Design engineers don’t have to be fluid dynamics experts to injection mold plastic parts without a hitch. The following rules will help engineers avoid problems when designing injection-molded plastic parts:
Rule 1: Keep Wall Thickness Consistent
Plastic part walls must be uniform in thickness. This is the most basic design parameter, and strict adherence to it will eliminate many manufacturing problems. Parts with uniform walls will not warp, will fill properly and will fit together because variable shrinkage is minimized. Wall thickness variations should not exceed 10% in high mold shrinkage plastics. In fact, even this slight disparity can introduce processing and quality problems.
Rule 2: Provide for Proper Gate Location
If varying wall thickness cannot be avoided, then designers should provide for proper gate location. If this is not supplied, then attaining uniform pack of the molded part will be nearly impossible. The most effective gate location is when the melt enters at the thickest part of the cavity and then flows to the narrower areas.
Rule 3: Determine Optimal Wall Thickness
Theoretically, there is no maximum wall thickness for injection-molded parts. But designers are more concerned with determining the minimum wall thickness because thinner is almost always less expensive. Two factors contribute to this: first, thinner parts require less raw plastic material, and second, they cool faster. To determine the most suitable wall thickness, engineers should first consider product requirements. Generally, strength dictates the wall thickness. Engineers can also rely on a finite analysis to select the optimal wall thickness.
Rule 4: Radius Corners Generously
During injection molding, the molten plastic has to navigate turns or corners. Rounded corners will ease plastic flow, so engineers should generously radius the corners of all parts. In contrast, sharp inside corners result in molded-in stress—particularly during the cooling process when the top of the part tries to shrink and the material pulls against the corners.
If the inside and outside radii of a part are each equal to half of the nominal wall thickness, a uniform wall around the corner can be achieved. Both sides of the corner will display equal amounts of shrinkage, and sink marks will be avoided entirely. Moreover, the first rule of plastic design—uniform wall thickness—will be obeyed. As the plastic goes around a well-proportioned corner, it will not be subjected to area increases and abrupt changes in direction. Cavity packing pressure stays consistent. This leads to a strong, dimensionally stable corner that will resist post-mold warpage.
Rule 5: Select Suitable Draft Angles
From a cost and manufacturability viewpoint, the ideal draft angle is the largest angle that will not lessen the customer’s satisfaction with the product. The minimum allowable draft angle is harder to quantify. Plastic material suppliers and molders are the authority on what is the lowest acceptable draft. In most instances, 1° per side will be sufficient, but between 2° and 5° per side would be preferable. If the design is not compatible with 1°, then allow for 0.5° on each side. Even a small draft angle, such as 0.25°, is preferable to none at all.
Guidelines for Plastic Injection MoldingPlastic Molding Design
our company has been manufacturing plastic parts for nearly 40 years now, and our clients trust our knowledge of the craft. Consideration needs to be put into each step of the injection molding process, and we strive to help you better understand how your plastic part is precision manufactured to create the result you need.
Our molded parts can vary in size, shape, and complexity, but Our CNC engineers know exactly how to produce high-quality custom plastic parts that meet client needs. To best manufacture a custom part, it takes following laid out design steps. That’s why we strive to help our clients better understand injection molding concepts so the product they receive best matches the intended use case.
Here are some important guidelines to consider when we manufacture your thermoplastic part:
Injection Molding Materials
Determining which thermoplastic resin is used for your molded part depends on what the part is used for. For example, certain plastics may have good tensile strength for strong impact resistance, while others are better at withstanding extreme temperatures like hot and cold.
Surface Finish – Industry-standard finishes are available for most plastic injection molded parts, including texturing and polishing. Whichever surface finish you have will depend on the material used on your plastic part.
The experts will help you make the best decisions regarding your custom plastic component’s material makeup.
Wall Thickness
When manufacturing plastic molded parts, the amount of plastic used in the mold’s walls helps them avoid potential issues such as sink marks and warpage, which can be damaging to the overall structure of the plastic. The wall thickness of a molded part can vary depending on the plastic resin used to create it and the size of the part in question.
Ribs – In-molded thermoplastic designs, ribs are often manufactured to reinforce the walls of a plastic part. Ribs are useful when wanting to increase the strength of a part without adding to the overall wall thickness, and ribs often need to be taken into consideration when designing a particular plastic part.
Walls designed to be too thick wastes material, money, and can negatively affect part performance. our designers will help your product strike the balance between proper function and cost-efficiency.
Draft
The angle at which plastic parts are removed from their mold is known as the draft, and it is vital to the overall structure of the plastic part. A “vertical” draft can be detrimental to the plastic part in question, therefore CNC engineers understand a degree of the draft is necessary when manufacturing and removing a plastic part from its mold.
Radius
The radius of a plastic part refers to the sharpness of the edges of each corner. Edges that are too sharp can greatly increase stress concentration, which could actually lead to eventual part failure. Sharp corners need to be rounded slightly in order to lessen the stress concentrations, meaning the edge’s radius needs to be taken into consideration when manufacturing a molded part.
Injection moulding is a versatile process and can be applied to almost any product. Although injection moulding is the industry standard for fabricating parts for products, it is not without its holdups. There are a few basic limitations to be taken into account. Here’s eight rules to follow when designing your product to ensure quality and durability:
Maximum wall thickness. The wall thickness of your part is directly proportionate to both the total materials needed to make the part and the cooling time required. By reducing the maximum thickness of the wall of your part, you reduce both these factors, resulting in lower cycle time, thus lower production costs. If the wall of your part is too thick or is inconsistent, problems can be caused involving sinkage and warpage, resulting in rejects and costly redesigns. Ensure your wall thickness is matched to the capabilities of the machine.
Corners. They can be a problem in a mould and will not always come out flush. It is almost impossible to force plastic into a perfect corner, and the result will look messy and amateurish, not to mention the strength of the part could be compromised. Round all corners where possible to enhance aesthetics and durability.
Applying a draft. A draft is a tiny angle — usually one or two degrees — applied to the mould on the face perpendicular to the parting line. This will allow for easy removal of the piece from the mould. Not including a draft in your design will mean the automatic ejection system of the injection moulding machine will not operate.
Ribs. Ribs are structural elements for your part, used for overall stability control. They are thin wall protrusions that extend perpendicularly from a wall or plane. Adding ribs rather than thicker walls will offer greater structural support.
Bosses. Bosses are hollow, cylindrical protrusions usually included in a design for accepting screws or other mating components of your deign. Ensuring these are secured by either attaching them to a wall or adding ribs will mean the bosses will remain straight and accept the part it was designed for without a problem.
External undercuts. A protrusion or depression in the outside of your mold — the cavity half — can create problems when trying to separate parts from the mold. Adjust your parting line to accommodate this.
Internal undercuts or overcuts. Similar to external undercuts, these protrusions or depressions are on the inside of your mold — on the core half. Adjust your parting line to accommodate this.
Threads. If your mould contains a thread, always arrange it perpendicular to the parting line. This will ensure that the fragile thread is not damaged. It is better, if possible, to not include a thread at all in your design. Simplifying your design will lower the chance of something going wrong.
Injection moulding design ensures a quality product and the countless possibilities far outstrip the limitations. Designing for a quality injection moulded product is the essence of the design process, and these limitations are the guidelines for creating a versatile end product.
When designing plastic parts that will be injection molded, you might have questions about design criteria. For example, what draft angle ensures the part’s easy ejection from the mold.
Below is a guide to the most common criteria for designing plastic parts.
Wall Thickness
The wall thickness of a thermoplastic part depends on several factors. These factors include the type of thermoplastic used, mechanical stresses on the part, electrical property requirements (if any), etc.
The following are general rules of thumb:
The part’s primary wall thickness should be uniform
Avoid quick thickness transitions. Instead, use gradual thickness transitions of 3 to 1.
Gating, where the injection of the thermoplastic occurs, should be in the thicker section
Wall Thickness Considerations
When designing plastic parts, several factors should be considered, determining the part’s wall thickness. These factors include:
Application requirements such as:
Structural strength
Fatigue
Deflection
Electrical loads
Moldability of the part includes
Part size
Material flow to the narrowest part
Number and location of gates
Design requirements for registering agencies such as UL or RTI
Parting Line
The part’s design should include considering the location of the parting line, also called a part line. The design of the part should also have the draft (angle of perpendicular components) and shutoff.
The location of the parting line should occur at a major feature plane. When a step parting line is required, a 7° draft angle is preferred, 5° minimum.
Radii and Fillets
The inside radius of a corner (r) should be 25% to 60% of the wall thickness. The outside radius of a corner (R) is the sum of the inside radius plus the material thickness (t). R = r+t.
The fillet radius is the same as the interior radius, with a minimum of 0.020 inches (0.508 mm). If the feature is load-bearing, a larger radius is required.
Sharp corners should have a break of at least 0.005 inches (0.127 mm).
Ribs
The thickness of the rib where it intersects the wall should be 50% – 60% of the wall thickness, with a fillet radius of 0.015 inches (0.381 mm). The maximum height of the rib is 3x the wall thickness.
The typical draft of a rib is 1° to 1.5°, with a minimum of 1/2° per side.
The minimum spacing between the ribs is twice the wall thickness.
Gussets
The recommended thickness of a gusset at the wall is 50% of the wall thickness.
The gusset height maximum is 95% of the height of its attached wall. Typically, the gusset height is less than four times the wall thickness, preferably two times the wall thickness. The fillet where the gusset meets a feature or wall has a radius of 25% of the wall thickness.
The spacing between gussets should be at least two times the wall thickness.
Bosses
Usually, the outside (OD) diameter is two times the inside diameter (ID). The boss’s base should be less than 60% of the wall thickness, and the maximum height of the boss is three times the OD. The boss’s radius at its base should be 25% to 50% of the wall thickness, with a minimum radius of 0.015 inches (0.381 mm). The bottom of the core should have a radius of 0.010 inches (0.245 mm).
The minimum draft of the OD is 1/2°. The draft of the ID is 1/4° minimum.
Bosses positioned near an external wall should be placed a minimum of 0.125 inches (3.175 mm) inboard from the wall to the OD.
The minimum space between two bosses is twice the wall thickness.
Bosses for Fastener
For a boss designed for fasteners, consider the following additional suggestions for designing plastic parts:
The boss ID should be 0.8 times the screw diameter
The screw engagement – 2 1/2 times the screw diameter
The core hole depth – 0.032 inches (0.813 mm) plus screw engagement length
Chamfer at the top of the boss – recommended for screw lead-in
Holes and Depressions
The following are guides for designing holes and depressions in a thermoplastic part:
Blind cores –
Hole ID ≤ 3/16 inches (4.763 mm) – height 3/16 inches (4.763 mm) – height < 3 times hole diameter
Through holes –
Hole ID ≤ 3/16 inches (4.763 mm) – height 3/16 inches (4.763 mm) – height < 6 times hole diameter
The spacing between two holes or one hole and the edge –
Twice the wall thickness or twice the hole diameter; whichever is greater, as a minimum.
While this guide gives the basic design information for injection molded parts, not all plastics have the same design criteria. For example, structural foam thermoplastics have different requirements and refer to the design specifications for the specific thermoplastic used.
5 Design Rules for: Problem Free Injection Molded Plastic Parts
In the last couple of decades, use of lightweight alternatives such as plastics and composites has dramatically increased, with its applications well entrenched in automotive, aerospace, consumer electronics industry.
Today’s car for example, has more than 150 kilograms of plastics on board in the form of seats, dashboards, bumpers, and engine components. Boeing 787 on the other hand has 50% carbon fibre-reinforced plastic and other composites in its airframe. The company says it has used more carbon composites and plastics than ever before in the 787 model.
Plastics offer good mechanical properties and are relatively lighter than metal, making products more efficient, while providing enough toughness to withstand the test of time. However, it is important that designers be sensitive to certain physical and mechanical properties of plastics as it is not as strong as metal, has relatively lower density than metal and is poor conductor of heat and electricity
Injection molding is the most commonly used manufacturing process but due to its intricacies product designers need to make adequate design considerations to ensure that part designs focus on maximizing molding performance and reducing tooling costs , an area that often plague the injection molding industry.
Plastics tend to have higher rates of thermal expansion than metals and thicker sections shrink more than a thinner section, resulting in warpage or sink mark during the molding process. Stress concentration is another area that is of particular concern for plastic part manufacturing. These stresses can be the result of a continuous load, warpage, or any other issue related to design, material, processing, or tooling factors. Additionally there can be many latent defects in plastic parts that can not be detected with routine quality control.
Plastic part performance and cost can be significantly enhanced by proper part design features. Through the use of simple designs and by following general moldability guidelines for plastic parts, design engineers can avoid problems occurring during manufacturing and also reduce the cost of parts. These factors require designers to introduce adequate design features that can lessen the stress level within a part and help develop low-shrinkage, warp-free parts.
Consider the following injection molding design considerations for designing better plastic parts
1. Radius
A design with corners always needs to accommodate large radii. Sharp corners spell stress thereby affecting the manufacturability of parts. Corners such as the attachment between bosses and surfaces which are often overlooked require scrutiny.
Sharp Corners in Injection MoldingThe radius should always be with regards to the part thickness thereby eliminating the prospects of high-stress concentration and resulting in the breakage of the plastic part. General guideline suggest that the thickness at the corner should be in the range of 0.9 times the nominal thickness to 1.2 times the nominal thickness of the part.
2. Wall Thickness
Given the different nature of the composition of plastics, plastic parts should always have walls with uniform thickness. Swerving away from the recommended would give rise to unfavorable results such as shrinkage and warpage. Apart from this, uniform wall thickness gives the assurance of minimum manufacturing cost. This further ensures quick cooling which in turn lets one produce more parts in a short span of time and optimum utilization of resources which is much sought after. And lighter parts have never been considered inconvenient.
Uniform Wall Thickness
General guidelines suggest that wall thicknesses for reinforced plastic materials should be between the range of 0.75 mm to 3 mm and those for unfilled materials should be 0.5 mm to 5 mm.
3. Determine an apt location for gate
While it is recommended to have a plastic design with a uniform wall thickness, we understand the need to have variations in few designs. In such unavoidable situations, having a proper gate location would decide the success of the part. Experts recommend designs with the gate at a location at which the melt enter the thickest section of the cavity only to flow out of a narrower region.
4. Draft
Plastic heavily relies on mold draft in the course of its removal from the mold. Due to which plastic parts are to be designed with a taper (or, draft) in the direction in which the mold moves. In such case, the lack of an appropriate draft would make the removal of plastic parts almost impossible.
Minimum Draft Angle in injection molded plastic parts
A design with sufficient draft is always considered to be a good practice. 1.5 degrees for a depth of 0.25mm is usually recommended by design experts. General guidelines suggests that a draft angle of 0.5 degrees is recommended for core and 1.0 degrees for cavity
5. Ribs
Recommended Rib ParametersA known aspect of plastic is its stiff nature. Given this, the inclusion of ribs in a design is often recommended which adds to the bending stiffness. Ribs are pocket-friendly and a convenient option, the end result of which is often well received by both the designer and the manufacturer.
But a plastic designer should always take the wall thickness into consideration at the time of including a rib in a plastic design. Thick and deep ribs can cause sink marks and filling problems respectively. Rib thickness of a part should never exceed the wall thickness.
General guidelines suggests that rib thickness at its base should be around 0.6 times nominal wall thickness of the part. Failing to include a proper rib would eventually lead to the distortion of the plastic part.
we have been manufacturing plastic parts for nearly 40 years now, and our clients trust our knowledge of the craft. Consideration needs to be put into each step of the injection molding process, and we strive to help you better understand how your plastic part is precision manufactured to create the result you need.
Plastic molded parts can vary in size, shape, and complexity, but CNC engineers know exactly how to produce high-quality custom plastic parts that meet client needs. To best manufacture a custom part, it takes following laid out design steps. That’s why we strive to help our clients better understand injection molding concepts so the product they receive best matches the intended use case.
DESIGN YOUR PLASTICS TODAY
Here are some important guidelines to consider when we manufacture your thermoplastic part:
Injection Molding Materials
Determining which thermoplastic resin is used for your molded part depends on what the part is used for. For example, certain plastics may have good tensile strength for strong impact resistance, while others are better at withstanding extreme temperatures like hot and cold.
Surface Finish – Industry-standard finishes are available for most plastic injection molded parts, including texturing and polishing. Whichever surface finish you have will depend on the material used on your plastic part.
Wall Thickness
When manufacturing plastic molded parts, the amount of plastic used in the mold’s walls helps them avoid potential issues such as sink marks and warpage, which can be damaging to the overall structure of the plastic. The wall thickness of a molded part can vary depending on the plastic resin used to create it and the size of the part in question.
Ribs – In-molded thermoplastic designs, ribs are often manufactured to reinforce the walls of a plastic part. Ribs are useful when wanting to increase the strength of a part without adding to the overall wall thickness, and ribs often need to be taken into consideration when designing a particular plastic part.
Walls designed to be too thick wastes material, money, and can negatively affect part performance.
Draft
The angle at which plastic parts are removed from their mold is known as the draft, and it is vital to the overall structure of the plastic part. A “vertical” draft can be detrimental to the plastic part in question, therefore CNC engineers understand a degree of the draft is necessary when manufacturing and removing a plastic part from its mold.
Radius
The radius of a plastic part refers to the sharpness of the edges of each corner. Edges that are too sharp can greatly increase stress concentration, which could actually lead to eventual part failure. Sharp corners need to be rounded slightly in order to lessen the stress concentrations, meaning the edge’s radius needs to be taken into consideration when manufacturing a molded part.
When manufacturing plastic molded parts, the amount of plastic used in the mold’s walls helps them avoid potential issues such as sink marks and warpage, which can be damaging to the overall structure of the plastic. The wall thickness of a molded part can vary depending on the plastic resin used to create it and the size of the part in question.
Ribs – In-molded thermoplastic designs, ribs are often manufactured to reinforce the walls of a plastic part. Ribs are useful when wanting to increase the strength of a part without adding to the overall wall thickness, and ribs often need to be taken into consideration when designing a particular plastic part.
Walls designed to be too thick wastes material, money, and can negatively affect part performance. the professional designers will help your product strike the balance between proper function and cost-efficiency.
Draft
The angle at which plastic parts are removed from their mold is known as the draft, and it is vital to the overall structure of the plastic part. A “vertical” draft can be detrimental to the plastic part in question, therefore CNC engineers understand a degree of the draft is necessary when manufacturing and removing a plastic part from its mold.
Radius
The radius of a plastic part refers to the sharpness of the edges of each corner. Edges that are too sharp can greatly increase stress concentration, which could actually lead to eventual part failure. Sharp corners need to be rounded slightly in order to lessen the stress concentrations, meaning the edge’s radius needs to be taken into consideration when manufacturing a molded part.