Effective Scrap Reduction Techniques in Mold Manufacturing

Table of Contents

In a proper injection molding setup parts produced are of high quality with no defects and zero scrap. Nonetheless this is normally not the case. Scrap is produced from various factors such as machine issues, improper screw design or part weight variations. Reducing scrap to zero may pose a challenge but you can use various techniques to minimize part defect

To promote sustainability in the manufacturing process, reducing scrap materials should be considered greatly.in addition cost reduction and better product quality is achieved through scrap reduction.

In the injection molding process, it is important to strive to limit waste and repurpose it whenever possible. To get a better understanding, in this article we will look into scrap reduction methods in injection molding.

Scrap Reduction 3

Plastic Injection Molding Scrap (Image Source: Shutterstock) 

Key Tips for Reducing Scrap

To reduce scrap in injection molding various various techniques are used. Below, we’ll look at the major techniques for scrap reduction. 

1. Preventive Use of Purging Compounds

During the production process residue piles up after material and color change leading to inefficiency. These factors impact various application which include extrusion, injection molding,blown film and extrusion blow molding 

On finished parts these issues are portrayed as streaks, spots, and discoloration. They result in higher scrap rates, more downtime and lower quality of finished parts. Through the use of preventive use of purging compounds  you can minimize these issues to reduce scrap.

Utilizing advanced purging compounds for preventive maintenance, ensures high-performance and high-quality characteristics remain constant.

The time and material required to clean a machine from color and polymer contamination or material degradation can greatly be reduced through the use of high quality purging compounds. This enhances efficient production process and long term financial gains. 

Through preventive maintenance using advanced purging compounds smooth transition between incompatible polymers and polymers with a wide processing temperature difference is achieved. Furthermore, they can be used to address contamination issues when used to transition to clear or transparent polymers and when changing color. This is necessary for hot runner molds and injection molding applications

2. Optimal Machine Shutdown Procedures

When proper shutdown procedures are followed, they can help reduce scrap in your injection molding process. Below are tips on how you should go about it.

  • Run out the screw: Close the hopper slide gate and discontinue the flow of material being fed into the feed throat. Reduce the screw speed to 15-20 rpm to avoid damage to contact surfaces of the screw and barrel. 
  • Rust protection: Add rust protection while the mold and machine are cooling. 
  • The purge: During machine shutdown remove unwanted resin from the barrel and screw unit by performing a surge to prevent resin from hardening  and clogging the barrel during startup..
  • Remove rust preventive:  Safely remove the coating of rust preventive by using a mold cleaner formulation.
  • Cleaning the mold: Use a flushing action with the mold cleaner by heavily applying it at the top of the mold cavity and core while allowing the cleaning solvents to flush down the face of the mold. 
  • Additional cleaning: Should any resin or gas deposits remain, or if you find color stains on the mold surface, additional mold cleaning is required.
  • Rust protection for storage: After all unwanted contaminants are removed, liberally apply a fresh coating of rust preventive to the mold cavity, platens, and mold base. 

3. Immediate Purging After Production

Purging technically means cleaning the inside of your extruder’s screw, barrel, and nozzle, by extruding a certain purging material. The material pushes the residue of the previously used polymer and cleans the inside of the machine, ready to be used again later. 

After the production process, purging is important to drive out all the processed material that remains inside the barrel. This process helps avoid resin accumulation, polymer degradation and ensure good machine condition.

Heat is the most primary factor for the extruder machine. After use if the polymer is left in the machine, it can result in the materials polymer chains breaking down which is a form of degradation..

Resin can accumulate when the extruder’s barrel and screw are not purged properly after use. This can also result in the flow of material to stagnate and contaminate your filament leading to scrap.

The accumulation of old material sticks to the extruders, screws, blocking the flow of new material. As a result, polymer degradation will occur because the material’s residence time is too long thus proper purging should be performed to avoid these issues. 

Scrap Reduction 1

Purging Process in Injection Molding (Image Source: Polyplastics)

4. Orderly Start-Up of Machines

Starting a shutdown injection molding machine should be sequential and be orderly to reduce scrap. Below are all the necessary steps you need to follow.  

  • Examine the molds: Inspect your molds for any signs of staining, oxidation or other unwanted contamination. If these issues are identified, remove the mold for cleaning.  
  • Testing ejector pins and plates: Test the ejector pins or the ejector plates for adequate lubrication to prevent seizing or galling of moving parts. 
  • Placing the mold: Place the mold in the machine and connect the water lines after cleaning and prepping the mold for production. 
  • Purge any remaining resin: It is important to purge the barrel and screw to remove unwanted ensuring only the correct color or resin will be molded.
  • Starting the run: When the desired temperature levels are met, add resin to the barrel and it will begin to run through the mold. Once a steady flow of polymer begins to exit the die or nozzle, you can increase the screw speed to the desired operating speed and parts can begin to be molded.
  • Removing parts from the mold: If you notice a difficulty with parts being removed from the mold, a mold release formulation may need to be applied. 

Strategies for Scrap Reduction in Injection Molding

Plastic manufacturing produces waste in the form of scraps.various strategies can be used by manufacturers to reduce scrap waste. Below are some strategies used.

Optimize Manufacturing Processes

This can be achieved through analyzing and streamlining production processes to reduce energy and material consumption, improve productivity and minimize waste. Manufacturers can reduce scrap, cost of production and resources through ensuring effective practices are incorporated in the production process.

To make this strategy effective, incorporating lean principles in the production process is important. To improve production efficiency and reduce waste automation and other other advanced technologies can be used.

Invest in Sustainable Technologies

To reduce the environmental impact of production, manufacturers can invest in sustainable technologies by using materials, processes and products that are environment friendly. These materials should be renewable, recyclable and biodegradable. In addition the use of energy conserving and efficient technology such as solar energy should be utilized.

Investing in sustainable technologies is significant in reducing carbon emission and water pollution caused during production. Additionally sustainable technology can help streamline production processes that need less energy and other resources. Through using sustainable technologies,you can reduce your waste output and provide an efficient production process that is suitable for the environment.

Enhancing Injection Molding Quality

To ensure that the final product is of high quality, it is crucial to maintain the proper melt temperature and pressure in your plastic injection molding process. Melt temperature is the temperature at which plastic material turns into a molten state. Pressure variation in injection molding is the pressure difference between the injection unit and mold cavity. 

Constant monitoring of these process parameters is important.  This is achieved through the use of sensors and softwares that monitor the temperature and pressure data in real time. This enables detection of deviation and the necessary adjustments are made.

Another factor that is essential in improving injection molding efficiency is controlling process and viscosity variation. Process variation is the changes in process parameters such as temperature and pressure. Viscosity variation on the other hand refers to changes in plastic resin flow behavior which could impact mold filling and part production quality.

To control process variation and viscosity variations, you need to implement process monitoring and control systems. This could include sensors and software that continuously adjust parameters based on collected data.

Scrap Reduction 2

Injection molding scrap (Image Source: Shutterstock) 


In the injection molding process scrap consideration is very important. To reduce scrap in manufacturing, you can use several techniques such as following proper machine shutdown procedures and the preventive use of purging compound.

Other strategies that can be used are, performing purging after production, and optimizing the manufacturing process. Investing in sustainable technologies also greatly contributes to reducing scrap. Through following these methods and strategies optimal production of high quality parts is achieved with minimal material usage and scrap.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

Contact Our Experts

Send us a Email, we will feedback to you ASAP!