to 200L industrial barrels, IBC ton barrels, auto parts, etc., the clamping force can reach hundreds of tons. This type of machine is usually equipped with single-cavity large molds. When designing the mold, the bearing capacity of the template should be fully considered, and the mold weight should be evenly distributed. At the same time, the mold should have sufficient rigidity to avoid deformation under high clamping force, resulting in flash and other problems. Large molds have high requirements for the cooling system, and should be matched with the cooling water circuit interface of the equipment to ensure sufficient cooling water flow.
For fully automatic multi-station rotary blow molding machines, the mold needs to rotate with the turntable, so the size and weight of the mold must strictly match the installation position and bearing capacity of the turntable. The positioning accuracy of the mold installation must be guaranteed to ensure accurate mold closing at each station. At the same time, the connection modes of air circuit and water circuit should be unified to realize automatic docking.
Apollo’s technical team can provide professional mold matching guidance for customers. Before customers make molds, they can provide detailed equipment installation dimensions and parameter specifications to guide customers’ mold design and ensure perfect matching between molds and equipment.
5.2 Price Range of Molds with Different Specifications
The price of blow molding molds varies greatly according to product size, complexity, material, number of cavities and precision requirements. The following is the price range of common types of blow molding molds for reference.
For small packaging bottle molds with a capacity of less than 1L, if made of 45# steel with single cavity and ordinary precision, the price is about 1,500 to 3,000 US dollars per set. If made of P20 steel with mirror polishing and 2 cavities, the price is about 4,000 to 7,000 US dollars per set. For 4-cavity high-speed production molds with high-precision thread and S136 material, the price can reach 8,000 to 15,000 US dollars per set.
For medium-sized product molds with a capacity of 5L to 30L, such as detergent barrels and chemical barrels, the single-cavity P20 steel mold is about 6,000 to 12,000 US dollars per set. If it is equipped with hydraulic core pulling and other mechanisms, the price will increase by 30% to 50% on the basis of the basic mold. For 20L standard chemical barrel molds with mature structure, the price is relatively transparent, usually about 8,000 to 10,000 US dollars per set.
For large product molds of 100L to 200L, such as 200L double-ring barrel molds, due to large size and high requirements for structural strength, the price of single-cavity P20 steel molds is about 25,000 to 45,000 US dollars per set. If it is made of higher-grade materials and has a cooling system with higher configuration, the price can reach 50,000 to 70,000 US dollars. For large molds with more complex structures such as automobile fuel tanks, the price is higher, usually more than 80,000 US dollars, depending on the specific structural complexity.
It should be noted that the above prices are only the basic mold body prices. If special surface treatment, hot runner system, automatic demolding mechanism and other configurations are added, additional costs are required. At the same time, the price level of different mold factories also varies greatly. It is recommended to comprehensively evaluate the technical strength, processing accuracy and after-sales service of the supplier, not just the price.
5.3 Return on Investment Analysis of High-Quality Molds
Although high-quality molds have a higher initial investment, they can bring more returns in long-term production. The following takes the common 1L detergent bottle production as an example to compare the investment returns of ordinary molds and high-quality molds.
Suppose a factory uses Apollo 4-station high-speed blow molding machine with 4-cavity mold, producing 1L HDPE detergent bottles, with an annual production time of 7000 hours. The price of ordinary 45# steel mold is 5,000 US dollars, the molding cycle is 12 seconds, the product qualification rate is 90%, and the mold service life is 200,000 shots. The price of high-quality P20 steel nitriding mold is 9,000 US dollars, the molding cycle is 10 seconds due to better cooling design, the product qualification rate is 98.5%, and the mold service life is 800,000 shots.
In terms of production efficiency, the ordinary mold produces 1200 pieces per hour, and the high-quality mold produces 1440 pieces per hour. The high-quality mold produces 240 more pieces per hour, and the annual output increases by 1,680,000 pieces. Calculated by a gross profit of 0.01 US dollars per piece, the annual profit increases by 16,800 US dollars.
In terms of raw material saving, the high-quality mold reduces the defective rate by 8.5%, saves about 119,000 pieces of raw materials every year, and saves about 5,950 US dollars in raw material costs calculated by 0.05 US dollars per piece of raw material cost.
In terms of mold replacement cost, the ordinary mold needs to be replaced 4 times a year, with an annual mold cost of 20,000 US dollars. The high-quality mold is replaced once a year, with an annual mold cost of 9,000 US dollars, saving 11,000 US dollars in mold cost every year.
Overall, using high-quality molds can increase annual comprehensive benefits by about 33,750 US dollars, while the initial investment difference is only 4,000 US dollars. The incremental investment can be recovered in less than 1.5 months, and the return on investment is very high. Therefore, for long-term mass production projects, choosing high-quality molds is a very cost-effective investment.
6. Common Mistakes to Avoid in Mold Procurement
6.1 Only Focus on Price and Ignore Quality Matching
The most common mistake in mold procurement is to only look at the price and choose the cheapest supplier, ignoring the processing quality, material authenticity and structural rationality of the mold. Low-cost molds usually cut corners in materials, heat treatment, processing technology and other aspects. The actual service life is far lower than expected, and various quality problems are prone to occur in the production process, resulting in high defective rate and frequent shutdown for maintenance.
Many enterprises only calculate the one-time purchase cost and do not calculate the losses caused by shutdown, defective products and frequent mold replacement. In fact, the losses caused by a set of low-quality molds in the production process are often much higher than the price saved when purchasing. When selecting mold suppliers, we should inspect their design capability, processing equipment, quality control system and past cases, and choose suppliers with good reputation and mature technology, rather than simply comparing prices.
6.2 Neglecting Matching with Existing Equipment
Another common mistake is to ignore the parameter matching with the existing blow molding machine when customizing molds. Many customers only provide product drawings to the mold factory and let the mold factory design independently, without providing the specific parameters of the equipment. As a result, the manufactured mold cannot be installed on the existing equipment, or cannot achieve the expected production effect after installation.
Common mismatches include that the mold size exceeds the tie bar clearance and cannot be installed, the mold mounting hole position does not match the template, the required clamping force exceeds the rated clamping force of the equipment, the mold cooling water interface does not match the equipment pipeline, the blowing interface does not match, etc. These problems will lead to the inability to use the mold normally, and the cost and time of rework and modification are very high.
Therefore, when customizing molds, be sure to provide the detailed parameter manual of the blow molding machine to the mold factory, including platen size, tie bar clearance, mounting hole size and position, clamping force, blowing air circuit interface, cooling water interface, etc. If necessary, you can let the equipment manufacturer participate in the review of the mold scheme to ensure the matching between the mold and the equipment. Apollo can provide free mold scheme review service for customers who purchase the company’s equipment to help customers avoid matching risks.
6.3 Overlooking Maintainability and After-sales Service
Many enterprises only pay attention to the initial performance of the mold when purchasing, and ignore the later maintainability and after-sales service. The mold will inevitably wear and fail in the process of use, and needs regular maintenance and repair. If the mold structure is unreasonable and difficult to disassemble and maintain, or the supplier cannot provide timely after-sales service and spare parts supply, it will cause long-term shutdown and bring great losses to production.
When selecting molds, we should consider the convenience of later maintenance, whether vulnerable parts are easy to replace, whether standard parts are general-purpose models, and so on. At the same time, we should choose mold suppliers with perfect after-sales service, which can provide timely maintenance, repair and modification services. For overseas customers, it is particularly important to choose suppliers with overseas service capabilities or mature technical support systems.
7. Daily Maintenance and Life Extension of Blow Molds
7.1 Daily Inspection and Regular Maintenance System
Establishing a scientific mold maintenance system is the key to extending the service life of the mold and ensuring stable product quality. The daily maintenance of the mold includes inspection before use, maintenance in use and maintenance after shutdown.
Before starting production every day, check the surface condition of the mold cavity, whether the cooling water channel is unobstructed, whether the guide post guide sleeve is well lubricated, and whether the fasteners are loose. After confirming that there is no abnormality, warm up the mold and then start formal production. During the production process, regularly check the product quality, and judge whether the mold is worn or damaged according to the flash condition, surface quality and dimensional change of the product. If abnormal conditions are found, stop the machine for inspection in time.
After the end of production, clean the surface of the mold cavity thoroughly to remove residual plastic and dirt, and then apply anti-rust oil to prevent the mold from rusting during storage. Close the cooling water valve and blow dry the residual water in the water channel to avoid rust inside the water channel. For molds that are out of service for a long time, they should be properly stored in a dry and ventilated environment, and regularly checked for rust.
Regular maintenance shall be carried out according to the production quantity. Generally, comprehensive maintenance shall be carried out after every 100,000 to 200,000 shots, including disassembly and cleaning of the mold, inspection of wear of each part, replacement of vulnerable parts, re-polishing of the cavity, etc. Regular maintenance can find hidden dangers in time and avoid sudden failures during production.
7.2 Treatment of Common Mold Failures
Some common faults will occur during the long-term use of blow molds. Mastering the correct treatment methods can reduce downtime losses. The most common faults include cavity wear, flash increase, product surface deterioration, cooling effect decline, etc.
Cavity surface wear is a normal phenomenon in long-term production, which will lead to the decline of product surface gloss and even obvious scratches. For slight wear, the surface can be re-polished to restore the finish. For serious wear or local damage, repair welding and then polishing can be used for repair. If the size has been out of tolerance due to serious wear, it is necessary to consider scrapping and remaking.
The increase of flash is usually caused by the wear of the parting surface or the decrease of clamping force. First, check whether the clamping force of the equipment is sufficient. If the equipment is normal, grind the parting surface of the mold to ensure close fit. If the flash land is seriously worn, repair welding or inlay treatment can be carried out on the flash land.
The decline of cooling effect is mostly caused by scaling and blockage of cooling water channels. Regularly use scale remover to clean the cooling water channels to keep the water channels unobstructed, which can restore the cooling effect. If the water channel leaks, find out the leakage point in time and repair it by welding or replacing the sealing ring.
