Extrusion blow molding is one of the most widely used forming processes for hollow plastic products, covering industrial packaging barrels, daily chemical bottles, automotive hollow parts, household appliance shells and many other fields. With the continuous improvement of downstream industries’ requirements for product dimensional accuracy, appearance quality and assembly performance, post-molding shrinkage deformation has become one of the most common and troublesome quality problems in blow molding production. Uneven shrinkage will not only lead to out-of-tolerance product dimensions, surface depression and warpage, but also cause problems such as poor sealing, unstable stacking and failed assembly, which directly increase the scrap rate, rework cost and customer complaint rate of enterprises. For mass production lines, even a small increase in the defective rate caused by shrinkage deformation will bring huge economic losses throughout the year, and even affect the long-term cooperative relationship with brand customers.
Essentially, post-molding shrinkage deformation is the result of the combined effect of material inherent properties, equipment performance, process parameters, mold design and post-processing conditions. To fundamentally solve this problem, it is far from enough to adjust a single process parameter. It is necessary to carry out systematic optimization from the whole production chain. Among all influencing factors, the performance of blow molding equipment is the core foundation for achieving stable shrinkage control. High-precision extrusion blow molding machines can achieve stable control of melt quality, wall thickness distribution, cooling process and mold closing accuracy, minimizing the root cause of shrinkage deformation from the hardware level.
As a professional manufacturer of plastic molding equipment, Wanplas has been deeply engaged in the research and development of extrusion blow molding equipment for many years. Its full series of blow molding machines are optimized for dimensional stability and shrinkage control. With high-precision plasticizing extrusion system, multi-point wall thickness control technology, uniform cooling shaping system and intelligent closed-loop control, it can help customers effectively solve various shrinkage deformation problems and achieve long-term stable mass production of high-precision blow molded products. This article will systematically analyze the root causes of post-molding shrinkage deformation, explain the targeted solutions from the dimensions of equipment optimization, process adjustment, mold improvement and material adaptation, provide detailed cost-benefit analysis of equipment upgrading, and share the best practices of long-term stable quality control, providing a comprehensive reference for blow molding production enterprises to improve product quality and production efficiency.
1. Understanding Post-Molding Shrinkage Deformation in Blow Molding
1.1 Definition and Common Manifestations of Shrinkage Deformation
Post-molding shrinkage refers to the phenomenon that the volume and size of plastic products gradually decrease during the cooling process from high-temperature molten state to room temperature. Its essence is that the polymer molecular chains change from a stretched and disordered state at high temperature to a curled and ordered state after cooling, resulting in volume reduction. For most thermoplastics, shrinkage is an inherent characteristic that cannot be completely eliminated, but can be controlled and balanced through reasonable process and equipment design. When the shrinkage of different parts of the product is uneven or the overall shrinkage exceeds the design tolerance range, it will form shrinkage deformation.
The specific manifestations of shrinkage deformation vary according to product types and application scenarios. For plastic packaging barrels and buckets, the common problems are shrinkage of the barrel mouth, uneven bottom, out-of-tolerance overall height, and warpage of the barrel body, which will lead to poor sealing with the barrel lid, unstable stacking and even collapse. For small packaging bottles such as cosmetics and medicines, the main problems are surface depression, out-of-tolerance thread size, and bottle body warpage, which affect the appearance grade and assembly accuracy. For automotive hollow parts and industrial accessories, dimensional deviation and assembly dislocation are the main hazards, which will directly lead to the failure of the entire assembly line. In addition, problems such as shrinkage marks at the rib position, sink marks on the thick wall surface, and internal stress warpage are also common shrinkage defects in blow molding production.
1.2 Negative Impacts on Production and Product Performance
The harm of shrinkage deformation runs through the whole value chain from production to end use. First of all, it directly leads to an increase in the scrap rate. Products with serious dimensional deviation and appearance defects can only be scrapped and recycled, resulting in waste of raw materials, energy and labor. For high value-added products such as engineering plastic parts and high-end packaging bottles, the cost of single scrapping is higher. Secondly, it increases the rework cost. Products with slight deformation need manual shaping, heating correction and other rework treatments, which not only consumes a lot of labor, but also may cause secondary damage to the product surface.
Thirdly, it affects the functional performance of products. For example, the shrinkage deformation of chemical packaging barrels will reduce the sealing performance, leading to the risk of liquid leakage during transportation and storage, and even safety accidents. The shrinkage deformation of automotive parts will lead to difficult assembly and reduced structural strength, affecting the safety performance of the whole vehicle. Fourthly, it damages the brand image of enterprises. Products with unstable dimensional quality are difficult to enter the supply chain of high-end brands, and can only stay in the low-end market for price competition, with meager profits. Long-term quality problems will also lead to customer churn and affect the long-term development of enterprises.
1.3 Why Shrinkage Control Is Critical for Mass Production
For small-batch trial production, slight shrinkage deviation can be solved by manual selection and rework, but for large-scale continuous mass production, stable shrinkage control is directly related to the profitability of the project. In the automated production mode, the dimensional consistency of products is the premise of supporting downstream automated assembly and packaging. Any batch of products with abnormal shrinkage will lead to the suspension of the entire downstream production line, causing huge chain losses.
At the same time, stable shrinkage control is also the basis for enterprises to reduce unit production cost. By accurately controlling the shrinkage rate, the wall thickness design can be optimized to the greatest extent on the premise of meeting the strength requirements, reducing raw material consumption per unit product. In the increasingly competitive plastic processing industry, the cost saved by accurate shrinkage control per unit product may be the main source of profit difference between enterprises. Therefore, mastering scientific shrinkage deformation control technology is not only a quality improvement measure, but also an inevitable requirement for enterprises to enhance their core competitiveness and achieve large-scale profitable production.
2. Root Causes of Post-Molding Shrinkage Deformation
To solve the problem of shrinkage deformation, we must first clarify its deep-seated causes. The formation of shrinkage deformation is not caused by a single factor, but the result of the joint action of multiple links such as materials, equipment, processes and molds. Only by finding the root cause can we take targeted measures to achieve twice the result with half the effort.
2.1 Inherent Shrinkage Characteristics of Polymer Materials
The inherent shrinkage of polymer materials is the internal cause of post-molding shrinkage. Different types of plastics have different molecular structures and crystallinity, and their shrinkage rates vary greatly. For example, high-density polyethylene, as the most commonly used blow molding material, has a typical shrinkage rate of 1.5% to 3% due to high crystallinity. Polypropylene has a shrinkage rate of 1.2% to 2.5%, and also has obvious crystallization shrinkage. Amorphous materials such as PETG and PVC have much lower shrinkage rates, usually between 0.2% and 1.5%, and the shrinkage is more uniform.
Even for the same type of material, different brands, melt indices and filler contents will lead to differences in shrinkage. Adding glass fiber, mineral powder and other fillers can significantly reduce the shrinkage rate, but if the filler is unevenly dispersed, it will lead to uneven shrinkage and increase the risk of warpage. The moisture and volatile content in the raw material will also affect the shrinkage effect. Excessive moisture will vaporize at high temperature to form bubbles inside the product, leading to local shrinkage depression after cooling. In addition, the thermal degradation of materials during processing will also change the molecular weight distribution, resulting in changes in shrinkage characteristics.
2.2 Improper Melt Plasticizing and Extrusion Stability
The uniformity of melt plasticizing directly affects the consistency of shrinkage. If the screw design of the extruder is unreasonable or the temperature control accuracy is poor, the melt will have uneven temperature and viscosity. The parts with high temperature have large shrinkage after cooling, and the parts with low temperature have small shrinkage, which will eventually lead to uneven shrinkage of the product. In severe cases, there will even be unmelted solid particles, resulting in local abnormal shrinkage and surface defects.
Unstable extrusion speed will lead to fluctuations in parison weight and wall thickness, resulting in differences in cooling speed and shrinkage degree of each product. The sagging effect of the parison during the extrusion process will lead to large differences in wall thickness between the upper and lower parts of the product. The thin wall part cools quickly and has small shrinkage, while the thick wall part cools slowly and has large shrinkage. This difference in wall thickness is one of the most important causes of warpage deformation. In addition, excessive shear heat generated during the extrusion process will lead to local overheating of the melt, increase the shrinkage after cooling, and even cause material degradation, which will further aggravate the instability of shrinkage.
2.3 Insufficient Blow Pressure and Holding Time
Blow pressure and holding pressure are important factors affecting the dimensional accuracy of products. In the blow molding stage, sufficient air pressure can make the parison closely fit the inner wall of the mold cavity, ensure that the product shape completely matches the mold cavity, and improve the dimensional reproduction accuracy. If the blow pressure is insufficient, the parison cannot fully fit the mold, resulting in unclear product outline, low surface gloss and large shrinkage deviation.
The holding stage after blow molding is the key to compensate for shrinkage. During the cooling process, the volume of the material will continue to shrink with the decrease of temperature. Sufficient holding pressure can make the melt always in a compressed state, supplement the volume gap caused by cooling shrinkage, and avoid surface depression and size reduction. If the holding time is too short or the holding pressure is insufficient, the material will shrink freely after cooling, resulting in large overall shrinkage, and thick wall parts are particularly prone to sink marks and internal shrinkage holes. For products with uneven wall thickness, insufficient pressure holding will also amplify the difference in shrinkage of each part, leading to warpage deformation.
2.4 Uneven Cooling and Internal Stress Accumulation
Uneven cooling is the most direct external cause of shrinkage deformation. The blow molding product cools from the outside to the inside. If the cooling speed of different parts is different, the shrinkage progress will be inconsistent, resulting in internal stress inside the product. After demolding, the internal stress will gradually release, leading to warpage and size change of the product. For example, if the cooling speed of the two halves of the mold is different, the side with fast cooling will shrink first and form compressive stress, while the side with slow cooling will form tensile stress, and the product will bend to the side with fast cooling after demolding.
Insufficient cooling time is another common problem. In order to pursue production efficiency, many enterprises shorten the cooling time, resulting in the product not being fully shaped when demolding. The internal temperature is still high, and the subsequent natural cooling process will produce greater shrinkage and deformation. At the same time, the product in the high-temperature soft state is prone to deformation under its own weight or external force. In addition, only relying on mold cooling and lack of internal cooling will lead to large temperature difference between the inside and outside of the product, long internal cooling time, and large post-shrinkage after demolding, which is particularly obvious for thick-walled products.
2.5 Unreasonable Mold Design and Structure
Mold is the forming carrier of products, and its design quality directly determines the shrinkage state of products. If the shrinkage rate is not accurately calculated in the mold design and the corresponding size pre-compensation is not carried out, the overall size of the finished product will be too small after cooling shrinkage, which cannot meet the tolerance requirements. For products with complex shapes, if the shrinkage characteristics of different parts are not considered respectively, the size deviation of each part will be inconsistent, resulting in shape distortion.
Unreasonable design of cooling water channel is an important cause of uneven cooling. If the water channel is far away from the cavity surface, the cooling efficiency is low, and the local temperature difference is large; if the water channel is unevenly distributed, the cooling speed of each part will be different, resulting in uneven shrinkage. Poor exhaust of the mold will lead to air trapping in the cavity, and the local parison cannot be blown in place, forming surface depression and size deviation. In addition, insufficient mold rigidity will produce micro deformation under the action of clamping force and blow pressure, resulting in product size deviation, which is often mistaken for shrinkage deformation.
2.6 Post-Molding Handling and Storage Conditions
The environment and mode of product after demolding will also affect the final shrinkage deformation. If the product is directly stacked randomly after demolding without cooling and shaping, it is easy to produce irreversible deformation under the action of its own weight and external extrusion. Especially for products with high demolding temperature, the deformation is more likely to occur. If the storage environment temperature is too high or directly exposed to the sun, the product will soften and produce secondary shrinkage and deformation.
Products with large internal stress will slowly release stress during long-term storage, resulting in gradual size change and warpage, which is called post-shrinkage phenomenon. For products with strict dimensional requirements, if no annealing treatment is carried out to eliminate internal stress, even if the delivery inspection is qualified, deformation may occur during transportation and storage, resulting in customer complaints. In addition, unreasonable stacking mode, such as too high stacking layers and uneven stress, will also lead to long-term creep deformation of the lower products.
3. Equipment Optimization: Core Solution from Wanplas Blow Molding Machines
To fundamentally solve the problem of shrinkage deformation, we must start with the equipment hardware to ensure the stability of the whole extrusion blow molding process. Wanplas series extrusion blow molding machines are designed with dimensional stability as the core goal. Through multi-dimensional optimization of extrusion system, wall thickness control, mold clamping system, cooling system and control system, it provides a solid hardware foundation for solving shrinkage deformation.
3.1 High-Precision Extrusion System for Uniform Melt Plasticizing
Uniform and stable melt quality is the premise of consistent shrinkage. Wanplas blow molding machine is equipped with a specially optimized barrier type screw, which is designed for common blow molding materials such as HDPE and PP. The unique barrier structure can realize gradual melting of materials, ensure sufficient plasticization and uniform mixing, and effectively control shear heat generation, avoiding local overheating leading to material degradation. The multi-stage mixing element makes the melt temperature and viscosity more uniform, and the temperature difference of the melt in the radial direction is controlled within ±1℃, which eliminates the shrinkage difference caused by uneven plasticization from the source.
The extruder adopts multi-zone independent PID temperature control system. Each section of the barrel and die head is equipped with independent heating and cooling units. The temperature control accuracy reaches ±0.5℃, which can accurately set the temperature curve according to the material processing requirements and maintain long-term stability. The servo drive system ensures stable screw speed with fluctuation less than 1%, and the extrusion output per unit time is highly consistent, ensuring the uniformity of parison weight and wall thickness. Stable extrusion performance makes the shrinkage rate of each product highly consistent, which greatly reduces the dimensional fluctuation between batches.
3.2 Multi-Point Axial Wall Thickness Control System
Uneven wall thickness is the primary cause of uneven shrinkage and warpage. Wanplas blow molding machine is equipped with 100-point axial wall thickness control system as standard. The high-precision servo oil cylinder drives the die core to move, and can adjust the die gap in real time during the parison extrusion process, so as to accurately control the wall thickness distribution of the product. According to the structural characteristics and shrinkage law of the product, the wall thickness of each part can be precisely adjusted. Appropriately increase the wall thickness at the parts with large shrinkage and fast cooling, and appropriately reduce the wall thickness at the parts with small shrinkage, so as to balance the shrinkage speed and shrinkage of each part and reduce the risk of warpage deformation.
For the parison sagging problem caused by gravity, the wall thickness control system can also compensate the sagging, so that the wall thickness of the upper and lower parts of the product is uniform, avoiding the problem of large bottom shrinkage and small mouth shrinkage caused by different wall thickness. The system supports storage of hundreds of groups of wall thickness curves. When changing products, you can call them with one key, which greatly shortens the debugging time and ensures the stability of product quality. Accurate wall thickness control can not only improve dimensional stability, but also save raw materials to the greatest extent on the premise of meeting strength requirements, reducing production costs.
3.3 High-Rigidity Clamping System with Stable Mold Closing
Stable and uniform mold closing force is the guarantee of product dimensional accuracy. Wanplas clamping system adopts high-rigidity frame structure, and the template is processed as a whole with high parallelism accuracy, which can ensure uniform distribution of clamping force on the whole mold surface, avoid local mold expansion and size deviation caused by insufficient clamping force. The double toggle clamping mechanism has the characteristics of stable operation, fast speed and large clamping force amplification ratio. The positioning accuracy of mold opening and closing is high, and the repeatability error is less than 0.1mm, ensuring that each mold closing state is consistent.
The clamping force adopts closed-loop control, which can be accurately adjusted according to product size and process requirements to avoid mold deformation caused by excessive clamping force or flash caused by too small clamping force. For large-scale blow molding products, the direct pressure clamping structure can be adopted, with more uniform clamping force and higher stability. The high-rigidity clamping system ensures that the mold cavity size remains stable during the whole blow molding and pressure maintaining process, and the product can accurately reproduce the mold cavity size, laying a good foundation for subsequent shrinkage control.
3.4 Optimized Cooling System for Uniform Temperature Field
Efficient and uniform cooling is the key to reducing shrinkage deformation. Wanplas blow molding machine is equipped with a constant temperature circulating cooling system as standard. The high-precision industrial chiller accurately controls the cooling water temperature with a fluctuation of less than ±1℃, ensuring the stability of mold temperature during long-term continuous production. The cooling pipeline adopts standardized interface design, which can be perfectly connected with the mold cooling water channel to ensure sufficient water flow and uniform heat exchange.
For products with high dimensional requirements, an internal air cooling system can be added. During the cooling process, low-temperature compressed air is continuously introduced into the product to cool the inner wall at the same time, realizing synchronous cooling inside and outside, reducing the temperature difference between inside and outside of the product, shortening the overall cooling time, and greatly reducing the post-shrinkage and internal stress after demolding. For large and thick-walled products, a multi-zone cooling control scheme can also be configured to independently control the cooling water temperature of different zones of the mold, actively adjust the cooling speed of each part, balance the shrinkage rate, and reduce warpage deformation caused by different cooling speeds.
3.5 Intelligent Closed-Loop Process Control System
Wanplas blow molding machine adopts PLC + touch screen intelligent control system, which integrates all process parameter control and has perfect closed-loop regulation function. The system monitors all key parameters such as melt temperature, melt pressure, screw speed, clamping force, blowing pressure and cooling time in real time. When affected by external factors such as ambient temperature fluctuation and power supply voltage fluctuation, the system can automatically compensate and adjust to ensure the stability of process conditions.
The system supports storage of hundreds of sets of production formulas. All process parameters can be saved with one click. When changing products, you only need to call the corresponding formulas, avoiding quality fluctuation caused by manual debugging errors. The production data recording function can save all process parameters and quality data in real time, which is convenient for quality traceability and process optimization. The perfect alarm mechanism can send out sound and light alarm in time when parameters are abnormal, avoiding batch defective products. The intelligent control system greatly reduces the dependence on operator experience and ensures the stability of product shrinkage rate during long-term mass production.
4. Process Parameter Tuning to Reduce Shrinkage Deformation
On the basis of excellent equipment performance, scientific process parameter debugging can further optimize the shrinkage control effect. For different materials and product structures, the process parameters should be adjusted to achieve the best balance between production efficiency and dimensional stability.
4.1 Melt Temperature Optimization Strategy
Melt temperature is the core parameter affecting shrinkage rate. Within the normal processing temperature range, appropriately reducing the melt temperature can reduce the thermal movement amplitude of molecules, reduce the volume shrinkage after cooling, and also reduce the internal stress of products. However, the temperature should not be too low, otherwise it will lead to insufficient plasticization, uneven material and greater shrinkage fluctuation. The specific temperature setting should be based on the material grade and product characteristics, and the optimal temperature point should be found through experiments.
The temperature of each section of the barrel and die head should be set reasonably to form a gradient temperature curve to ensure that the material is gradually heated and plasticized, avoiding local overheating. The die head temperature should be kept uniform to avoid temperature difference in the circumferential direction, which will lead to uneven parison wall thickness and inconsistent shrinkage. For materials sensitive to temperature, it is necessary to strictly control the temperature fluctuation range and avoid long-time stay of materials in the barrel at high temperature, so as to prevent thermal degradation from changing the shrinkage characteristics. When changing raw material batches, the process temperature should be fine-tuned in time according to the melt index and shrinkage rate of the new batch to maintain stable product size.
4.2 Blow Pressure and Holding Pressure Adjustment
Appropriately increasing the blow pressure can make the parison fit the mold cavity more closely, improve the dimensional accuracy and surface finish, and at the same time enhance the heat transfer effect between the product and the mold, speed up the cooling speed, and help reduce the shrinkage difference. However, the pressure should not be too high, otherwise it will increase the burden of the clamping system and even lead to mold expansion. The specific pressure value should be determined according to the product size, wall thickness and material type.
The holding pressure and holding time have the most direct impact on compensating shrinkage. After the parison is blown into shape, maintaining a certain pressure during the cooling process can force the material to fill the shrinkage space in time when the material is cooled and shrunk, and avoid surface depression and size reduction. For thick-walled products, it is necessary to extend the holding time appropriately to ensure that the thick-walled parts are fully shaped. The step-by-step pressure holding process can also be adopted. The pressure gradually decreases with the cooling process, which can not only compensate for shrinkage, but also reduce internal stress and reduce the risk of post-shrinkage deformation.
4.3 Cooling Cycle and Temperature Control
Cooling time is a parameter that needs to be weighed between production efficiency and dimensional stability. Sufficient cooling time can ensure that the product is fully shaped before demolding, and the post-shrinkage after demolding is small and the size is stable. Too short cooling time will lead to high product temperature when demolding, large post-shrinkage and easy deformation. However, too long cooling time will reduce production efficiency and increase unit cost. The best cooling time should be the shortest time on the premise of ensuring that the product size meets the standard and there is no obvious post-deformation.
Cooling water temperature also has an important effect on shrinkage. Appropriately reducing the water temperature can speed up cooling, but too low water temperature will increase the temperature difference between inside and outside of the product, resulting in greater internal stress and increasing the risk of later warpage. For crystalline materials, too fast cooling will lead to insufficient crystallization, and post-crystallization will occur in the later storage process, resulting in secondary shrinkage. Therefore, the cooling water temperature should be set reasonably according to the material characteristics and product requirements. In addition, it is necessary to regularly clean the cooling water channel of the mold to remove scale and impurities, ensure smooth water flow and stable cooling efficiency, and avoid the decline of cooling capacity affecting product size stability.
4.4 Extrusion Speed and Parison Control
Extrusion speed affects the temperature uniformity and wall thickness consistency of the parison. Appropriate increase of extrusion speed can shorten the parison extrusion time, reduce the temperature drop of the parison during suspension, and make the temperature of the upper and lower parts of the parison more uniform, which is conducive to consistent shrinkage. However, too fast extrusion speed will increase shear heat, lead to melt temperature rise, increase shrinkage, and even cause material degradation.
At the same time, the extrusion speed should be matched with the wall thickness control curve to compensate the parison sag caused by gravity. By optimizing the wall thickness curve, the wall thickness deviation caused by sag is minimized, so that the axial wall thickness of the product is uniform, the cooling speed is consistent, and the shrinkage is balanced. For multi-cavity molds, it is also necessary to ensure that the discharge of each die head is uniform, and the weight deviation of each parison is controlled within a small range to ensure the consistency of shrinkage of each cavity product.
4.5 Post-Molding Sizing and Annealing Treatment
For products with high dimensional accuracy requirements, shaping treatment after demolding can effectively reduce post-shrinkage deformation. Use special shaping fixture to clamp the product when it is just demolded and still has a certain temperature, and maintain a fixed size during the cooling process, which can forcefully limit the shrinkage deformation of the product and obtain more accurate size. The shaping time is usually equivalent to the cooling time in the mold, which can greatly reduce the post-shrinkage rate and improve the dimensional stability.
For products with large internal stress or high long-term dimensional stability requirements, annealing treatment can be carried out. Place the product in a constant temperature environment, keep it warm for a certain time, and then cool it slowly to room temperature, so that the internal stress can be fully released, the molecular structure is more stable, and the deformation in the later storage and use process is greatly reduced. The annealing temperature is usually set between the glass transition temperature and the thermal deformation temperature of the material, and the holding time is determined according to the wall thickness of the product. Annealing treatment will increase the process and cost, but it is very effective for improving the long-term dimensional stability of engineering plastic products.
5. Mold Design Improvements for Dimensional Stability
Mold is the direct carrier of product forming. Reasonable mold design can fundamentally reduce the difficulty of shrinkage control and achieve twice the result with half the effort. Optimizing the mold from the aspects of shrinkage compensation, cooling system, exhaust and structure is an important part of solving shrinkage deformation.
5.1 Shrinkage Rate Pre-Compensation Design
Accurate shrinkage rate compensation is the primary premise of mold design. According to the selected material type, product wall thickness and process conditions, combined with empirical data and CAE simulation analysis, the actual shrinkage rate of the product is predicted, and the corresponding amplification is carried out on the mold cavity size to ensure that the product size after cooling shrinkage is just within the tolerance range. For products with complex structures, different shrinkage compensation values should be adopted for different directions and different parts, rather than a single amplification coefficient.
For example, the axial shrinkage and radial shrinkage of cylindrical products are different, and the shrinkage of thick-walled parts and thin-walled parts are also different. It is necessary to adjust them respectively. For products with high precision requirements, it is recommended to make a test mold first, measure the actual shrinkage rate through trial production, and then modify the formal mold according to the measured data, so as to avoid the problem that the size is out of tolerance after the formal mold is completed, resulting in rework loss. With the help of professional blow molding simulation software, the shrinkage and deformation of the product can be predicted more accurately, the number of mold modifications can be reduced, and the development cycle can be shortened.
5.2 Optimized Cooling Channel Layout
The design of cooling water channel directly determines the uniformity of cooling. The principle of water channel design is to be close to the cavity surface, evenly distributed, and ensure that the cooling speed of each part of the product is consistent. The distance between the water channel and the cavity surface should be appropriate. Too far will reduce the cooling efficiency, and too close will lead to uneven surface temperature. The spacing between water channels should be uniform to avoid local cooling dead corners.
For products with complex shapes, conformal cooling water channels can be used, which change with the shape of the cavity to ensure consistent distance from the cavity surface and more uniform cooling. For thick-walled parts, the density of water channels can be appropriately increased to strengthen cooling and balance the cooling speed with thin-walled parts. Large molds can adopt partition cooling design. Each partition has independent water inlet and outlet, and the water flow and temperature can be adjusted respectively to actively control the cooling speed of each part, balance the shrinkage rate, and reduce warpage deformation. In addition, the diameter and flow rate of the water channel should be reasonably designed to ensure that the cooling water is in a turbulent state and improve the heat exchange efficiency.
5.3 Exhaust System and Venting Optimization
Good exhaust can ensure that the parison is fully blown and close to the mold cavity, which is very important to ensure the dimensional accuracy and surface quality. If the exhaust is not smooth and the air in the cavity cannot be discharged in time, it will be compressed between the parison and the mold wall, resulting in local blow molding dissatisfaction, surface depression and size deviation, which is often misjudged as shrinkage deformation.
Exhaust grooves are usually set on the parting surface, and the depth and width are designed according to the viscosity of the material, so as to ensure smooth exhaust without producing flash. For the parts where air is easy to be trapped at the bottom and top of the product, exhaust plugs or breathable steel inserts can be added to strengthen local exhaust. Regularly clean the exhaust groove and exhaust plug during production to avoid blockage by material debris, which will lead to the decline of exhaust effect. Good exhaust can not only reduce surface defects, but also improve the fitting degree between parison and mold, improve cooling efficiency and dimensional stability.
5.4 Wall Thickness and Structural Optimization
From the perspective of product structure design, trying to make the wall thickness uniform is the most fundamental way to reduce uneven shrinkage and warpage. Under the condition of meeting the strength and use requirements, try to avoid sudden change of wall thickness. The transition between thick wall and thin wall should be smooth to reduce the shrinkage difference caused by different wall thickness. For the position where reinforcing ribs need to be set, the thickness of ribs should be controlled to avoid too thick ribs leading to sink marks on the back.
Reasonable setting of rounded corners and transition structures can reduce stress concentration and shrinkage unevenness. For large flat products, appropriate micro arc design can be adopted to offset the influence of shrinkage warpage. Good product structure design can greatly reduce the difficulty of later process adjustment, improve the qualified rate of products at one time, and also help to shorten the cooling cycle and improve production efficiency. In the product development stage, the equipment and mold technical team should participate in advance to optimize the product structure from the perspective of blow molding process, which can achieve twice the result with half the effort.
6. Targeted Solutions for Common Blow Molding Materials
Different materials have different shrinkage mechanisms and characteristics, and the solutions are also different. Targeted process and equipment configuration should be adopted according to the material type to achieve the best shrinkage control effect.
6.1 HDPE Products: Balancing Shrinkage and Stiffness
HDPE is the most widely used blow molding material. It has high crystallinity and large shrinkage, and the shrinkage is mainly caused by crystallization. To control the shrinkage of HDPE products, we should first control the crystallization process. Appropriately reducing the melt temperature and mold temperature can speed up the cooling rate, refine the crystal grains, and reduce the total shrinkage to a certain extent. However, too fast cooling will lead to low crystallinity, and post-crystallization will occur in the later stage, resulting in large post-shrinkage.
Therefore, for HDPE products with high long-term dimensional stability requirements, the mold temperature should be appropriately increased to slow down the cooling, so that the crystallization is more sufficient in the mold, reduce the post-crystallization after demolding, and improve the dimensional stability. At the same time, increase the holding pressure and holding time to compensate for the volume shrinkage caused by crystallization. Wanplas extrusion system can accurately control the melt temperature and shear strength, stabilize the crystallization state of HDPE, and ensure the consistency of shrinkage rate between batches. With multi-point wall thickness control, the wall thickness distribution can be optimized, and the shrinkage difference of each part can be balanced.
6.2 PP Products: Reducing Warpage and Low-Temperature Deformation
PP material also has high crystallinity, large shrinkage, and is very sensitive to cooling uniformity. It is very prone to warpage deformation due to uneven cooling. To solve the shrinkage deformation of PP products, the primary task is to ensure uniform cooling on both sides of the mold and consistent cooling speed, so as to avoid warpage caused by asymmetric shrinkage on both sides.
Appropriately reduce the processing temperature, reduce the molecular orientation and internal stress, and help reduce warpage. Extend the cooling time appropriately, fully shape in the mold, and reduce post-shrinkage deformation. Adding nucleating agent to the formula can refine crystal grains, improve dimensional stability and surface gloss, but it is necessary to ensure uniform dispersion of nucleating agent. Wanplas multi-zone independent temperature control system can accurately control the temperature of each section of the die and each cooling zone of the mold, reduce the temperature difference, and effectively reduce the warpage of PP products. The high-rigidity clamping system ensures uniform mold closing and avoids size deviation caused by uneven stress.
6.3 PETG / PET Products: Controlling Shrinkage and Clarity
PETG and PET are amorphous or low crystallinity materials with low shrinkage and good transparency. They are mostly used in high-end packaging bottles and containers. Such materials are sensitive to temperature and are prone to thermal degradation and hydrolysis, resulting in bubbles and haze, which will also affect the dimensional stability.
To produce PETG products, the raw materials must be fully dried to control the moisture content below the standard, so as to avoid bubbles and voids caused by hydrolysis, which will lead to local shrinkage and depression. Strictly control the processing temperature, avoid excessive temperature leading to material degradation, and reduce the generation of volatile matter. Strengthen the exhaust system to discharge volatile gas in time. The cooling should be uniform and moderate to maintain high transparency and dimensional stability. Wanplas exhaust optimized design and precise low-temperature plasticizing system are very suitable for processing PETG and other heat-sensitive materials, which can not only ensure product transparency, but also effectively control shrinkage and improve dimensional accuracy.
6.4 PVC Products: Minimizing Thermal Degradation Shrinkage
PVC has poor thermal stability and is easy to decompose when overheated, releasing corrosive gas, and degradation will lead to changes in material properties and unstable shrinkage. Therefore, the core of PVC shrinkage control is to avoid thermal degradation. Low temperature plasticizing process should be adopted to control the processing temperature within a reasonable range and avoid local overheating.
The screw should be specially designed for PVC, with low shear and good plasticizing effect, to reduce shear heat generation and avoid material degradation. The temperature control system should have high precision to avoid temperature fluctuation. The mold and screw should be made of corrosion-resistant materials to prevent the impact of corrosion on the dimensional accuracy. Wanplas provides a special PVC blow molding configuration. The low-shear special screw and precise temperature control system can ensure uniform plasticization of PVC materials without degradation, and the shrinkage rate is stable and controllable. It is also equipped with anti-corrosion treatment to extend the service life of the equipment.
7. Cost-Benefit Analysis of Shrinkage Control Upgrade
Many enterprises worry that upgrading equipment and processes to improve shrinkage control will increase costs. In fact, solving the problem of shrinkage deformation can bring rich economic returns, and the investment payback period is very short.
Take a medium-sized double-station blow molding production line producing 20L HDPE chemical barrels as an example to calculate the loss caused by uncontrolled shrinkage. Suppose the production line produces 9000 finished barrels per day, the original scrap rate caused by shrinkage deformation is 5%, and the rework rate is 8%. The raw material cost of each barrel is about 1.2 US dollars, and the manual rework cost is 0.1 US dollars per piece.
The daily scrap loss is 9000 * 5% * 1.2 = 540 US dollars, and the daily rework loss is 9000 * 8% * 0.1 = 72 US dollars. Calculated by 300 working days per year, the annual direct loss caused by shrinkage deformation is (540 + 72) * 300 = 183,600 US dollars. This does not include the increased management cost, customer return loss, order reduction and other hidden losses caused by quality problems. For high value-added products such as automotive parts and high-end cosmetic bottles, the loss caused by shrinkage deformation is even greater.
