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.
Rapid molding is a key player as the “digitization” of nearly every process to make products continues. The latest industry to experience this shift is molding. Here’s a look at how digitization may affect traditional molding service providers.
Pierre Viaud-Murat | Senior Vice President of Sales
Digital manufacturing offers many benefits across multiple touch points: it can reduce time to market, labor overhead and asset use, plus it helps customers control quality and inventory. These advantages allow users to explore new revenue streams, develop and enhance innovative designs and respond to market demands. The digital age can enhance designers freedom to create and develop through one streamlined methodology.
The traditional manufacturing process The traditional manufacturing process usually consists of a several-step sequence within the manufacturing flow. Team members monitor and ensure that safeguards are in place throughout the entire part lifecycle. Throughout the process, various testing for form, fit and function are required to discover any part flaws. Each team member should be aware of these critical-path processes, as the traditional method of manufacturing requires more manual than automated production.
The traditional injection molding manufacturing process usually consists of several steps. Throughout the process, various testing for form, fit and function are required to discover any part flaws. Usually, the traditional method of manufacturing requires more manual than automated production.
For example, generating a quote using a DFM analysis and confirming an order manually can take approximately a week, if all of the process steps are accurate and on track the first time through. In the event that any modification is required, the same process steps would be repeated before finalizing the order.
Once an order has been placed, the mold design is reviewed for viability. When that design is finalized, it will go onto the next phase of the traditional manufacturing process. This can be a lengthier portion of the process, ranging from three to eight weeks. Additional delays may also occur if project issues are not identified early in the quoting process, which will result in re-quoting, redesigning or redefining the project. Any of the three can result in significant delays.
With all of the production factors in place, the part will then be molded. After the first shots, a visual and dimensional inspection will be done. If the part passes inspection, it is shipped out to the customer for review and feedback.
Once the customer inspects the part, they have the opportunity to approve or reject the run. In the event of a non-approval, the process would be modified and revisited again with continual iterations until the optimal part is achieved. Any combination of these factors can impact the cost, quality and timing in a traditional manufacturing process.
The digital manufacturing process Another approach to injection molding involves the digitization of as many injection-molding steps as possible. This approach is referred to as rapid or digital manufacturing, and is a natural progression of traditional manufacturing. The streamlining possible by digitizing many traditional molding steps can reduce total costs.
An evolution of traditional injection molding manufacturing is referred to as digitization. It involves digitizing as many injection-molding steps as possible to streamline many traditional molding steps to reduce total costs.
Digital manufacturing begins with the upload of a 3D CAD file to a service provider’s servers. The geometry and part requirements are analyzed, usually with proprietary software. Then the customer receives an interactive quote. Once the customer approves the quote, an order is generated and the mold design finalized and a tool-path created, often within hours.
The initial mold can often be created with modular components, which is another cost savings. Then, the mold is usually machined, a process that takes a couple of days. After benching and finishing, the mold tool is assembled for first production shots and inspection.
This process of rapid mold development can shorten development lead-time to weeks instead of months.
Digital manufacturing can accelerate every step of a part creation process. In traditional manufacturing, the retooling investment could push the break-even manufacturing numbers into the thousands – a cost-prohibitive change for many smaller businesses. Digital manufacturing, however, can implement that small design change for approximately a quarter of the investment in a quarter of the time.
Digital manufacturing is scalable and flexible. Should demand rapidly increase, the digital manufacturing process can move between low-volume to mass production. Thus, designers can react to market behaviors quickly and easily. If demand drops, rapid manufacturing enables adjustments while still achieving the lowest total cost. Thus, regardless of a market’s volatility, digital manufacturing lets users respond accordingly.
The digital manufacturing process allows customers to quickly go through multiple iterations easily. Low-cost tooling makes low-production runs economical enough to test on select markets and use the feedback as a learning curve. Customers can maintain their existing quality controls, while developing good parts that can get to market faster. Some service providers offer optional services, such as inspection and project management consulting. Digital inspection allows for rapid feedback and tool modifications, if necessary. Service providers usually have experienced tooling experts who can consult during the mold development process to solve any problems that arise.
Mold considerations Mold service providers strive to ensure a mold tool is available over the lifecycle of a project. Often, the tool is made from a high-grade aluminum base material, like QC-10, which offers an excellent strength-to-weight ratio.
Compared to steel, aluminum is softer and less dense, dissipates heat quickly and efficiently and costs up to 75% less. It’s also a recyclable material, an attractive feature when material waste can be higher than 50% per part. Recyclability and material reuse can help recoup initial material costs.
Although aluminum may not be the right material for every type of mold, it works well for prototyping, bridge tooling and low volume production. Depending on the size and structure of the parts, the heat dissipation within an aluminum mold can be up to 50% higher than steel or other metals, creating faster production turnaround times. Aluminum molds will usually last through production runs of several thousands of parts. DW.
We are frequently asked about different plastic molding processes and how each effect the process of part design and production. In this post we explain the basics of 4 plastic molding processes: plastic injection molding, blow molding, rotational molding, and vaccuum molding.
Plastic Injection Molding
Xcentric Mold specializes in plastic injection molding for low-volume and protyping. Injection molding is one of the most versatile manufacturing processes. It is used for producing simple and complex plastic parts for nearly every industry.
The injection molding process involves injectiong molten material into a mold. The, material is fed into a heated barrel, mixed (using a helical shaped screw), and injected into a mold cavity. Finally, the material cools and forms the plastic part.
Not all materials heat (or cool) the same.
When designing for plastic injection molding, material selection is a critical for success. After all, materials do not all perform the same – during or after the injection molding process. To achieve the intended fit, form, and function of your part design, work with your supplier to choose the optimal material.
The injection molding process can be performed with a host of materials including: plastic, metal (for which the process is called die-casting), glass, 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 Mol. However we may be able to provide you with recommendations of other suppliers who can assist you.
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
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 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 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 molding 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 Application Engineers today at 586-598-4636 or firstname.lastname@example.org.
Our wall thickness guide will come in handy when you’re designing plastic parts for the injection molding process. Regardless of industry or application, designing plastic parts can be a challenge. Investing time early in process to optimize your design for manufacturability can help to save time and money.
Top 5 design tips for optimal wall thickness
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.
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
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.
Designing Plastic Parts For Injection Molding? Ribs Can Help Add Support And Strength.
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. Here are some helpful design guidelines:
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 experienced injection molding manufacturer can not only lessen your risk with production issues but also save costs and time in the long run.
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One of the more common reasons to utilize the overmoldingprocess 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.