Chapter 1 Mold Order and solution 1.1 Request for Mold Quotation Table 1-1: request for mold quotation from a customer Request for Mold Quotation PART NAME NO CAV. RFQ DATE DRAWING NUMBER REVISION QUOTE DUE DATE MATERIAL TYPE SHRINK RATE STANDARD FEATURES SPECIAL FEATURES IX Locating ring nm diameter IX]Sprue bushing mm radius [X Guided ejector system [X Balanced Lift Hole in Mold Strap [X Early ejector return [X Cycle Counter [X Lift holes on All Mold Plates IX Parting line locks X Pry slots"NO Corners TX Cavity I.D. on molded part(for multi-cavity) TX Unscrewing cores IX Recessed cooling connectors 1/4"NPT(P-252) TX Center Ko is NOT used in our shop [X Engrave Part number and Mold Number MOLD DESIGN MOLD CONSTRUCTION X Mold maker Ⅺ Customer TX Standard 2 plate X Hot Sprue Bushing KX Full detai T Lay-out only [X Two Shot Mold [X Hot Runner TXMUD Inserts EJECTION GATETYPE RUNNER MOLD BASE MATERIAL IX Knock Out Pins X Full Round [X] Stripper Plate IX Ejector Blade TX] Sprue [ X Runner Shut-offs [XAir Assist [X Pin Point SIDE ACTION COOLING CAVITY [X Mechanical Slides [INserts [X Hardened Tool Steel TX Hardened Tool Steel [X Wear Plates(Lamina) K Retainer Plates [X Hardened Stainless []Hardened Stainless [X] Positive Locks(Superior)[]Support Plate XIPre-Hardened Steel [X Pre-Hardened Steel [X Hydraulic Cylinder TX Bubblers TXJ Air Cylinder Ⅺ] Finish TXFin
Chapter 1 Mold Order and Solution 1.1 Request for Mold Quotation ` Table 1-1: request for mold quotation from a customer 1. Request for Mold Quotation PART NAME NO. CAV. RFQ DATE DRAWING NUMBER REVISION QUOTE DUE DATE MATERIAL TYPE SHRINK RATE . STANDARD FEATURES SPECIAL FEATURES [X] Locating ring: mm diameter [X]Sprue bushing mm radius [X] Guided ejector system [X] Balanced Lift Hole in Mold Strap [X] Early ejector return [X] Cycle Counter [X] Lift holes on All Mold Plates [X] Parting line locks [X] Pry slots “NO Corners” [X] Cavity I.D. on molded part (for multi-cavity) [X] Unscrewing cores [X] Recessed cooling connectors 1/4" NPT (JP-252) [X] Center KO is NOT used in our shop [X] Engrave Part number and Mold Number MOLD DESIGN MOLD CONSTRUCTION Design by: [X] Mold maker [X] Customer [X] Standard 2 plate [X] Hot Sprue Bushing Type of design: [X] Full detail [X] Lay-out only [X] 3 plate [X] Two Shot Mold [X] Hot Runner [X] M.U.D. Inserts EJECTION GATE TYPE RUNNER MOLD BASE MATERIAL [X] Knock Out Pins [X] Sub-gate [X] Trapezoid [X] Sleeves [X] Edge Gate [X] Full Round [X] Stripper Plate [X] Hot Bushing [X] Hot Manifold [X] Ejector Blade [X] Sprue [X] Runner Shut-offs [X] Air Assist [X] Pin Point SIDE ACTION COOLING CORE CAVITY [X] Mechanical Slides [X] Inserts [X] Hardened Tool Steel [X] Hardened Tool Steel [X] Wear Plates (Lamina) [X] Retainer Plates [X] Hardened Stainless [X] Hardened Stainless [X] Positive Locks (Superior) [X] Support Plate [X] Pre-Hardened Steel [X] Pre-Hardened Steel [X] Hydraulic Cylinder [X] Bubblers [X] [X] [X] Air Cylinder [X] Slides [X] Finish [X] Finish
IX Lube grooves on slides [X Core Pins X] Individual Core Inserts [X Individual Core Inserts ⅤENTS x Venting where practical [X] Cavities [X] Cores [X]P TX Pilots SLIdes Ⅺ Runners Primary vent depth:0003”/0005”, Secondary vent depth:003”/005” Additional Requirements [X Mold Status Report due each week IX Stamp steel type and Rockwell hardness on component [X] Preliminary drawings to be approved by the customer []One set of final mold drawings to be supplied to the customer. [X Electronic copy of final mold drawing to be supplied to the customer. IX Mold sampling required and parts approved by the customer prior to mold shipping. X]Try Out material to be supplied by [X] Customer [X Mold maker IX Spare parts required Slides Table l-1 is a typical format of request table for mold quotation from a customer. The mold signer starts with the design of a new mold when he receives a part drawing, its CAD model and its request for mold quotation. Additional information includes the machine the mold will be run in, the number of cavities required, and, if it is not shown on the drawing, the type of plastic that will be used for this product. While the above information is important, it is incomplete. There are other concerns which should be addressed before going into the mold design Molding characteristics of the specified plastic? How many parts will be molded? This is, Anticipated molding cycle times? Where and how is the product used? Must the product fit with other parts? Tolerances? Draft angles? What type of feed system is required? Basic mold structure(2 plates/3 plates)? Gate location, flow and weld lines, ejector marks? Surface finish? Spares required with mold? Is indicated machine suitable: Tonnage? Shot size? Plasticizing capacity? Is mechanical product removal planned Timing of project? Date for T1? Some of these questions and concerns may have been answered with the accompanying the request for mold quotation from the customer. There may be other questions which will have to be answered by dialogue with the customer. After a request for mold quotation has been received, it is important to find out how the quoted price for this mold has been arrived at. Mold prices are usually estimated by an experienced mold designer or estimator, often by the owner of the mold making business. At the time when the estimator receives a request for a quotation, he or she may sketch a mold design which is believed to be appropriate for the product, based on experience or from records of similar molds. The
[X] Lube grooves on slides [X] Core Pins [X] Individual Core Inserts [X] Individual Core Inserts VENTS [X] Venting where practical [X] Cavities [X] Cores [X]Pins [X] Pilots [X] Slides [X] Runners Primary vent depth : 0003” /.0005” , Secondary vent depth: 003” /.005” Additional Requirements: [X] Mold Status Report due each week [X] Stamp steel type and Rockwell hardness on components. [X] Preliminary drawings to be approved by the customer. [X] One set of final mold drawings to be supplied to the customer. [X] Electronic copy of final mold drawing to be supplied to the customer. [X] Mold sampling required and parts approved by the customer prior to mold shipping. [X] Try Out material to be supplied by [X] Customer [X] Mold maker. [X] Spare parts required: % Cores % Cavities % Ejectors % Slides % Other Table 1-1 is a typical format of request table for mold quotation from a customer. The mold designer starts with the design of a new mold when he receives a part drawing, its CAD model and its request for mold quotation. Additional information includes the machine the mold will be run in, the number of cavities required, and, if it is not shown on the drawing, the type of plastic that will be used for this product. While the above information is important, it is incomplete. There are other concerns which should be addressed before going into the mold design: Molding characteristics of the specified plastic? How many parts will be molded? This is, Anticipated molding cycle times? Where and how is the product used? Must the product fit with other parts? Tolerances? Shrinkage? Draft angles? What type of feed system is required? Basic mold structure (2 plates/3 plates)? Hot runner? Gate location, flow and weld lines, ejector marks? Surface finish? Cavity numbering? Spares required with mold? Is indicated machine suitable: Tonnage? Shot size? Plasticizing capacity? Is mechanical product removal planned? Timing of project? Date for T1? Some of these questions and concerns may have been answered with the accompanying the request for mold quotation from the customer. There may be other questions which will have to be answered by dialogue with the customer. After a request for mold quotation has been received, it is important to find out how the quoted price for this mold has been arrived at. Mold prices are usually estimated by an experienced mold designer or estimator, often by the owner of the mold making business. At the time when the estimator receives a request for a quotation, he or she may sketch a mold design which is believed to be appropriate for the product, based on experience or from records of similar molds. The
designer then bases all cost figures on this sketch. The designer may, according to the complexity or novelty of the product, add a safety factor before arriving at an estimated mold price There is a risk that the quoted mold price is lower than it might have been if all the parameters had been considered at the time of the quoting. The quoted mold price must be competitive with that quoted Another problem is that many more molds are estimated than will eventually result in It is common practice in all industries to send out requests for at least three quotations from makers before a mold order is placed It is important to understand that the mold designer must know the type of mold that the estimator had in mind when quoting a price. If not, he or she may design a mold that greatly exceeds the quoted price. Note that the purpose of any industry is to make money with their product The mold envisaged by the estimator is not necessarily the best design. There is a good chance that in the course of the design process a better design will be found. But it is necessary that the mold designer is aware of the quoted mold price at the start of a mold design project. It is quite possible that the estimator has erred, has underestimated some difficulties in molding the product, or was missing some important information which was subsequently supplied with the mold order. If, after a preliminary study by the mold designer of the product and the final specifications, it becomes apparent that sometimes some parameters have been either inadvertently or deliberately changed, the customer must be immediately advised of any increase in the mold cost caused by such changes before more time is spent on the project. In this way, much aggravation can be avoided On the other hand, if the error was due to poor estimating, there is nothing that can be done by the mold maker but to try to stay within budget and to design whatever is necessary to build the mold as quoted. Under no circumstances must the quality or the performance of the mold be compromised Since the reputation of the mold maker is at stake, any loss suffered due to poor estimating must be written off as learning experience and credit standing. There is a possibility that the customer may agree to carry some or all of the extra costs involved, but it is a policy decision of the mold maker whether to approach the customer for an increase of the contracted mold price Occasionally, a mold price may be deliberately quoted low as the result of a sales policy. For example, to win a new customer or to enter a new field of products in which the molder has little or no experience. Regardless of the low price, the mold maker must still devolop the best possible mold to perform as specified, at a reasonable cost We will here consider only characteristics which directly affect the mold design 1. Flow characteristics. Easy flowing materials usually present no problems, but"stiff materials require higher injection pressure and, therefore, heavier construction of the mold. This will also affect the need for more accuracy and strength of the alignment elements 2. Melt processing temperature. The higher this temperature, the more important becomes the heating and cooling design and, sometimes, the method of heat insulation between hot and cooled portions of the mold 3. Material degradation. Every thermoplastic is to some degree heat sensitive or subject to degradation when exposed to high temperatures over a length of time ome materials, give off poisonous gases when heated for even a short time above their upper permissible temperature and corresponding safe time, if even the material does not have visible signs of degradation at the very first. Other materials give off highly corrosive gases and will require specially selected mold materials and finishes
designer then bases all cost figures on this sketch. The designer may, according to the complexity or novelty of the product, add a safety factor before arriving at an estimated mold price. There is a risk that the quoted mold price is lower than it might have been if all the parameters had been considered at the time of the quoting. The quoted mold price must be competitive with that quoted by other mold makers. Another problem is that many more molds are estimated than will eventually result in orders. It is common practice in all industries to send out requests for at least three quotations from different mold makers before a mold order is placed. It is important to understand that the mold designer must know the type of mold that the estimator had in mind when quoting a price. If not, he or she may design a mold that greatly exceeds the quoted price. Note that the purpose of any industry is to make money with their product. The mold envisaged by the estimator is not necessarily the best design. There is a good chance that in the course of the design process a better design will be found. But it is necessary that the mold designer is aware of the quoted mo1d price at the start of a mold design project. It is quite possible that the estimator has erred, has underestimated some difficulties in molding the product, or was missing some important information which was subsequently supplied with the mold order. If, after a preliminary study by the mold designer of the product and the final specifications, it becomes apparent that sometimes some parameters have been either inadvertently or deliberately changed, the customer must be immediately advised of any increase in the mold cost caused by such changes before more time is spent on the project. In this way, much aggravation can be avoided. On the other hand, if the error was due to poor estimating, there is nothing that can be done by the mold maker but to try to stay within budget and to design whatever is necessary to build the mold as quoted. Under no circumstances must the quality or the performance of the mold be compromised. Since the reputation of the mold maker is at stake, any loss suffered due to poor estimating must be written off as learning experience and credit standing. There is a possibility that the customer may agree to carry some or all of the extra costs involved, but it is a policy decision of the mold maker whether to approach the customer for an increase of the contracted mold price. Occasionally, a mold price may be deliberately quoted low as the result of a sales policy. For example, to win a new customer or to enter a new field of products in which the molder has little or no experience. Regardless of the low price, the mold maker must still devolop the best possible mold to perform as specified, at a reasonable cost. We will here consider only characteristics which directly affect the mold design: 1. Flow characteristics. Easy flowing materials usually present no problems, but "stiff” materials require higher injection pressure and, therefore, heavier construction of the mold. This will also affect the need for more accuracy and strength of the alignment elements. 2. Melt processing temperature. The higher this temperature, the more important becomes the heating and cooling design and, sometimes, the method of heat insulation between hot and cooled portions of the mold. 3. Material degradation. Every thermoplastic is to some degree heat sensitive or subject to degradation when exposed to high temperatures over a length of time. Some materials, give off poisonous gases when heated for even a short time above their upper permissible temperature and corresponding safe time, if even the material does not have visible signs of degradation at the very first. Other materials give off highly corrosive gases and will require specially selected mold materials and finishes
All materials show degradation by changing color to yellow or brown. The ultimate form of gradation is when the plastic becomes charred and black. All products containing even small portions degraded material must be discarded, not only because of poor appearance of the product but because physical properties may have been lost and the product will not perform as expecte There are several reasons why the estimator and the designer should know the anticipated molding cycle. It is always desirable to build a mold with the shortest possible molding cycle, but this can only be achieved at a cost. Special cooling methods, added ejection features, special mold materials, lubrication, etc, will certainly add to the cost of the mold. From one to the other, differences in the plastic, wall thicknesses, draft angles, methods of gating, and other factors may have a significant impact on the molding cycle This is probably the most important area to be checked before the mold design should be starte The estimator should check and question the tolerances at the time when the job is quoted. The final drawing released with the order must be compared with the product drawing used for the quotation to make sure that there were no changes made. Quite often, the drawings used for quotations are only preliminary and incomplete, and tolerances may not have been shown. Since a large portion of the machining cost of them old components is directly related to the tolerances. If the new tolerances will affect the mold cost, the mold maker must immediately approach the customer to bring the tolerances back to what was quoted or to have the contract requoted Often, the product designer assigns close tolerances to the product that are not necessary for the function of the product. The proper method is to give a relatively large general tolerance and tighten up dimensions only where it is necessary for the function of the product or where it is required in the assembly with matching product Whichever method is specified, the designer must make sure that the tolerances shown on the product drawing make sense and can be achieved. The mold designer must not forget that the sizes also depend on the operating temperatures. In some cases, the product keeps shrinking hours and even days fter molding. With some critical dimensions and certain materials, it must be established before hand when, and under what conditions, the product will be measured. This study of the tolerances at the ginning of the job will prevent arguments later on. If the tolerances specified are unreasonable, the mold maker must discuss them with the customer and get a release in writing so that the mold maker will not be held responsible for sizes of the molded product which are outside the specified ones Where and how is the product to be used? This question must be asked not just out of idle curiosity It will give the mold designer some idea of the importance of certain aspects and critical areas of the product such as required fits with other products, finish, strength, location of gate, ejectors, etc. The designer may then suggest changes, especially in the areas of fragile mold cores or thin ribs, not only to make the mold easier to build but also to extend the life of the mold and to improve its serviceability 1.2 Plastic Part Manufacturability The design of plastic parts is largely determined by its purpose, functionality, and appearance Most plastic parts are made with injection molding. The molding process has its own unique features and limitations. Thus, the manufacturability of each part design must be thoroughly considered for its economics,as well as its application and aesthetic expectation. To each mold designer, it is critical to acquire a basic understanding of the plastic part design
All materials show degradation by changing color to yellow or brown. The ultimate form of degradation is when the plastic becomes charred and black. All products containing even small portions of degraded material must be discarded, not only because of poor appearance of the product but because physical properties may have been lost and the product will not perform as expected. There are several reasons why the estimator and the designer should know the anticipated molding cycle. It is always desirable to build a mold with the shortest possible molding cyc1e, but this can only be achieved at a cost. Special cooling methods, added ejection features, special mold materials, lubrication, etc., will certainly add to the cost of the mold. From one to the other, differences in the plastic, wall thicknesses, draft angles, methods of gating, and other factors may have a significant impact on the molding cycle. This is probably the most important area to be checked before the mold design should be started. The estimator should check and question the tolerances at the time when the job is quoted. The final drawing released with the order must be compared with the product drawing used for the quotation to make sure that there were no changes made. Quite often, the drawings used for quotations are only preliminary and incomplete, and tolerances may not have been shown. Since a large portion of the machining cost of them old components is directly related to the tolerances. If the new tolerances will affect the mold cost, the mold maker must immediately approach the customer to bring the tolerances back to what was quoted or to have the contract requoted. Often, the product designer assigns close tolerances to the product that are not necessary for the function of the product. The proper method is to give a relatively large general tolerance and tighten up dimensions only where it is necessary for the function of the product or where it is required in the assembly with matching products. Whichever method is specified, the designer must make sure that the to1erances shown on the product drawing make sense and can be achieved. The mold designer must not forget that the sizes also depend on the operating temperatures. In some cases, the product keeps shrinking hours and even days after molding. With some critical dimensions and certain materials, it must be established before hand when, and under what conditions, the product will be measured. This study of the tolerances at the beginning of the job will prevent arguments later on. If the tolerances specified are unreasonable, the mold maker must discuss them with the customer and get a release in writing so that the mold maker will not be held responsible for sizes of the molded product which are outside the specified ones. Where and how is the product to be used? This question must be asked not just out of idle curiosity. It will give the mold designer some idea of the importance of certain aspects and critical areas of the product such as required fits with other products, finish, strength, location of gate, ejectors, etc. The designer may then suggest changes, especially in the areas of fragile mold cores or thin ribs, not only to make the mold easier to build but also to extend the life of the mold and to improve its serviceability. 1.2 Plastic Part Manufacturability The design of plastic parts is largely determined by its purpose, functionality, and appearance. Most plastic parts are made with injection molding. The molding process has its own unique features and limitations. Thus, the manufacturability of each part design must be thoroughly considered for its economics, as well as its application and aesthetic expectation. To each mold designer, it is critical to acquire a basic understanding of the plastic part design
1.2.1 Plastic Part Dimensional accuracy Plastic parts are molded under elevated temperature. Their dimensional accuracy is affected by accuracy of the mold and shrinkage of the plastic material, which is affected by the following factors 1. Each plastic material has a unique shrinkage rate, which may further vary by its manufacturer, batch number. and water and volatile contents 2. During the injection molding process, the change in the injection conditions such as in force, time, and temperature all directly affect its shrinkage. For example, plastic parts usually less at a higher injection pressure 3. The wall thickness and shape of plastic part also affects its shrinkage. For example, the plastic part with a thin wall shrinks less 4. The mold structure also directly affects plastic parts shrinkage. a larger gate size leads to a small shrinkage. Parallel flow leads to a larger shrinkage. Thus it can be seen that the shrinkage of plastics is unstable, which shall inevitably influence the dimension precision of plastic parts. Considering this factor as well as other complications such as the draft, the flash on parting line and the abrasion of molding parts etc, it is unnecessary to select high dimension precision when designing plastic structural rts if not need be Table 1-2: recommend grades of molding part precision Recommend Selecting Precision Class High PrecisionGeneral Precision General Purpose Polystyrene(Ps) Methacrylate(PMMA) Polycarbonate(PC) 30% of Glass Fiber Reinforced Plastics Polyamide6、66、610.9.1010 2 Chlorinated Polyethers(CPT) 6 PVC-U High density ethylene(HDPE) 6 Polyoxymethylene( POM) PVC-F Polyethylene(LDPE) Recommend grades of molding part precision are shown in Table 1-2. Aside from dimensional accuracy, the surface quality of plastic parts depends on whether there exist defects such as spots, wrinkles, porosity, dents, welding marks, and their surface luster and finishes. Defects occur during the
1.2.1 Plastic Part Dimensional Accuracy Plastic parts are molded under elevated temperature. Their dimensional accuracy is affected by accuracy of the mold and shrinkage of the plastic material, which is affected by the following factors. 1. Each plastic material has a unique shrinkage rate, which may further vary by its manufacturer, batch number, and water and volatile contents. 2. During the injection molding process, the change in the injection conditions such as injection force, time, and temperature all directly affect its shrinkage. For example, plastic parts usually shrink less at a higher injection pressure. 3. The wall thickness and shape of plastic part also affects its shrinkage. For example, the plastic part with a thin wall shrinks less. 4. The mold structure also directly affects plastic part’s shrinkage. A larger gate size leads to a small shrinkage. Parallel flow leads to a larger shrinkage. Thus it can be seen that the shrinkage of plastics is unstable, which shall inevitably influence the dimension precision of plastic parts. Considering this factor as well as other complications such as the draft, the flash on parting line and the abrasion of molding parts etc, it is unnecessary to select high dimension precision when designing plastic structural parts if not need be. Table 1-2: recommend grades of molding part precision Recommend Selecting Precision Class Category Plastic Types High Precision General Precision Low Precision ABS General Purpose Polystyrene(PS) Polymethyl Methacrylate(PMMA) Polycarbonate(PC) Phenylene oxide(PPO) 1 30% of Glass Fiber Reinforced Plastics 3 4 5 Polyamide6、66、610.9.1010 2 Chlorinated Polyethers(CPT) PVC-U 4 5 6 High Density Polyethylene(HDPE) Polypropylene(PP) 3 Polyoxymethylene(POM) 5 6 7 PVC-F 4 Low Density Polyethylene(LDPE) 6 7 8 Recommend grades of molding part precision are shown in Table 1-2. Aside from dimensional accuracy, the surface quality of plastic parts depends on whether there exist defects such as spots, wrinkles, porosity, dents, welding marks, and their surface luster and finishes. Defects occur during the
injection process. Surface luster and finish relate to mold surface, wear, plastic material brand and quality, and injection molding conditions. The molding surface quality must be higher than the surface requirement for the plastic parts. Transparent plastic parts typically require a very high surface finish, which is normally a Ra of0. 025 um or bette 1.2.2 Wall thickness Plastic parts require a reasonable thickness. It should not be too thin, as it needs to have sufficient strength for its application, fastening during its assembly, filling out its cavity during molding, and impact during ejection. It should not be too thick, for it increases the shrinkage, further varies dimensions, prolongs the cooling time, and wastes the material. It may also cause porosity, shrinkage cavity, dent and warpage. The thickness decision needs to carefully strike a balance between economics and part quality and strength, though it also depends on the material type, part size, and molding conditions. thermoplastic materials are good for parts with a think wall, which usually varies between 1.5-4 mm. though it could be as thin as 0.6-0.9 mm or even 0.25 mm at times The thickness should be homogeneous all around the part, to avoid residual forces and defects, caused by uneven shrinkage during the solidification and cooling process. Fig 1-1(a) shows an undesired part structure, while Fig 1-1(b)is a reasonable structure. It is a practice to vary the thickness of a part design, in order to probably locate the welding mark. Fig 1-2 shows an effort of assuring the part quality on the top, by increasing the thickness of the top area, thus avoid leaving welding marks in the top area. Fig 1-1: wall thickness design of plastic parts Welding marks 接痕 4:>1湖门 Gate Fig 1-2: non-uniform wall thickness of plastic parts
injection process. Surface luster and finish relate to mold surface, wear, plastic material brand and quality, and injection molding conditions. The molding surface quality must be higher than the surface requirement for the plastic parts. Transparent plastic parts typically require a very high surface finish, which is normally a Ra of0.025μm or better. 1.2.2 Wall Thickness Plastic parts require a reasonable thickness. It should not be too thin, as it needs to have sufficient strength for its application, fastening during its assembly, filling out its cavity during molding, and impact during ejection. It should not be too thick, for it increases the shrinkage, further varies its dimensions, prolongs the cooling time, and wastes the material. It may also cause porosity, shrinkage cavity, dent and warpage. The thickness decision needs to carefully strike a balance between economics, and part quality and strength, though it also depends on the material type, part size, and molding conditions. thermoplastic materials are good for parts with a think wall, which usually varies between 1.5~4 mm, though it could be as thin as 0.6~0.9 mm, or even 0.25 mm at times. The thickness should be homogeneous all around the part, to avoid residual forces and defects, caused by uneven shrinkage during the solidification and cooling process. Fig.1-1(a) shows an undesired part structure, while Fig.1-1(b) is a reasonable structure. It is a practice to vary the thickness of a part design, in order to probably locate the welding mark. Fig.1-2 shows an effort of assuring the part quality on the top, by increasing the thickness of the top area, thus avoid leaving welding marks in the top area. Fig.1-1: wall thickness design of plastic parts Fig.1-2: non-uniform wall thickness of plastic parts Welding marks Gate
1. 2.3 Ribs Ribs are often used to enhance the plastic part strength and stiffness without a thicker wall. Ribs could also improve material flow conditions during molding Air bubbles Fig 1-3 adding ribs to reduce wall thickness Fig 1-3(a)shows a thick and uneven wall design. Fig 1-3(b) shows a wall design of even thickness It saves material and enhances its strength and stiffness while avoiding air bubbles shrinkage cavities dents and warpages. Rib dimensions are shown in Fig. 1-4. Their thickness is usually smaller than the wall thickness When considering adding ribs to a plastic part design, the focus is to minimize the concentration of the material in an area, to avoid air bubbles and shrinkage cavities. Fig. 1 -5 shows a rib arrangement Fig 1-5(a) shows an undesired arrangement where material concentrates in one area. Fig 1-5(b)shows a better arrangement. Rib should not be too big; they should be short and more in quantity. The distance between ribs should be equal or greater than twice the wall thickness. As shown in Fig. 1-6, a good lesign can avoid shrinkage cavities and increase part strength and stiffness. The orientation of rib arrangement should be in line with the material flow direction, to ease filling out the cavity and to avoid disturbing the flow. There should be a gap between rib's end and the part support surface <(0.50. R=t/8
1.2.3 Ribs Ribs are often used to enhance the plastic part strength and stiffness without a thicker wall. Ribs could also improve material flow conditions during molding. Fig.1-3: adding ribs to reduce wall thickness Fig.1-3(a) shows a thick and uneven wall design. Fig.1-3(b) shows a wall design of even thickness. It saves material and enhances its strength and stiffness, while avoiding air bubbles, shrinkage cavities, dents and warpages. Rib dimensions are shown in Fig.1-4. Their thickness is usually smaller than the wall thickness. When considering adding ribs to a plastic part design, the focus is to minimize the concentration of the material in an area, to avoid air bubbles and shrinkage cavities. Fig.1-5 shows a rib arrangement. Fig.1-5(a) shows an undesired arrangement where material concentrates in one area. Fig.1-5(b) shows a better arrangement. Rib should not be too big; they should be short and more in quantity. The distance between ribs should be equal or greater than twice the wall thickness. As shown in Fig.1-6, a good design can avoid shrinkage cavities and increase part strength and stiffness. The orientation of rib arrangement should be in line with the material flow direction, to ease filling out the cavity and to avoid disturbing the flow. There should be a gap between rib’s end and the part support surface. Fig.1-4 Rib dimensions Air bubbles < (0.5~0.7) t < 3 t t R = t / 8
Fig 1-5: rib arrangement Fig 1-6: rib design 1. 2.4 Support Surface When plastic parts need to have a support surface, it is not desirable to use the whole part bottom surface as the support. Fig 1-7 shows the bottom surface becomes uneven, when the plastic part deforms a bit. A Frame, three or four bottom feet are usually a better support surface Fig. 1-7: design of support surface 1. 2.5 Draft Draft is used to protect plastic part surface from scratch during ejection. It is a draft reserved along with the ejection direction. The draft depends on the material shrinkage, part shape, wall thickness, and ejection location For inner holes on a plastic part, the smaller end of core is used as reference. Its draft is shown along with the rising direction. For exterior of a plastic part design, the larger end of cavity is used as reference. Draft is dimensioned along with the reducing direction Please refer to Table 1-3 for various draft designs. A typical draft ranges between 30-1 30. Normally a tall or large plastic part design has a smaller draft. When there is a special need or higher precision requirement, a smaller daft is used
Fig.1-5: rib arrangement Fig.1-6: rib design 1.2.4 Support Surface When plastic parts need to have a support surface, it is not desirable to use the whole part bottom surface as the support. Fig.1-7 shows the bottom surface becomes uneven, when the plastic part deforms a bit. A Frame, three or four bottom feet are usually a better support surface. Fig.1-7: design of support surface 1.2.5 Draft Draft is used to protect plastic part surface from scratch during ejection. It is a draft reserved along with the ejection direction. The draft depends on the material shrinkage, part shape, wall thickness, and ejection location. For inner holes on a plastic part, the smaller end of core is used as reference. Its draft is shown along with the rising direction. For exterior of a plastic part design, the larger end of cavity is used as reference. Draft is dimensioned along with the reducing direction. Please refer to Table 1-3 for various draft designs. A typical draft ranges between ' ' 30 1 30 ° − . Normally a tall or large plastic part design has a smaller draft. When there is a special need or higher precision requirement, a smaller daft is used
External draft can be as small as 5. Internal draft can be as small as 10 -20. A complex plastic part which is difficult to eject, should adopt a larger draft. Usually a draft of 4-5 on each side is considered for a plastic part design with ribs or bosses. Larger draft should be chosen for a plastic product with large wall-thickness; during mold opening, in order that the product is retained on the side of the moving mold, draft of the inner surface should be smaller than that of the outer surface and the other way round, draft of the outer surface should be smaller than that of the inner surface so as to retain the product on the side of the fixed mold Fig 1-8: draft Table 1-3: drafts for common plastic materials Draft Draft Plastic Types Plastic Types Cavity PMMA PP&PVC-F 20′~50 PA PVC-U 50′~1°45′ 30′~50′ 30 ABS 30′-1°35′-1°30 1.2.6 Hole design k Holes may exist on a plastic part, including through holes, blind holes, screwed holes, and irregular shaped holes. In principle, holes should be as simple as possible. a complex hole increases the difficulty of mold making. As shown in Table 1-4, a sufficient gap should be reserved between holes or between a hole and the wall. The diameter of a hole also relates to its depth as shown in Table 1-5. When the distance between two holes or from the hole to the edge is smaller than the one specified in Table 1-4 please refer to the pattern design shown in Fig 1-9 Table 1-4: holes'pitch, space to wall and diameter of thermoset plastics Hole diameter mm 3~6 6~10 10~18|18-30 Holes pitch, space to wall /mm 3~4 4~5 5~7 Table 1-5: relations of hole's diameters and depths Molding types Hole depth Through hole Blind hole Horizontal hole 2.5d <1.5d Compression molding Vertical hole Extrusion or injection molding 4~5d
External draft can be as small as ' 5 . Internal draft can be as small as ' ' 10 − 20 . A complex plastic part, which is difficult to eject, should adopt a larger draft. Usually a draft of ° ° 4 − 5 on each side is considered for a plastic part design with ribs or bosses. Larger draft should be chosen for a plastic product with large wall-thickness; during mold opening, in order that the product is retained on the side of the moving mold, draft of the inner surface should be smaller than that of the outer surface and the other way round, draft of the outer surface should be smaller than that of the inner surface so as to retain the product on the side of the fixed mold. Fig.1-8: draft Table 1-3: drafts for common plastic materials Draft Draft Plastic Types Core Cavity Plastic Types Core Cavity PE 20′~45′ 25′~45′ PMMA 35′~1° 35′~1°30′ PP&PVC-F 20′~50′ 50′~1° PA 20′~40′ 25′~40′ PVC-U 50′~1°45′ 50′~2° PC 30′~50′ 35′~1° PS 30′~1° 35′~1°30′ CPT 20′~45′ 25′~45′ ABS 35′~1° 40′~1°20′ POM 30′~1° 35′~1°30′ 1.2.6 Hole Design Holes may exist on a plastic part, including through holes, blind holes, screwed holes, and irregular shaped holes. In principle, holes should be as simple as possible. A complex hole increases the difficulty of mold making. As shown in Table 1-4, a sufficient gap should be reserved between holes or between a hole and the wall. The diameter of a hole also relates to its depth as shown in Table 1-5. When the distance between two holes or from the hole to the edge is smaller than the one specified in Table 1-4, please refer to the pattern design shown in Fig.1-9. Table 1-4: holes’ pitch, space to wall and diameter of thermoset plastics Hole diameter mm ＜1.5 1.5~3 3~6 6~10 10~18 18~30 Holes’ pitch, space to wall /mm 1~1.5 1.5~2 2~3 3~4 4~5 5~7 Table 1-5: relations of hole’s diameters and depths Molding types Hole depth Through hole Blind hole Horizontal hole 2.5d ＜1.5d Compression molding Vertical hole 5d ＜2.5d Extrusion or injection molding 10d 4~5d
白④ Fig 1-9: improvement design for too small holes' pitch and space to wall The holes used for fastening on the plastic products as well as other holes under stress should be reinforced with a convex edge designed thereon, as indicated in Fig. 1-18. The fixing hole can be designed in the form of a screw hole with sinking head as illustrated in Fig. 1-19a), whereas that in Fig 1-19b)is generally not recommended. The form as shown in Fig. 1-19c)can be used instead so that the core can be better set m山 Fig1-10: reinforcement of holes 国國 Fig. 1-11; types of fixing holes 1.2.7 Screw Screws on a plastic part can be formed during the injection process. They may be created by machining after the injection process. For a plastic part which is subject to frequent assembly and For design of screws on a plastic part, the following principles are followed 1. The diameter of injected external screws should not be smaller than 4 mm. Internal screws sl not be smaller than 2 mm. Length of fit should be reduced to less than 1.5-2.0 times of the diameter. in order to minimize the accumulative error on the screw pitch
Fig.1-9: improvement design for too small holes’ pitch and space to wall The holes used for fastening on the plastic products as well as other holes under stress should be reinforced with a convex edge designed thereon, as indicated in Fig. 1-18. The fixing hole can be designed in the form of a screw hole with sinking head as illustrated in Fig. 1-19a), whereas that in Fig. 1-19b) is generally not recommended. The form as shown in Fig. 1-19c) can be used instead so that the core can be better set. Fig.1-10: reinforcement of holes Fig.1-11: types of fixing holes 1.2.7 Screw Screws on a plastic part can be formed during the injection process. They may be created by machining after the injection process. For a plastic part which is subject to frequent assembly and disassembly, the part may be inserted with a metal screw. Fig.1-12: screw design on plastic parts For design of screws on a plastic part, the following principles are followed: 1. The diameter of injected external screws should not be smaller than 4 mm. Internal screws should not be smaller than 2 mm. Length of fit should be reduced to less than 1.5~2.0 times of the screw diameter, in order to minimize the accumulative error on the screw pitch
