Designing plastic molded parts in our factory involves a precise process of engineering and craftsmanship, ensuring each piece meets stringent quality and performance standards.

Designing plastic molded parts focuses on material choice, part geometry, and mold design to boost durability, performance, and cost-effectiveness, while minimizing material waste and optimizing manufacturing processes.

While this introduction outlines the essentials of plastic part design, delving deeper into each component offers insights into improving manufacturing outcomes. Discover how material selection and advanced mold design techniques elevate part quality and production efficiency.

What is the Structural and Dimensional Design of Plastic Molded Parts?

Structural and dimensional design is crucial in molding plastic parts, influencing their function, longevity, and manufacturability across diverse applications.

Designing plastic molded parts requires attention to dimensions, tolerances, and structural integrity to ensure effective performance. Important elements include appropriate wall thickness, rib design, and draft angles to enhance strength and molding efficiency.

Structure Design

• Mold size:When designing the mold size, you need to consider the size requirements of the product and the shrinkage rate of the plastic material. Generally, the size of the mold should be larger than the size of the final molded product. Different plastic materials have different shrinkage rates, generally between 0.1% and 2%.

• Mold splitting method¹:Choose the appropriate mold splitting method according to the shape and requirements of the product. Common mold splitting methods include fixed upper mold, movable lower mold, movable upper mold, fixed lower mold, left and right mold splitting, etc. The mold splitting method should consider the shape of the product, manufacturing difficulty, mold structure, cost, and other factors, and strive to split the mold smoothly and have high production efficiency.

• Das Kühlsystem²:The cooling system in the mold has a big impact on the quality of plastic products and production efficiency. The cooling water arrangement and size should be reasonably designed. The cooling water channel should be as close as possible to the contour of the product to improve the cooling effect. At the same time, attention should be paid to avoid deformation caused by uneven cooling, shorten the cooling time, and improve production efficiency.

• Exhaust system:When designing a mold, you need to consider the gas generated during the filling and cooling of the plastic material in the mold. You need to have a way to get rid of the gas. The design of the exhaust system should try to avoid the production of bubbles and short shots and other defects, and improve the quality of the product.

• Hopper design:The hopper design should be reasonably arranged, with the hopper and nozzle positioned to ensure that the plastic material flows evenly into the hopper, avoiding material blockage and nozzle bubbles. At the same time, consider the connection between the hopper and the mold, making it easy to disassemble and clean.

Dimension Design

• Wall thickness design:The thickness of the wall of the product directly affects the quality and performance of the molded product. If the wall is too thick, it will cause shrinkage and deformation of the molding, and if the wall is too thin, it will cause the product to lack strength. In the design process, the wall thickness should be reasonably controlled according to the use and requirements of the product to improve the quality of the product.

• Clamping force design³:Clamping force is the force needed to separate the mold, and it should be determined based on the size of the product, the structure of the product, and the nature of the material. If the clamping force is too high, it will increase the load on the equipment. If the clamping force is too low, the mold will not separate completely.

• Sliding parts design⁴:When molding parts with a sliding structure, you need to design the slider’s position and shape properly. This will make sure the slider moves smoothly and doesn’t get stuck or damaged during use. Also, when you design the mold, you need to pay attention to the dimensions of the sliding parts and the mold. This will make sure the mold opens and closes smoothly.

• Tensile design⁵:To make sure the tensile rod doesn’t get bent or broken when you’re stretching something with a lot of tensile strength, you need to put it in the right place and make sure it’s the right shape. You also need to make sure the stretching rod and the mold are the right size so you can stretch things right.

What is the Design and Development Process of Plastic Molded Parts?

Designing and developing plastic molded parts entails a systematic approach to ensure efficiency, functionality, and quality in production across various industries.

The plastic molded parts design process encompasses concept creation, material selection, mold design, prototyping, and testing, focusing on manufacturability, cost-effectiveness, and achieving specific mechanical properties.

Figure out the Demand

First, you need to know what this plastic part is for. Is it a toy part or a part in a car? Once you know what it’s for, you’ll know what performance requirements you need to meet. Does it need to be strong? Does it need to be wear resistant? Does it need to be chemically resistant? And so on. At the same time, we also need to think about any special requirements the customer or the market has for this plastic part. Does it need to look good? Does it need to be a certain color? Does it need to be cheap? And so on.

Start Sketching

Based on what the designer has told me, I would sketch out a simple plastic part on paper or with computer software. I would show the shape and roughly how big it is. I wouldn’t worry about being too accurate at this point. I just want to get the general idea across. I want to see if it looks like it will work. For example, if there are some places where the shape is really weird and it can’t be made, or if the size doesn’t look like it will work for what it’s going to be used for.

Detailed Design

The sketches are further refined and the 3D model of the plastic part is accurately drawn using specialized design software⁶ (e.g. CAD, ProE, etc.). In this process, it is necessary to determine the specific dimensions of the plastic part, the details of the shape of each part, how much wall thickness is appropriate and so on. It is also necessary to consider how the plastic part fits with other parts, for example, whether it is to be matched with screw holes or spliced together with other parts.

At the same time, choose the right plastic material. Different plastic materials have different characteristics. Some are very hard, some are very soft. Some are transparent, some are opaque. According to the use of plastic parts and performance requirements, choose.

Analyzing and Checking

Do a bunch of tests on the plastic parts you designed to see if they’ll actually work. For example, use some fancy software to see if the plastic part will get all bent or broken when you push on it, check how hot it gets, see how well the plastic flows into the mold, and so on. If you find any problems, you have to go back and change your design until all the tests come out good.

Making the Mold

Once you’ve designed a plastic part, you need to make a mold for mass production. The mold designer designs the structure of the mold based on the design drawing of the plastic part, including the mold cavity (where the plastic part is formed), the gate (where the plastic melt enters the mold), the cooling system (to allow the plastic to cool down quickly for molding), and so on. Then you find a factory to process and manufacture the mold based on the mold design drawing[^7]. This process requires high precision because the quality of the mold directly affects the quality of the plastic part.

Trial Mold

Once the mold is ready, the first thing you do is to test the mold on the injection molding machine. You heat up the plastic material, melt it, inject it into the mold cavity through the injection molding machine, then you open the mold after the plastic cools and solidifies, and you take out the plastic parts.

You see if the plastic parts are the same as the design, if there are no defects, like the surface is not smooth, there are bubbles, the size is not accurate, and so on. If there is a problem, you need to adjust the mold or the injection process parameters (like temperature, pressure, injection speed, etc.), and then you try the mold again, until you make a satisfactory plastic part.

Massenproduktion

Once you’ve successfully tested your mold, you can start mass producing plastic parts. During production, you need to strictly control the quality of production and regularly check the size, appearance, and performance of the plastic parts to make sure each product meets the requirements.

Quality Inspection and Improvement

Quality testing is necessary for plastic parts. Use different methods (like measuring dimensions, testing strength, checking appearance, etc.) to find out which products don’t meet the standards. Also, pay attention to feedback from customers and problems that come up during production. Improve the design or production process for plastic parts so that the quality keeps getting better.

What are the Key Points for Designing Plastic Molded Parts?

Designing plastic molded parts requires a balance of functionality, manufacturability, material selection, and cost-effectiveness to ensure high-quality production and performance.

Designing plastic molded parts requires understanding material properties, ensuring proper wall thickness, incorporating draft angles, and planning for manufacturability to enhance product performance, durability, and cost-efficiency.

Loch

The holes should be as simple as possible, generally round. The hole should be in the same direction as the mold opening direction, which can avoid the core pulling. When the length-to-diameter ratio of the hole is greater than 2, the demolding slope should be set.

At this time, the diameter of the hole should be calculated according to the small diameter size (the largest solid size). The L/D ratio of blind holes is generally not more than 4. The distance between the hole and the edge of the product is generally larger than the hole size.

Abgerundete Ecken

If the rounded corners are too small, the product may crack due to stress concentration. If the rounded corners are too small, the mold cavity may crack due to stress concentration. Setting a reasonable rounded corner can also improve the

machining process of the mold, such as directly milling the cavity with an R cutter, avoiding inefficient electrical machining.Different rounded corners may cause shifting of parting lines, and different rounded or cleaned corners should be selected in conjunction with the actual situation. Marking

Product marking is generally set on the inner surface of the product is relatively flat, and the raised form, select the normal direction and the molding direction ruler may be consistent with the face of the marking, you can avoid straining.

Precision of injection-molded parts; due to the non-uniformity and uncertainty of shrinkage during injection molding, the precision of injection-molded parts is obviously lower than that of metal parts, and appropriate tolerance requirements should be selected in accordance with the standard (OSJ1372-1978)

Einsätze

You can put inserts in injection molded parts to make them stronger, harder, more accurate, and to put small threaded holes (bosses) in them for whatever reason you want. It also makes the part more expensive. Inserts are usually made of brass, but they can be made of other metals or plastic. Inserts that are molded into the plastic part should be designed so they don’t rotate or pull out.

For example: knurls, holes, undercuts, flats, shoulders, etc. Inserts that are around the plastic should be made thicker so the plastic doesn’t crack. When you design an insert, you have to think about how it will be held in the mold (holes, pins, magnets, etc.).

Mold Release Tilt

Good mold release slope can avoid product hair pulling. The mold release slope should be more than 0.5 degrees for smooth surfaces, more than 1 degree for fine textured surfaces, and more than 1.5 degrees for coarse textured surfaces. Good mold release slope can avoid the product top injury.

When designing deep cavity structure products, the slope of the outer surface should be less than the slope of the inner surface to ensure that the core of the mold does not deviate when injection molding, to get uniform wall thickness of the product and to ensure the strength of the material density of the open part of the product.

Wanddicke

Different plastics have a certain range of wall thickness, generally 0.5 ~ 4mm. When the wall thickness is more than 4mm, it will cause excessive cooling time, resulting in shrinkage and other issues. You should consider changing the product structure. Uneven wall thickness will cause surface indentation. Uneven wall thickness will cause air holes and fusion marks.

Verstärkung

Using reinforcement properly can make the product more rigid and reduce deformation. The thickness of the reinforcement should be less than 1/3 of the wall thickness of the product, otherwise it will cause surface indentation. The slope of one side of the reinforcement bar should be more than 1.5° to avoid top injury.

One-Piece Hinge

By using the toughness of PP material, the hinge can be designed to be integrated with the product. The size of the film used as hinge should be less than 0.5mm and kept uniform. When injection molding one-piece hinge, the gate can only be designed on one side of the hinge.

Spritzgießen mit Gasdruckunterstützung

Gas-assisted injection molding can make products more rigid and less likely to deform. Using gas-assisted injection molding can prevent shrinkage. Using gasunterstütztes Spritzgießen⁷ can save raw materials and reduce cooling time.

Mold Opening Direction and Parting Line

When designing an injection molded product, the mold opening direction and parting line should be determined at the beginning of the design process. This will help minimize the need for complex core pulling mechanisms and eliminate any negative impact on the appearance of the product caused by the parting line.

Once you know which way the mold opens, you can design the reinforcements, clips, ribs, and other features to match the mold opening direction as much as possible. This will help you avoid undercuts, reduce knit lines, and make the mold last longer. For example, if the mold opens in the X direction for the part, and you design a feature that opens in the Y direction, you need to call that out on the drawing. Once you know which way the mold opens, you can pick the best parting line to make the part look and work the best.

Core Extraction Mechanism of Injection Mold and Avoidance

When the molded part can’t be demolded smoothly according to the molding direction, the core extraction mechanism⁸ should be designed. The core pulling mechanism can mold complex product structure, but it is easy to cause product stitching line, shrinkage and other defects, and increase the cost of the mold to shorten the life of the mold.

Design of injection molded products, such as no special requirements, try to avoid the core structure. Such as the direction of the hole axial and tendon to the direction of the mold, the use of cavity core touch through and other methods.

Schweißen (Heizelementschweißen, Ultraschallschweißen, Vibrationsschweißen)

Welding makes connections stronger. Welding can make product design easier.

Reasonable Consideration of the Contradiction Between Process and Product Performance

When designing injection molded products, you have to think about how the product looks, how it works, and how it’s made. Sometimes you have to give up a little bit of how it’s made to get a really good look or a really good performance.

What are the Common Mistakes in the Design of Plastic Molded Parts?

Designing plastic molded parts requires attention to detail to avoid errors that can impact functionality and manufacturability.

Common design mistakes in plastic molding include inadequate wall thickness, poor draft angles, and ignoring material flow, causing defects, higher costs, and reduced performance.

Choosing the Wrong Material

Another thing I see people mess up is picking the right material. You should pick the material based on where the part is going to be. If it’s going to be outside, you need to put UV stabilizers in it so it doesn’t crack. If it’s going to be a load-bearing part, you need to put fillers in it like fiberglass to make it stronger. If it’s going to be a part that has to slide on something, you need to put additives in it like lubricants.

Uneven Wall Thickness

Different wall cross sections are common in part design and can lead to unexpected part characteristics when molded. It is difficult for plastics to flow and transition between uneven wall thicknesses.

Thick wall cross sections produce varying shrinkage, which can lead to part distortion. Try to make wall thicknesses thicker at least in the gate area and thinner at the end of filling. Aim for a wall thickness of 0.080-0.120 inches and utilize rib and angle brace plate features in part design.

Not Adding Mold Pull Slope

You need an extractor taper in your mold to get the plastic part out of the mold. The plastic usually shrinks toward the middle of the part. If you don’t have an extractor taper, the part will stick in the mold and not come out right. If you can, put a 1° extractor taper on each side. Any extractor taper will help get the part out.

Unnecessary Depressions

Because of the complexity of today’s part designs, depressions are designed for, but often overlooked. Depressions in the mold need to be dealt with before the part is released or the part will be damaged. There are many ways to address depressions in mold handling through-core design, slides, cores, or lifters. A simple rule for design: dents + complex molds = higher cost.

Excluding Fillet Radii

People often forget about fillet radii when they’re designing parts. But they’re important because they make the part stronger. A radius reduces stress concentrations and fracture in plastic parts. It also makes the part look better and reduces the risk of injury when you handle it.

Schlussfolgerung

Designing plastic molded parts requires considering both structure and dimensions. When it comes to structure, you need to think about the size of the mold, how you’re going to separate the parts, how you’re going to cool the mold, how you’re going to get the air out of the mold, and how you’re going to feed the plastic into the mold. All of these things are important to make sure you can mold the part efficiently and make sure the part is good.

When it comes to dimensions, you need to think about the Wandstärke⁹, how much force you’re going to use to clamp the mold together, how you’re going to get the parts to slide, and how you’re going to make sure the part doesn’t break. All of these things are important to make sure the part doesn’t get deformed and to make sure the part is good.

The design process includes requirements analysis, sketching, detailed design, mold making, mold testing and mass production, while focusing on quality inspection and improvement. Common design mistakes include unreasonable material selection, uneven wall thickness, and lack of draw slope, etc., which should be avoided in order to reduce cost and improve the quality of finished products.