The screw is the base of the plasticizing unit that melts, mixes, and dispenses the polymer melt with precision into the mold. As an injection molding screw starts to wear out, problems may not happen immediately, but inefficiencies will eventually cause major issues.
If you’re looking for reasons why product quality is decreasing, shot sizes are unstable, or scrap is increasing, check your injection molding screw first. Detecting issues early will help you avoid downtime, excessive material cost, and barrel damage down the road.
This blog will go over five common signs that your screw needs replacement and why it’s crucial to service it on schedule. If you’re looking for custom screw replacements designed for long service life, Deer Machine is here to help you. Our injection molding screws are precision engineered for performance across all your industrial plastic molding applications.
What Are Injection Molding Screws?
Purpose and Function of Injection Molding Screws
Injection molding screws are used to convey, melt, and meter resin into the mold. It is done through three zones on the screw:
- Feed
- Compression
- Metering
Each zone is designed to optimize these functions while minimizing shear heat.
For that reason, most screws are three zones separated into conveying plastic pellets, melting the material, and mixing the melt to a uniform temperature. Wear to any zone will disturb this balance and hurt performance.
Injection molding screws are available in several materials and configurations. Most screws are alloy steel and have been nitrided or bimetallic coated to increase wear resistance. Specific screw materials are recommended based on the polymer being processed.
Common Materials and Designs
The injection molding screws can be made of nitrided or bimetallic coated alloy steels to ensure greater wear resistance. The screw material and surface treatment would also depend on the type of polymer the company is processing.
As an illustration, general-purpose screws can be suitable for polypropylene (PP) or polyethene (PE). Abrasive materials like glass-filled nylon or mineral-filled compounds, however, demand very wear-resistant screws that will ensure performance in the long run.
Special screws for improving melt homogeneity are also available. Among these screws, the barrier screws are widely preferred for high-speed processes because they isolate the solid bed from the molten polymer during the plasticizing process. This isolation improves the melting consistency and eliminates the effect of “un-melt,” which is normally encountered during high-speed processes with conventional screws.
Mixing screws are used for improving color mixing and additive mixing, making them suitable for processes requiring high precision and engineering grade material properties.
The impact of Wear on Performance.
Wear gradually increases the clearance between the screw flights and the barrel wall. The larger this gap, the more probable will be a backflow in the process of injection. It lowers the efficiency of plasticizing, poor pressure control and uneven plastic melt delivery.
And it is more than mechanical degradation; it has a direct effect on product quality, energy consumption, and speed of production.
5 Signs Need to Replace Your Injection Molding Screw
Sign #1: Product Quality is Declining
Losing part quality is always a major concern for injection molding users. When molded parts suddenly begin failing inspections, you should check your screw.
Surface Imperfections on Molded Parts
Are your parts coming out of the mold with burn marks, black specs, flow lines, or silver streaks? One of the most common causes of melt degradation is poor mixing from a worn screw.
As the screw wears, there can be dead spots of stagnant melt. If the polymer sits in these locations for too long, it can degrade before it even reaches the injection mold.
Dimensional Changes in Parts
Another easy way to tell if your screw needs to be replaced is if your parts start coming out of weight or their dimensions begin to change. Because of backflow during injection, excessive wear allows molten plastic to slip backward when pressure is applied.
Dimensional changes can lead to variation in wall thickness, shrink, and part geometry. It is obviously a problem if you pride yourself on tight tolerances and precise parts.
Sign #2: Material Throughput is Reduced
Among the most important signs of screw wear is seen in the compression zone, where the material is compressed and melted. This is where the screw wears down, resulting in increased flight depth. This wear will then cause a decrease in compression ratios.
This will then affect the compression ratios directly. Compression ratios are important in the generation of shear heat necessary for melting the material. However, when compression ratios fall below optimal levels, the screw will be unable to generate the sufficient pressure necessary for melting the material uniformly.
It causes unmelted granules or “cold slugs” in the melt, inconsistent shot weights, poor mixing of additives or colors, and surface defects on molded parts. These issues can be explained as insufficient compression ratios resulting in insufficient shear heating and forcing operators to increase barrel temperatures to compensate for insufficient shear heat. However, this can have negative effects on materials.
When these symptoms persist despite changes in processing parameters, it is a clear indication that screw compression zone wear has exceeded acceptable limits and needs replacement.
Sign #3: Increased Backflow and Losing Shot Size Consistency
The clearest technical sign of screw wear is excessive backflow when injecting material into the mold. Wear on the flight edges decreases the sealing ability of the screw.
Operators will usually notice this when they are struggling to get consistent shot weights between parts. You may even notice you have to increase injection pressure to reach proper filling volumes.
Continued backflow will degrade your part quality and can tax your machines’ hydraulic and clamping systems.
Sign #4: Rising Costs and Product Waste
Operating costs slowly creeping up? That may be due to a worn injection molding screw as well. Decreased melting efficiency forces the machine to increase temperatures and screw speeds.
Not only will this increase energy costs per cycle, but poor melt quality will increase scrap rates as well. That small percentage of scrap can add up on high-volume production machines.
Often times, simply replacing a worn screw can restore your machine’s efficiency and minimize energy use and wasted material.
Sign #5: You Can Visually See Wear or Measure it
This may seem like a no-brainer, but if you can measure the diameter of your screw and see that it has worn past specifications, it’s time for a replacement. Some screws will even have visible scoring or corrosion.
You should not wait until your screw has damaged the barrel to replace it, and continued use with that kind of wear will almost always lead to irreversible barrel damage.
Why Timely Screw Replacement Matters for Your Production
Preventive maintenance is one of the best ways to ensure your injection molding machine is operating at peak performance. Replacing your injection molding screw when it needs to be replaced can save you time, money, and headaches.
Proactive screw maintenance is not just good practice — it is a direct investment in the long-term reliability and profitability of your operation.
Damaged screws can hurt your part quality, waste material, and, if left long enough, damage the barrel. Partnering with a reliable screw manufacturer like Deer Machine will ensure your screws are made to match the exact specifications of your machine and the polymer you’re processing.
For more abrasive materials, we can even recommend custom screw designs to maximize your screw service life. If your business depends on keeping your downtime to a minimum, keeping screws on a maintenance schedule can drastically improve overall equipment efficiency.
Injection Molding Machine FAQs
What are signs of wear and tear on injection molding screws?
Fluctuations in shot size, longer cycle times, cosmetic defects, and changing injection pressures are good indications that your screw may be wearing out. To confirm your suspicions, you should physically inspect the screw and measure the diameter.
When should I service or replace my injection molding screw?
Screw wear should be inspected anytime you notice inconsistencies in your production. However, if you wait until your part quality is affected, you risk damaging the barrel as well. Once you identify that your screw is wearing but has not yet affected your parts, it is the ideal time to replace it.
How does a worn injection molding screw affect my plastic injection quality?
Decreased melting efficiency creates inconsistencies in melt temperature and pressure. As a result, you’ll see variations in part dimensions, cosmetic flaws on parts, and parts that are out of weight.
Can preventive maintenance help me extend the life of my injection molding screw?
Yes! Proper maintenance goes a long way when extending the life of your screw. Regular maintenance, proper purge practices, material selection, and processing parameters can keep your screws spinning for a long time.
How often should I inspect my injection molding screws?
That depends on the type of plastic you’re processing. If you process highly abrasive or filled materials, you should inspect your screws more frequently. A good rule of thumb is to inspect your screws during PM or after every recommended service interval per your machine manufacturer.
What happens if I continue to run my injection molding machine with a damaged/worn screw?
The immediate thing you will notice is an increase in scrap, loss of efficiency, and your energy bills going up. If the screw gets too damaged, you can even snap your screw off in the middle of production.
