Inox Cast Weld Line Injection Molding

Inox Cast can fully supply your limited range of desired weld line injection molding for your plants. We have a wide plant that assures you numerous types of production of weld line injection moldings that suit your finished products. 

By Inox Cast, your finished products will improve. We have been importing our injection molding even to other countries for a long time. We are so professional to handle and take care of your orders. We can make possible the fast yet safe packaging and shipping.

Inox Cast assures you that you can receive the exact models and designs you ordered. Inox Cast has senior managerial and technical engineers who can assure you of excellent finished works. We can give you the most satisfying services, no-hassle, issues, and complications.

Our weld line injection molding is essential for the best result of finished molded products. This type of injection molding resolves any trouble caused by undesired welding lines. We have various types and models available. Just check out some of it on the above lists.

Inox Casting is a reliable company that never leaves your partner in any trouble with processing. Instead, we make sure everything is settled and well processed for our customers’ benefit and peace of mind. We conduct high-standard weld line injection molding analysis before offering.

Be with our company and experience satisfaction as long as productive businesses or plants include Inox Cast weld line injection molding. High-quality and performance are what we provide for our clients. Just let us know your details or drawings, and we will make it for you.

Please feel free to contact us now!

Weld Line Injection Molding: The Ultimate Guide

This guide covers all information you are looking for about weld line injection molding.

So, if you want to learn more about weld line injection, read this guide.

1.  What Is Weld Line In Injection Molding?

Weld line is a line that occurs around the holes.

That is, where two flow fronts meet when there is the inability of more flow fronts hence, they cause weak areas during molding process.

weld line injection molding

Weak weld line occurs when there is a little polymer chain entanglement.

Hence, the ribs split the melt flow and the separate flows do not meld completely when they meet.

2.  How Weld Line Appear In Injection Molding

Weld line usually appears in the part with opening where the two resin flows meet hence at this point, the temperature of the two resin is usually lower.

Where the two flows re-join, there is always interfusing of the plastic due to some cooling and re-solidification of the plastic that has occurred.

weld line injection molding

Weld line can occur on a molecular level the molecules that are not oriented in the same direction of the flow path.

The weld line appears as dark line when inorganic decorative effect pigments are used.

Thus, conspicuous in dark, transparent or brilliant mouldings having smooth surfaces polished.

They also appear around holes or obstructions thus causing weak areas in the moulded part.

3.  Parting Line Vs. Weld Line Injection Molding

Parting line is the border line in which draft angles change direction while weld line is a line that occurs around the holes where two flow fronts meet.

Parting line does not affect the whole part but can be noticeable depending on mold finish, material type, processing parameters and material colour.

parting line

Weld line can be more or less visible depending on the condition of the mold hence most of the time they do not weaken the part unless the part is processed incorrectly.

Parting lines cannot be avoided completely due to the injection molding that relies on a mold that has two halves.

weld line

Weld lines are unavoidable hence the part designer always shifts the location of the weld line by moving the gate before the mold is produced.

4.  Causes Of Weld Lines In Injection Molding

Weld lines are commonly cause by inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic.

There following are some of the causes of weld line in injection molding.

Low pressure.

The two molten resin streams are squeezed at the weld point where the bonding conditions depend on the pressure applied there.

There is always lower strength and more weld line when the holding pressure is also lower because the pressure transfer becomes more difficult as solidification progresses.

This can be controlled by raising the resin temperature, raising the mold temperature, use materials with great fluidity, raising the injection speed.

Poor Ventilation

The appearance and the strength of the weld line will deteriorate if the vent is not provided at the weld line point hence ensure sufficient ventilation at this position.

Insufficient ventilation will cause burn marks thus check the vent thickness and size to ensure it is smooth or contaminated by mold deposit.

Low Resin Temperature.

If the temperature is low, the surface layer of the weld line will become thicker hence the strength and the texture will be reduced.

The lower the temperature the more the weld line hence when the temperature of the two resin is higher, the adhesion will increase and the lines will be less.

Ensure you increase the temperature of the resin at the weld line section thus to achieve maximum results while the pressure holding effect will increase.

Mold Design.

Improper wall thickness and improper placed gates are some of the errors in mold design that might cause weld line in different areas in your design.

Wall should not be too thin or too thick hence for resin to be able to pass through it as quickly as possible.

Low Speed.

The two fronts must cool at equal times thus the speed should be at a high rate for resin to travel through the mould quickly hence to avoid formation of knit lines or meld lines.

Improper Use Of Release Agent.

A small amount of agent is usually used thus it is determined according to molding conditions, variety of raw materials and shape of the plastic parts.

When large amount of release agent is used, it causes welding marks on the surface of plastic part.

Poor Structural Design Of Plastic Parts.

When thin-wall is formed, the strength of plastic parts is lowered hence when the walls are too thin and the insert is too much, it may cause poor welding.

When designing plastic part structure ensure that the thinnest part of the plastic parts is larger than minimum wall thickness allowed during molding.

5.  Why Avoid Weld Lines In Injection Molding

You should avoid knit lines since they are undesirable in injection molding when surface appearance and part strength are significant.

Weld lines should be avoided by all mean because they make the product look ugly and also compromise the strength of the product.

You should avoid them because of the following reasons;

Deformity On The Surface Of The Product.

A tiny weld line can deform the surface of the product thus it is very important to take all the precautions to avoid weld lines.

A single knit line might deform your design thus you must me very keen when making your design hence to protect the surface appearance of your design.

They can also affect the durability of the product like auto plastic parts.

They Make The Products Fragile.

Weld lines are a very big problem when the product demands strength.

Weld lines are the weakest area on your part thus the fragility caused by weld lines can make the material to easily break off.

Therefore, weld lines should not be taken for granted as they affect the products strength to at bad state.

6.  Fixing Weld Line In Injection Molding

You can fix the weld line by increasing the injection speed, pressure and temperature.

Hence, avoid weld lines in areas which need strength or the area that needs to appear smooth.

This can be done by changing the polymer injection location or changing wall thickness to set up a different fill time.

Fixed weld line

When the speed is adjusted, the flow fronts may meet at a different spot and move the weld line location.

Hence, the material flow will fill the mold more quickly and cools down maturely.

Optimize runner system design by reducing runner dimensions and maintain the same flow rate to use shear heating to increase the melt temperature at the flow front.

After the injection molding is completed, consider machining features like holes to avoid weld lines.

Improve the flow properties and minimize weld line by changing the raw material to a plastic with lower viscosity or a lower melting point.

Just have one flow front and avoid multiple flow fronts thus by adjusting the design to be a single source flow.

A flow front can meet at different spot and move the weld line location for a shorter or longer fill time hence consider to change the product wall thicknesses during design,

Moreover, the wall thickness affects the time it takes to fill the mold.

The positioning of the gates will also affect where the weld lines will appear on the part hence it should be considered before the mold fabrication.

7.  Weld Lines Vs Meld Line In Injection Molding

The main difference between the two lines is considered by the angle at which the converging flow fronts meet.

Weld Lines

Weld lines occurs where two flow fronts meet and the polymer molecules are misaligned hence when it forms, the thin frozen layers at the front of each flow path meet.

The flow path is perpendicular to the weld line at the orientation of the plastic hence it is a sharp difference in molecular orientation which causes the decrease in strength.

Meld Lines.

They are usually stronger than weld lines thus they occur when two flow fronts blend together at an oblique angle.

meld lines

The orientation of the plastic molecule is usually more uniform after a weld line has formed compared to before the weld line is formed.

8.  How Weld Lines Affects Injection Moulded Parts

Weld lines are the common defects to get rid of thus it can weaken the structural integrity of a part.

However, most of the time the weld lines do not weaken the part unless the part is processed incorrectly.

Weld lines can affect the injection mold parts through the following factors;

Design Of The Surface Roughness Of Cavity And Core.

Designers must be careful to specify finish that is good for the plastic material to be moulded hence it will determine the moulded parts appearance.

Design Of Exhaust System And Cold-slug.

Increase the thickness of the wall hence to help facilitate required pressure and also ensure that the temperature remains high.

Injection Pressure And Holding Pressure.

Increase the injection pressure to help you solve the problem of weld lines in the design which can lead to functional failure of the end product.

Design Of The Gating System Hence It Should Not Be Too Much.

Since weld lines appear due to faulty mold design, weld lines can be easily dealt with by increasing the size of the runner or gate.

Low Temperature Control System.

Make adjustments in the moulding conditions like temperature to an appropriate level hence to prevent weld lines in the design.

The polymers work just like human body thus the hotter the environment becomes, the less performance we can perform.

9.  Troubleshooting Weld Lines Injection Molding Defect

When a polymer resin is injected into a mold, it flows through all parts of the mold hence the defect occurs when the injected plastic meets within a mold.

Avoiding these defects is actually by using the right mold manufacturer and having a part manufacturer that has the right quality assurance.

To avoid bottlenecks, the number of gates, location and the designing of the mold should be replaced hence the speed can also be adjusted.

Pressure and temperature can be increased at the machine to avoid other molding defects hence optimize holding pressure time.

Check if the mold vent is blocked or not then make it larger at the location with the weld line when the gate is too small or the location of the gate is not good.

Make the size of runner channel wider or shorter if the main runner and sub-runner are too thin or too long.

Increase the cold-slug well size or set flash gate at location with the weld line when the old-slug well is too small.

Decrease the mold release agent or try not to use it if not needed.

Check the material quality to make sure it hasn’t been exposed to external pollutants or change to a different material.

10.  Knit Line Vs Weld Line In Injection Molding

Knit Line

This line is when two flow fronts meet hence it’s like a head-on collision at a4-way stop and when they meet, there is no cavity geometry to flow through.

It is weaker than a meld line due to it difficulty to pressurize the area of the part during molding process.

knit line

Knit line can weaken the structural integrity of a part hence they occur when melt flow fronts collide in a mold cavity.

Weld Line.

Weld lines occurs where two flow fronts meet and the polymer molecules are misaligned.

Hence, when it forms, the thin frozen layers at the front of each flow path meet.

The flow path is perpendicular to the weld line at the orientation of the plastic.

Hence, it is a sharp difference in molecular orientation which causes the decrease in strength.

11.  How To Reduce Weld Line In Injection Molding

Here is how you can reduce weld line in injection molding;

• Increase nozzle diameter hence to increase the flow speed and prevent early cooling.
• Keep the flow rate consistent to the thicker sections by rounding the corners of the mold where wall thickness increases hence to reduce flow lines.
• Increasing the injection pressure, material temperature or speed helps the product move more quickly, filling the mold before it begins to cool thus reducing flow lines.
• Increase the cooling time and reduce weld line by providing further separation between the injection point and cooling apparatus.

12.  How To Cope With Weak Weld Line In Injection Molding

Weak weld lines occur when there is no polymer chain entanglement across the flow front boundaries.

Therefore, you can cope with it by minimizing flow interruptions and place.

The ribs should be oriented in the direction of the flow hence for ease of venting and filling.

Hence, the flow fronts are allowed to meet and flow some distance after interruption.

Designers and moldmakers usually work together to ensure the weld lines at cold-runner Molds are minimized by employing valve-gated hot runner system.

Pick a gate location that allows flow for a long distance after the flow interruption by optimizing strength.

It allows more of chains to entangle across the weld line.

Glass filler improves strength over net resin.

Thus, add 30% glass and the weld line strength retention will drop to 34% hence the weld line strength will reduce but not improve.

Lower the viscosity of the polymer during fill by decreasing the fill time and increasing the shear rate which will allow better chain entanglement and better packing.

Weld lines are very hard to eliminate hence their strength, cause and appearance are influenced by;

• Part design.
• Testing.
• Processing.
• Material selection and handling.
• Tool design and construction.

Minimizing of weld lines deals with all these hence the increase of volatiles coming off the polymer usually decreases weld line strength.

13.  Determining Injection Molding Weld Line Strength

Weld line strength for plastics can vary from 10% to 100% of the strength of the plastic.

The decrease or the strength of weld lines can be explained by the lack of interdiffusion of the molecular chains across the interface.

Strong molecular orientation in the weld line is due to the higher shear rates and concentration through a V-shaped notch at the plate surface along the weld line.

There are many factors that affect the strength of weld line during molding process such as;

V-notches.

When the compressed gases are trapped where the flow fronts meet, it may result to notches hence improper venting may also lead to this.

Notches usually act like a stress amplifier thus they reduce cross-sectional thickness area.

Poor Bonding.

When the polymer mobility is poor where flow fronts recombine then it is up to secondary polar bonds to hold the two sides together.

Therefore, the quality of polymer bonding at the interface affects the meeting angle hence the bonds can be 10 to 50 times weaker than the chain bonds.

Orientation.

If a crack propagates at the time when fountain flow orients polymer chains where the two fronts meet, the strength of the plastic is neutralized by orientation.

For all your inquiries or questions on weld line injection molding, contact us now.