How to Achieve Rapid Color Change

01

Principles of Cleaning and Color Changing in Hollow Blow Molding Machines  <<<<

1. Principle of High Efficiency: While ensuring thorough cleaning, the time for cleaning and color change should be shortened as much as possible. This not only reduces production interruption time and improves equipment efficiency, but also effectively reduces economic losses caused by equipment downtime.

2. Principle of Thoroughness: Cleaning and color changing must be comprehensive and thorough to ensure that the old colorant is completely removed and to prevent contamination of the new colorant. This ensures color consistency in the product, so that the products produced after cleaning and color changing are free of impurities, black spots, and other quality defects, meeting quality standards.

3. Cost control principle: While ensuring the cleaning and color change effect, it is necessary to reasonably control the color change cost, and minimize the input of new raw materials, labor and electricity, so as to achieve the minimum cleaning and color change cost.

1. First, squeeze out as much of the remaining old color material as possible from the extruder barrel. Then, use compressed air to blow away any remaining plastic particles and color impurities from the extruder barrel (storage hopper) to effectively reduce the amount of residual material in the barrel. When using compressed air for cleaning, ensure that the equipment and its surrounding environment are free from contamination. In cleanrooms, industrial vacuum cleaners can be used for cleaning.

2. Selection of Transition Material for Color Change. Before the formal color change, transition material can be added to clean the equipment. Transition material is generally white or low-value mixed-color crushed material; some low-density polyethylene can also be added initially. This material is then fully plasticized and extruded within the equipment to remove most of the residual old color material. When the extruded color is similar to the new color material to be replaced, the equipment is thoroughly cleaned with the new material until completely clean.

3. During start-up cleaning and color change operations, the heating temperature of each section of the die head can be increased by 5-10℃ compared to the normal process temperature, especially at the connection between the extruder and the die head, and at the die. For storage-type hollow blow molding machines, increasing the die head temperature by 10-15℃ during cleaning and color change will significantly improve the color change effect.

4. For large multi-layer hollow blow molding machine heads that are difficult to change colors, a combination of methods can be used during the cleaning and color-changing process.

First, a transition material containing a small amount of moisture, such as crushed recycled plastic, is used. These materials generate numerous bubbles during the melting process inside the accumulator head, increasing internal pressure and effectively shortening the cleaning and color-changing time. Subsequently, approximately 5% PPA masterbatch is added to the transition material for deep cleaning, which better removes pigment buildup in the runner and die, significantly shortening color-changing time and also providing some screw cleaning functionality.

5. Screw cleaner is a chemical cleaning agent specifically designed for extruder barrels and screws. It is suitable for deep cleaning of carbides and organic pigment agglomerations within the barrel. During chemical cleaning and color change processes, the usage time and dosage of the cleaning agent must be strictly controlled to prevent damage to the equipment. Therefore, long-term reliance on chemical cleaning methods is not recommended.

1. Equipment defects make it difficult to clean residual material. Specifically, dead corners in the barrel and screw channels, excessively high feed section temperature, unreasonable die head flow channels, insufficient pressure, and poor surface gloss of the die can all lead to the accumulation of plastic raw materials during normal production. This not only prolongs the cleaning and color-changing time and increases the cost of color change, but also makes it difficult to clean thoroughly, thus affecting the purity of the new color. It is recommended to contact the equipment manufacturer to optimize the equipment structure and improve areas with material dead corners to reduce the accumulation of plastic raw materials in the equipment.

2. Some equipment has a filter screen designed at the front end of the extruder. This location is usually where masterbatch (pigment) and impurities are most likely to accumulate, and relying solely on transition material during color changes is often insufficient for thorough cleaning. Therefore, it is recommended to select equipment equipped with a quick-change screen device when ordering, so as to quickly replace the filter screen without disassembling the die head, which can effectively reduce color change problems caused by filter clogging.

3. Storage-type die heads are generally designed with overflow holes to facilitate the discharge of carbon deposits generated by friction between the pressure ring and the material cylinder. Regularly check the overflow hole for blockages to prevent obstruction and ensure proper carbon deposit discharge. During cleaning and color changing, injecting an appropriate amount of lubricant into the overflow hole and increasing the pressure ring stroke can promote the discharge of material retained on the inner wall of the material cylinder, thereby shortening the cleaning and color changing time.

Direct extrusion die heads are usually designed with a material escape hole at the junction of raw materials. This hole is opened during cleaning and color changing to quickly remove the weld line formed by the old and new color materials, thereby achieving efficient color changing.

4. The cleaning and color-changing time for multi-layer extrusion heads is typically more than twice that of single-layer extrusion heads, while the cleaning and color-changing time for accumulator-type extrusion heads is generally more than three times that of direct extrusion heads. The efficiency of cleaning and color-changing is determined by selecting the appropriate extrusion method based on the product manufacturing process requirements. Therefore, when selecting equipment, both production needs and cleaning and color-changing efficiency must be comprehensively considered to achieve optimal production benefits.

5. Raw material conveying, scrap recycling, and auxiliary equipment are crucial components of a closed-loop production system for blow molding machines. During the cleaning and color-changing process, ease of operation must be fully considered to ensure rapid cleaning and efficient operation, which is essential for reducing color contamination and guaranteeing product quality.

A reasonable raw material transportation plan should be developed to minimize the distance of colored raw material transportation pipelines, reducing their bends and length to lower the amount of raw material residue in the pipelines. During edge material recycling, a dedicated machine or a one-color-one-use model can be adopted to minimize the frequency of cleaning and color changing of the crusher and related auxiliary equipment.

1. Personnel Management

(1) Develop work instructions, specifying the standard operating procedures and related precautions for the equipment cleaning and color changing process, and have experienced operators provide on-site guidance.

(2) Strengthen on-site training, covering equipment principles, color change process, selection and treatment of cleaning materials, clarify the responsibilities of each position, and improve the efficiency and quality of cleaning and color change operations.

(3) Strengthen safety supervision, provide safety knowledge training to operators, ensure that personal protective measures are in place, and ensure the safe operation of equipment.

2. Equipment Management

(1) Before cleaning and color changing, conduct a comprehensive inspection of the blow molding machine and its supporting equipment to confirm whether they meet the conditions for cleaning and color changing. Pay special attention to checking whether the temperature of the heating components of the equipment is appropriate to ensure that the equipment can operate normally, thereby avoiding malfunctions and unnecessary waste.

(2) During the color change process, the equipment must be kept clean, and effective protective measures must be taken to prevent any damage. After the color change is completed, be sure to clean up the area thoroughly to prepare for the next production run.

(3) Standardize equipment status management, set up labels on the equipment to clearly mark the equipment's operation, maintenance, standby status information, and establish color change records to facilitate analysis and optimization of cleaning and color change processes and improve equipment maintenance efficiency.

3. Materials Management

(1) Strictly control the cleanliness of the transition material to prevent foreign matter from entering, so as not to affect the cleaning and color change and the normal operation of the equipment. When the transition material and cleaning material are reused, they must be properly stored to prevent contamination.

(2) Strengthen material recycling and processing, and classify and recycle residual raw materials and cleaning waste. Recyclable raw materials should be crushed and granulated for reuse to achieve resource recycling. Non-recyclable waste should be treated in accordance with environmental protection requirements to avoid environmental pollution.

02

1. Prevention is key: In production planning, try to produce batches of products with similar colors or good compatibility adjacently to reduce cleaning difficulties caused by large color differences. For example, produce light blue buckets first, then transition to dark blue buckets, and finally produce black buckets to avoid dark colors staining light colors.

 2. High-efficiency cleaning: First, understand the composition and chemical compatibility of the corresponding color masterbatch, and then select appropriate cleaning materials, cleaning agents and cleaning tools to ensure that the old color materials remaining in the equipment can be removed quickly and thoroughly without damaging the equipment.

 3. Standardize Operations: Develop a "Quick Color Change Operation Guide," clearly defining the steps, tools, and acceptance criteria. Operators must strictly follow the "Quick Color Change Operation Guide" to ensure that each step is performed correctly, minimizing extended cleaning time due to operational errors.

Before cleaning, we need to understand the reasons for the difficulty in changing colors, or the location of residual old color materials. These locations are mainly in the following areas: the feed pipes, dead corners inside the barrel and screw, and dead corners in the flow channels of the die head storage cylinder.

1. Use compressed air to clean the inside of the pipes, especially the dead corners.

2. For products that undergo minor color changes (such as light blue to dark blue), the raw material mixture of this product can be used for cleaning in principle. The cleaned raw materials can be further crushed and reused to reduce waste caused by raw material contamination.

3. For products undergoing color changes (from black to light blue or off-white), specialized cleaning materials are required. For example, when our company changes from black to white, we use specialized cleaning materials that are compatible with both the equipment and raw materials (such as a specialized coupling agent + titanium dioxide). Adding the cleaning material to the equipment and allowing it to flow freely within the screw and barrel removes residual old color materials, effectively shortening the color change time.

4. In terms of process, temperature and speed can be increased: Within permissible ranges, appropriately increasing the barrel temperature and screw speed can improve the flowability of the raw material and facilitate discharge. However, it is important to control the temperature and speed to prevent overheating and decomposition of the raw material. For example, when changing colors in the production of polyethylene products, the barrel temperature can be increased by 10-20℃ and the screw speed by 10%-20%.

5. Segmented cleaning: The entire color-changing process is divided into several stages, and cleaning is carried out step by step. For example, a rough wash is performed first, using a large amount of cleaning material to initially remove most of the old color material; then a fine wash is performed, using a small amount of cleaning material to remove the remaining fine color impurities.

 6. Equipment: For older equipment that is particularly difficult to clean, it is recommended to use specialized equipment to produce dark-colored or natural-colored products, if conditions permit. Alternatively, choose newly developed equipment such as continuous extrusion equipment (without a storage tank) to reduce dead zones in the equipment's flow channels. Regularly maintain the equipment, cleaning accumulated material and scale from components such as the screw and barrel, to ensure better material flow and cleaning during color changes.

03

(I) Structural Design of the Screw

Generally speaking, when the material, heat treatment, and surface finish are guaranteed during screw manufacturing, the screw itself has a relatively fast material and color change speed. When designing the screw structure, considering the plasticizing output and mixing effect required for normal extrusion production, we optimize the screw structure and appropriately increase the screw extrusion pressure. This allows the melt to enter the die channel and produce a stronger squeezing and rinsing effect on residual old materials that easily adhere to the inner wall of the channel, thereby accelerating the color change speed. Simultaneously, it also improves the melt density and plasticizing quality.

(II) Structural Design of the Die Head Flow Channel

For improving color-changing efficiency in the blow molding machine industry, whether it's a storage-type or continuous extrusion machine, the internal flow channel structure design of the die head is paramount. One of the core design elements is achieving pressure balance throughout the entire flow channel, eliminating dead zones. In flow channels with pressure imbalances, color changing often occurs rapidly in areas of high pressure and slowly in areas of low pressure, especially at the point where the melt enters and converges, easily forming blanks. Most areas complete color changing in a short time, leaving only one or two symmetrical areas requiring a longer or more complete color change. We can use software such as fluid finite element analysis to rationally design the flow channel structure, optimizing and balancing the pressure of the melt at various points within the flow channel, avoiding dead zones. Simultaneously, it's crucial to avoid excessively high flow channel pressure, which can cause the melt to overheat as it passes through the channel. Due to the die head's heat dissipation method (generally natural cooling), a continuous, constant temperature rise can actually cause melt degradation and adhesion to the flow channel surface, having the opposite effect.

Secondly, when selecting the flow channel material for the die head, factors such as heat resistance, thermal stability, and mirror finish should be comprehensively considered. Materials with good heat resistance and thermal stability can maintain stable performance over long periods at high temperatures, exhibiting minimal thermal deformation and preventing dimensional deformation due to temperature changes. In particular, inconsistent transition dimensions at flow channel component connections can create misaligned or uneven surfaces, affecting the stability and efficiency of the color-changing process. The surface finish of the flow channel is crucial to the machinability of the selected material. Materials with good mirror finish can make the flow channel surface smoother, contributing to improved overall efficiency in the color-changing process. Especially when producing corrosive raw materials, corrosion-resistant flow channel materials must be selected, or the screw and flow channel surfaces must undergo special processes such as chrome plating or titanium plating to prevent these raw materials from corroding the smooth flow channel surface, thus affecting stable production and color changing.

(III) Optimization of extrusion die temperature control method and rational layout of heating sections

During equipment preheating, a one-time heating method often causes oxidation and yellowing of the material surface in the flow channel closest to the heater, resulting in poor fluidity and adhesion to the flow channel surface. This causes streaks on the extruded billet surface, affecting the surface smoothness of the product. By optimizing the equipment's heating control method, and using a discontinuous and gradual heating control process for different heating zones of the screw and die head at different heating time periods according to the plasticizing temperature corresponding to different raw materials, the material remaining in the screw and die head flow channels can be gradually and evenly heated from the outside to the inside to reach a melt state ready for operation. Secondly, the design and layout of the number of heating stages in the die head and the power of the heater must also follow the principle of reasonable number of stages and uniform heating power; otherwise, it will also have an adverse effect on color changing efficiency.

Based on feedback from various product manufacturers, the main factors causing difficulties in color changing, under the constraints of the extrusion screw and die flow channel structure, are as follows:

(a) Influence of color masterbatch and raw material factors

Different manufacturers' color masterbatch formulations and resin grades used as the main raw materials have different characteristics in terms of flowability, compatibility, and high-temperature decomposition coefficient, which will also affect the efficiency of color change. Under the premise of ensuring the color of the product and the performance of the container, trying to change the color masterbatch and raw materials with suitable melting point, good compatibility and less decomposition can significantly help shorten the color change time.

(ii) Adjustment of temperature and extrusion pressure during color change

When changing colors, the extrusion pressure can be increased by appropriately lowering the screw temperature and increasing the screw speed, while also appropriately increasing the temperature of each area of the die head. This allows the screw to extrude the lower-temperature melt and apply greater pressure to the relatively higher-temperature melt in the die head flow channel, creating a hard extrusion and flushing state, which can effectively shorten the color change time.

(III) Reasonable arrangement of changing different colors

Manufacturing companies can rationally arrange the production of color-change orders based on the color characteristics of their products and order delivery timelines. It is recommended to gradually arrange the production of transitional order colors from dark to light or from light to dark, which can simultaneously meet order requirements and reduce color-change costs. Changing from dark to light colors immediately often takes a long time, resulting in significant waste of raw materials, labor, time, and electricity.

(iv) Avoid prolonged heat preservation or standby time.

When the equipment is preheated and kept at a production temperature, it should be started up in time to avoid the melt remaining in the screw and die for a long time. If it is necessary to stop the machine during normal production due to various factors, try to shorten the standby time. If the actual standby time is long, the temperature of each heating zone can be appropriately reduced first. When production is resumed, the temperature can be increased to the normal production temperature. The machine can be started up again after the temperature is reached.

04

1. Extrusion system and die head flow channel design

The extrusion system and die head flow channel design of an EBM blow molding machine play a crucial role in color change efficiency. A well-designed flow channel reduces the residence time of the extruded material within the die head, preventing material buildup in dead zones and thus reducing cleaning difficulty and time during color changes. For example, the internal flow channel design of the die head should be free of dead zones to ensure smooth material flow and prevent residue buildup. This design not only helps improve color change speed but also reduces product color contamination caused by residual material.

2. Gradient Design and Fluid Dynamics Principles

Multi-layer co-extrusion EBM blow molding machines utilize a gradient design with multiple dies to effectively improve the self-cleaning properties of the runner. The runner cross-section gradually transitions from wide to narrow, leveraging material flow inertia to reduce residue. By optimizing the shape and size of the runner, uniform back pressure is created during the flow of the rubber compound, reducing material retention within the runner due to uneven pressure. Furthermore, the die head runner structure should conform to fluid mechanics principles to ensure balanced back pressure during extrusion. For example, employing a spiral runner, or a composite runner design tailored to different application scenarios, enables the rubber compound to achieve a stable flow state within the runner, reducing turbulence and eddies, thereby improving both fluid stability and color-changing efficiency.

3. Advantages of center-feed type feed head

Center-feed die heads are a key design feature for achieving rapid color changes. Compared to traditional side-feed methods, center-feed die heads distribute the adhesive more evenly throughout the flow channel, improving flow channel balance and reducing adhesive buildup caused by uneven feeding. This design not only increases color change speed but also ensures product quality stability. Yaqi has been fully implementing center-feed die head designs since 2005, improving color change efficiency by over 50%. For example, dark color changes take less than 20 minutes, while transitions from extremely dark to light colors typically take about 60 minutes.

In recent years, with the increasing maturity of headstock flow channel simulation design and analysis software, and especially with the continuous improvement of machining equipment and processing technology, the level of flow channel design and process technology has been continuously improved.

4. Surface treatment of flow channels and application of new materials

In recent years, flow channel surface coating technology has been widely used. By coating the flow channel surface with a special material, the coefficient of friction between the adhesive and the inner wall of the flow channel can be significantly reduced, thus decreasing the residence time of the adhesive within the flow channel. This coating technology not only improves color-changing efficiency but also extends the service life of the die head and reduces equipment maintenance costs. For example, nanoscale non-stick coatings (such as ceramic coatings or polytetrafluoroethylene) are used. Compared to traditional electroplating processes, this greatly reduces melt adhesion and makes delamination less likely.

Modular flow channel components are used. Physical cleaning is achieved through quick-disassembly flow channel modules, reducing downtime.

The dispersibility and flowability of color masterbatch directly affect color change efficiency. If low-cost formulations are pursued blindly, it may lead to color masterbatch aggregation and poor migration, which will increase the difficulty of cleaning.

1. Improvements in decentralization and liquidity

The dispersibility and flowability of color masterbatches are crucial factors affecting color-changing efficiency. Optimizing the color masterbatch formulation and increasing pigment dispersibility allows for more uniform pigment distribution within the compound, reducing color-changing difficulties caused by pigment aggregation. Simultaneously, improving the flowability of the color masterbatch reduces resistance during extrusion, minimizing color-changing time delays due to compound retention in the runner. The following measures are provided for reference:

Optimize carrier resin compatibility. The color masterbatch carrier needs to have high compatibility with the substrate (PE/PP). It is recommended to choose a low molecular weight resin with a melt index similar to that of the substrate to shorten the melting time. For example, use a low-viscosity PP carrier for PP substrates to avoid uneven dispersion due to poor compatibility.

Applications of dispersants and lubricants. Superdispersants (such as modified fatty acid esters): reduce the surface energy of masterbatch particles and prevent agglomeration.  

Synergistic effect of internal and external lubricants. Internal lubricants (such as zinc stearate) reduce friction within the melt, while external lubricants (such as silicone) improve the slippage of the melt against the runner wall and accelerate the removal of residual adhesive.

High-concentration masterbatch and nanotechnology. Using nanoscale pigments (such as carbon black dispersed to below 50nm) can improve tinting strength while reducing the amount of masterbatch added, thus lowering the risk of residue during color changes. However, attention must be paid to preventing the agglomeration of nanoparticles.

2. Formula adjustments after increasing extrusion volume (This is an important point that deserves attention).

In multi-cavity high-speed blow molding applications, the masterbatch formulation needs to be adjusted accordingly as the extrusion volume increases. To meet the demands of high production volumes, the proportion of dispersants and lubricants in the masterbatch formulation should be appropriately increased to improve pigment dispersibility and compound flowability. When adjusting the formulation, color-changing efficiency should not be sacrificed simply to reduce costs. For example, some low-cost dispersants may lead to uneven pigment dispersion, thus increasing the difficulty of color changing. Therefore, a balance between cost and color-changing efficiency should be comprehensively considered during the formulation optimization process.

Even with optimized equipment and color masterbatch, operator skills and the selection of auxiliary tools remain crucial.

1. Systematic preparation before color change  

Phased shutdown: First switch to the transition color, then gradually transition to the target color to reduce the color difference range;  

Pre-cleaning process: Use a low-melting-point cleaning material (such as high-flow PP) to fill the flow channels and remove residual adhesive.

2. Innovative Applications of Cleaning Agents and Foaming Agents

Chemical cleaning agents: Special cleaning materials containing surfactants can dissolve carbides, but care must be taken to avoid their corrosiveness to equipment;  

Foaming agent: Add micro-foaming agent (such as sodium bicarbonate) to the cleaning material to use expansion pressure to flush out dead corners of the flow channel.

3. Data-driven operation management  

Establish a database of color-changing parameters to record the optimal combination of temperature, pressure, and cleaning agent dosage for different colors and materials, thereby reducing human trial-and-error time.

Rapid color changeover for EBM blow molding machines relies on three key elements: equipment design as the foundation, raw material color formulation as the link, and standardized operation as the key. Only through the synergy of these three elements can efficiency be maximized. In the future, with the maturation of technologies such as intelligent temperature control, self-cleaning coatings, and new material formulations, color changeover cycles are expected to be further shortened to the minute level, helping companies reduce costs and increase efficiency.