7 Common Elements Caught in the Design for Manufacturing Process

Getting products to market fast and on budget are two critical factors in the manufacturing process. The design for manufacturing process is cited by both product manufacturers and injection molders as the step that can have the greatest impact on production outcomes. When plastic injection molders are involved early in the part design process including prototype development and mold flow analysis, many cost and time efficiencies will be realized.

Designing a plastic part for manufacturability from the outset involves several considerations that can ultimately have a significant impact on key variables. While some manufacturers don’t account for design adjustments in their timelines, early collaboration with your molder may uncover aspects of a design that can be optimized to improve the efficiency of part production and performance. Here are a few of the most common elements caught in the design for manufacturing process:

1. Draft

As an essential requirement in injection molding, draft angles make it easier for a finished, cooled part to be released from a mold. Minimizing friction during the part release process is important to prevent damage to the parts, provide a uniform surface finish and reduce wear and tear on the mold.

Draft angles are calculated as a degree measurement from the direction of pull. Designing a part with sufficient draft is critical, which is why design engineers typically recommend minimum draft angles of 0.5 degrees for core and 1.0 degree for the cavity. More draft is also needed if a textured surface is desired and if there are steel shut off surfaces in the tool design.

2. Wall Thickness


Another important factor in part design is wall thickness. A proper and uniform wall thickness reduces the risk of structural and cosmetic defects in injection molded parts.

While typical wall thickness ranges from .04 – .150 for most resins, it is recommended that you work with a knowledgeable injection molder/design engineer to verify thickness specifications for the material(s) you are considering for your part.

Analyzing wall thickness is an essential step in the design process to avoid producing parts that have sink, warp or are ultimately non-functional.

3. Ribs

Ribs are used to strengthen the walls of your part without increasing wall thickness, making them a valuable element in injection molded parts. Particularly in complex parts, good rib design should shorten the mold flow length while ensuring the proper connection of ribs to enhance the strength of the part.

Since thickness and location are essential in rib design, ribs should be no greater than ⅔ of the wall thickness, depending on the material used. Using wider ribs may create design and sinking issues. To mitigate this, a design engineer will typically core out some of the material to reduce shrinking and maintain strength.

Rib length should not exceed 3 times the length of the wall thickness, as anything over this could lead to part shorting/not being able to fill the part completely. Identifying the proper placement, thickness and length of ribs in the early phases of part design is an important element to the viability of a part.

4. Gate Location


A gate is the location where molten plastic material flows into the mold part cavity. While every injection molded part has at least one gate, many parts are manufactured using several gates. Because gate location affects the orientation of the polymer molecules and how the part will shrink during the cooling process, gate location can either make or break your part design and functionality.

For example, if a part is long and narrow and must be absolutely straight, it is best to place the gate at the end of the part. For parts that need to be perfectly round, a centrally located gate is recommended.

Sharing preliminary part designs with your injection molding engineering team, leveraging their knowledge and expertise in material flow, will result in optimal gate placement and injection points.

5. Ejector Pin Location


After a plastic part is molded, ejector pins (located within the B-side/core of the mold) apply just the right amount of force required to eject the part from a mold. Ejector pin location is typically a relatively minor concern in the early phases of design, but marks and indentations can result from improperly placed ejector pins, which is why design and positioning should be considered as early as possible in the process.

The location of ejector pins depends on a number of factors, including draft and texture of sidewalls, depth of walls and ribs, and the type of material used. Reviewing part designs will either confirm that your initial ejector pin placement is correct or may generate further recommendations to improve production outcomes.

6. Sink Areas

When the material in the area of thicker features, such as ribs or bosses, shrinks more than the material in the adjacent wall, sink marks may result in the injection molded plastic part. This occurs because thicker areas cool at a slower rate than the thinner ones, and the different rates of cooling leave a depression on the adjacent wall that is commonly referred to as a sink mark.

Several factors contribute to sink mark formation, including the processing methods used, part geometry, material selection and tooling design. Depending on the part specifications, it may not be possible to adjust geometry and material selection, but there are many options available to eliminate sink areas.

Depending on the part and its final application, tooling design (e.g., cooling channel design, gate type and gate size) can be leveraged to influence sink. In addition, manipulating process conditions (e.g., packing pressure, packing time, length of packing phase and conditions) offers several options to reduce sink. Finally, minor tooling modifications, such as retrofittable components or process modifications (e.g., gas assist or foaming) are also available to combat sink. As a result, it is best to collaborate with your injection molder to determine which methods will work best to mitigate sink in your specific injection molded parts.

7. Parting Lines


Parting line location is worth noting and planning for when producing more complex parts and/or when complicated shapes are required.

Since part designers and molders tend to evaluate parts differently, sharing your design with your injection molder can dramatically affect the production and function of your finished product. If parting line challenges are found, there are several ways to address them.

Being aware of the significance of the parting line in your initial design is a good first step, but that may not be your only option. By leveraging CAD software and mold flow analysis, you may be able to determine other possible locations. Working with a knowledgeable injection molder will keep your part end use top of mind and will guide you to the best possible location for parting lines.

There is no question, engaging your plastic injection molder early in the design for manufacturability process and working closely with a design engineer to identify efficiencies will help get your product to market quicker and on budget. What challenges are you currently facing with the plastic part design process?

Learn how Nicolet Plastics can help you reduce lead times and identify turn-key solutions for every product.

8 Factors in Plastic Part Design for Manufacturability

Plastic Injection Molding DesignDesigning a plastic part for manufacturability involves many important factors that touch on all areas of part design, tooling, material selection and production. First, it is essential to build parts around functional needs by keeping design intent or the end use in mind. Consider weight reductions, the elimination of fabrication and assembly steps, improving structural components, reducing costs and getting products to market quicker. Here are 8 important factors to consider to meet your plastic part design goals for a successful production process.

  1. Material Considerations

Manufacturers often select a familiar grade of plastic from a similar application or rely on recommendations from suppliers. Resins chosen this way may be adequate, but are rarely optimal. Plastic selection is a complex task that involves many considerations, such as:

  • Temperature: Thermal stress that may occur during normal and extreme use conditions, as well as during assembly, finishing and shipping.
  • Chemical resistance: The effects that occur when any solid, liquid or gas come in contact with the part.
  • Agency approvals: Governmental and private standards for properties such as heat resistance, flammability, and electrical and mechanical capabilities.
  • Assembly: The plastic’s cooperation with all assembly steps like bonding, mechanical fasteners and welding.
  • Finish: The material’s ability to produce the desired finish such as gloss, smoothness and other appearance values as it comes from the mold.
  • Cost: Resin pricing as well as the cost calculations for manufacturing, maintenance, assembly and disassembly to reduce labor, tooling, finishing and other costs.
  • Availability: The resin’s availability in regard to amount needed for production.
  1. Radius

Radius should always be a consideration in regard to the part’s thickness – eliminating the likelihood of areas of high stress and possible breakage of the part. A general rule of thumb is that the thickness at a corner should be in the range of 0.9 times the nominal thickness to 1.2 the nominal thickness of the part.

  1. Wall Thickness

Designing your part so that wall thickness is consistent can help avoid many part defects that can occur during the manufacturing process. When plastic melts, it flows to the areas of leas resistance. If your part has inconsistent thicknesses throughout, the melt may flow into the thick areas first (depending on gate locations). When this occurs, the thin areas may not fill properly. Additionally, thicker areas tend to cool more slowly and are at risk for voids or sinking defects. Designing your part with rounded corners will also aid in the proper filling of the part during the molding process.

  1. Gate Location

Gates are critical to ensuring the resin flows properly into the mold. These small components of your design are what directs the flow of resin from the runners to then be distributed through the part. Type of gate and placement has an important impact on the part’s overall quality and viability.

  1. Draft

Draft is the amount of taper on the vertical walls of the plastic part. Without draft, a part may not eject from the mold, or may sustain damage during ejection. Typically, draft angles between 1° and 2° are required, but can vary depending on part restrictions and specifications. 

  1. Inclusion of Ribs

A plastic part that has been designed with a minimal wall thickness will not be as strong as a thicker part – which is why the inclusion of ribs may be needed to help reinforce the part’s strength. Depending on the material used, rib thickness should be between 50 – 70 percent of the relative part thickness to avoid sink marks. To avoid sinking, designers may core out material to reduce defect risk.

  1. Mold Shrinkage

The shrinkage that occurs during the plastic part molding process can be as much as 20 percent by volume. Crystalline and semi-crystalline materials are most prone to thermal shrinkage. Amorphous materials are known to shrink less. Here are a few easy ways to avoid molding shrinkage issues:

  • Adjust the formulation
  • Adjust the mold design to get the dimension you want based on the expected shrinkage that will occur
  • Optimize the processing parameter such as molding temperature, melt temperature, and injection speed/pressure/time, cooling time.
  1. Special Features

Plastic parts should be designed so that mold tools open and eject the part easily. When a part is released, the two sides of an injection mold separate in the opposite direction. When special features like holes, undercuts or shoulders prevent the release from happening, it may be required that side actions be incorporated into the design.

Side actions pull coring in a direction other than the direction of the mold separation. This adds flexibility to the part design and at times, may increase the cost of the mold.

Working with an experienced plastic injection molder and engineering team is a critical component to avoiding many issues that can occur during the design and development process. If you keep these factors in mind during the design process, and align with a knowledgeable plastics engineer, you will be on track to get your product to market quicker and within your budget.

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3 Tips for Calculating the Right Press Size for Your Plastic Injection Molded Project

Injection Moulding Machine

Are you a product designer or engineer that is looking for more information on how to make your plastic part design more efficient in regard to cost and production time? One important consideration during the part design process is to have a good understanding of plastic injection press basics including the size of machine needed for your part.

“Bigger the better” is not always the case when determining the press size needed. In the molding process, plastic is injected into the mold at an exceptionally high-pressure rate, which creates a natural pull to force the mold open. A press is designed to keep the mold shut with larger parts requiring more tonnage and force, and smaller parts requiring less. A general calculation for determining press size needed is as follows:

Pressure (lb/in2) x Projected Area (in2) = Force (lb.)

Here are a few other important tips for calculating the right press size for your plastic injection molded part. 

  1. Understand press size tonnage.

Your plastic injection molder should help you determine the size of machine needed to help you achieve the best result for your product. Knowing an approximate size of what will be needed can help you determine the best injection molding partner based on the press capacity they have available. For example, larger presses cannot accommodate smaller molds because they can’t close far enough and the injection process will not work.

Additionally, smaller presses have tie bar spacing too narrow to accommodate larger products. If the mold doesn’t fit between them horizontally or vertically, you must move up in press size. Many injection molders offer press sizes ranging from 68 ton up to 400 ton.

  1. Calculate your total projected shut-off area and shot volume.

When determining press size for your plastic part, it’s important to calculate the total projected shut-off area. This area consists of only the space that is 90 degrees to the direction of the injection molding machine platens. Thickness does not have any implication on the clamp tonnage and the general rule is to have 2 to 5 ton of clamp tonnage per square inch of projected area.

Calculating shot volume to make sure your barrel has enough capacity can be accomplished by working with your injection molder to run a mold flow analysis. On some engineered materials, the increased residence of the material in the barrel can cause the material to degrade, resulting in poor part quality. Mold flow analysis will help you determine the volume of your part and runner while determining any factors that would cause safety issues. 

  1. Know how much clamping force or pressure is required. 

Pressure plays a significant role in the overall quality of a plastic part. Pressure keeps the mold closed during the injection process. Too much or too little pressure can cause various issues such as flashing and viscosity. One important consideration in regard to pressure is that plastic compounds react differently from one another based on their Melt Flow Index (MFI). MFI measures the ease of flow of a thermoplastic polymer and the higher the MFI, the higher pressure needed to create a successful part.

“Safety factor” is an additional percentage added to your calculation as a buffer to help reduce defects in your part. Most injection molders will recommend 2.5 times the surface square inches of the part and an additional 10% as a safety factor. If you have a part that is 120 square inches, you would need a press size with 300 tons of pressure. When you add the 10% safety factor, the required press size would have 330 tons of clamping force.

Having a general understanding of how to calculate press size is a good first step in determining what injection molding partners are available to you. Strong partners will make recommendations on how to appropriately tweak your part to ensure the final design fits your manufacturing needs and reduces upfront tooling costs. Learn about Nicolet Plastics quick response manufacturing processes and press sizes up to 400 tons.

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