Injection moulding Process

INTRODUCTION

Many of the things you own and come into contact with every day are made from plastics which have been injection moulded into shape. These things range in size from pens, up to computer and television cases. Many products have become very cheap and widely available because of the injection moulding 'revolution'. Injection moulding can produce very large or small products with great precision, in very large numbers, and at low cost.

Injection moulding is suitable processing method for following materials:

• Thermoplastics
• Elastomers
• Rubbers
• Thermosets
• Composites
• Foamed plastics

The most used materials are thermoplastics. Hence from here onwards injection moulding refers to thermoplastic injection moulding.

BASIC CONCEPT OF INJECTION MOULDING

The basic concept of injection moulding is the ability of a thermoplastic material to be softened by heating, formed under pressure, and hardened by cooling.

PRINCIPLE OF INJECTION MOULDING

Injection moulding, as its name suggests, involves injecting fused (softened) plastic into a mould. In an injection moulding machine, plastic granules are fed into a heated barrel. They are forced through this barrel either by a piston or a screw (similar to one in a mincing machine). The granules fuse together into a mass like soft chewing gum and this is forced from the end of the barrel into a metal mould.

The mould itself is made from two or more metal parts which fit tightly together leaving only a small hole for the fused plastic to enter. This hole is called the sprue entry. If you look at an injection moulded product carefully, you may see the point at which the plastic entered the mould. It will be a small mark where the sprue has been broken or cut off the moulding.

• Advantages of Injection Moulding Process:
• Parts can be produced in large volume at high production rates.
• Process can be automated and hence increases in production
• Relatively low labor cost per unit is obtainable.
• Many different colors and finishes are available, and good decoration is possible.
• Close dimensional tolerances can be maintained.
• Parts can be molded with metallic and non-metallic inserts.
• Parts can be molded in combination of plastics and additives such as fillers.
• Molded parts require little or no finishing.
• Minimal scrap loss result as runner, gate and rejection can regrind and reused.
• Process creates consistent parts.
• High return on investment.

Limitations of Injection Moulding Process:

• High mold cost.
• Moulding machine and auxiliary equipment cost are high.
• Lack of knowledge about the fundamentals of process causes problem.
• Lack of knowledge about the long term properties of the material may result in long term failure.
• Process requires extremely high temperature.
• Process requires extremely high clamping force.

INJECTION MOULDING PROCESS

In its broadest terms, thermoplastic injection moulding forms a part by forcing a liquid resin into a closed mould, under pressure, until the part has cooled or cured and can be ejected from the mould. This process consists of the following five distinct operations.
1. Feeding of raw materials
2. Plasticizing (mixing (mechanical work), external heating)
3. Injection (filling of the mould cavity)
4. Holding pressure
5. Cooling (cooling of the material in the mould cavity)
6. Ejection of the injection moulded part
The main factors in the injection moulding are the temperature and pressure history during the process, the orientation of flowing material and the shrinkage of the material. This means that the structure and the properties of injection moulded parts are inhomogeneous and the products have always internal stresses.

Feeding of Raw Material
First the molder receives plastic resin in the form of small chopped pellets. These are fed into the hopper of an injection-moulding machine, where they fall into an augur-type screw channel, which feeds the pellets forward inside the heated barrel.


Plasticizing

The cycle begins with the extruder plasticizing the resin and accumulating it in the forward section of the barrel. The heater bands maintain the melt's temperature as the shot it built up. The mould is closed. The cycle is typically timed so that there is minimal time between the closing of the mould and the next shot. Injection of the Resin into the Closed Mould.

Injection
After the mould closes, the screw (not rotating) pushes forward to inject melt into the cooled mould. The air inside the mould will be pushed out through small vents at the furthest extremities of the melt flow path.


Holding pressure

When the cavity is filled, the screw continues to push forward to apply a holding pressure. This has the effect of squeezing extra melt into the cavity to compensate for the shrinkage of the plastic as it cools. This holding pressure is only effective as long as the gate(s) remain open.

Cooling the Resin inside the Closed Mould
Cooling starts immediately when polymer mass flows into the mould cavity including the injection and holding phases, and continues after these too. Therefore the cooling time is the longest phase in the injection moulding process. Because the injection and holding phases are included in the cooling phase, they also effect on the cooling of the moulded part. Cooling time depends on the raw material, wall thickness of the part and the heat transfer capability of the mould. In order to have a nonporous and unstressed part the polymer melt should solidify uniformly.
Opening the Mould and Ejecting the Moulded part

Once the gate(s) freeze, no more melt can enter the mould and so the screw-back commences. At this stage the screw starts to rotate and draw in new plastic from the hopper. This is conveyed to the front of the screw but as the mould cavity is filled with plastic, the effect is to push the screw backwards. This prepares the next shot by accumulating the desired amount of plastic in front of the screw. At a pre-set point in time, the screw stops rotating and the machine sits waiting for the solidification of the moulding and runner system to be completed. When the moulding has cooled to a temperature where it is solid enough to retain its shape, the mould opens and the moulding is ejected. The mould then closes and the cycle is repeated.
THE PROTOMOLD PROCESS
At Protomold we have automated the process of designing and manufacturing molds based on customer supplied 3-D CAD part models. Due to this automation, we typically cut the lead time for the initial parts to one-third of conventional methods. Cost saving varies with the number of parts being produced, but Rapid Injection Molding may also have a substantial cost advantage in runs of up to thousands of parts.

The Protomold Rapid Injection Molding process gives design engineers a fast and affordable way to get real injection molded parts in prototype or low-volume quantities.
So from an overall market perspective we see Rapid Injection Molding as in between Rapid Prototyping and Conventional Injection Molding as illustrated in the figure.
THE INJECTION MOULDING PROCESS CYCLE

The typical process cycle time varies from several seconds to tens of seconds, depending on the part weight, part thickness, material properties, and the machine settings specific to a given process.

Process control of injection molding has a direct impact on the final part quality and the economics of the process. The various components of process control must be fully understood to maximize profit and part quality.
The various stages are:

Injection phase (filling phase)
Packing phase
Holding phase
Cooling phase

Fill time

At the beginning of the injection molding cycle (point 1), the mold has just closed and the molten polymer, which is maintained at a fairly uniform temperature inside the barrel of the injection machine, is forced to flow through the nozzle, runner, gate, and then into the cavity under controlled flow rate or pressure, depending on the control scheme of the injection unit. The fill time is defined as the time needed for the polymer to fill the entire cavity (duration between points 1 and 4.

Post-fill time
After the mold is completely filled (point 4), more material is packed into the cavity and the polymer continues to cool. The post-filling stage ends when the polymer temperature is sufficiently low and the part is rigid enough to be removed from the cavity without significant deformation (point 7). The post-fill time is defined as the time between the moment when the cavity is completely filled and the instant when the mold opens (duration between points 4 and 7).

Mold open time

The mold-opening stage begins when the mold is opened (point 7) and ends when the mold is closed (point 8) to start the next cycle. The mold-open time includes the time taken for mold opening and closing actions as well as part ejection (duration between points 7 and 8). Because this can be a significant portion of the cycle time in processes with extremely short cycles, each action of the mold clamp and ejection systems should be analyzed for possible time delays and wasted energy. During this stage, additional heat transfer occurs between the mold and ambient air.

To further illustrate the machine motion within the process cycle, the hydraulic (cylinder) and cavity pressure traces, screw position, and mold face separation position are shown below
1. Filling (injection stage) 2.Packing and cooling stage 3.Mold opening 4.Part ejection 5.Mold closing.

ULTRASONIC WELDING

ULTRASONIC WELDING

When bonding material through ultrasonic welding, the energy required comes in the form of mechanical vibrations. The welding tool (sonotrode) couples to the part to be welded and moves it in longitudinal direction. Ultrasonic welding involves the use of high frequency sound energy to soften or melt the thermoplastic at the joint. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations usually at a frequency of 20, 30 or 40 kHz.

Differences in the process for welding plastics and metals with ultrasonic

1. Anvil
2. Parts to be welded
3. Sonotrode
4. Ultrasonic oscillation

Ultrasonic welding of plastics

Oscillations are introduced vertically

Ultrasonic welding of plastics is a state-of-the-art technology that has been in use for many years. When welding thermoplastics, the thermal rise in the bonding area is produced by the absorption of mechanical vibrations, the reflection of the vibrations in the connecting area, and the friction of the surfaces of the parts. The vibrations are introduced vertically. In the contraction area, frictional heat is produced so that material plasticizes locally, forging an insoluble connection between both parts within a very short period of time.
The prerequisite is that both working pieces have a near equivalent melting point. The joint quality is very uniform because the energy transfer and the released internal heat remains constant and is limited to the joining area. In order to obtain an optimum result, the joining areas are prepared to make them suitable for ultrasonic bonding. Besides plastics welding, ultrasonic can also be used to rivet working parts or embed metal parts into plastic.
Benefits of the process include: energy efficiency, high productivity with low costs, ease of automated assembly line production and fast joining times.

Core and cavity generation method in injection mould design M. W. FU, J. Y. H. FUH and A. Y. C. NEE

Core and cavity generation method in injection mould design M. W. FU, J. Y. H. FUH and A. Y. C. NEE

In a computer-aided injection mould design system, the generation of parting surfaces and the creation of core and cavity blocks is usually a bottleneck. The parting surfaces and core/cavity blocks are created based on the parting direction and parting lines. Here, the architecture of an injection mould design system is proposed on the basis of the practical information flow and processing steps in mould production lifecycle. In this architecture, the methodology to generate the parting surfaces and the core/cavity blocks is proposed. To generate the parting surfaces, the parting line edges are classified and the extruded directions specified to the different groups of parting line edges. Extruding the parting line edges to the boundary of the core/cavity-bounding box generates the parting surfaces. To create the core/cavity blocks, the Boolean regularized difference operation (BRDO) is used and the related algorithms are presented. The criterion to identify whether the undercut features need local tools for moulding is proposed. The case studies illustrate and validate the methodology to generate the parting surfaces and core/cavity blocks.

In injection mould design, the main design activities include the determination of parting direction, parting lines and surfaces, selection of mould types, cavity layout, gating, ejection, venting, heating/cooling types, mould materials, and the temperature control system. After the parting direction and lines are determined and the design scheme is decided, the rest of the detailed design activities mentioned above can proceed. Based on the known parting direction and parting lines, the methodologies related to the generation of parting surfaces and core/cavity blocks are presented.

To generate the core/cavity blocks automatically, two methods known as the Boolean-based approach (BBA) and the Euler-based approach (EBA. In BBA, the core/cavity blocks are generated using the Boolean regularized difference operation (BRDO) between the core/cavity bounding box and the moulding. In EBA, the Euler operation is the key process to generate the related core/ cavity block surfaces.

Generation of core/cavity blocks

Core and cavity blocks can be generated based on the following steps.
Step 1. Identify all the through-hole undercut features and `patch’ up all these features.
Step 2. Generation of a bounding box. The bounding box is defined as the material stock that fully contains the moulding with enough space for assembly of other components .Figure shows the bounding box of a moulding.
Step 3. Generation of parting surfaces and `sewing’ all the parting surfaces together.
Step 4. Subtracting the bounding box with the patched part solid by the BRDO and splitting the bounding box using the generated parting surfaces.
The procedures to generate the core and cavity blocks are shown in the figure . After patching all the through-hole features in Step 1, it is necessary to determine the maximum dimensions Lx, Ly and Lz of the moulding in X, Y, Z directions and the bounding box thicknesses a, b, c in three dimensions. The bounding box thicknesses can be determined based on the required mould strength and the moulding parameters. The final bounding box dimensions are (Lxa, Lyb, Lzc). In Step 3, the parting surfaces, generated based on the parting line edges, are sewn together. In the last step, the BRDO is performed between the bounding box and the patched moulding. After the operation, the bounding box has an empty space inside. The sewn parting surface is then used as the splitting surfaces to split the box into two mould halves. One is the core block; the other is the cavity block. The procedures are illustrated in figure.
In an injection mould, if the core and cavity blocks and their inserts cannot mould the undercut features, the incorporation of local tools in the mould structure will be needed. It is hence necessary to identify the undercut features that cannot be moulded by the core and cavity blocks based on the following equation:

Where is the undercut direction and is the parting direction. Above equation means that if the undercut direction is not in the parting direction, the undercut features will become the `real’ undercut features and local tools are needed. The method to generate the local tools is not covered here.


Conclusions
Here, the architecture of an injection mould design system is proposed based on the practical mould design procedures. An efficient methodology for the creation of parting surfaces and the creation of core/cavity blocks is presented according to the generated parting direction and parting lines. Three types of parting, namely flat, stepped or complex partings are proposed. To generate the core/cavity blocks, the Boolean regularized difference operation is adopted. Case studies are used to illustrate the procedures of the methodology and the related algorithms. The results have shown that the methodology is effective in providing a solution for the computer aided injection mould design system automatically to generate the parting surfaces and create the core and cavity blocks.

Decision criteria for computer-aided parting surface design by B Ravi and M N Srinivasan

Decision criteria for computer-aided parting surface design by B Ravi and M N Srinivasan

A scientific approach is presented and the related logic developed for design of parting surfaces of patterns, moulds and dies used in the manufacture of cast, forged, injection-moulded and die-cast components. This has enabled computer-aided generation of parting surfaces and the determination of projected area, flatness and draw for a parting surface, identification of surfaces to which draft is provided, recognition of component segments causing undercuts, testing for dimensional stability, and location of flash, machined surfaces and feeders. Influencing criteria for parting-surface design have been formulated and developed into algorithms implemented on a personal computer. This approach greatly aids the engineer in rational decision making, paving the way for a systematized code for parting surface design.

Castings are manufactured by pouring molten alloys into various shaped mould assemblies. Moulds are in turn prepared by compacting sand around patterns in segments or halves, one each for the bottom and the top mould. A forged component is manufactured by compressing a heated blank between two shaped dies. In die-casting and plastic injection moulding the material is forced under high pressure into a cavity formed by bringing together two die-halves. In all these manufacturing processes, the design of patterns, moulds and dies, the crucial tooling, directly affects productivity and component quality. The most significant design aspect is the choice of the surface separating the two halves of the mould or die, referred to as the parting surface. A combination of several mechanical, metallurgical and process parameters influences parting-surface location, rendering the design exercise complex.

Decision criteria

Nine influencing parameters for decision making in parting-surface design for a component have been identified: projected area, flatness, draw, draft, undercuts, dimensional stability, flash, machined surfaces and directional solidification.

Projected area

To facilitate removal of the pattern from the mould or the manufactured component from the die, the cross-sectional area should gradually decrease from the parting surface to points farthest from the parting surface. The basal plane of an upright cone or the diametrical planes of a sphere satisfy this requirement. This condition is applicable to flat as well as irregular parting surfaces.

Flatness

Considering technological aspects such as side thrust, dimensional stability, sealing off, flash, and complexity in tooling and mould making, a flat parting surface is preferred over an irregular one.

Draw

Draw is the minimum distance through which a component is linearly translated in order to clear it from the mould. Design considerations such as application of draft to the vertical surfaces of mould and metallurgical and technological problems such as grain flow in forgings, flask size in castings and machine draw capability in injection moulding, as well as increase of cycle time and reduced productivity, are caused by deep draw.

Draft

A conical or pyramidal part is drawn along its axis from a mould with ease compared to a straight cylinder or rectangular part. Surface quality, interracial interaction between the pattern or component and the mould or die, the extent of draw and related factors also contribute to this effect. All such faces of the component that are parallel to the draw vector are given a small taper or draft to aid in easy withdrawal. Surfaces that are not parallel to the draw direction are considered to have a natural draft or form undercuts. Application of draft results in the alteration of the part geometry and additional machining may be required to restore the shape of the component.

Undercuts

Projections or recesses in the component unfavorably inclined with respect to the draw vector hinder the removal of the pattern from the mould or component from the die. It is not always possible to design the parting surface to avoid undercuts completely, so to overcome this, cores or other manufacturing devices like inserts and loose-pieces have to be incorporated. This directly affects the process cycle time and tooling and manufacturing costs.

Dimensional stability

Tile possibility of mismatch between the cope and drag portions of the mould or at the joint between two die halves, results in dimensional reliability across the parting surface being considerably lower compared to that in the portions of the mould lying on one side of the parting surface. Hence the parting surface is designed such that any two points between which high dimensional tolerance is required occur on the same side of it.

Flash

A material flowing into the gaps at the plane of separation of the two mould halves or the interface between mould and a core produces fin-like protrusions or flash. This is generally trimmed after manufacture. However, flash leaves surface imperfections, and in some cases trimming may not be feasible or economically viable. The designer may also specify certain surfaces to be free of flash. Such surfaces must not intersect the parting surface.

Location of surfaces to be machined

Critical surfaces of a cast component requiring machining are preferably located as the bottom or the vertical walls of the mould, which are relatively free of defects. In general, a casting may have several machined faces, all of which cannot be located as the bottom or vertical surfaces in the mould. When several alternative parting surfaces are considered, their relative merits are quantitatively assessed for location of machined surfaces using the criterion proposed below:
Feeders and directional solidification
Solidification of metal in a mould is accompanied by shrinkage, which can manifest in the form of micro pores or cavities. This is prevented by promoting controlled solidification initiating in thin sections to proceed towards thicker sections. The last freezing section is fed by a reservoir of molten metal (a feeder). This introduces definitive conditions on geometric interfaces and interactions between component sections, from which a number of design recommendations on variation of cross section and joining sections have emerged. Geometry of the casting and its disposition in the mould, particularly on access to the feeder, is considered an important parameter. The parting surface is chosen so that the hot spots (regions of mass concentration) are at the top of the casting.

Conclusion

A decision-making scheme on a scientific basis has been developed to assess the design of a parting surface for components manufactured in moulds or dies. Nine different criteria influencing the design have been identified and delineated to allow them to be analyzed by computer. The logic developed has been implemented in algorithms on a PC and tested on typical mechanical components.

An Intelligent Cavity Layout Design System for Injection Moulds by Weigang Hu and Syed Masood

An Intelligent Cavity Layout Design System for Injection Moulds by Weigang Hu and Syed Masood

This paper presents the development of an Intelligent Cavity Layout Design System (ICLDS) for multiple cavity injection moulds. The system is intended to assist mould designers in cavity layout design at concept design stage. The complexities and principles of cavity layout design as well as various dependencies in injection mould design are introduced. The knowledge in cavity layout design is summarized and classified. The functionality, the overall structure and general process of ICLDS are explained. The paper also discusses such issues as knowledge representation and case-based reasoning used in the development of the system. The functionality of the system is illustrated with an example of cavity layout design problem.

Cavity Layout Design in Injection Moulds

Current practice for injection mould design, especially cavity layout design, depends largely on designers’ ex- pertinences and knowledge. It would therefore be desirable to use knowledge engineering, artificial intelligence and intelligent design techniques in generating an acceptable cavity layout design in injection mould accurately and efficiently. In mould design, most of patterns of cavity layout and rules and principles of cavity layout design can also be easily represented in the form of knowledge, which can be used in most of knowledge-based design systems.

For example, for the layout patterns shown in Fig, the criteria to select the suitable layout pattern for design are mainly dependent on working environments, conditions and requirements of customer and are mainly based on designer’s skill and experience. To make a choice of contradictory factors will rely obviously on designer’s knowledge and experiences. It is rather suitable for intelligent design techniques to be used in systems designed for such situations, especially for routine or innovation design.

Design of injection mould mainly involves consideration of design of the following elements or sub-systems

(1) Mould type

(2) Number of cavities

(3) Cavity layout
(4) Runner system
(5) Ejector system
(6) Cooling system
(7) Venting
(8) Mounting mechanism
Most of the elements are inter-dependent such that it is virtually impossible to produce a meaningful flow chart covering the whole mould design process. Some of the design activities form a complicated design network as shown in Fig.
Obviously, in injection mould design, it is difficult for designer to monitor all design parameters. Cavity design and layout directly affects most of other activities. The application of advanced knowledge based techniques to assist designer in cavity layout design at concept design stage will greatly assist in the development of a comprehensive computer-aided injection mould design and manufacturing system.

It is noted from Fig a number of different layout patterns are possible with multiple cavities inside a mould. Higher the number of cavities of mould, higher the productivity of the injection mould. But this may lead to difficulties with issues such as balancing the runners or products with the complicated cavity shapes, which in turn may lead to problems of mould manufacturability. It is also possible that the number of cavities and the pattern of cavity layout will influence the determination of parting line, type of gate, position of gate, runner system and cooling system. Most of the main activities of mould design are therefore linked to cavity layout design. Fig. 3.5 shows the relations between cavity layout design and other design activities. The cavity layout design problem therefore depends upon a number of functionalities of the overall mould design system, which includes:

(1) Definition of design specifications including analysis and description of characteristics of design problem
(2) Determination of mould type
(3) Determination of number of cavities
(4) Determination of orientation of product
(5) Determination of runner type and runner configuration
(6) Determination of type and position of gate
(7) Cavity layout conceptual design
(8) Evaluation of ejection ability, manufacturing
(9) Ability and economic performances
(10) Determination of cooling system
(11) Graphic results display and output


Relationship diagram between cavity layout design and other modules of mould.

Example of Application

An application example, “determination of cavity layout pattern” of the “conceptual design for cavity layout” provided by Intelligent Cavity Layout Design System
(ICLDS) is given below:
If the initial design conditions are:
(1) What type of mould is used? Two plate
(2) What type of runner is used? Cold runner
(3) How many cavities are there in mould?
(4) How long is it required for product to clear the moulding area? Small
(5) What shape of product does moulding make? Rectangle
Then the result is given by: (this is shown in Fig.) Pattern of cavity layout design is:
Y-Rectangular-Layout
The knowledge base is developed using features of ECLIPSE language, such as ‘defrelation’, ‘deftemplate’, ‘defruleset’, and ‘goal’ generation. Part of the program, which describes the overall format of knowledge base development, is listed below:
Conclusion
The problem of design of cavity layout in multiple cavity injection moulds has received relatively little attention in computer aided design support systems for injection moulding. A computer based design system will offer great savings in time and cost in arriving at the best possible layout from a number of alternatives. The development of Intelligent Cavity Layout Design System (ICLDS) is believed to be the first attempt in this direction using knowledge-based approach. The development of ICLDS for injection mould is based on RETE++ in Windows environment on PC. From a practical point of view, ICLDS can be used as a tool for designer to implement cavity layout design of injection mould at concept design stage. It provides a positive step towards the development of a fully automated injection mould design process from product model to mould manufacturing.

Polymeric Materials

Polymeric Materials

Introduction
Joining together thousands of small molecular units known as monomers makes synthetic large molecules called Polymer. The process of joining the molecules is called polymerization and the number of these units in the long molecule is known as the degree of olymerization.

The words polymers and plastics are often taken as synonymous but in fact there is a distinction. The polymer is the pure material, which results from the process of polymerization and is usually taken as the family name for materials, which have long chain-like molecules (and this includes rubbers). Pure polymers are seldom used on and it is when additives are present that the term plastic is applied.

Classification of Polymeric Material
Polymers are classified as
• Thermosets
• Thermoplastics
Thermosets are cross-linking polymers in which the final macromolecules are formed by chemical reaction under the influence of heat and pressure. Once this reaction is complete, thermosets cannot be altered from this state by further application of heat and pressure. Phenol (PF), Urea (UF), Melamine (MF) formaldehyde resins, Polyester (UP) resins and epoxy (EP) resins are typical Thermosets.

Thermoplastics consist of long chain macromolecules, which are not interlinked. Their characteristics property is that they may be moulded when the temperature is increased beyond their softening range, and on cooling revert to the solid state in its new moulded shape. This process may be repeated indefinitely, but it is in fact limited by the ageing stability of the particular material. This means that after undergoing a certain number of processing operations, the original properties of the material are altered as a result of excess thermal stress. HDPE, LDPE, PP, PS, ABS, NYLON, PVC, PMMA, PBT, etc are thermoplastics.

Polymer Blends

By alloying of two polymers it is possible to get in one material advantage of two or more polymers. At present, the following alloys are available.

PVC / Acrylic	Tough with good flame and chemical resistance.
PVC / ABS	Easily processed with good impact and flame resistance.
PC / ABS	Hard with high heat distortion temperature and good notch impact strength.
ABS / Polysulphane	Less expensive than unmodified Polysulhone.
PPO / HIPS	Improved processability and reduced cost.
SAN / Olefin	Good weatherability.
Nylon / Elastomer	Improved notch impact strength.

General properties of plastic materials

Important characteristics of plastic materials to be considered while designing and manufacturing are:
1. Dimensional Stability: Dimensional stability is defined as the ability of a material to maintain its size and shape under various temperatures and stresses, which is necessary for satisfactory part performance in many applications. The complexity, size of the mould cavity and the tendency of the material being moulded to shrink as it cools in the mould determine the final dimensions of a moulded part.
2. Drying: Under adverse high humidity conditions or large temperature fluctuations from cold temperatures to hot temperatures, moisture pickup may occur and cause splay marks or bubble formation in formed parts. Drying the resin for about two hours at 71 - 82°C will eliminate condensed moisture on the granules and assure introduction of constant temperature granules to the fabrication equipment.
3. Compatibility: Equipment should be thoroughly purged with respective resins while fabricating. Few resins are physically compatible with themselves. Delaminating, streaking or haze will occur if incompatible resins are mixed with the material to be fabricated.
4. Outdoor Weatherability: Most of Plastics are not considered to be weather resistance plastics. Continuous long-term outdoor exposure results in both discoloration and reduction in strength and toughness properties. Weatherability can be improved by the addition of certain pigments or additives. Best results are obtained with finely dispersed carbon black or UV stabilizers.
5. Use of Regrind: Many thermoplastic resins can be reground for use as 100% regrind or blends of regrind resins with virgin resin. When you are fabricating with regrind resins, you need to use experienced judgment and screening. The use of degraded or contaminated regrind product may result in lower quality parts and performance.

Application of Plastics

Polymers will continue to replace other materials on an increasing scale. The polymers already perform satisfactorily in many applications previously employing metal, wood, paper, glass etc. Usage of polymers is already well established in Automobile, electronics, Telecommunication, Computer, Toys, Medical application, clock, house ware, plumbing, footwear, electrical switch gears, luggage, etc. New polymers with specific properties and applications are being developed. With the result, number of polymers with properties suitable for specific applications is now available.
Reasons for replacement of traditional materials:
• Availability of stronger, stiffer polymers.
• Development of processing techniques to exploit the properties,
• Design possibilities of plastics,
• Availability of accurate, meaningful data on the mechanical properties of polymers.
• Greater willingness on the part of engineers to consider plastics as raw materials in their own right, rather than substitute,
• Increasing awareness of the cost saving, energy saving, labor saving and ease of manufacturing technique.
It is not correct to say that PLASTICS will generally and universally replace all materials. Need for replacement of any part should arise from functional requirement, ease of fabrication, cost with out compromising functional needs, lower weight, lower energy requirement. New polymers with specific properties and applications are being developed.
The part should be designed with plastics material, by considering the
• Functional needs
• Service condition
• Mechanical loading and
• Duration of loading
• Polymer melt behavior ( flow, shrinkage, response to shearing )
• Strength of material Properties of plastics and
• Processing (conversion- fabrication) technique.