Reasonable product design and mold design are key technologies that affect the success of injection molding. This article introduces some basic principles of injection molded product design and mold design.
The basic conditions of plastic product design
1.1. Record the product usage conditions as detailed as possible.
These conditions are:
A. Use environment
Outdoor conditions (sunlight, rain, ice, snow, dust), sunlight (ultraviolet rays, radiation, etc.), contact with gas (corrosive gas, water vapor), contact with liquid (water, detergent, oil, medicine, etc.), other (plasticizers), copper);
B. Use intensity
Common temperature, maximum temperature, minimum temperature (usually, special circumstances, outdoor, warehouse, window, car, etc.), repeated cold and hot
C. Use load
Common load, maximum load (normal, special circumstances. Static, dynamic, repeated, continuous, impact, drop, etc.).
1.2. Assembly situation
How to install and use the product, what are the key dimensions that need to be controlled, which related parts are matched, and what degree of cooperation?
1.3. Secondary processing requirements
What post-processing should be done after the plastic is formed, such as painting, bonding, printing, hot stamping, electroplating, etc?
Product design procedure
2.1. Product planning
When the new product plan report was made, it was decided to use plastic materials. The new product development report mainly discussed the properties of plastics and molding processing issues.
2.2. Product idea
From the beginning of drawing the design sketch of the product and conceiving the shape of the product, the question of whether it can be formed should be considered. In this process, the relevant technical issues should be frequently contacted with plastic craftsmen and mold design technicians.
2.3. Record the use environment, use requirements, and assembly conditions of the product in detail and make a checklist to check whether the designed product meets user requirements in the final stage.
2.4. Choose plastic varieties
Focus on the necessary characteristics of the products in the inspection table, and consider other factors (such as cost, etc.) to select plastic varieties.
2.5. From the appearance and shape of the product.
Dimensional accuracy, material, batch size, delivery date, cost, and other constraints determine the forming and secondary processing methods
2.6. Determine the parting line
Set the parting line at the largest diameter of the product. Design the mold slope with the parting line as the limit, and the shape of the parting line is as simple as possible. After the parting line is determined, the basic structure of the mold is roughly determined, and the parting line is closely related to mold manufacturing and molding processing.
2.7. Ejection method
First, consider which direction to eject from. The trace of the ejector pin should be located in a position that does not affect the appearance of the product. Products that are not allowed to have ejector marks cannot be ejected by ejector pins, and other demoulding methods should be considered.
Generally, the ejector mechanism is set in the movable mold. According to the shape of the product, when it is foreseen that the product may stick to the mold, measures or improvement measures should be taken to force the product to stick to the movable mold, especially in fully automatic molding, to ensure product reliability fr mold ejection, the ejection method must be considered in the product design stage.
2.8. Loss mode slope
In order to improve the production efficiency and the qualification rate, the draft angle of the product should be designed. The draft angle is generally 1/40. For precision plastic products with no draft angle on the side, the slider core method must be used. The non-slope surface is made on the slider. After the mold is opened, the slider is released first, and then the product is ejected.
2.9. Wall thickness
The uniformity of the wall thickness should be considered when designing the product, and the cooling time is determined by the maximum wall thickness. Too thick walls not only take too long to cool down but also tend to shrink. The problem with thin walls is that the strength is insufficient and easy to crack and difficult to form. Within a reasonable range, the thinner the wall, the better.
2.10. Fillet connection
Whether it is to prevent plastic parts from cracking or from the perspective of mold manufacturing, sharp corners are not allowed on the product. After each surface is connected with rounded corners, stress concentration can be avoided and the fluidity of the baked material can be improved.
Pay attention to the design of product appearance
In the design of product structure, it is necessary to focus on avoiding shrinkage from the perspective of appearance. Shrinkage is the biggest problem in appearance quality in plastic injection molding. The design conditions to prevent shrinkage are:
3.1. The wall thickness of plastic products is generally 2.5~3 mm for electronic engineering shells. Daily necessities shell 1.5~2mm, and thin-walled shell 0.5~0.8 mm (not too thick, otherwise it will affect the processing efficiency)
System wall thickness. Change the design of the wall thickness disparity, and uniform the wall thickness by designing process holes and other means; Thinning particularly thick parts;
3.2. Simulate the flow state of the melt and improve the part that hinders the flow of the melt (thin wall part);
3.3. In order to increase the strength and rigidity of plastic products and prevent deformation, stiffeners are often used. The thickness of the ribs is about 0.5~0.8 mm
3.4. Corresponding surfaces of ribs and bosses are prone to shrinkage, so attention must be paid when designing.
Design to ensure dimensional accuracy
4.1. First of all, it should be realized that the dimensional accuracy of plastic products is different from that of metal parts because there are many reasons that affect the accuracy of plastic products.
- the thermal expansion coefficient of plastic is large;
- the plastic has molding shrinkage;
- Plastic products will undergo aging changes (post-shrinkage); d. The size of plastic products is affected by humidity;
- Part of the dimensional accuracy of plastic parts is difficult to guarantee by the accuracy of mold parts;
- The accuracy of plastic products is affected by the accuracy of mold manufacturing.
4.2. There are three main factors affecting the dimensional accuracy of plastic products:
A. Mold manufacturing error accounts for 50%;
B. Hangers in molding conditions account for 30%;
C.The error between material batches and within batches accounts for 20%.
4.3. Some high-precision dimensions on plastic parts cannot be achieved by molding.
For example, the size of the shaft hole matched with the metal shaft must be embedded in the plastic part with a metal sleeve in the form of an insert, and a combination of plastic and metal is used. It can not only ensure the dimensional accuracy of a certain part, but also improve the production efficiency, and the economic benefits are very obvious.
When plastic and metal are combined, due to the difference in the thermal expansion coefficient of the two materials (the linear expansion coefficient of polystyrene is 7×10-5/℃, and aluminum is 4×10-8/℃), it is necessary to prevent the temperature from changing drastically Stress cracks have occurred.
Design to ensure product strength
The disadvantage of plastic products is poor rigidity, and deformation and cracking should be considered in advance when designing plastic parts.
The principle of ensuring product strength is:
- Do not make the wall of the product too thin
- Remove sharp corners
- Investigate the impact strength of the plastic used at low temperatures. In fact, plastic parts often have strength problems in winter
- The punch-out test method should be close to the actual use of the product, such as a drop test
The specific measures to increase the strength of the product are:
(1). Set reinforcement ribs.
Reinforcing ribs can effectively increase the rigidity and strength of the product
It is more economical and reasonable than simply increasing the wall thickness. The ribs on the large flat products also have the function of preventing warping.
In order to achieve a better reinforcement effect, the design of the reinforcement must be paid attention to.
- The height of the ribs should not exceed 3 times the wall thickness
- The width of the bottom of the rib cannot exceed 2/3 of the wall thickness,
- Design a 2~5 degree inclination of the mold on the stiffener;
- The distance between the two reinforcing ribs shall not be less than twice the wall thickness.
(2). Circle corner connection.
In plastic products, there are many cases of cracking at the sharp corners. The reason is that during the injection process, the flow of the bakery material at the sharp corners changes sharply, resulting in natural stress, and remains at the sharp corners.
The design of the stiffener is for strengthening. If the bottom of the stiffener is not rounded, the stiffener will not only fail to strengthen but will reduce the strength of the product.
Except for the mold structure reasons such as the inlay core and parting surface, the other surfaces should be connected with rounded corners. Due to the determination of the corner size, set the wall thickness as T and the radius of the rounded corner is R, then R/T20.25
The specific measures to increase the strength of the product are:
(3). Determine the safety factor.
The safety factor based on tensile strength is 3 for steel and 10 for plastic under static load. Although the safety factor of plastic is set very large. However, due to molecular orientation and other reasons, the strength of each part of the product is very different. It is better to use a larger full coefficient.
(4). Self-tapping thread strength.
The strength of self-tapping threads is determined by the rigidity of the material, and the strength of self-tapping threads of high-rigidity materials is greater. The self-tapping threaded boss should be as large as possible without causing shrinkage.
Product design to facilitate mold manufacturing
6.1. Change the parting surface of the inclined plane or other shapes to the parting surface perpendicular to the ejection direction
6.2. The mold structure of the inlay core is often used, and the inlay method should be considered when designing the product.
6.3. Do not design rounded corners for the parts that are to be inlaid
6.4. The core mating surface looks very well inlaid, and it will deform slightly after being subjected to molding pressure, and the inlaid traces will become obvious. Therefore, the inlay part and the cavity forming the outer surface of the product should not be a flat surface, and a step should be provided to cover the traces of the inlay line.
6.5. The small holes on the product are difficult to mold because the slender small core is easy to bend or break under injection pressure. The improvement method is to design rounded corners at the root of the thin core and thicken the lower half.
6.6. The groove on the product should not be a thin blade or a thin-walled shape. The blade-shaped core is not only difficult to process but also easy to damage
6.7. Symmetrical products, easy to manufacture molds
6.8. The straight knurls on the side of the product should not extend to the parting surface, and the upper mouth of the cavity should be straight to simplify the shape of the parting surface.
6.9. Turning processing is faster than milling, and the cost is low, try to use a circular design
6.10. Embossed characters on the product, the mold is easy to engrave
6.11. With the vertical and horizontal design steps on the lattice surface products, the mold is easy to finish processing
Product design for spraying and printing
7.1. Design of plastic parts:
7.1.1. If the surface of the plastic part is stained with a release agent, abrasive, paraffin, etc., it will not be painted or cause the paint layer to fall off. When designing spray-painted plastic parts, the key considerations are to make the plastic parts better out of the mold, such as increasing the inclination of the mold, improving the surface finish of the mold cavity, and plating the surface of the mold cavity with hard chrome.
7.1.2. There should be no defects such as flashing and welding lines on the painted surface.
7.1.3. The designed product should have a shape that can disperse internal stress. Products with concentrated internal stress will produce stress cracks under the action of the paint thinner.
7.1.4. Chromatic spray paint or partial spray paint must have obvious boundaries. It is difficult to chromatic in the deep groove and the design must be changed
7.1.5. The apex of the two-color spray paint surface should be a right angle, and the bottom should be rounded
7.2. Printed wood grain
7.2.1. When shrinkage is caused by uneven wall thickness such as stiffeners and bosses, wood grains are often printed to cover up the defects.
7.2.2. The printed and non-printed surfaces must have obvious steps (above 2mm), and the steps smaller than 2mm shall be pasted on the non-printed surface with pressure-sensitive adhesive.
7.2.3. The design conditions are the same as those of spray-painted plastic parts, with special attention to chamfering (about 0.5mm) and removing flash.
7.2.4. Print wood grains on the circumference or three sides of the box-like plastic parts. In order to maintain the continuity of the printed pattern, the fillet radius of each connecting surface should be less than 8mm.
7.3. Screen printing
7.3.1. The screen-printed surface should be flat. The cost of screen printing on curved surfaces with a radius of less than 200mm is very high.
7.3.2. The recessed part on the product will damage the screen.
7.3.3. There should be no shrinkage on the screen-printed surface (it is better not to have ribs and bosses on the back of the screen-printed surface, as the shrinkage part will be missed).
7.3.4. The small concave and convex shapes such as knurling on the plastic parts can only obtain unclear print patterns.
Design of bonding, hot welding, and hot stamping products
8.1. Design of bonded products
8.1. 1. The design conditions are the same as the spray-painted plastic parts, and the demolding angle should be large.
8.1. 2. If the bonding surface shrinks, it will affect the bonding fastness, and the uneven wall thickness must be completely changed. During solvent bonding, part of the solvent remains in the shrinking part, causing cracks on the surface of the plastic part.
8.1.3. Use double-sided adhesive tape to bond with good reliability, but poor bonding force
8.1.4. The use of solvents or adhesives to bond plastic products is a manual operation. Reliability becomes a problem in mass production. It is not as reliable as screw mechanical bonding.
8.1.5. From an economic point of view, try to replace the bonding process with mechanical methods such as snap connection, which can simplify the process.
8.1.6. The assembly of bonded parts must be designed with gaps
8.2. Design of hot welding products
8.2.1. Hot soldering is to use a soldering iron with a hemispherical concave front end to heat the plastic solder foot to deform it and firmly bond with other parts (the solder foot diameter is less than 20 mm with a 6oW soldering iron and above 20 mm with a 100W soldering iron).
8.2.2. The diameter of the welding foot is at least 2mm, the root is small fillet, the height of the welding foot is insufficient, the surfacing is not large enough, the strength is poor, the thickness of the hot welded piece is at least 1mm, and the length of the welding foot is calculated by the formula:
The thickness of the hot welded piece +1.5×the diameter of the welding foot.
8.2.3. Hot welding requires that the hot welding parts do not move during and after the hot welding, and the positioning structure should be designed according to the design.
8.2.4. Hot welding methods are commonly used for high production efficiency and good reliability. Rotary friction welding, ultrasonic welding, high-frequency welding, etc. (the welding ribs of ultrasonic welding should be small and large)
8.3. Design of hot stamping products
Hot stamping is the use of a hot mold engraved with pixels or cross-characters under a certain pressure to transfer the colorful blessing foil on the hot stamping material to the surface of the molding material to complement the pattern or cross-learning.
8.3.1. The back of the hot stamping surface should not have a convex shape with uneven wall thickness. No pattern can be stamped in the recess, and the stamping surface must be flat.
8.3.2. The hot stamping surface is higher than other parts. If the hot stamping part has a shape just higher than the hot stamping surface, the foil will be wrinkled. There should be a fixed pitch.
8.3.3. Hot stamping at the weld line, the pattern is easy to peel off, try to move the weld line to another place.
8.3.4. The design conditions are the same as those of sprayed products.
8.3.5. The hot-stamped product is placed flat and cannot be warped.
8.3.6. The products to be hot stamped should pay attention to the process and hygiene, and cannot touch the hot stamping surface with your hands.
8.3.7. The back of the hot stamping surface should be supported by force. If the hot stamping force is insufficient, the lines will peel off.
8.3.8. Hot stamping on the entire surface, no part of the printed surface is allowed to protrude.
8.3.9. Fully hot stamping. The junction of the two faces cannot have rounded corners.
Design of metalized plastic parts and plastic parts for installation of nameplates
9.1. Design of vacuum evaporation plastic parts
The metallization of plastic parts not only maintains the original characteristics of the plastic, such as lightweight, easy processing, and high production efficiency, but also overcomes some shortcomings in the material properties of the plastic itself, such as heat resistance, non-conductivity, easy aging, and moisture absorption, etc. The plastic surface has a beautiful metallic texture.
The metallic luster will make the original shrinkage of the plastic part more obvious, so the uneven wall thickness of the product must be completely changed to avoid shrinkage.
Plastic plating parts used in cold areas consider the bonding force between plastic and plating. Because the thermal expansion coefficient of plastic and metal is an order of magnitude, at low temperatures, the plastic shrinks, and the plating floats, looking like bubbles.
Take the following measures to improve the quality of evaporated products
- Use electroplating grade plastic.
- Fully dry the plastic particles to avoid the defects of silver wire and air bubbles in the product. In the molding temperature range, try to increase the melting temperature.
- It is strictly forbidden to use mold release agents. In order to ensure the smooth ejection of plastic parts without using mold release agents, the mold should increase the angle of ejection and improve the surface finish.
- Do not use beryllium-cyan alloy as the molding material. If the surface of the plastic part is attached with a very thin copper film, it will hinder the adhesion of the metal coating.
- Scratches and scars are not allowed on the surface of the mold cavity. Any defects left in the mold will become more obvious due to metalization. Therefore, the appearance inspection of semi-finished plastic parts should be strengthened.
9.2. Design of electroplated plastic parts
9.2.1. The basic requirements are the same as vacuum evaporation.
9.2.2. There must be no weld lines on the plastic parts.
9.2.3. There must be no parting line and core inlay line on the plating surface.
9.2.4. Avoid hundreds of holes. The plating solution remaining in the blind holes is not easy to clean and cause pollution in the next process.
9.2.5. The depth of the groove should not exceed 1/3 of the groove width. The deeper the groove, the higher the voltage required for electroplating, and the higher the electroplating voltage, the poor quality of the coating.
9.2.6. The wall thickness of electroplated plastic parts should be moderate, and the rigidity of plastic parts that are too thin (less than 1.5mm) is poor, easy to deform during electroplating, and the adhesion of the coating is also poor.
9.2.7. The electroplating process has the phenomenon of thickening of the sharp edges. During electroplating, the sharp edges bow and discharge at the tip, causing the corner plating to bulge, and the fillet radius is at least 0.8mm. Flat plate-shaped plastic parts are difficult to electroplate.
The central part of the plated part has a thin coating. The closer the edge is, the thicker the coating. The entire coating is uneven. The flat shape should be changed to a slightly rounded surface, or an orange peel Glossy. The larger the surface area of electroplating, the greater the difference in gloss between the center and the surrounding edges. A slightly parabolic surface can improve the gloss uniformity of the plating surface.
9.3. Install the plastic part of the nameplate
9.3.1. The nameplate is flat on the plastic part, and the edge is easy to turn up. The nameplate should be buried in the nameplate prefabricated frame of the plastic part. There must be a certain gap between the nameplate and the frame to prevent the thermal expansion of the two materials from deforming.
9.3.2. When using adhesive to bond the nameplate, there must be adhesive holes on the plastic part
9.3.3. The larger the bonding surface between the nameplate and the plastic, the better
9.3.4. It is difficult for the end face of the stretched nameplate to be in a square shape. The frame should be designed on the plastic part so that the edge of the nameplate is buried in the frame to cover the end face
9.3.5. In order to make the nameplate and the plastic part fit well, the nameplate burr groove should be set on the plastic part to make the burr on the nameplate empty
Product design to prevent shrinkage
10.1. The wall thickness of the entire product should be uniform, and the wall thickness change should not exceed 20%.
10.2. Change the big stiffener into several small stiffeners, see figure 3.36. Connecting lattice shapes can significantly increase the strength.
10.3. Change the thick wall to a thin wall supported by stiffeners, and the stiffeners are in the direction of the force.
10.4. The brighter the surface of the product, the more obvious the shrinkage. The surface of the product is made into a matte surface with dermatoglyphics or pulse patterns, which can effectively cover the shrinkage.
Product design to prevent deformation
11.1. The upper edge of boxes, basins, corners, and other products is easy to deform, and frame reinforcement is required
11.2. The bottom of the box products is easy to deform, and it is reinforced with uneven deformation or framed.
11.3. Asymmetrical products are easily deformed due to uneven shrinkage. The approximate symmetrical design can prevent deformation
11.4. The main causes of deformation are uneven shrinkage (including molding shrinkage and post-shrinkage) and stress effects (such as excessive injection pressure, unbalanced force during demolding, etc.). Products with less deformation should be simple in shape and uniform in wall thickness.
11.5. Reinforcing ribs or frames on thin-plate products will increase deformation, which is caused by uneven shrinkage.
11.6. The anti-deformation measure for large plane products is to change the plane shape to a large arc (such as a radius of 10M) curved surface. In order to improve the appearance and impact strength, the large plane should be designed as a parabolic surface of about 0.5 mm per 100 mm.
Product design to prevent cracking
12.1. The stress of the product is often concentrated at the sharp corners or notches. The primary condition to prevent cracking is to maintain continuity in the shape of the product, change the sharp corners to rounded corners, and the gaps to arc transitions.
12.2. The distance between the hole and the hole is too close or the hole is too close to the edge of the product will weaken the strength, and the distance should be increased.
12.3. The orifice is a stress concentration area, and the hole is the most prone to cracking, so the orifice should be reinforced with a frame.
12.4. The convex steps in the paint products are prone to stress cracking due to the action of the paint thinner, and must be connected with rounded corners.
Product design to reduce molding failure
13.1. The holes, grooves, and text marks on the side of the product all form an undercut for the product, and any undercut shape needs to be formed with a horizontal core-pulling mold structure. The horizontal beat core mold is not only complex in structure, but also poor in molding stability, prone to failure, and low production efficiency. If it is changed to a U-shaped hole or a hole formed by the upper and lower core slopes, the core pulling structure can be eliminated.
13.2. The convex ribs inside the product also form a concave cut, which needs to be formed by the inner core pulling method. The structure of the inner core-pulling mold is more complicated, and the corresponding bottom surface has a through-hole, which can eliminate the core-pulling action.
13.3. It is difficult to form small and deep holes on plastic parts because the core is easy to break or bend under injection pressure. Post-processing drilling is more economical and effective than molding holes.
It is recommended to mold a cone pit in the drilling position of the plastic part for post-processing drilling positioning.
13.4. The lower butt hole will be eccentric, and there should be obvious steps on both sides.
13.5. Products with sharp corners are difficult to demold, and the products are easily scratched during demolding. It is a design to eliminate sharp corners.
13.6. Simplify the shape of the product and try not to have a high or slender core in the fixed mold.
13.7. The sidewall of the transparent product must have sufficient out-of-mold oblique hair to prevent the sidewall from fuzzing.
13.8. Simulate the flow of molten material and determine the position of the weld line to determine the gate position. Cup products should have a gate at the bottom.
13.9. The gate is located in the thick-walled part of the product for easy molding. When the material flow flows from the thick-walled part to the thin-walled part, the injection pressure loss is small, and it is conducive to maintaining pressure.
Prevent waste caused by metal inserts
14.1. The surrounding edges of metal inserts should be chamfered. The large internal stress concentrated around the inserts with sharp edges will cause the product to crack.
14.2. The threaded metal insert should make the thread protrude out of the plastic part.
14.3. Due to the difference in thermal expansion between metal materials and plastics, plastic products with metal inserts have greater internal stress. The larger the metal insert, the greater the internal stress of the product. The plastic around the insert is often cracked, affecting the use. It is best to avoid inserts.
14.4. The embedded round nut should be flush with the installation surface of the plastic part, and lower than the plane, the insert will hang out during installation.
14.5. In order to prevent the insert from rotating or falling off in the product, the straight knurled cylindrical insert is fixed with a groove in the middle, the groove depth is 1~2mm, and it has a strong resistance to tension and torsion. Small inserts are under very small force, and diamond knurling can be used without grooves.
14.6. The price of metal inserts is relatively high, the inserts are time-consuming, the production efficiency is low, and the cost of products increases. Therefore, try to use butterfly bolt connections or self-tapping screws to replace inserts.
Avoid problems caused by product assembly
15.1. Screwing the metal rod into the plastic part is prone to stress cracking.
15.2. If two plastic parts are butt-connected, if there is no barrier, it is not easy to connect flat, and the apparent effect is poor. The product designed to be connected should be left with a gap.
15.3. The lid and box body is often not closed due to deformation and high seams. It is recommended that the edges should be framed or reinforced to prevent deformation.
15.4. Avoid applying large loads to the plastic parts, for example, the bottom of the dark bar has no rounded corners and cannot withstand lateral impact.
15.5. Where other parts are installed, you must study in detail what problems will occur after assembly, and think about improvement measures in advance, such as the parts that are set on the boss, because the bottom of the boss is rounded, it cannot fit closely with the assembly parts.
Other issues to be considered in product design
16.1. Add a border around the words and trademarks. The words or trademarks can be separately made into the molding core and inserted into the mold cavity. The mold is easy to process and grind, and the words and trademarks can be exchanged easily.
16.2. According to the use and assembly function of the product, dig out the redundant function part and hug it to make the product lighter and save raw materials.
16.3. Carefully design the position of the parting surface so that even if a flash occurs, the appearance is not affected, or it is easy to repair the flash.
16.4. In plain acrylic-body hinges, it may be better to make the arc of the hinge into two stages.
16.5. The use of a one-time molding method for some products will make the shape of the cavity very complicated or weak. The products are difficult to process and are prone to failure during molding. It may be more economical and reasonable to adopt post-processing on part of the shape of the product.
16.6. The boss of the self-tapping thread must have a certain strength, and the chamfer of the orifice is of great significance to facilitate the screwing of the self-tapping screw and to enhance the mold core.
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