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Quali sono le cause dello stampaggio a iniezione a stampa corta? Principali ragioni e soluzioni

Esplora le cause e le soluzioni dello stampaggio a iniezione a stampaggio corto, un difetto in cui le parti sono riempite in modo incompleto. Impara le tecniche per prevenire e affrontare questo problema.
HITOP / Blog / <a href="https://hitopindustrial.com/it/short-shot-injection-molding/" title="Quali sono le cause dello stampaggio a iniezione a stampa corta? Principali ragioni e soluzioni">Quali sono le cause dello stampaggio a iniezione a stampa corta? Principali ragioni e soluzioni

Stampaggio a iniezione is an extremely versatile process used to manufacture thousands of products today. These include everything from plastic forks to automotive dashboards

Although there are many ways to accomplish injection molding, generally, it involves injecting molten plastic into a cavity. This is formed by a mold to produce a precise and reproducible part. However, problems occur sometimes. 

In this article, we’ll focus on one of the problems that can occur: short-shot injection molding. A short shot occurs when enough plastica isn’t injected into the mold to completely fill it. There are several root causes for short shots.

After discussing the root causes of short shots in injection molding, we’ll review the remedies for each of these causes.

Stampi di precisione in plastica dura per la produzione industriale

What Is Short Shot Injection Molding?

A short shot is when the wonderful science of injection molding doesn’t quite go as planned. Instead of the molten plastic filling the cavità dello stampo, the plastic will not make it all the way and will leave a void. 

Short shots are problematic and terrible for efficiency. Incomplete parts must be rejected, which means valuable material and time are wasted. And, of course, once a short shot is identified, time is then wasted on identifying what caused the short shot. 

However, the effects of short shots go further than just wasted time and material. Parts that have short shots in them will not perform to the structure they were designed for. A chair leg that is short shot will be wobbly and unstable, and a housing for a medical device well could get nasty.

Victims of short shots are all around us. A bottle cap with a missing tab, a toy car with an incomplete fin, and a power tool handle with a hollow section. All these could be due to short shots occurring during the processo di stampaggio

Material Issues

Injection molding is very dependent on selezione del materiale and how the material behaves during the injection molding process. Let’s look at some of the material properties and handling issues that can cause short shots:

Material Viscosity

Which one is easier to contain when pouring water versus honey? Viscosity is the action of resistance to flow in a material. 

Materials with high viscosity tend to flow slowly and have more difficulty getting into the mold cavity before the part solidifies. Low-viscosity materials, on the other hand, tend to exhibit excessive jetting (non-uniform flow) or turbulence in the mold.  Proper selection of the material viscosity in relationship to the progettazione di stampi is very critical.

Humidity

Certain materials like nylon are hygroscopic in nature. They tend to absorb moisture from air whenever they are exposed to it. This moisture, when absorbed, is vaporized when injected in the mold, resulting in the formation of voids. This may also lead to incomplete fill of the mold.  It is important to dry such hygroscopic materials before use. The moisture in the material is often the culprit for short shots.

Material Handling

Good material handling is like a recipe. If you follow the steps, you always get the same results. Material handling is the same. 

If you handle the material properly, you will get the desired results every time. Here is why it is so important:

  1. Storage: If not properly stored, materials can be contaminated by moisture in the air or degraded by UV light.  Stored in sealed containers and out of sunlight or other potentially degrading environments.
  2. Preheating:  Most materials for injection molding should be preheated before they enter the machine. This lowers their viscosity and enables them to flow more easily and fill the mold cavity better.

Solutions

Luckily, there are ways to overcome material short shots:

  1. Drying: If the material is hygroscopic, proper drying will remove any moisture prior to processing.  The most common types of dryers used for this purpose are dehumidifying dryers or desiccant dryers.
  2. Additivi:  There are some additives that can be added to the material to give it different properties.  Flow modifiers, for instance, enhance flow properties and reduce viscosity which can minimize short shot opportunities.

Understanding the relationship between material characteristics, their distribution, and injection molding operations allows us to take proactive measures. 

These measures will ensure plastic flow in an injection mold cavity proceeds smoothly, preventing short shots.  Next, we will look at parametri di processo that also affect injection molding success.

Process Parameters: Fine Tuning The Flow

Speaking of flow, there are other causes for short shots. These have nothing to do with the solidified properties of the plastic but instead with the injection molding machine settings. 

Let’s look at some of the ways the most common process parameters can contribute to a short shot:

Injection Speed and Pressure

Squeeze your toothpaste tube. If you don’t apply enough pressure, very little paste will emerge.  The same is true in molding. 

If the injection pressure is not enough to overcome the material and mold resistance, the part will be short shot.  However, injecting at too high a pressure can create other difetti, such as flash.

Injection speed is related to pressure.  Very slow speed may allow the material to cool before it fills the mold.  

Too fast speed can create air swirl and turbulence that prevents the material from filling the mold completely.  The molding process optimizes the balance of speed and pressure.

Macchina per lo stampaggio ad iniezione che mostra uno stampo in metallo con parti in plastica

Controllo della temperatura

Let’s talk about honey again.  Stick it in the microwave, and it runs out easily.  Now put it in the refrigerator, and it gets slow.  Same with injection molding, material flow is highly affected by temperature. 

Barrel temperature is the heat added to the material going through the machine.  Too low of a barrel temperature will cause the viscosity to be high and create short shots.  

On the other hand, a very high barrel temperature can cause the material to degrade.  Mold temperature is another factor.  

A cold mold can cause the material to solidify quickly and not allow time for it to fill properly.  It is a balance between temperature to allow proper flow and prevent short shots.

Injection Time

This is the length of time that the plastica fusa will be allowed to remain in the mold cavity.  If the injection time is too short, then insufficient plastic may fill the mold before it begins to solidify.  

However, if the injection time is too long, then excess plastic may flood the mold. This results in an unnecessary increase in material usage and cycle time.  

There is often a sweet spot in terms of injection time that should be determined by the mold design and material used.

Plastic injection molding process with molds and finished parts

Solutions: Optimizing the Process

The good news is that process parameters related to short shots are preventable:

  • Optimization: Optimizing the process parameters is something that should be done regularly.  Material properties, mold design, and part geometry all play a role in establishing optimal parameters.  Using statistical process control (SPC) methods helps uncover trends and ensures ongoing quality.
  • Training: Proper training for operators on optimizing process parameters should be part of the injection molding process.  An experienced operator will recognize signs that a short shot could occur. They will be able to adjust the process to avoid the short shot.

Mold Design: The Blueprint for a Flawless Fill

A well-designed road will efficiently direct traffic. In the same vein, a well-designed mold will efficiently flow molten plastic through its pathways. 

However, just as a pothole in the road can divert traffic, mold design flaws can cause short shots. Here’s why:

Design Flaws

Many design factors can prevent full mold filling.  One of the most common is inadequate venting.  Air trapped in the mold cavity obstructs the flow of plastic and often results in a short shot.  Other factors include poor runner design.

Narrow runner channels or an improper gate location can also cause obstruction and problematic short shot concerns.  Other common design flaws include thin walls with limited flow area or sharp corners which create cold spots, obstructing mold filling.

Flow Path and Gate Location

Flow path refers to the route that the molten plastic material takes in the mold from the sprue to the cavity.  The material likes to flow easily, and an optimized flow path with minimal viscosity drop ensures the plastic flows smoothly.  

The location of the gate can also have a big impact.  A poorly located gate can create turbulent flow or wind sheer, which increases the chance of a short shot.  

Proper flow path and gate placement can be the difference between a well-running injection mold tool and a production nightmare.  Always consider this when designing your component for injection molding. 

Manutenzione

Plastic injection mold for bottle production

The best-engineered mold will give you years of service, but mold wear and tear is inevitable.  Proper mold maintenance and inspection should be performed on a regular schedule to ensure the mold is in optimal working condition.  

A common issue with mold wear is it often causes flow restrictions. It many also prevent proper venting in the mold, which can result in short shots.  Performing a preventive maintenance routine on your injection molds will help you detect potential mold problems. Before they affect production.

Solutions: Building a Successful Mold Design

There are, thankfully, ways to mitigate the effects of potential short shots in mold design:

  • Simulation: Prior to building the mold, a mold designer can use mold flow simulation software. This can work to simulate the performance of the mold design in a virtual environment.  

The software predicts areas where flow problems might occur.  Such as short-shot areas or air traps, and allows the designer to make changes before the mold is built.

  • Redesign: Using information from the mold flow simulation or from production, when problems are encountered, the mold can be redesigned. This will help to achieve better flow and eliminate short shots.  Runner sizes may be changed, gates moved, and better venting channels added.

Machine Problems: The Engine of Production

We’ve discussed material, process parameters, and even mold design. But sometimes, despite everything going right, the machine can have a bad day. Here are some ways machine-related problems can cause short shots.

Industrial injection molding machine in operation

Machine Calibration

An injection molding machine relies on many settings, such as injection pressure and speed.  Like anything subject to wear and tear, these settings can slowly drift off, resulting in an inconsistent molding process.  

Machine calibration is the process of bringing the machine back to the proper settings. It is recommended on a regular basis to prevent short shots from occurring due to unintentional changes.

Wear and Tear

Injection molding machines are made up of many moving parts, and like a car, they will require maintenance.  When parts such as valves, pumps, or seals in the machine begin to wear.  It can affect the machine’s ability to provide the part with proper pressure or flow rate. 

Solutions: Keeping the Machine Running Smoothly

Luckily, there are methods to reduce the risk factors that cause short shots to occur due to machine issues:

  • Manutenzione: A good maintenance program for the injection molding machine is essential.  This program should include preventive maintenance activities such as lubrication, filter changes, and inspections of worn components. 
  • Upgrades: Technology is always evolving, and the latest generation of injection molding machines provides even greater accuracy and tighter control.  Sometimes, it is cost-effective to upgrade older machines that may have a history of short-shot problems with new machines.  New machines provide more stable operation and fewer short shots.

Bottom Line

In conclusion, short shots are often a bothersome problem in injection molding. However, by understanding the general causes and applying the solutions in this article, you have been able to avoid this problem. 

Combine material selection, processing parameters adjustment, well-designed mold, good machine maintenance, etc. You will enjoy a well-run injection molding process and obtain high-quality injection molded parts stably.

Need a Trusted Injection Molding Partner?

HITOP Industrial is always here waiting for your consultation. With rich experience in injection molding, HITOP will be a good assistant for you in realizing your injection molding ideas. 

Sommario

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Ciao, sono Sparrow Xiang, COO della società HiTop, io e il mio team saremmo felici di incontrarti e imparare tutto sulla tua attività, requisiti e aspettative.