Plastic Injection Mold Design | Complete Guide

The first step in plastic injection mold design is to design the part and pick plastic. Then, plan the mold and how plastic flows. 3rd step is to design the tools to shape the plastic. To design the tools, you can use SOLIDWORKS and CATIA. Add parts like pins and cooling tubes. Last, test the design on a computer. This helps find problems early. Then, you can build the real mold.

Let’s discuss mold design factors, like drafting, part lining, or advanced considerations, in detail.

Plastic Injection Mold Design Important Rules

1.   Parting Line Selection

The parting line in mold manufacturing is the point where both halves of the injection mold meet. It divides the mold into the top half (cavity) and bottom half (core). However, there may not always be a strict top-and-bottom configuration.  This line is important in the mold to determine the ejection, shape, or mold opening factors.

Also Read: Automotive Injection Molding

How the Parting Line Works in Plastic injection mold design?

The manufacturer chooses the mold and flows the molten material into the cavity, carefully filling the space to form the exact shape part. After the cooling process, they remove the part by opening the mold along the parting line.

They separate the two halves and pop out the molded product for ejection. It is important to choose the right parting line. Otherwise, you can face difficulties in manufacturing, and these can be caused by defects like flash (wasted material around the seam) and improper ejection.

Steps to Select the Parting Line for Custom Injection Molds

A.  Analyze the Part Shape:

Analyze the part shapes and address where the parting line would appear on the design. It is the point at which it divides the part into sections. For example, the flat surface parting line would be in the middle, and for complex-shaped parts, it depends on the curve or angle.

B.  Ensure Proper Draft Angle:

Remember to add draft relative to partining line. They will make the ejection process easier, or the part will stick inside the mold. Typically, the recommended draft angles are 1 to 3 degrees.

C.  Avoid Undercuts in Critical Areas:

If your mold designs are complicated and undercuts are unavoidable, then you can make them non-critical areas. So that you can address them by adding mechanisms like sliders or lifters.

2. Draft in Plastic Injection Mold Design

The manufacturers add a slight angle to the vertical walls of the molded part. The purpose of this draft angle is to make it a lot easier to pull out of the mold. Because the plastic shrinks as it cools by a little bit, and if all of the lines are perfectly straight, it can shrink against the mold and be very hard to get out of the mold.

How to Design Draft:

The kinds of plastics with high shrinkage chances, like polypropylene, need larger draft angles, commonly 2 to 3 degrees. Conversely, low-shrinkage materials (e.g., ABS) can be fitted with only 1-degree angle.

Part Geometry:

Find out which deep cavities or sharp edges of mold that really require larger drafts. A 3 to 5-degree drat angle can work for parts, and do not let them stoic against it.

Molding Process:

If you are using high-pressure processes in injection molding, select smaller drafts (1 degree), but low-pressure molds use larger angles. For example, the mold of a plastic cup contains a draft angle of 1-2 degrees for easy ejection.

3. Wall Thickness in Plstic Injection Mold Design

The wall thickness of mold designs can make the manufacturability of molded parts easy and efficient. It provides excellent performance, ensures uniform cooling, reduces stress, and prevents defects.

Key Considerations:

• Make all fairly uniform thickness thicker or as uniform as possible so improve part strength. But you must optimize them in order to eliminate excessive thickness. That might cause sink marks (surface dents) and internal stresses. The recommended range can be 1.5–4 mm, depending on the material.
• Thicker walls of mold take more time to cool down the injected material. That results in increased production time and cost. Therefore, if you try to maintain thinner walls as possible, you can make efficient production without compromising functionality.
• A variety of contexts cause warpage during the manufacturing process. The frequent reasons can be uneven thicknesses. That does not get even cooler and shrinks the part, leading to distortion. Try to add accurate thickness or make gradual changes in sections to avoid warping. Example: A plastic lid typically uses 1-2 mm walls for strength and efficient cooling.

4. Radii

Start by creating a radius (rounded corners) of the corners and intersections in mold design initially if you are not sure what a good size is so you can change that later.  These parameters improve the performance and aesthetic of parts.

Key Benefits:

• Sharp corners of the mold will not fill appropriately, blocking the smooth flow and causing air entrapment. So if you transmit that sharp edge into rounded ones, then they make this flow better.
• Sharp corners do not compensate for stress and crake the parts or break them. Adding a radius will allow the material to spread evenly and distribute stress.
• Rounded edges in parts also make them look beautiful and give a polished impact.

The pro tip for adding the right radius here is. Match the external radius of mold with part designs and give edges at least 0.5 to 1 mm roundness to their internal corners. If your design has thicker sections, you can add larger radii to maintain the strength and avoid defects like sink marks.

5. Ribs and Bosses

Ribs (kind of thin walls) and bosses (used for screws, locators, brass threaded inserts, fasteners, etc.) in mold design are structural features. They improve strength and support stability.

Key Considerations

• RIbs features in mold design support thin or flat wall sections. It removes the defect of bending or busking. On the other side, bosses give anchor points to screws or fasteners. You can modify the rib thickness according to the mold by keeping them at 50 to 70% of the wall thickness. It will prevent the sink marks.
• It also seems overly thick ribs or bosses can slow the cooling channels. They also increase the production time and may warp the part. For this, try to use consistent thickness and add rounded transitions.
• If your design contains sharp edges, then adjust the ribs or bosses, adding fillets (rounded edges). Add a radius of 10.25 to 0.5 times the ribe thickness to spread stress evenly. 

6. Undercuts

To make the mold halves totally clock during the injection process and then open them cleanly along the parting line, all you need to do is add undercut features. These can be holes, grooves, or projectors. However, they improve the performance of mold but somewhat increase complexity and manufacturing cost.

Solutions for Undercuts:

• It’s what’s known as a side action (movable components) moving sideways in the mold to free the undercut features before part ejection. These are supportive for undercuts like side holes or recessed places.
• Sliders are the other mechanical components that you can use in a mold. They slide into position when you do molding. Then the undercut is formed and the sliders retract to open the mold.
• They are not really needed in simple designs, but for complex shapes, they simplify mold construction, minimize undercuts, and reduce overall cost.

7. Textured Surfaces

Textured surfaces are part of designs that the designers add to increase the overall visual impact or functionality of molded parts. These can be in the form of patterns or deep grooves that serve more than simple aesthetics.

How Textures Work:

• Texture additions in mold design allow manufacturers to hide welded lines or any kind of scratches. In this way, the appearance of parts becomes clear and more attractive.
• Through textures, you can give a better grip to handles and reduce glare on surfaces. Plus, these textures resist wear impressively.

Common Methods:

Mold Inserts: There are different types of mold inserts. One type is pre-textured inserts, which you can place in the mold cavity to transfer specific patterns onto the part. The other is the chemical etching technique, which creates as intricate a uniform texture as possible on the mold surfaces.

Workable Tip:

It is good to use shallow textures (0.05 to 0.1 mm depth) in small parts for better injection and to get consistent or idencticla results. However, the deeper textures need a larger draft angle, approximately 3 to 5 degree angles so that they won’t stick in mold. 

8. Cooling Channels

Cooling channels and their types are designed as pathways where the operators flow the liquid or water or circulate the coolant. These channels escape the heat from the molten material. It is necessary to give sufficient cooling to create part error-free.

Design and Placement:

• Placement: Place the cooling channels evenly near high-heat areas of the thick sections. Focus on giving a consistent distance, like 1.5 to 2 times the channel diameter of the mold surfaces. These distances eliminate the warping and hotspots in parts.
• Diameter: Basically, channel diameters range from 6 to 12 mm about, according to the mold size and as cooling needs to spread out. Large cooling channels create resistance in localized cooling but give better flow.
• Flow Rate: Apply a flow rate of 1–2 m/s. This will maximize heat transfer without causing turbulence.
• For the part that has thicker walls, you need closely spaced channels (8 mm in diameter). This will reduce cooling time by up to 30%, improving production efficiency.

9. Ejection System

The ejection process in manufacturing starts after the molted material is transformed into a solid shape. The manufacturer makes sure they remove the part safely without damaging its features.

Components and Design:

• Ejector Pins: These pins are for removing the part from mold. Attach or place these pins in that place where they won’t damage the molded part to avoid leaving marks on critical surfaces. Plus, try to maintain easy ejection without using these pins on thin or fragile sections.
• Ejector Pin Force: The force of the ejector pin is determined by identifying the material nature and part geometry, and it typically ranges from 0.5 to 3 tons. Very low amounts of force cause stickiness, or very large forces use risk deformation. The calculation of the right forces is here based on surface area and friction.
• Tip: You can use an ejector plate. They will synchronize multiple pins for even force distribution. It further reduces the risk of defects.

10. Mold Base Selection

Every manufactured part has a substrate called the base. Similarly, the mold base is the area where the engineer assembles or attaches all the main components. They ensure the parts remain stable and in alignment.

Key Points:

• Mold Base Size: the manufacturer pinks the size of the base, comparing all component dimensions (cooling channels and ejection system) to make it fit on the mold. Add extra space for later modifications. You can choose a 150 x 150 mm mold base for small parts, while larger parts may need over 500 x 500 mm, obviously depending on mold size. For instance, small plastic connectors have good workability with a 150 x 200 mm standard mold base.
• Mold Base Types: You can use ready-made or budget-friendly bases that are good for simple molds. Custom bases also fit the need for complex or precise parts. Plus, hardened bases are suitable for high-performance parts and long-run production due to their high durability.

Conclusion

We just wanted to give you an idea of what’s involved in building a mold design. That is really helpful for you if you are doing in-place modeling with different components. Carefully focus on each parameter, like parting line sections, drafts, wall thickeners, radii, etc. So that you achieve the right design at cost-effective prices.

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