Ideal design
In order to overcome problems that may be caused by large wall thickness, using is an effective method that can reduce wall thickness and increase rigidity. In general, rigidity of a part can be enhanced by following methods.
§ increase wall thickness;
§ Increase modulus of elasticity (such as increasing content of reinforcing fibers);
§ Considered in design.
If material used for design does not meet required stiffness, a material with a higher modulus of elasticity should be selected. Simple way is to increase content of reinforcing fibers in plastic. However, for a given wall thickness, this approach only yields a linear increase in stiffness. A more efficient approach is to use an optimized design. Due to increased moment of inertia, rigidity of component increases. When optimizing size, not only issues that should be considered in engineering design, but also technical issues related to production and appearance should be considered.
Optimized size
Large moments of inertia can be easily achieved by setting thick and high. However, for thermoplastic engineering plastics, this method often causes problems such as product surface dents, internal voids and warping. Also, if height is too high, structure will likely expand under load. For this consideration, dimensions must be kept within reasonable proportions (see Figure 1).

 

In order to ensure that stripped products are easily ejected, an appropriate stripping taper must be designed (see Figure 2).

 

Prevent material buildup
For components with very high surface requirements, such as automotive wheel covers, size is very important. Proper design can reduce possibility of surface sinking of components to improve quality of components. Bottom material accumulates in circle shown in Figure 1. Size of this circle is related to size of circle, and should be as small as possible, so as to reduce or avoid dents. If circle is too large, internal voids may be formed, and mechanical properties of product will be very poor.
Reduce stress on the bottom
If a load is applied to a certain component, bottom of component may be stressed. Without rounding in this area, very high stress concentrations can occur (see Figure 3), often resulting in fracture and failure of component. Remedy is to create a circular arc with a radius large enough (Fig. 1) to create a better stress distribution at the bottom of rib.

 

But if radius of arc is too large, it will also increase diameter of circle mentioned above, which will lead to problems mentioned above.

 

Among plastic designs, cross structure is the best because it can handle many different load arrangements (Figure 4). Properly designed cruciform structures to withstand expected stresses can ensure uniform distribution of stresses throughout article. Nodes formed at intersections of crisscross (Fig. 5) represent accumulations of material, but nodes can be hollowed out in the center to prevent problems. Care must also be taken not to create material buildup at intersections and where edges of components meet (Figure 6).

 

 

Implicit cost elements and their cost implications
Injection molded products cannot be same as machined products. While most are aware of this, it is often specified beyond reach, or makes cost-effective production impossible.
A Injection molding is generally divided into 3 quality grades, namely general purpose injection molding, technical injection molding and high precision injection molding. DIN16901 standard states that they are classified according to size and size of injection molded products within allowable range (range 1 and 2).
§Injection molding for general purposes requires a low level of quality control and is characterized by low return rates and fast production cycles.
§Technical injection molding can be more expensive because it places higher demands on mold and production process, requires frequent quality checks, and thus increases return rates.
§Third type, namely high-precision injection molding, requires precise molds, optimal production conditions and 100% continuous production monitoring. This will affect production cycle, increase unit production cost and quality control cost.
Designers play a key role in determining cost of injection molded products. They must determine that it is commercially feasible. Although selection does not have to be as strict as possible, it must be strict enough. Commercially acceptable products are generally, deviation of product from standard size is not higher than 0.25-0.3%, but this needs to be judged in combination with specific requirements of application (Figure 1).

 

Thermoplastics are generally highly ductile and elastic, and do not need to be specified in same strict ranges as metals, which have high stiffness, low ductility, and low elasticity.
Factors affecting
In order not to impose unduly strict limits on plastic parts, it is important to be aware of several factors that affect dimensional accuracy of injection molded parts (Figure 2).

 

Mold manufacturing must be relatively strictly followed. Designers should keep in mind that importance of draft angle lies in its ease of release and resistance to warpage (see Figure 3).

 

A related problem is when moldings are made of different materials or different wall thicknesses. Mold shrinkage values are direction and thickness dependent. This property of glass reinforced materials is even more pronounced. Orientation of glass fibers can produce significantly different shrinkage in horizontal and vertical directions, resulting in dimensional inaccuracies.
Geometry of plastic part also has an effect on shrinkage and thus on shrinkage (Fig. 4).

 

If complex molding process is very demanding, it is necessary to obtain accurate data on shrinkage value and warpage behavior of mold prototype.
Production and use
Because thermoplastics are affected by conditions of use, it is important to decide whether they only need to be produced or used at the same time. For example, thermal expansion of thermoplastics may be 10 times higher than that of metals (Figure 5), and water absorption of some plastics (such as nylon) has a very important impact on reliability of product in use.
Post-mold shrinkage must be considered when using semi-crystalline plastics. This phenomenon is mainly affected by processing conditions of injection molding, which can cause dimensional changes in product after demoulding.
Quality control does not have to be performed immediately after demoulding. DIN16901 standard states that quality control can only be carried out after 16 hours of storage under standard climatic conditions (23℃, 50% relative humidity) or after proper pretreatment.
Suggestion
The lower limit specified in DIN 16901 can be used as a cost-effective production of plastic products. Technology of modern injection molding machines allows us to obtain values that are more precise than those specified in this standard.
For high-precision injection molding, because DIN16901 is no longer applicable, various industrial sectors have developed their own tables.
At any time, if it is necessary to determine precise, be sure to consult with injection molding plant or material supplier to determine whether required is technically feasible and commercially applicable (Figure 6).

 

Right choice
Generally speaking, there are no bad materials, only wrong materials are used in specific areas. Therefore, designers must thoroughly understand properties of various alternative materials, carefully test these materials, and study influence of various factors on properties of molded products.

 

Traditional Thermoplastics
Thermoplastics are most commonly used in injection molding. It can be divided into amorphous plastics and semi-crystalline plastics (see Figure 1). These two classes of materials differ markedly in their molecular structure and properties affected by crystallization (see Figure 2).

 

Generally speaking, semi-crystalline thermoplastics are mainly used for components with high mechanical strength, while amorphous thermoplastics are often used in housings because they are less flexible.
Fillers and reinforcements
Thermoplastics are available in unreinforced, glass fiber reinforced, mineral and glass filled products. Glass fibers are used primarily to increase strength, sturdiness and application temperatures; mineral and glass fibers have less reinforcement and are used primarily to reduce warpage.

 

Fiberglass can affect molding process, especially shrinkage and warpage of part. Therefore, glass fiber reinforcements cannot be replaced by unreinforced thermoplastics or low reinforcement content without dimensional change (see Figure 3). Orientation of glass fibers is determined by flow direction, which will cause a change in mechanical strength of part.

 

To demonstrate these effects, 10 test strips were cut from transverse and longitudinal directions of injection molded sheet and their mechanical properties were compared on same tensile tester (see Figure 4).
For thermoplastic polyester resins reinforced with 30% glass fibers, tensile strength in transverse direction is 32% lower than that in longitudinal direction (flow direction), flexural modulus and impact strength are reduced by 43% and 53%, respectively. These losses should be taken into account in strength calculations that take into account safety factors.

 

A range of reinforcements, fillers and modifiers are added to some thermoplastics to modify their properties. In process, performance changes caused by these additives must be carefully checked from manuals or databases (such as Campus), and it is better to listen to technical advice of experts from raw material manufacturers (see Figure 5).

 

Effect of humidity
Some thermoplastic materials, especially PA6 and PA66, are very hygroscopic. This may have a large impact on their mechanical properties and dimensional stability. When proceeding, special attention should be paid to this property (see Figures 6 and 7).
Other selection criteria
Some requirements relate to processing considerations and assembly. It is also important to study integration of several different functions into one component, which can save expensive assembly costs.
This criterion is very useful for calculating production costs. It can be seen from price calculation that not only price of raw materials should be considered, but also that materials with high performance (rigidity, toughness) can make wall thickness thinner, thereby shortening production cycle. Therefore, it is important to list all criteria and evaluate them systematically.
A ductile material selection process is shown in Figure 8.

 

Design Checklist
Goal of new product development or product improvement is to make product have excellent performance while obtaining low production cost. Here, design task mainly refers to selection of raw materials, selection of suitable processing, strength calculation and mold design.
Only when these steps are considered holistically and followed up systematically can high-quality, commercially viable molds be produced. Design departments are often just looking for practical solutions. However, it must be emphasized that practicality and cost-efficiency of plastics are not inevitable, designers must pay great attention to developing right solutions for raw materials and processing.

 

Plastic properties are not eternal
Properties of plastics are affected by use environment, processing process, mold design and operating conditions (as shown in Figure 1). Plastic properties obtained from tests in a laboratory environment. Test chart is produced by testing a high-gloss mold with optimized parameters under standard conditions of specified pressure. However, in reality, it is impossible for plastics to be produced under such conditions, nor can they be under same pressure in use. Therefore, precise requirements and defining conditions must be carefully analyzed and laid out when developing any plastic design scheme, and design checklists can provide useful assistance in this regard.

 

Production of samples
To develop a product, from design stage to market preparation stage, it is necessary to prepare samples for testing and correction. Make sure that sample production method is broadly applicable to mass production scenarios. Partially injection molded samples are also made from injection molds. If no tooling is available, it is necessary to use a similar material or sheet and cut it into a test sample. However, there are always problems of one kind or another, for following reasons:
§ It is not possible to examine influence of weld lines on injection molded parts.
§In some cases, grooves created during machining can reduce strength properties of component compared to injection molded parts.
§Extruded rods and sheets are stronger and harder than injection molded parts due to high crystallinity.
§ Effect of fiber orientation cannot be examined.
A sample for a light switch made of extruded material that can withstand 180,000 cyclic stresses. In same case, injection molded parts showed fatigue failure after 80,000 cycles. This difference is due to difference in crystal structure during injection molding process.

 

Sample mold
At present, molds for producing samples are all made by simple machining or using low-cost materials (such as aluminum or copper as raw materials). However, it should be noted that parameters that are very important for production, such as temperature, pressure, etc., cannot be represented by such a mold. In addition, their different cooling properties lead to different shrinkage and thermal transformation behavior. It is recommended to use high-hardness steel to make molds, and molds can be designed with a single cavity arrangement.
Assay design
With development of modern computer simulation techniques, potential malpractice in design and process can sometimes be identified at an early process stage, as done by strength analysis and process analysis. However, these simulation analyzes cannot fully ensure performance and quality of final product under actual operation. Only testing samples under actual operating conditions can provide the most reliable information. This inspection is a non-negligible requirement to obtain a product of higher quality and functionality.
If actual sample detection is difficult, detection under simulated conditions can also be carried out. However, value of such testing depends on representativeness of simulated operating conditions.
It is neither feasible nor economical to use time-consuming tests to infer long-term behavior of finished plastic products under influence of mechanical stress and heat. On the other hand, accelerated aging test under harsh conditions may not be reliable as a long-term performance prediction, and more attention should be paid to it. Plastics also perform quite differently under long-term stress tests than they do under short-term, rapid tests.
Innovative ideas
Many different industrial applications show that future of plastics industry is bright. If raw material properties of polymers can be cleverly utilized, multifunctional products will be produced, which will have better commercial and functional value than previous designs.
Today's designs require increasingly complex geometries and materials. Plastics can solve many different types of problems. However, fit between plastic and application is also crucial. Raw material (resin) manufacturers have extensive experience in this area. Their expertise must be used to translate new design concepts into actual products.

 

Plastic not metal
Many plastic designs still follow "metal parts" design concept. At the beginning of topic, author puts forward points that need to be paid attention to when designing plastic raw material parts compared with other traditional raw materials.
Basic characteristics of various raw materials
Properties of plastic raw materials can vary more widely than any other industrial raw material. Its properties can be greatly transformed by addition of fillers, reinforcements and modifiers. Basic properties of most plastics are significantly different from metals. For example, by direct comparison, metals have higher .
§density
§Maximum operating temperature
§ Rigidity/Strength
§Thermal conductivity and
§ Conductivity
while in
§ Mechanical shock absorber
§Heat stretchability
§Elongation at break
§toughness
On the one hand, engineering plastics have a larger range. In order to produce functional plastic parts while saving costs, it is necessary to use innovative designs to replace metal with plastic. In process of redesigning components, it is very possible to achieve effect of functional integration and structural simplification.

 

Different raw material properties
Under different conditions, plastics behave completely differently than metals. Therefore, a cost-effective functional design for cast metal will easily fail if it is hastily applied to plastic. Therefore, plastic designers must be very familiar with properties of these raw materials.
Deformation properties as a function of temperature and time
When temperature of material is closer to its melting point, temperature and time will directly affect deformation performance of raw material. Most plastics exhibit changes in mechanical properties when exposed to stress at room temperature or for short periods of time. On the other hand, unless a metal approaches its recrystallization temperature (>300℃), its mechanical properties do not change substantially.

 

If use temperature and deformation rate change greatly, properties of engineering thermoplastics can also change from hard and brittle to elastic. For example, when door of an air bag is opened, its deformation performance is completely different from that of slow assembly parts during practical application (such as explosive opening) (see Figure 2). Similarly, snap fittings must also choose different assembly methods as temperature changes. Here, effect of temperature is much larger than that of loading rate.

 

Factors Affecting Material Properties
Properties of plastics are not just those of pure raw material. Under different operating environments, basic properties of plastic components will change with different factors (such as UV radiation, Figure 3). Even the best design can fail if raw material is processed in an inappropriate range. Likewise, artifacts cannot be processed to address design weaknesses. Therefore, only an optimized process that takes into account all factors can guarantee quality of plastic parts.

 

Unlike metals, plastics do not have much tolerance for mistakes in design, and when designing plastic parts, designs that match their characteristics are required. Therefore, a complete and detailed analysis of all requirements and constraints of product must be performed prior to design.

Optimal Assembly Techniques - Part 1
Some simple assembly techniques recognized by all designers, such as snap-fit assembly, press assembly and thread assembly, can greatly save production costs by assembling components easily and quickly.
Assembly technology is divided into "separate" and "integrated" two types. Following items are included in integrated assembly process.
§ Welding
§ Fixed
§ Bonding
§ Embedded technology
§90 degree angle buckle
Separate assemblies include:
§ Less than 90 degree angle buckle
§ Screw assembly
§ Center assembly
§ Press assembly
Snap fit design
The biggest advantage of snap fit is that no additional assembly parts are required. The most common types of clips used in plastics processing are:
§ Barb buckle
§ Cylindrical buckle
§ Ball Seat Buckle
§ In all of these snap fit designs, designer must ensure that geometry of fitting avoids stress relaxation that would cause the assembly to loosen.

 

Basic Design Principles
Design of snap fit depends on allowable deformation of materials used. For example, due to fact that polyamides allow lower deformation in dry state than in conventional state, it is necessary to pay more attention to application of this material. Allowable inclination of hook also has an effect. (see picture 1)

 

In a barb-style snap fit, pointed barb tip reduces stress on barb as it deforms (see Figure 2), and this design distributes stress evenly throughout barb shank. Stress concentration at the base of barb is relatively reduced. Assembly pressure is also reduced considerably. Ignoring issue that radius of curvature at junction between base of barb and body of member should be sufficiently large often results in weak points. In principle, a sufficiently large radius of curvature should be provided to avoid pressure concentrations. Cylindrical or ball-and-hole snap-fitting systems are often slotted to facilitate assembly, so slot ends must not be designed with pointed edges.

 

Press assembly
Press-fit assembly enables high-strength assembly of plastic components at minimal cost. For snap fits, for example, tensile strength of high pressure assemblies decreases over time due to stress relaxation (see Figure 3). Design calculations must take this into account. In addition, it is necessary to conduct experiments with temperature cycle changes to ensure feasibility of design.
Threaded assembly
Threaded assemblies consist of use of split, combined screws or integral screw inserts. Flexural modulus of material provides guidance for reasonable assembly of screw. For example, flexural modulus of a threaded screw can reach 2800Mpa. Metal threaded inserts are required if metric screws are to be used, or if threaded assembly needs to be done multiple times.

 

Ensuring correct bushing size is critical to avoid rejecting components (see Figure 4). Screw manufacturers have a lot to offer in this regard.

 

Use of screws with conical countersinks in plastic assemblies should in principle be avoided due to resulting pressure that will open nut mouth (see Figure 5). One possible consequence of this extra pressure is that weld line of nut is prone to cracking.

Optimal Assembly Techniques - Part 2
In addition to assembly techniques described in Part 7 of this series, there are many other different welding methods that can be used in joining plastic parts. To ensure a cost-effective, functional design, it is essential to select an appropriate welding method at an early stage of design and to give due consideration to required connection geometry.
Welding may be used for assembly of plastic parts that are permanently joined without other assembly parts. Welding method is selected according to following criteria: according to geometry of molded part, raw materials used, combination of cost, total production cycle, required mechanical properties and appearance quality of assembly part.

 

Different welding methods
In mass production, there are many different cheap welding methods. The most commonly used welding methods for plastic engineering parts are (Figure 1):
§High temperature tool welding
§ Spin welding
§ Vibration welding
§Ultrasonic welding
Other methods are:
§High frequency welding
§Induction welding
§Hot gas welding
There are also new methods being developed (such as laser welding), but not yet widely used in industry.
In all methods, heat (to melt plastic surface for bonding) and pressure are used. Heat is usually provided by means of contact or radiant heating, internal or external frictional heating, or electrical heating.
Choose right method
In order to obtain high-quality and repeatable welding quality, it is necessary to select an appropriate welding method to optimize welding parameters and ensure that parts to be welded are designed correctly and match selected welding method. Manufacturers of welding machines have to offer not only standard equipment, but also special welding equipment for a wide variety of welding tasks. Before deciding to use a certain welding method, it is a very wise choice to discuss with equipment manufacturer or resin supplier.
Different Welding Characteristics
In theory, all thermoplastics are weldable. But sometimes welding properties of plastics are quite different. Amorphous polymers and semicrystalline polymers cannot be welded together. Since water vapor will affect welding quality, water-absorbing plastics such as nylon need to be pre-dried before welding. For the best welding quality, nylon parts are best welded immediately after injection molding, or placed in a dry environment before welding. Resin additives such as glass fibers and stabilizers can also affect weld quality. With proper selection of process parameters and part design, strength of welded assemblies of unreinforced plastics can be comparable to its raw materials. In the case of glass fiber reinforced plastics, loss of strength due to fiber separation and reorientation at welded area must be accounted for in design.

 

Proper Weld Design
Basic guarantee of high-quality welding lies in correct design of welding profile. Correct basic design is shown in cross-sections shown in Figures 2 and 3. If there are special aesthetic requirements for appearance of welded area, special geometries need to be considered.

 

This image shows passage of grooves to absorb excess material, thus avoiding flashing (Fig. 4). When designing thin-walled parts, it is necessary to add a guide groove between two sides of part, so that when a certain welding pressure is used, part wall will not move and deviate from predetermined position.

 

 

Features of Ultrasonic Welding
Semi-crystalline polymers have a sharp change in melting point, and when heated, polymer transitions from a solid to a liquid phase instantaneously. For this reason, shear welding is best used for ultrasonic welding of semi-crystalline plastics (Fig. 5). Since amorphous plastics have a softening range, welding design of amorphous plastics is relatively minor. Fig. 6 shows schematic diagrams of near-sound field and far-sound field welding methods. Depending on distance between contact points, ultrasonic waves transmit vibrations into workpiece and bond contact surfaces together. In general, near-sound welding will work well with all plastics. For welding of low elastic modulus plastics, it is best to use near-sound field welding method.

 

As much as needed, as little as possible
In design of engineering plastic parts, experience has shown that there are some design points that should be considered frequently, so these points can be distilled into simple design guidelines. One of these points is the design. have a significant impact on part quality.
Impact on Special Parts Standards
Changing a part will have a significant impact on following main properties:
§Part weight
§Achievable flow length in molding
§ Production cycle of parts
§ Rigidity of molded parts
§ Tolerance
§ Part quality such as surface finish, warpage and voids
Ratio of process to
In initial stage of design, it is necessary to consider whether materials used can meet requirements. Ratio of flow to mold has a great influence on cavity filling in injection molding process. If process is to be long and thin in injection molding process, it is very necessary that polymer should have a relatively low melt viscosity (easy to flow and melt). To gain insight into flow behavior of polymers when they melt, a special mold can be used to determine flow (Fig. 1, Fig. 2).

 

Flexural modulus as a function of
Flexural stiffness of a slab is determined by material-specific modulus of elasticity and moment of inertia of block's cross-section. Automatic additions to improve rigidity of plastics without any validation usually lead to serious problems, especially for crystalline materials. For glass fiber reinforced materials, change will also affect orientation of glass fibers. Near mold wall, fibers are oriented in direction of fluid flow. In the center of cross-section of mold wall, fiber orientation is disordered, resulting in turbulent flow.

 

For glass fiber reinforced plastics, there is a boundary area that can accurately distinguish stiffness of product, and this boundary area will be reduced accordingly. This explains why flexural modulus decreases as it increases (Fig. 4). Strength value determined according to standard test strip (3,2 mm) cannot be directly used for determination, otherwise deviations will occur. In order to estimate performance of product, it is necessary to use a safety factor.

 

Therefore, if it is increased without considering consequences, it will increase cost of materials and production, but rigidity has not increased.
Do you want to increase?
Increase not only determines mechanical properties, but also quality of finished product. In design of plastic parts, it is very important to make it as uniform as possible. Variations in same part can cause different shrinkage of part, depending on part rigidity, which can lead to serious warpage and dimensional accuracy problems (Figure 6). In order to obtain uniformity, thick-walled part of molded product should be provided with a mold core (Figure 5). This prevents voids from forming and reduces internal pressure, which minimizes distortion. Voids and micropores formed in part will narrow cross section, increase internal stress, and sometimes have a notch effect, thereby greatly reducing its mechanical properties.

 

Correct gate location
Wrong selection of type and type of gate system will not only cause processing problems, but also have a certain impact on quality of plastic products. Therefore, design department must never underestimate importance of gate location.
Designer not only needs to carry out design calculation of plastic product, but also must pay special attention to gate design of mold. They must choose correct gating system as well as number and location of pouring points. Type and location of gate will have a greater impact on quality of product.
Choice of gate location will determine following properties of plastic products:
§ Filling Behavior
§ Final dimensions of product (tolerances)
§ shrinkage behavior, warping
§ Mechanical performance level
§ Surface quality (appearance)
If designer chooses wrong gate, it is almost impossible to correct resulting consequences by optimizing processing parameters during molding.

 

Performance measurement of products in different directions
During injection molding, orientation of long-chain plastic molecules, fibrous fillers, and reinforcements is primarily determined by flow direction of molten plastic, which leads to a direction-dependent dependence (anisotropy) of part properties. For example, stretch performance in direction of flow is much better than that in perpendicular direction (see Figure 1). Components containing fiber reinforcement are much more affected than components without fiber reinforcement. Orientation of fibers also causes a difference in the shrinkage of part in horizontal and vertical directions, which will cause part to warp.

 

 

Part quality degradation due to weld lines and air traps
A weld line occurs when 2 or more melt streams in a mold come together. For example, weld lines occur when melt needs to flow through insert, or when part is poured at several points simultaneously (see Figures 2a and 2b). Also, different wall thicknesses in same part can cause melt front to separate, creating weld lines. Air trapping (occurrence of air bubbles) occurs when air that should be removed from mold is trapped in mold by melt and cannot escape. Weld lines and air pockets are often identified as surface defects. In addition to making surface unsightly, they can significantly reduce mechanical properties of affected area, especially impact strength (see Figures 3 and 4).

 

Strength reduction due to weld lines

 

Adverse consequences caused by improper gate location selection
Because gate often leaves obvious traces, it cannot be installed in areas with high requirements on appearance of surface. High stress (shear) will be generated in any gate area, which will significantly reduce performance of plastic resin (Figure 5). Weld line quality of plastics without reinforcements is significantly higher than that of plastics with reinforcements. Mass attenuation factor in fusion line area has a great relationship with type and content of fillers and reinforcing materials. Additives such as processing aids and flame retardants have adverse effects on quality of fusion line. Thus, it is difficult to assess how much these factors affect final strength of part. Also, fact that weld line region has a high load-carrying capacity under tension does not mean that it has good impact or fatigue resistance.

 

Due to inclusion of fiber reinforcement, alignment direction of fibers in fusion line region is perpendicular to flow direction. This will significantly degrade mechanical properties of part at this point (see Figure 6).

 

Correct gate location
It is impossible for a complex mold to have no fusion line. If number of weld lines cannot be reduced, they should be placed in unimportant positions of mold in terms of surface quality and mechanical strength. This can be accomplished by changing gate location or increasing / decreasing wall thickness of part.
Basic design principles:
§ Do not place gates in high pressure areas
§ Try to avoid or reduce fusion line
§ Try to keep fusion line away from high pressure area
§ For reinforced plastics, gate location determines part warpage performance
§ Provide adequate exhaust openings to avoid air traps

Low cost design
Price is a Design Element
Designer bears most of responsibility for final cost of plastic part. His decisions predetermine cost of production, tooling and assembly. Later corrections and optimizations are often expensive and infeasible.
Raw material performance affects cost
Taking full advantage of characteristics of plastic raw materials can save costs in many ways.
§Multifunctional integrated design
Combining several functions in one component reduces number of parts.
§ Use of low-cost assembly technology
Snaps, welding devices, fixing devices, double injection molding technology, etc.
§ Utilize self-lubricating properties
Reduces need for additional and continuous lubrication
§ No need for surface treatment procedures
Plastics have properties such as coloring, chemical and corrosion resistance, electrical and thermal insulation.
§ Tuberculosis
Raw materials of same series have different crystallization cycles, which is due to accelerated crystallization effect of nodulant in melting cooling stage.
Finished Product Design Affects Cost
In addition to above mentioned, pay attention to following points can further save costs.
§Wall thickness
Optimizing wall thickness distribution can impact raw material costs and save production time.
§ Mold
Double-sided mold can reduce number of splits.
§Tolerance
Excessively demanding tolerances will increase product failure rates and quality management costs.
§Raw material
Use low-deformation polymers to reduce warping and deformation problems (such as adding an appropriate amount of minerals to glass fiber materials), choosing fast-setting or fast-curing raw materials can reduce molding cycles and cooling time.

 

Cost comparison by each step of production
When an injection molded part comes out of injection machine, it should be ready for assembly immediately without any additional handling. If post-processing is required, the overall plastic cost can often equal metal cost.

 

Design determines production cost
Increased wall thickness does not always increase strength, but means increased production and raw material costs.
Semi-crystalline thermoplastics shrink considerably when solidified. During holding phase, this shrinkage must be compensated by continuous molten feed. Holding time per mm of wall thickness is approximately:
§ Delrin: 8 seconds
§ Non-reinforced polyamide 66: 4-5 seconds
§ Reinforced polyamide 66: 2-3 sec (for up to 3mm wall thickness)
Typical Application Examples
In contrast to metal designs that must be machined and often pass through many assembly steps to complete a single part, plastic processing technologies offer considerable cost-saving opportunities.

 

In Fig. 3, traction bar, spring, sawtooth, buckle and bearing are all injection molded at one time. Same metal design requires no less than 5 separate components to assemble and requires lubrication when shaft connects to piston. In fact, use of polyoxymethylene resin in this link can eliminate need for additional lubricants.

 

In design of cable buckle shown in Figure 4, buckle is combined with design of living joint, which reduces production cost and makes assembly easier. If a relatively brittle material is used, another buckle can be used to replace joint. Hinge design can be.

 

During design process, it is necessary for designer to specify design of cavity. He decides on ejector and number of mold movable blocks required. Through ingenious arrangement, core can be used to replace movable block.