If you’ve ever picked up an injection molded part, you can find a line around the circumference of the part. Why is it there, and how is it created as the part is molded? It’s a simple answer, but it can be quite complicated to explain. Let us give it a try…
What is the Parting Line in Injection Molding?
Injection molding is the process of forcing molten resin into an injection mold under pressure. The mold is comprised of two halves: one side is called the cavity, and the other is called the core. These two halves of the mold come together for the injection portion of the process, where they touch is called the ‘parting line’. Think of the parting line as where the molds come a(part) if that helps you remember what it is.
The parting line (abbreviated PL on drawings) can also be seen around tooling inserts, shutoffs, and action pins inside the mold. There are several categories of parting lines, which include straight or flat areas and stepped or curved areas. Areas that require tight sealing surfaces or exacting precision may require additional flash (leftover molded resin on parting lines) to be removed. These removal options can include hand trimming, vibratory tumbling in a rough media, media blasting, and cryogenic deflashing.
Where Is the Parting Line After Molding?
The parting line on an injection molded part is unfortunately unavoidable. The two halves must come together at some point on the part to enclose the resin while it is being injected. While it’s unavoidable, that doesn’t mean that it can’t be hidden in an inconspicuous spot. The goal with the parting line it to strategically place it on the part in a location that isn’t as noticeable and won’t decrease the function or intended design of the part.
One of the deciding factors must include the cost associated with the mold design. The placement of the parting line can have a dramatic effect on the investment cost, so choosing the location of the parting line is critical. The simplest and lowest cost typically designs a parting line on a flat surface or flat plane. On the other side of that, complex or curved parts can add parting lines that must follow the complex curves of the molded part. Complexity adds cost, plain and simple.
What Types of Parting Lines Are There in Injection Molding?
Parting lines occur where the two halves of an injection mold meet. Sometimes that is directly in the center of the part, and other times it’s in a different location entirely. The design should meet the functionality and form required of the molded part, and there are many options to ensure that happens. These are typical options available:
- Vertical parting line: The line is on a plane perpendicular to the mold opening direction
- Beveled parting line
- Curved parting line
- Stepped parting line
- Comprehensive parting line: The line is a combination of stepped, curved, or beveled
How Can a Parting Line Affect Your Quality?
There’s no doubt that poorly designed parting lines can affect the visual quality of the finished part. What most don’t think about is that a poor parting line can also affect the strength and durability too. Many times the wall thickness of a finished part is only a few millimeters or maybe 1/8-inch, and poor parting line quality can affect how well the parts are formed together. There are a few factors that need to be considered when designing the injection mold to ensure high quality standards:
- The parting line location on the injection mold will have an effect on how the cavity and core (both halves of the injection mold) come together and seal. There needs to be a balance between fit of the halves, function of the part, visual quality expectations, and the cost to produce the mold. Very high quality and an extreme fit may cost more than the planned budget for the mold. On the opposite side of that, low quality standards cost less initially but require more work after molding to produce a higher quality part.
- The mold design will also correlate to the surface finish of the final part. Surface finish can be used strategically to blend or partially hide the parting line. You will need to make sure the design can accommodate the parting line type. A vertical parting line may not work in for all mold designs, so another may be the best option.
- The mold design is related to how the molten resin flows through the mold and cools to the shape of the finished part. The cooling rate can cause friction (also called shearing rate) that can contribute to stress in the final part and a loss of function and durability.
- The ejection process after the part has cooled in the mold will leave small amounts of flash where the ejector pins are located. The design of the mold and the ejection process should be considered to ensure they don’t affect the finished product.
Conclusion
Choosing where to locate the parting line and what style to use is a difficult design decision if you don’t have experience in injection mold design. If you need a partner to discuss your options, look no further than SEA-LECT Plastics. At SEA-LECT Plastics we pride ourselves in tool and die manufacturing that allows us to deliver high quality products free from defects. We have multiple options for manufacturing to help your business succeed. SEA-LECT Plastics has an elite team that can discuss the available options for your new project, we produce world-class prototypes and products, and we have decades of experience with plastic injection molding operations. Our tooling experts can create molds from 3D designs and make fine adjustments to ensure smooth molding operations on a daily basis. We can offer support to determine what type of tooling you need, what materials to choose, and can support the generation of documentation to win new business in your industry. Give us a call at (425) 339-0288 or email us at mattp@sealectplastics.com.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.