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Knit Lines in Injection Molding

  • Prototype Tool In Soon As 10 Days
  • Production Tool In As Soon As 3 Weeks
  • 10k Parts in 10 Days
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Knit Lines in Injection Molding

The line where two resin flows “knit” or “weld” together during injection molding is called a weld or knit line. A knit line may be unnoticeable or may look like a crack. The type of resin used during injection molding plays a large role in whether or not a knit line will be present. For example, filled resins and ABS are more likely to have knit lines. There are two common areas where a knit line may form: behind a boss or between gates. The boss is a hole where a threaded fastener will be inserted. A gate is where the resin is injected into a part. Knit lines may potentially cause cosmetic or structural issues but may also be no problem at all. There are various ways to prevent knit line problems, including adjusting the temperature, injection speed, pressure, material selection, etc.

How to Prevent Knit Lines in Injection Molding / /


When the temperature is too low, the material may solidify prematurely. Some areas of the part may begin to solidify while other areas are still flowing. Using a higher temperature for both resin flows will prevent them from cooling too fast and forming a knit line.

Injection speed

Resins moving slowly through the mold will cause the material to solidify at different times. When one resin cools before it meets the other, a knit line is caused. Using materials that flow quickly will prevent them from cooling too soon.

Injection pressure

If the machine has an inappropriate setting or is faulty, it may not apply enough pressure to push the meld and flow together, causing a weld line. Low injection pressure may be resolved by increasing the pressure at the nozzle or adjusting the machine settings.


A material with a lower melting temperature will allow for a quicker flow and slower cooling. Materials that are not filled will have stronger knit lines than those that are filled. Materials with longer fibers and higher filler content will have weak knit lines. Some resins with flame retardants, lubricants, and other additives will also have weak knit lines.


Adjusting the design of the mold can help to eliminate any partitions between areas, eliminating knit lines. Using a thicker wall will help slow the resin’s cooling rate, strengthening the knit line while improving the appearance of the part. Moving the gate to an area where knit lines are minimized or located in a less critical area will help to prevent any issues.

Why You Should Use Injection Molding / /

Injection molding is a versatile, simple, and efficient manufacturing method that creates high-quality plastic products.

Injection Molding Services Offered by Fathom / /

Fathom is ready to work with you on your injection molding project. Let us utilize our single-source global network to support your low- and high-volume projects. By leveraging both additive and advanced technologies, Fathom can produce high-quality parts that will satisfy our customers.

Injection Molding is one of 28 manufacturing processes offered by Fathom. Additional manufacturing services include:

Additive Manufacturing

  • FDM
  • SLS
  • SLA
  • Polyjet
  • MJF
  • DMLS
  • DLS
  • STEP (Evolve)

CNC Machining

  • CNC (3 & 5 Axis)
  • CNC Lathe
  • Precision Grinding
  • Wire & Sinker EDM

Injection Molding

  • Injection Molding
  • Tooling
  • Compression Molding

Metal Cutting and Forming

  • Thermoforming
  • Sheet Metal Punching
  • Sheet Metal Cutting
  • Metal Forming
  • Laser Cutting
  • Water Jet Cutting
  • Chemical Etching
  • Welding
  • Micro Cutting


  • Die Casting
  • Urethane Casting
  • Composite Layups
  • Extrusion
  • Post Processing & Finishing
    • Pad Printing
    • Powder Coating
    • Bead Blasting
    • Surface Grinding
    • Polishing
    • Anodize
    • Powder Coat Passivate
    • Chemfilm
    • Vacuum Metallization
    • Silkscreen
    • Plating
    • Tumbled

Injection Molding Quote Times / /

Take advantage of Fathom’s SmartQuote online platform and receive a quote in as little as 1 hour, usually within 24 hours. Do you need an injection molding quote, or would you like to learn about other manufacturing services? Fathom’s injection molding lead times are as follows:

  • Prototype Tool in Soon As 10 Days
  • Production Tool in As Soon As 3 Weeks
  • 10k Parts in 10 Days

Injection Molding Machines Used by Fathom / /

Fathom’s manufacturing facilities are powered by over 530 machines, including mills, lathes, presses, press brakes, turret punches and additive systems. Our machines include 15 injection molding presses ranging in size from 40 Ton to 350 tons. We can process multi-shot tooling, bi-injected tooling, insert over molding and conventional molds.

Fathom provides not just injection molding services but also hybridized services leveraging injection molding. Need injection molding as part of a more complex project? Fathom can handle it. A recent case study is listed below where Fathom delivered 20,000 parts in 27 days using 55+ tools, including injection molding services:

Case Study //

Real Customers. Real Results.

A customer came to Fathom with a complex project that spanned the technologies and processes of rapid production—and needed it done fast. Fathom delivered.

20,000 Parts // Within 27 Days
55+ Tools to Injection Mold 15,000 Parts First Article in 2 Weeks

55+ Tools to Injection
Mold 15,000 Parts

First Article in 2 Weeks

3,000+ 3D Printed Parts

3,000+ 3D
Printed Parts

2,000+ Metal Fabricated Parts

2,000+ Metal
Fabricated Parts

Hybridized Services Used //

3D Printing / Additive Manufacturing—PolyJet, SLS, MJF.

DFM Analysis + CAD Mods.

CNC Machining. Laser Cutting.

Stamping. Die Cutting. Post-Opp Drilling.

24-hour Turnaround Urethane Casting.

Injection + Compression Molding.

Model Finishing. Insert Assembly.

Advanced Project Management.

Fathom has formed the Fathom Tooling+ Team, building on more than eight decades of tooling and injection molding experience. Fathom offers a single-source global solution that supports any size company’s low- to high-volume molding needs. In-house experts working side-by-side are improving the injection mold business model—accelerating the quoting process, as well as including Design for Manufacturing (DFM) overview with every initial quote.

  • Prototype Tool in As Soon As 10 Days
  • 10K Parts In 10 Days
  • Production Tool in As Soon As 3 Weeks
  • Up To 25K Parts in As Soon As 21 Days (Tools Built for 250k to 1 Million+ Shots)
  • DFM Overview Included with Every Initial Quote

Fathom’s Locations for Injection Molding / /

Fathom has 2 US-based locations that offer injection molding, offering nationwide coverage to the entire United States. Fathom’s advanced prototyping and manufacturing services include wire EDM, CNC machining, chemical etching, metal 3D printing, and more. We have provided over 200,000 quotes to leading US companies and are ready to provide you with a quote today.

Locations include:

1050 Walnut Ridge Drive
Hartland, WI 53029
ISO 9001:2015

1207 Adams Drive
McHenry, IL 60051

Injection Molding Specifications and Tolerances / /

Injection molding projects are done on CNC machines with tolerances of +/- 0.005 inches. Designs with tighter tolerances are more expensive to manufacture than parts with normal tolerances. A tight tolerance is +/- 0.002 inches and very tight is +/- 0.001 inches.

Rapid Tool (Master Unit Die Base) 1.5-3 Weeks 5,000 Shots 0.010in 0.010in ±0.003 + 0.002in/in Pantone Colors A2 Yes
Prototype Tool (Stand Alone) 2-4 Weeks 5,000-50,000 Shots 0.010in 0.007in ±0.005 + 0.002in/in A2
Production Tool 5-10 Weeks (Depends on Size and Complexity) 250,000 - 1,000,000 Shots (Depending on Class of Tool Selected) 0.005in 0.005in SPI (Commercial) A1

Injection Molding Materials List / /

Selecting the best material for your unique project is important because it can affect the part’s performance. When picking a material, consider the environment in which the part will function and how the part is expected to function. A material’s durability, color, flexibility, strength, and cost are all important. Here are some of the materials offered by Fathom for the injection molding process.

  • Crystalline Plastics
  • Amorphous Plastics
  • Imidized Plastics
  • Polyethylene (PE)
  • Polypropylene (PP)
  • Acrylonitrile Butadiene Styrene (ABS)
  • Polyoxymethylene (POM)
  • Polystyrene (PS)

Details Considered When Quoting an Injection Molding Job / /

Several important factors determine the cost of injection molding, including the design, material, quantity ordered, and turnaround time. To begin your injection molding project, you will need to provide a CAD drawing or product sample. Detailed drawings that show the complexity and size of the part are important. A prototype can show how to adjust the design of the part for mass manufacturing. Parts with more complex designs will cost more to produce. Selecting the best type of resin for an injection molding project is important. Fathom can work with you and your team to find the most cost-effective plastic material and additives for your part. A large part of the cost of injection molding is tooling, so it is important to determine the quantities needed beforehand. Smaller quantities may use an aluminum mold, while large volume projects should use a high-quality stainless-steel mold. The time spent executing an injection molding project also contributes to cost; making the mold, quality control, adjustments, assembly, and shipping can affect the turnaround time.

Knit Lines in Injection Molding Quotes FAQs / /

Q: Does Fathom offer Injection Molding machining?

A: Yes, Fathom offers injection molding machining.

Q: What is Injection Molding?

A: Injection molding is a manufacturing process commonly used to make a high volume of uniform parts. Injection molding makes a part by injecting molten plastic into a custom mold. Once the part has been formed, it is released. Injection molding is a fully automated process. As the mold can be used repeatedly, injection molding can copy the same part thousands of times.

Q: Is Fathom ISO certified?

A: Fathom’s certifications include ISO 9001:2015, ISO 9001:2015 Design, and ISO 13485:2016. For site-specific certifications, please visit https://fathommfg.com/fathom-manufacturing-certifications.

Q: Is Fathom ITAR certified?

A: Yes, Fathom is ITAR certified. For site-specific certifications, please visit https://fathommfg.com/fathom-manufacturing-certifications.

Q: Is Fathom AS9100:2016 certified?

A: Yes, Fathom is AS9100:2016. For site-specific certifications, please visit https://fathommfg.com/fathom-manufacturing-certifications.

Q: Is Fathom NIST 800-171 certified?

A: Yes, Fathom’s certifications include NIST 800-171. For site-specific certifications, please visit https://fathommfg.com/fathom-manufacturing-certifications.

Q: How do you get rid of knitting lines?

A: Knitting lines can be prevented by selecting the right material, paying careful attention to the design, using higher temperatures, and monitoring injection speed and pressure.

Q: What is the difference between knit lines and flow lines?

A: Flow lines on the surface of a part appear in a wavy or ring-shaped pattern. Flow lines appear when thinner areas of a part cool faster than thicker areas or when molten resin flows past areas of a part that have already cooled. Knit lines appear on the surface of the part as a straight line. Like a flow line, knit lines occur when two resins meet at different temperatures.

Q: Difference between knit lines and blush in injection molding?

A: Blush appears as discoloration around the gate but may be found in other areas of the part. Blush may be caused by a high injection speed, low injection pressure, wrong temperature, or improperly sized gates. Blush lines have a cloud-like shape, whereas knit lines are lines.

Q: How can you reduce my flow marks?

A: Flow marks can be reduced by adjusting the injection speed or pressure, raising the resin’s temperature, relocating gates, increasing the diameter of the nozzle, and selecting the best material for your unique project.

How to Get Injection Molding Quotes / /

Are you looking to get Injection Molding quotes or calculate costs for an upcoming project? Fathom’s SmartQuote application provides fast quotes for any file.

Manufacturing Locations Across National Time Zones
Manufacturing Locations Across National Time Zones
The Fathom Advanced Manufacturing Platform
new map update 3

1050 Walnut Ridge Drive
Hartland, WI 53029
ISO 9001:2015


444 W. 21st St. Ste. 101
Tempe, AZ 85282
ISO 9001:2015
NIST800-171 Compliant


7770 Washington St.
Denver, CO 80229
ISO 9001:2015


13758 Johnson Street NE
Ham Lake, MN 55304


1801 Rowe Lane
Pflugerville, TX 78660
ISO 9001:2015

1513 Sam Bass Rd
Round Rock, TX 78681
ISO 9001:2015
ISO 13485:2016


46758 Lakeview Blvd,
Fremont, CA 94538
ISO 9001:2015 Design Certified
NIST 800-171 Compliant


14000 N.W. 58th Court
Miami Lakes, FL 33014


1920 Slaterville Rd
Ithaca, NY 14850

401 W. Shore Blvd.
Newark, NY 14513
ISO 9001:2015


1207 Adams Drive
McHenry, IL 60051
ISO 9001:2015

1401 Brummel Ave
Elk Grove, IL 60007
ISO 9001:2015 Design Certified

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