How to Eliminate Knit Lines In Injection Molding

How to Eliminate Knit Lines In Injection Molding

Knit lines are formed when two or more plastic flow fronts collide and solidify or “knit” together during the molding process.

Overall, injection molding is a relatively simple process. A thermoplastic resin is heated to its melting point and injected into the cavity of an injection mold to produce a specific part geometry. The part is cooled in the mold until it reaches a temperature where it is solid enough to be ejected.

Knit lines most commonly occur around holes or other obstructions to the melt flow such as bosses. A boss is a feature with a hole that designed for a threaded fastener. A gate is an area where the resin is injected into the cavity.

Some thermoplastic resins with lower flow rates such as ABS and filled resins are more prone to having knit line issues. There are approximately 85,000+ thermoplastics available in the marketplace. Within the vast material options available, there are approximately 40 polymer blends or families.

While the presence of knit lines does not always compromise the structural integrity of the plastic part, they are almost always a cosmetic issue.

Changing the injection profile parameters – modifying the fill time for instance – may cause the knit line to move to a more favorable location.

Material selection, part design, tool design, and process parameters all also affect knit lines.

How to eliminate Knit Lines

  • Select resins that are less susceptible to knit line formation.
  • Change the boss or gate locations.
  • Thicken part walls to slow down the resin cooling process however be careful not to make them too thick that it causes sink marks.
  • Place knit line causing features farther from the edge of parts when the design allows for it to do so.

Do you have a question regarding knit lines? Send your design to one of our Technical Specialists for review at 586-598-4636 or

Plastic Injection Molding & Other Molding Types

Plastic Injection Molding & Other Molding Types

Plastic Injection Molding

We are frequently asked about different molding types and how each effect the process of part design and production. At Xcentric Mold & Engineering, we specialize in plastic injection molding.  Below briefly highlights some of the more popular molding types:

Injection molding is a manufacturing process process for producing parts by injecting molten material into a mold. Material for the part is fed into a heated barrel, mixed (using a helical shaped screw), and injected into a  mold cavity, where it cools and hardens to the configuration of the cavity.

Material selection is critical for plastic injection molding, which can be performed with a host of materials including plastic, metals (for which the process is called die-casting), glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers.

Common examples of plastic injection-molded parts include medical equipment and medical devices, automotive, marine, industrial, agriculture, aerospace and tight tolerance parts.

Other Molding Types

Below are some molding types which are not specialties of Xcentric Mold & Engineering however we may be able to provide you with recommendations of other suppliers who specialize in the following:

Blow Molding

Blow molding is a specific manufacturing process by which hollow plastic parts are formed and can be joined together. In general, there are three main types of blow molding: extrusion blow molding, injection blow molding, and injection stretch blow molding. The blow molding process begins with melting down the plastic and forming it into a parison or in the case of injection and injection stretch blow molding (ISB) a preform. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass.

The parison is then clamped into a mold and air is blown into it. The air pressure then pushes the plastic out to match the mold. Once the plastic has cooled and hardened the mold opens up and the part is ejected.

Common examples of blow molding products include bottles, containers and other hollow shapes.

Rotational Molding

Rotational molding is comprised of a heated hollow mold which is filled with a charge or shot weight of material. It is then slowly rotated (usually around two perpendicular axes), causing the softened material to disperse and stick to the walls of the mold. In order to maintain even thickness throughout the part, the mold continues to rotate at all times during the heating phase and to avoid sagging or deformation also during the cooling phase.

Common examples of rotational molding include parts larger than 2’ such as containers, utility carts, storage bins, car parts, tanks (oil, septic, water) and leisure products such as kayaks.

Vacuum Molding

Vacuum forming is a process by which a sheet of plastic is heated until it becomes pliable, stretched onto a single-surface mold and forced against the mold by a vacuum to create a shape.

This process can also include thick-gauge thermoforming, a type of vacuum molding, that is known for producing a variety of products including disposable cups, containers, lids, trays, blisters, clam shells, and other products for the food, medical, and general retail industries.

Common products produced with the vacuum forming application include industrial containers and crates, pallets, exterior door panels, plastic totes, plastic trailers, passenger cabin window canopies for winged aircraft, and lawn mower hoods.

Would you like additional information about the plastic injection molding process and its capabilities?  Contact our Technical Specialists today at      586-598-4636 or

How Wall Thickness Impacts Injection Mold Part Design

How Wall Thickness Impacts Injection Mold Part Design

Maintaining uniform wall thickness throughout your plastic injection molding part design is critical.  Without uniform wall thickness, many issues can occur such as sink, warping, short shot (meaning the material in tool does not fill correctly), and cosmetic imperfections.

Below are some general guidelines on wall thickness:

  • A 10% increase in wall thickness provides approximately a 33% increase in stiffness with most materials
  • Walls should be no less than 40%-60% that of adjacent walls
  • Core out all unneeded thickness and wall stock
  • Sharp internal corners and long unsupported part spans should be avoided
  • Use ribs as stiffening features and supports to provide equivalent stiffness with less wall thickness

plastic injection molding

Material Selection

Selecting the proper material for your part design has a significant impact on wall thickness.  How the part is expected to perform and under what conditions will play a considerable role in material selection.

There are thousands of materials available to choose from.  Material properties not only effect wall thickness but also effect strength and durability.  Below are some recommended wall thickness guidelines with some common materials.

wall thickness

Refer to the materials page on our website for additional material options click materials .  

Do you have a question regarding your part design? Contact one of our Technical Specialists at 586-598-4636 or  We will be happy to review your design and provide you with recommendations.



Ribs Can Enhance Your Injection Molded Part

Ribs Can Enhance Your Injection Molded Part

Injection molding: Rib design

Ribs are a feature in plastic injection molded parts. They are thin extensions that run perpendicular from a wall or plane.  They are commonly used to provide additional support and strength to a part. Thickness and location are essential to rib design.  Below are some helpful guidelines when considering the use of ribs:
  • They should be designed with a thickness that is ½ the wall thickness to avoid a thick section at the wall base
  • To minimize sink marks, design ribs that are approximately 60% of the joining wall thickness for minimum risk
  • Best practices include spacing at a distance of at least twice the wall thickness
  • Glossy materials require a thinner rib (40% of wall thickness)

Parts can be designed with tall or multiple ribs.

Replace large, problematic ribs with multiple shorter ones to provide better performance.  Taller ribs can also provide greater support however if they are not sized properly, they can cause moldability issues.  (See Figure 1.) Figure 1-Tall and Multiple Ribs The material or materials being used for the part should also be considered when designing for resins. Depending upon the resin, rib thickness should be a percentage of the wall thickness.  Refer to the chart below for some general resin allocations. An advantage of using ribs is that they increase the strength of the part without increasing wall thickness.  Walls that are too thick can sink, warp or result in other defects.  Their integration into a part results in less material usage; therefore a more cost effective solution. Working with an injection molding manufacturer who is experienced in working with ribs can not only lessen your risk with production issues but also save costs and time in the long run.

Ready to get started? Xcentric makes it easy!

Submit a design to one of our Technical Specialists today for assistance at 586-598-4636 or
Use Overmolding To Create A Soft Grip

Use Overmolding To Create A Soft Grip

How to Create a Soft Grip

One of the more common reasons to utilize the overmolding process is to create a soft grip. Pulling from the example used
below, the green part is used functionally as a twist-able nozzle that will help direct the flow of a liquid. However, in this case,
the plastic chosen to facilitate the flow and to keep its chemical resistance for lasting part integrity happens to be very rigid and
hard to the touch; not ideal for tightening and loosening with human hands.

The solution is to design for an overmolded rubber-like grip to aid the user in the twisting of this product.
But, tactile functionality is not limited to human grip. Overmolding can also be cleverly used to add rubber-like grips to
clips designed to grab inanimate objects.

3 Types of Manufacturers: How To Choose

3 Types of Manufacturers: How To Choose

Here are the Top 3 Types of Manufacturers and Tips for Choosing The Best Option

Choosing a competent vendor to help you produce your part is a critical step for the success of your project.  Let’s examine some of the different types of vendors currently available to source from.

Conventional Manufacturing

Conventional manufacturing is Unless your design is highly niched, extra large in size or requires more than 100,000 parts run at a time, conventional manufacturing, let’s face it, is just too damned slow. You are looking at a 6-12 week lead time, at minimum. And changes to your design or the tool will take even longer. In this faced paced economy where being first to market is the difference between winning and losing, you just can’t afford to go that route.

Overseas Manufacturing

You are probably thinking that China can produce your part faster and cheaper than conventional manufacturing. And you’d be right. But faster than 6-12 weeks isn’t saying much and cheaper goes both ways…pricing and quality.
Here are some more potential problems when working with a manufacturer overseas:

  • Hidden Fees– often times, shipping costs, duty and insurance are not included in a quote and will be added after the fact.
  • Reduced Quality– it’s been reported over the years that, among other problems, many Chinese manufacturers will reduce part quality to increase profit margins. One technique they might employ is to use a substandard material than the one you specified to save on their costs.
  • Delays– due to logistics and the distances for shipping, delivery can be unpredictable.
  • Copyright– the laws are very different abroad. There is little you can do to protect your design from being copycatted.

The bottom line is that you are really going to have to take the extra time to vet your overseas vendor to make sure you are getting what you are paying for. But we know your time is precious.
There is a third option.

Digital, On-Demand, Rapid Manufacturing

There are some companies right here in the US that specialize in true rapid manufacturing. A true rapid manufacturing company will be able to provide a quote very quickly, will have in-house production capabilities to ensure fast turn-around and be readily available for answering questions.
Some companies who claim to be a true rapid manufacturing company are simply brokers. They will take your order within the States and get their Chinese manufacturing partner to produce the mold and parts. No matter what they claim, this sacrifices speed.
Xcentric Mold & Engineering is a true rapid manufacturing company. Because we understand the custom nature of all projects, quotes that come through our 24 hour, online, interactive quote system are thoroughly reviewed by a highly trained engineer to determine the best possible way to manufacture it. The result is a quote that represents the best possible price for your part and delivered within mere hours. Combined with our on-site, state-of-the-art, fully-equipped manufacturing facility and along with our Advanced Mold Making System and Proprietary Process Engine, we are routinely able to produce some of the most complex parts that other suppliers will “no quote” or request design concessions.
We don’t find it acceptable to require that you change your design. After all, the features you put into that part were put there for a reason, right? It doesn’t make sense to us that you remove them. Instead, we’ll do everything possible to think “outside the box” to get the part that you designed manufactured as you designed it.
That is what makes Xcentric Mold & Engineering the best choice for manufacturing your project.
We can produce your prototypes and samples very quickly and accurately, but when production parts are your end game, understand that we’ve perfected the injection molding process to:

  • provide from the lowest to the highest of runs
  • serve the most economical of budgets
  • produce the most complex of designs
  • deliver the most immediate and urgent of projects

lifetime-mold-guaranteeAnd finally, with our Lifetime Mold Guarantee, we will guarantee your mold for life. That means there’s no quantitily limit for the life of your project.
With a onetime, upfront tooling fee, we guarantee that we will supply unlimited part quantities for the life of your project.
Start your project off right with the manufacturer who can produce more parts, faster, cheaper and more consistently than anyone else.