Plastic Injection Molding Design guidelines

This article focuses primarily on plastic part and mold design, but also includes the design process; designing for assembly; machining and finishing; and painting, plating, and decorating.

Good design combines concept with embodiment. Unless the two are considered together, the result will be an article that cannot be made economically or one that fails in use. This is particularly important for plastics. It is vital to choose the right material for the job. When that is done, it is equally important to adapt the details of the design to suit the characteristics of the material and the limitations of the production process.

Material Selection:

Engineering Plastics A comprehensive look at material testing and the issues to consider when selecting a plastic material.

Joining Techniques:

Includes information and guidelines on the methods for joining plastics including mechanical fasteners, welding techniques, inserts, snap fits, and solvent and adhesive bonding.

Snap-Fit Joints for Plastics:

Contains the engineering formulas and worked examples showing how to design snap fit joints for plastic resins.

Plastic material

Plastics come in a bewildering variety. There are a hundred or more distinct generic types. On top of that, advanced techniques with catalysts and compounding are creating new alloys, blends and molecular forms. All of these materials can have their properties modified by control of molecular weight and by additives such as reinforcements. The number of different grades of plastics materials available to the designer now approaches 50,000. The importance and the difficulty – of making the right choice is obvious.

Plastics can be grouped into categories that have roughly similar behavior. Thermoplastics undergo a physical change when processed; the process is repeatable. Thermosets undergo a chemical change; the process is irreversible. A key distinction between thermoplastics relates to the molecular arrangement. Those with random tangled molecules are called amorphous. Those with a degree of molecular arrangement and ordering are called semi crystalline. The difference is significant. For example, most amorphous materials can be fully transparent. And the more crystalline a material is, the less likely it is to have a wide ‘rubbery’ processing region, so making it less suitable for stretching processes like blow molding and thermoforming Designers must design for process as well as purpose and material. In single-surface processes for example, there is only indirect control over the form of the second surface. Design must take this limitation into account.

Caused by the macromolecular structure and the temperature-dependent physical properties plastic materials are distinguished into different classes. Below Figure gives an overview of the classification of plastics with some typical examples. Thermoplastics are in the application range of hard or tough elasticity and can be melted by energy input (mechanical, thermal or radiation energy). Elastomers are of soft elasticity and usually cannot be melted. Thermosets are in the application range of hard elasticity and also cannot be melted.

Classification of plastics

Thermoplastic resins consist of macromolecular chains with no crosslinks between the chains. The macromolecular chains themselves can have statistical oriented side chains or can build statistical distributed crystalline phases. Thermoplastic resins can be reversibly melted by heating and resolidified by cooling without significant changing of mechanical and optical properties. Thus, typical industrial processes for part manufacturing are extrusion of films, sheets and profiles or molding of components.

Plastic Design

When designing plastic parts, your team should consist of diverse players, including conceptual designers, stylists, design engineers, materials suppliers, mold makers, manufacturing personnel, processors, finishers, and decorators. Your chance of producing a product that successfully competes in the marketplace increases when your strategy takes full advantage of team strengths, accounts for members’ limitations, and avoids overburdening any one person. As the designer, you must consider these factors early in strategy development and make adjustments based upon input from the various people on the design team.

When designing and developing parts, focus on defining and maximizing part function and appearance, specifying actual part requirements, evaluating process options, selecting an appropriate material, reducing manufacturing costs, and conducting prototype testing. For the reasons stated above, these efforts should proceed simultaneously.

Thoroughly ascertain and evaluate your part and material requirements, which will influence both part design and material selection. When evaluating these requirements, consider more than just the intended, end-use conditions and loads: Plastic parts are often subjected to harsher conditions during manufacturing and shipping than in actual use. Look at all aspects of part and material performance including the following.

       

PART DESIGN PROCESS: DEFINING PLASTIC PART REQUIREMENTS

Mechanical Loading

Carefully evaluate all types of mechanical loading including short-term static loads, impacts, and vibrational or cyclic loads that could lead to fatigue. Ascertain long-term loads that could cause creep or stress relaxation. Clearly identify impact requirements.

Temperature

Many material properties in plastics impact strength, modulus, tensile strength, and creep resistance to name a few vary with temperature. Consider the full range of end-use temperatures, as well as temperatures to which the part will be exposed during manufacturing, finishing, and shipping. Remember that impact resistance generally diminishes at lower temperatures.

Chemical Exposure

Plastic parts encounter a wide variety of chemicals both during manufacturing and in the end-use environment, including mould releases, cutting oils, de-greasers, lubricants, cleaning solvents, printing dyes, paints, adhesives, cooking greases, and automotive fluids. Make sure that these chemicals are compatible with your selected material and final part.

Electrical Performance

Note required electrical property values and nature of electrical loading. For reference, list materials that are known to have sufficient electrical performance in your application. Determine if your part requires EMI shielding or UL testing.

Weather Resistance

Temperature, moisture, and UV sun exposure affect plastic parts’ properties and appearance. The end-use of a product determines the type of weather resistance required. For instance, external automotive parts such as mirror housings must withstand continuous outdoor exposure and perform in the full range of weather conditions. Additionally, heat gain from sun on dark surfaces may raise the upper temperature requirement considerably higher than maximum expected temperatures. Conversely, your requirements may be less severe if your part is exposed to weather elements only occasionally. For example, outdoor Christmas decorations and other seasonal products may only have to satisfy the requirements for their specific, limited exposure.

Radiation

A variety of artificial sources such as fluorescent lights, high-intensity discharge lamps, and gamma sterilization units emit radiation that can yellow and/or degrade many plastics. If your part will be exposed to a radiation source, consider painting it, or specifying a UV-stabilized resin.

Dimensional Tolerances

Many applications have features requiring tight tolerances for proper fit and function. Some mating parts require only that mating features have the same dimensions. Others must have absolute size and tolerance. Consider the effect of load, temperature, and creep on dimensions. Over-specification of tolerance can increase product cost significantly.

Processing

Determine if your part design places special demands on processing. For example, will the part need a mold geometry that is particularly difficult to fill, or would be prone to warpage and bow. Address all part-ejection and regrind issues.

Production Quantities

The number of parts needed may influence decisions, including processing methods, mold design, material choice, assembly techniques, and finishing methods. Generally, for greater production quantities, you should spend money to streamline the process and optimize productivity early in the design process.

Assembly

Address assembly requirements, such as the number of times the product will be disassembled or if assembly will be automated. List likely or proposed assembly methods: screws, welds, adhesives, snap-latches, etc. Note mating materials and potential problem areas such as attachments to materials with different values of coefficient of linear thermal expansion. State any recycling requirements. The “Part Requirements and Design Checklist” in the back of this manual serves as a guide when developing new products. Be sure not to overlook any requirements relevant to your specific application. Also do not over-specify your requirements. Because parts perform as intended, the costs of over specification normally go uncorrected, needlessly increasing part cost and reducing part competitiveness.

PART DESIGN PROCESS: GENERAL DESIGN

Wall thickness

Cost savings are highest when components have a mini-mum wall thickness, as long as that thickness is consistent with the part’s function and meets all mould filling considerations. As would be expected, parts cool faster with thin wall thicknesses, which means that cycle times are shorter, resulting in more parts per hour. Further, thin parts weigh less, using less plastic per part. On average, the wall thickness of an injection moulded part ranges from 2mm to 4mm. Thin wall injection moulding can produce walls as thin as .05mm.

Non-uniform wall thickness can lead to air traps.

Many designs, especially those converted from cast metal to plastic, have thick sections that could cause sinks or voids. When adapting these designs to plastic parts, consider the following:

Avoid designs with thin areas surrounded by thick perimeter sections as they are prone to gas entrapment problems

  • Maintain uniform nominal wall thickness
  • Avoid wall thickness variations that result in filling from thin to thick sections.
  • Core or redesign thick areas to create a more uniform wall thickness
  • Make the outside radius one wall-thickness larger than the inside radius to maintain constant wall thickness through corners
  • Round or taper thickness transitions to minimize readthrough and possible blush or gloss differences. Blending also reduces the molded in stresses and stress concentration associated with abrupt changes in thickness.

Core out thick sections as shown on right to maintain a more uniform wall thickness.

RIBS

Proper rib design involves five main issues: thickness, height, location, quantity, and moldability. Consider these issues carefully when designing ribs. Many factors go into determining the appropriate rib thickness. Thick ribs often cause sink and cosmetic problems on the opposite surface of the wall to which they are attached. The material, rib thickness, surface texture, color, proximity to a gate, and a variety of processing conditions determine the severity of sink.

BOSSES

Bosses find use in many part designs as points for attachment and assembly. The most common variety consists of cylindrical projections with holes designed to receive screws, threaded inserts, or other types of fastening hardware. As a rule of thumb, the outside diameter of bosses should remain within 2.0 to 2.4 times the outside diameter of the screw or insert. To limit sink on the surface opposite the boss, keep the ratio of boss-wall thickness to nominal-wall thickness the same as the guidelines for rib thickness. To reduce stress concentration and potential breakage, bosses should have a blended radius, rather than a sharp edge, at their base. Larger radii minimize stress concentration but increase the chance of sink or voids.

GUSSETS

Gussets are rib-like features that add support to structures such as bosses, ribs, and walls. As with ribs, limit gusset thickness to one-half to two-thirds the thickness of the walls to which they are attached if sink is a concern. Because of their shape and the EDM process for burning gussets into the mold, gussets are prone to ejection problems. Specify proper draft and draw polishing to help with mold release. The location of gussets in the mold steel generally prevents practical direct venting. Avoid designing gussets that could trap gasses and cause filling and packing problems. Adjust the shape or thickness to push gasses out of the gussets and to areas that are more easily vented.


Contour lines show flow front position at incremental time intervals. Squared gussets can trap air in the corners.

DRAFT

Draft providing angles or tapers on product features such as walls, ribs, posts, and bosses that lie parallel to the direction of release from the mold eases part ejection. Figure below shows common draft guidelines. How a specific feature is formed in the mold determines the type of draft needed. Features formed by blind holes or pockets such as most bosses, ribs, and posts should taper thinner as they extend into the mold. Surfaces formed by slides may not need draft if the steel separates from the surface before ejection.

Common draft guidelines

UNDERCUTS

Some design features, because of their orientation, place portions of the mold in the way of the ejecting plastic part called “undercuts,” these elements can be difficult to redesign. Sometimes, the part can flex enough to strip from the mold during ejection, depending upon the undercut’s depth and shape and the resin’s flexibility. Undercuts can only be stripped if they are located away from stiffening features such as corners and ribs. In addition, the part must have room to flex and deform. Generally, guidelines for stripping undercuts from round features limit the maximum amount of the undercut to a percentage defined as follows and illustrated in figure below.

Generally, avoid stripping undercuts in parts made of stiff resins such as polycarbonate, polycarbonate blends, and reinforced grades of polyamide 6. Undercuts up to 2% are possible in parts made of these resins, if the walls are flexible and the leading edges are rounded or angled for easy ejection. Typically, parts made of flexible resins, such as unfilled polyamide 6 or thermoplastic polyurethane elastomer, can tolerate 5% undercuts. Under ideal conditions, they may tolerate up to 10% undercuts.

Most undercuts cannot strip from the mold, needing an additional mechanism in the mold to move certain components prior to ejection. The types of mechanisms include slides, split cores, collapsible cores, split cavities, and core pulls. Cams, cam pins, lifters, or springs activate most of these as the mold opens. Others use external devices such as hydraulic or pneumatic cylinders to generate movement. All of these mechanisms add to mold cost and complexity, as well as maintenance. They also add hidden costs in the form of increased production scrap, quality problems, flash removal, and increased mold downtime.

Undercut features can often successfully strip from the mold during ejection if the undercut percentage is within the guidelines for the material type

References
  • LANXESS Part_and_Mold_Design_Guide
  • RTP-Part-Design-Guidelines-for-Injection-Molded-Thermoplastics
  • Design Guides for Plastics – Clive Maier, Econology Ltd
  • GE Engineering Thermoplastics- DESIGN GUIDE
  • Material Properties of Plastics Wiley-VCH Verlag GmbH & Co. KGaA

About the authors

Davide de Brouwer

Founder / Managing Director Engibex

Premanandh Kumaravelu

Senior mechanical engineer

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