Utilization of Sprue Material for Hollow Blow Molded Products

01

Definition and source of sprue material  <<<<

First, we need to understand the specific meaning of the term "sprue" in blow molding. A sprue typically refers to the excess plastic portion that extends beyond the mold, bottle neck, and handle during the blow molding process. This excess plastic is removed after cooling, hence the name "sprue." Depending on the size and shape of the product, this portion typically accounts for 25-50% of the total weight of the preform.

The term "sprue material" usually includes sprue, defective products in the process, products used for machine adjustment, and blanks used for machine debugging.

During the product development stage, the proportion of sprues can be effectively reduced by controlling the distance between the blow needle, mold, and die; the weight of the sprues can also be reduced by adjusting the wall thickness of the sprue sections at both ends of the product.

How to utilize sprue material  <<<<<

The amount of sprue added depends on the product's function, stress conditions, and appearance requirements.

1. Reuse of the same product in production:

After proper processing (such as crushing and screening), sprue can be reused as part of the raw materials in the production of blow-molded products. This not only reduces production costs but also reduces the need for new raw materials.

It is important to note that the reuse ratio of sprue needs to be controlled within a certain range to avoid negatively impacting product performance. Generally speaking, excessive sprue may lead to problems such as the product becoming brittle and fragile.

2. Downgrade the use of sprue material:

When it cannot be used for high-quality or high-requirement products and can only be used as a downgraded sprue for other products:

(1) Classification and screening:

Sprue materials are classified according to material, color, shape, etc., in order to make better use of them.

By screening, impurities and substandard sprue materials are removed, ensuring the stable quality of sprue materials used in downgraded applications.

(2) Formula adjustment:

Adjust the raw material formula according to the requirements of the downgraded product to ensure that the performance and quality of the downgraded product meet the requirements.

Increase the proportion of sprue material in the formula appropriately to reduce production costs.

(3) Production process optimization:

By improving the production process, such as increasing the melt temperature and extending the melting time, it is ensured that the sprue material can be fully melted and mixed during the production process.

Precautions for using sprue material<<<<<

1. Quality Control:

When using sprue, its quality must be strictly controlled. This includes ensuring the purity of the sprue, avoiding contamination, and performing appropriate pretreatment (such as drying and impurity removal).

2. Proportional control:

During the production process, it is necessary to strictly control the mixing ratio of sprue material. Excessive sprue material may affect the performance and quality of the product.

The utilization ratio of sprue material should be reasonably determined based on the specific requirements of the product and the production process.

3. Storage and Management:

Sprue material should be stored away from direct sunlight and humid environments to prevent it from deteriorating or aging.

Establish a sound management system for sprue materials, including classified storage, regular cleaning, and statistics.

The significance of utilizing sprue material  <<<<<

1. Reduce costs:

By making reasonable use of sprue material, production costs can be reduced and the economic benefits of enterprises can be improved.

2. Conserve resources:

The reuse of sprue helps reduce the demand for new raw materials, thereby conserving natural resources.

3. Environmental protection and waste reduction:

By reducing waste emissions, the reuse of sprue material helps mitigate pollution and damage to the environment.

02

Methods to reduce sprue material  <<<<

1. Control material waste during use;

2. Control the recycling and reuse of materials from non-conforming products;

3. Control the stable weight of the product to vary within the specified minimum range.

Collection of sprue material  <<<<<

1. Sprue material generated during production debugging

During production debugging (especially the process of switching production and changing colors), the extruded or injected molten plastics may stick together after cooling, making recycling difficult. This is to avoid the inability to separate and break the plastics after cooling.

At this point, clean tools should be used to break the extruded and injected plastic into small pieces and place them on a clean iron rack to cool for a period of time before recycling. The hot-melt tube of the extrusion blow molding machine can be broken off from the die head using a defective "bottle" produced in real-time. Avoid using cotton gloves to pull the tube (to prevent cotton threads from sticking to the tube or debris from dirty gloves from transferring to the tube). It is recommended to quickly send the uncooled sprue (clean sprue material in various hot-melt states) generated from pulling, etc., to a real-time crusher for pulverization.

2. During normal production, the following principles shall be followed:

(1) The remaining material of the product after overflow is placed in the "edge material recycling turnover box" or directly conveyed online to the machine side for crushing and recycling;

(2) Products that are found to be non-material-related and continuously fail to meet standards during self-inspection shall be directly recycled.

(3) If the random inspection finds that the products are occasionally unqualified, it is necessary to confirm that the waste materials can be directly recycled and placed in the waste plastic basket;

(4) If non-conforming products are found in the next process, it is necessary to confirm whether they can be directly recycled or centrally processed.

Use of sprue material  <<<<<

After one or more cycles of high temperature, compression, expansion, and crushing, the physical properties of sprue material gradually decrease, such as impact resistance, rigidity, and adhesion. At the same time, the production process also becomes unstable, such as the uniformity of extrusion/injection blanks, quality changes, and mold expansion. Furthermore, the recycling process is prone to contamination, resulting in crystal points, black spots, and impurities (sand particles). In addition, most coloring pigments fade or decompose or disperse again after repeated high temperature compression, causing significant changes in product color. Therefore, the proper use of sprue material directly affects many aspects of product quality and requires strict control.

1. Usage ratio

In principle, all sprue material should be used for each product, but it must be added according to the predetermined formula ratio during mixing. Before changing the ratio, the impact on the production process and product quality must be considered, and relevant technicians and quality inspectors must be notified for process adjustments and quality testing.

2. Calculation Method

Plastics extruded due to machine adjustments during production and various reasons for defective products need to be processed and used. Under normal circumstances, it is advisable to use them according to the scrap rate of the product (about 3%).

For example, the weight of the product after removing the sprue is the weight of the sprue produced by the molding process. Dividing this weight by the molding process yields the normal sprue ratio.

Sprue weight / (Sprue + Product) weight = Sprue ratio + 3%

Note: Normal production means that the product weight meets the standard, the shape and size of the blank are appropriate, and the length of the tail and head scraps can meet the normal production control. Generally, the head scrap length is 20-25mm and the tail scrap length is 30-40mm.

Coloring sprue usage  <<<<<

Because hollow blow molding products are produced in many varieties and batches, different colors of sprue material are generated after each order is completed. Based on the recycling status, they can be roughly divided into three situations for handling.

1. Solid color material

Solid-color sprue material (of the same color, i.e., sprue material produced by this machine) from normal production processes is used directly according to the specified ratio.

2. Light-colored material

The color change cleaning process may produce a lot of sprue material. During the cleaning process (generally using a raw material cleaning machine of the same color, note that due to changes in extrusion parameters, carbonized substances that may remain in the screw and die may be extruded), the sprue material does not contain colorant (masterbatch) and needs to be separated, crushed and labeled. Colorant will be added according to the weight ratio when it is used next time.

3. Mixed colorants

Color matching and slab cutting will produce a small amount of colored sprue material of different colors; after the production conversion is completed (the crusher has been cleaned), a small amount of non-natural-colored products or unqualified products will become sprue material; a small amount of returned non-natural-colored products, etc. When they accumulate to a large extent, they can be crushed according to the three primary color principle (see the figure below) and then used in suitable products by color matching.

03

Key measures to reduce sprue material generation  <<<<<

The generation of sprue waste is closely related to product design, mold structure, and production process. Our company systematically reduces the proportion of sprue waste through the following methods:

1. Optimize mold design

A. Width of the overflow groove at the head and tail of the mold: The sprue material at the head and tail of a normal mold accounts for about 35% of the weight of the barrel. Optimize the width of the overflow groove of the mold so that the width of the overflow groove meets the thickness of the joint line between the top and bottom of the barrel, and reduce the other parts as much as possible. This can reduce the proportion of sprue material from 35% to about 30%.

B. Precision length/weight precise control: Photoelectric sensors can prevent the preform from being too long or too short, ensuring that the preform weight error is controlled within ±1%, and avoiding the generation of sprue material due to excessive preform.

2. Lightweight design of product structure

A. Lightweight design: By optimizing the layout of the reinforcing ribs in the barrel through finite element analysis, the average weight is reduced from 1500g to 1350g while ensuring stacking strength, and the amount of sprue material per barrel is reduced by about 10%.

B. Process adjustment of head and tail blank thickness: When formulating the product wall thickness curve, consciously reduce the thickness of the overflow part at the head and tail (provided that the thickness of the joint line is guaranteed).

Practical solutions for recycling and utilizing sprue material  <<<<<

The recycling of sprue feedstock needs to balance economic efficiency and quality stability. Our company adopts a strategy of "graded processing + scientific formulation":

1. Crushing of sprue material: Our company uses on-line crushing at the machine. The centrifugal crusher crushes the sprue material at about 100 degrees Celsius into 6-8mm particles, and the cyclone dust collector removes the powder to ensure particle uniformity.

2. Ingredient ratio and process control

A. Ratio of virgin to recycled materials: Classified application according to the intended use of the product.

For non-load-bearing components (such as bucket lids): add approximately 50% sprue material;

Load-bearing barrel body: Add 35%-40% sprue material (30% of the sprue material generated by the barrel itself + 5% crushed material from the inspection and scrapping barrels).

B. Production process adjustment: Appropriately increase the extruder temperature (+5℃) to compensate for the decrease in the fluidity of recycled material, and extend the cooling time by about 10% to avoid excessive product shrinkage caused by excessive sprue material.

3. Actual Results and Data:

A. Through the above measures, our company's comprehensive utilization rate of sprue material has increased from 98% to 99.5%, saving approximately RMB 1.2 million in raw material costs annually.

B. According to our quality control department, the finished drum with 40% recycled material passed the drop test (-20℃/1.8m) with a 100% pass rate, and its performance fully complies with GB/T 13508-2011 standard.

C. The proportion of sprue material has decreased from 40% seven years ago to 30% currently, saving a significant amount of energy.

Sprue material management and color matching principles  <<<<<

Mixed colors are the core problem restricting the high-value utilization of sprue material.

1. Management process for sprue material:

A. Ensure timely use of sprue material: Ensure that the sprue material on the production machine is used immediately to avoid accumulation. This can solve 90% of the sprue material problem.

B. Storage of sprue material: Dedicated silos are marked with color codes and raw material codes. They must never be mixed up, as this will cause serious quality accidents.

C. Plastic drums using sprue material should be subject to enhanced testing: Plastic drums using crushed material should be re-inspected using a colorimeter to ensure color consistency within the same batch.

2. Basic Principles of Color Matching

A. Prioritize materials of the same color family: Light-colored crushed sprue (such as white) should only be used for the production of light-colored products;

B. Dark colors covering light colors: Crushed sprue material can be mixed into dark blue or gray products in small amounts (≤5%);

C. Transition color control: The ratio of mixed pigments needs to be determined through small-scale testing. For example, a light gray bucket can be produced by using recycled materials in a 7:3 ratio of "white + blue" without the need to add additional color masterbatch.

04

Methods to reduce sprue material <<<<<

1. Product design stage:

Optimize product structure by simplifying shape and structure while ensuring product functionality, reducing complex connections, protrusions, and other areas prone to sprue material buildup. Accurately calculate preform dimensions and weight to ensure more rational material distribution during blow molding and reduce excess scrap.

2. Mold design and manufacturing:

Designing appropriate gate locations and quantities ensures uniform mold filling of the plastic melt, preventing excessive sprue material due to uneven flow. Improving mold manufacturing precision ensures uniform mold clearance, reducing flash and burrs.

3. Production process optimization:

Precise control of blow molding process parameters, such as temperature, pressure, and blowing time, ensures the preform is fully formed in the mold, reducing sprue buildup caused by process instability. Regular maintenance and upkeep of production equipment ensures stable operation and prevents production abnormalities and increased sprue buildup due to equipment malfunctions.

Precautions for adding sprue material to new product production  <<<<<

1. Crushing and recycling of sprue material:

Use appropriate crushing equipment (paying special attention to the material and spacing of the blades) to crush the sprue material into uniform granules for easier subsequent batching. Control the crushing time carefully during the crushing process to avoid over-crushing, which can generate dust that easily carbonizes and forms slag during plasticizing, and to prevent excessively small particles from affecting the proportions and product performance.

2. Ingredient ratio:

Based on product quality and performance requirements, a reasonable ingredient ratio is determined through experiments (if automatic ingredient dispensing equipment is used, the equipment needs to be calibrated regularly to avoid dispensing errors). Generally, the initial addition ratio can be controlled at 10%–20%, and then gradually adjusted based on product test results. For products with high performance requirements, the addition ratio can be appropriately reduced; for some products with relatively low performance requirements, the ratio can be appropriately increased.

3. Production process:

After adding sprue, it may be necessary to fine-tune the production process parameters, such as appropriately increasing the processing temperature to better integrate the sprue with the virgin material. Strengthening the mixing of the mixture is also crucial to ensure uniform mixing of the sprue and virgin material and to avoid localized performance differences.

Classification and Color Matching Principles of Different Colored Sprue Materials  <<<<<

1. Categorized Management:

Use dedicated storage containers of different colors to promptly collect sprue materials of different colors on the production floor. Clearly label the collected sprue materials with information such as color, source, batch, material, and weight for easy management and use later. Store them according to color, keeping the storage environment dry and well-ventilated to prevent moisture, aging, and other factors that could affect their performance.

2. Color matching principles:

Using white or transparent sprue as a base, small amounts of other color masterbatches can be added for color matching. Dark-colored sprue should be used as much as possible for producing dark-colored products, avoiding its use in light-colored products where color matching is difficult. Neutral colors, such as red and green, or blue and yellow, can be obtained by mixing complementary colors in certain proportions using the color wheel principle. In practice, a small-scale test should be conducted first, adjusting the proportions according to color deviations until a satisfactory result is achieved.