Demolding angle: To facilitate demoulding, wall of plastic product should have an draft angle α in demoulding direction, and its value is expressed in degrees.
Key points for determining demoulding angle
(1) The higher product precision requirement, the smaller demoulding angle should be.
(2) For large products, a smaller demoulding angle should be used.
(3) For products with complex shapes that are difficult to demould, a larger angle should be used.
(4) If product shrinkage rate is large, angle should also be increased.
(5) Reinforced plastics should have a large angle, and plastics containing self-lubricants can use a small angle.
(6) The thicker product wall, the larger angle should be.
(7) Direction of angle. Inner hole is based on small end, meets size requirements of drawing, and slope is obtained in direction of expansion; outer shape is based on large end, meets requirements of drawing, and slope is obtained in direction of reduction. In general, demoulding slope is not limited by tolerance zone of product. Demoulding slope of high-precision plastic products should be within tolerance zone.
Plastic is hard and brittle, and rigidity is large, so demoulding slope is required to be large.
A smaller demoulding slope can be used for cores that meet following conditions:
(1) Rigidity of product is sufficient during ejection.
(2) Friction coefficient between product and mold steel surface is low.
(3) Roughness value of core surface is small, and polishing direction is consistent with demoulding direction of product.
(4) Shrinkage of product is small and sliding friction is small.
Plastic parts structural design - recommended demoulding angle values for various commonly used plastic parts

Flash
1. Excessive injection pressure
2. Loose mold closing
3. Dirty mold parting surface
4. Excessive plastic temperature
5. Projected area of plastic part on parting surface exceeds allowable range of machine tool
6. Mold plate bending and deformation
Deformation
1. Insufficient cooling time
2. Mold temperature is too high or uneven
3. Unreasonable ejector position
4. Uneven thickness of plastic parts
Bubbles
1. Raw material contains water or other volatile substances
2. Plastic temperature is too high or heating time is too long
3. Injection speed is too fast
4. Injection pressure is too low
5. Mold temperature is too low
6. Injection piston is retracted too early
7. Air is mixed in barrel
Insufficient molding
1. Insufficient feeding amount
2. Injection speed is too slow
3. Injection pressure is too low
4. Mold temperature is too low
5. Barrel and nozzle temperature are low
6. Projection area of plastic part on parting surface is too large
7. Too much return material
8. Small cross-sectional area of gating system
9. Poor mold exhaust
10. Injection piston retracts too early
11. Barrel nozzle is clogged by debris
Cracks
1. Insufficient mold withdrawal angle
2. Mold temperature is too low
3. Plastic cooling time is too long
4. Ejector device is tilted or unbalanced
5. Total cross-sectional area of ejector is too small
6. Insert is not preheated or temperature is not enough
Dents
1. Uneven or too thick wall thickness of plastic part
2. Insufficient feed amount
3. Barrel temperature is too high
4. Injection pressure is too low
5. Injection speed is too slow
6. Cross-sectional area of gating system is too small or gate position is unreasonable
7. Injection and holding time are too short
Surface ripples
1. Barrel temperature is too low
2. Injection speed is too slow
3. Injection pressure is too low
4. Mold temperature is too low
5. Cross section of pouring system is too small
Peeling, stratification
1. Mixing of different plastics
2. Mixing of different grades of same plastic
Welding marks
1. Plastic temperature is too low
2. Mold temperature is too low
3. Injection speed is too slow
4. Injection pressure is too low
5. Too many gates
6. Poor mold venting
Silver wire, streaks
1. Moisture content of raw material is too high
2. Plastic temperature is too high
3. Injection pressure is too low
4. Cross section of pouring system is too small
5. Resin contains low volatiles
Black spots and streaks
1. Plastic has decomposed
2. Plastic debris is stuck between injection piston and barrel
3. Main runner of mold does not fit well with nozzle
4. Mold has no venting holes
Vacuum bubbles
1. Low mold temperature
2. Uneven wall thickness of plastic parts
3. Too short injection time
Cold or stiff blocks
1. Too low temperature, uneven plasticization
2. Mixed impurities or plastics of different grades
3. Too low nozzle temperature
4. No cold material hole
5. Weight of plastic part is close to rated value of equipment
6. Too short molding time
Dimensional instability
1. Unstable electrical or hydraulic system of equipment
2. Inconsistent molding cycle
3. Gate cross section is too small
4. Uneven feeding amount
5. Too short cooling time of plastic parts
6. Unstable process parameters (temperature, pressure, time)
7. Plastic particles of different sizes
8. Uneven mixing ratio of recycled materials and new materials
Strength reduction
1. Plastic decomposition or depolymerization
2. Molding temperature is too low
3. Plastic is recycled too many times
4. Plastic has high water content
5. Plastic is mixed with impurities
6. Mold temperature is too low

1. Function of reinforcing ribs
(1) Enhance strength and rigidity of products without increasing wall thickness of products, so as to save plastic, reduce weight and reduce costs.
(2) It can overcome distortion and deformation of products caused by uneven stress due to difference in wall thickness of products.
(3) It facilitates flow of plastic melt and provides channels for filling of melt in some parts of plastic product body where wall is too thin.
2. Shape and size of reinforcing ribs

(1) Use ribs with lower height and slightly more number to replace single reinforcing ribs with higher height to avoid surface depression when thick rib bottom cools and shrinks (Figure 2-17, Figure 2-18). When depression appears on the back of rib and affects appearance, it can be covered with decorative structure.
(2) Arrangement direction of ribs should preferably be consistent with filling direction of molten material.
(3) Root of rib is transitioned with an arc to avoid stress concentration and damage caused by external force. However, if radius of root fillet is too large, depression will occur.
(4) Generally, no parts are placed on ribs.
(5) Boss located on inner wall of product should not be too close to inner wall to avoid insufficient local melt filling of boss.

Determining the appropriate product wall thickness is one of main contents of product design.
1. Role of product wall thickness
(1) Make product have a certain structure and certain strength and rigidity to meet use requirements of product.
(2) Have a good flow state during molding (such as wall cannot be too thin), filling and cooling effects (such as wall cannot be too thick)
(3) Reasonable wall thickness enables product to be ejected smoothly from mold.
(4) Meet strength requirements of insert fixation and part assembly.
(5) Prevent product warping and deformation.
2 Design of product wall thickness
Basic principle-uniform wall thickness. That is: uniform filling, cooling shrinkage, good shape, high dimensional accuracy, and high productivity.
Under condition of meeting product structure and use requirements, use a smaller wall thickness as much as possible.
(2) Wall thickness of product should be designed to withstand impact and vibration of ejector device, etc.
(3) Sufficient thickness should be provided at connection and fastening points of product, embedded parts, and convergence of plastic melt at hole window (weld mark).
(4) Wall thickness required for strength during storage and transportation should be guaranteed.
(5) Wall thickness required for melt filling during molding should be met, both thin walls that are insufficiently filled or easily burned and thick walls that are easily dented or ruptured by melt should be avoided. Relationship between adjacent wall thickness differences on product (thin wall: thick wall) is:
Thermosetting plastics: pressing 1:3, extrusion 1:5
Thermoplastic plastics: injection molding 1:1.5
When uneven wall thickness cannot be avoided, wall thickness design of product can adopt form of gradual transition, or be modified into two products and then assembled into one product.

In structural design of plastic products, it can be said that support surface and fillet problem are an important part of the entire design link that cannot be ignored. Therefore, when we design related plastic products, we should have a certain understanding of their relevant knowledge so that we can design reasonable and quality products.
1. Support surface
Support surface of product cannot be the entire bottom surface, but should adopt a convex edge or raised foot structure, such as three-point support, frame support, etc.
2. Rounded corners
Two intersecting planes of product should be transitioned with arcs as much as possible to avoid stress concentration caused by sharp angles. Functions of rounded corners of products are:
(1) Disperse the load, enhance and give full play to mechanical strength of product.
(2) Improve fluidity of plastic melt, facilitate filling and demolding, and eliminate defects such as depressions at turning points of wall.
(3) It is convenient for machining and heat treatment of mold, thereby improving service life of mold.

There are many forms of holes, which can be mainly divided into two categories: circular holes and non-circular holes.
Depending on the difference in hole diameter and hole depth, the holes can be formed by the following methods:

(1). General holes, shallow holes, molding.

(2). For deep holes, part of hole depth is molded first, and the rest of hole depth is obtained by machining (such as drilling).

(3). For small-diameter deep holes (such as hole diameter d＜1.5mm), machining is used.

(4). For small-angle inclined holes and complex holes, use split core molding to avoid side core pulling.

(5). For thin-walled holes and holes (hole systems) with high center distance accuracy, use mold punching to simplify mold structure.

 

(1) Center distance between holes should be greater than 2 times hole diameter (the smaller hole between two), and distance from center to edge of hole should be 3 times hole diameter.
(2) Wall thickness around hole should be increased, and its value should be 20%~40% larger than outer diameter of assembly part to avoid adverse effects caused by shrinkage stress.
(3) Hole on product wall (i.e. hole axis is perpendicular to mold opening direction) can be replaced by a side wall groove to avoid lateral core pulling. However, processing difficulty of m-n surface in figure increases..
(4) When plastic melt flows around core and converges to form a hole in plastic product, a weld mark will be formed at edge of hole where melt converges. Existence of weld mark weakens strength of product. Solution is:
1) Distance between holes should be appropriately increased to avoid overlapping connections of weld marks.
2) Hole is designed as a blind hole, leaving a 1/3 wall thickness skin to allow melt to pass over core head so that there is no weld mark where melt converges. Finally, drill (punch) off hole skin. Thin-walled hole-shaped parts (such as radiator windows) of thermoplastics and laminated phenolic plastics can be punched out with a punching die.
(5) For products that need to be drilled, positioning or guiding part of drill bit should be shaped when holes are molded.

1. Inserts in plastic products
Metal or other material parts such as bolts and terminals embedded in plastic molding process or pressed in after molding are collectively referred to as inserts in plastic products. Inserts can increase function of products or decorate them. Molding of inserts makes operation more complicated, cycle longer, and productivity lowered (robots or automatic lines with automatic clamping of inserts are not included).
2. Concave and concave patterns (knurling) in plastic products
Role of convex and concave patterns:

(1). Increase contact area and prevent sliding during use.

(2). Decorate or cover certain parts of product.

(3). Increase firmness of the assembly.

Product size error composition: δ＝δs+δz+δc+δa Wherein δ--total molding error of product; δs--product error caused by plastic shrinkage fluctuation; δz--product error caused by mold molding parts manufacturing accuracy; δc--product error caused by mold wear; δa--product error caused by mold installation and matching clearance. Generally, δs＝1/3δ, δz＝1/3δ, δc＝1/6δ. There are many factors that affect dimensional accuracy of products, and they have cross-influences on each other:
(1) Molding materials. Material factors that affect dimensional accuracy of products include shrinkage fluctuation value, raw material moisture and volatile content, raw material preparation process, raw material production batch number, molecular weight distribution, crystal form, storage method and time.
(2) Molding conditions. Molding conditions that affect dimensional accuracy of products include: barrel temperature, mold temperature, injection volume, injection speed, injection pressure, holding time, cooling time, and molding method (injection, pressing).
(3) Shape and size of products. Complex shapes lead to uneven shrinkage; large changes in wall thickness lead to uneven shrinkage; large-sized products have a large total shrinkage; large slopes lead to low precision.
(4) Mold structure. The larger size of feed port, the smaller shrinkage; the larger size parallel to material flow direction, the smaller size perpendicular to material flow direction; parting surface determines position and direction of burr, affecting dimensional accuracy perpendicular to parting surface. Mold assembly, such as method of fixing core and ejector pin, mold assembly, and mold processing, directly affects dimensional accuracy of product. Mold wear, such as wear of cavity and core, directly affects accuracy of product.
(5) Manufacturing error. Mold manufacturing error will be directly reflected in product.
(6) Post-molding conditions. Mainly refers to measurement error and storage error. Measurement error is mainly caused by factors such as measurement tools, measurement methods, and measurement time; improper storage methods cause products to bend and twist, improper storage temperature and humidity cause products to change in shape and size.

 

Many plastic containers have a small ID card - a triangle symbol, which is usually printed on the bottom of plastic container. There are 1-7 numbers in triangle, and each number represents a plastic container. They are made of different materials and have different taboos in use.
"No. 1" PET/PETE
Material: Polyethylene terephthalate
Common uses: mineral water bottles, carbonated beverage bottles, etc.
Precautions and hazards: Heat-resistant to 65℃, cold-resistant to -20℃, only suitable for warm or frozen drinks, easy to deform when filled with high-temperature liquids or heated, and substances harmful to human body melt out. In addition, scientists have found that No. 1 plastic products may release carcinogen DEHP after 10 months of use, which is toxic to testicles. Therefore, beverage bottles should be thrown away after use, and should not be used as water cups or storage containers to carry other items, so as not to cause health problems that are not worth loss.
"No. 2" HDPE
Material: High-density polyethylene
Common uses: cleaning supplies, bath products
Precautions and hazards: It is recommended not to recycle if cleaning is not thorough. It can be reused after careful cleaning, but these containers are usually difficult to clean, and original cleaning products remain, becoming a breeding ground for bacteria. You'd better not recycle them.
"No. 3" PVC
Material: Polyvinyl chloride
Common use: Food packaging, rarely used now.
Precautions and hazards: It is best not to buy this kind of packaged food. This material is prone to produce harmful substances at high temperatures, and it will even be released during manufacturing process. After toxic substances enter human body with food, they may cause breast cancer, congenital defects and other diseases. At present, containers made of this material are rarely used to package food. If used, never let it heat up.
"No. 4" LDPE
Material: Low-density polyethylene
Common use: Cling film, plastic film, etc.
Precautions and hazards: Do not wrap cling film on the surface of food into microwave. It is not heat-resistant. Usually, qualified PE cling film will melt when temperature exceeds 110℃, leaving some plastic preparations that cannot be decomposed by human body. In addition, when food is wrapped in cling film and heated, fat in the food can easily dissolve harmful substances in cling film. Therefore, before putting food into microwave, you must first remove plastic wrap that wraps it.
"No. 5" PP
Material: Polypropylene
Common uses: Microwave lunch boxes
Precautions and hazards: Remove lid when putting it into microwave. The only plastic box that can be put into microwave can be reused after careful cleaning. It is important to note that some microwave lunch boxes are made of No. 5 PP for box body, but lid is made of No. 1 PE. Since PE cannot withstand high temperatures, it cannot be put into microwave together with box body. To be on safe side, remove lid before putting container into microwave.
"No. 6" PS
Material: Polystyrene
Common uses: Bowl instant noodle boxes, fast food boxes Precautions and hazards: Do not use microwave to cook bowl instant noodles. It is both heat-resistant and cold-resistant, but it cannot be put into microwave to avoid releasing chemicals due to excessive temperature. It cannot be used to carry strong acids (such as orange juice) and strong alkaline substances, because it will decompose polystyrene that is not good for human body and is easy to cause cancer. Therefore, you should try to avoid using fast food boxes to pack hot food.
"No. 7" PC　　
Material: All other resins and mixtures not listed　　
Common uses: Kettles, cups, baby bottles　　
Precautions and hazards: PC glue releases bisphenol A when heated. A material that is used in large quantities, especially in baby bottles, and is controversial because it contains bisphenol A. Lin Hanhua, associate professor of Department of Biology and Chemistry at City University of Hong Kong, said that in theory, as long as bisphenol A is 100% converted into plastic structure during production of PC, it means that product does not contain bisphenol A at all, let alone release. However, if a small amount of bisphenol A is not converted into plastic structure of PC, it may be released and enter food or drinks. Therefore, it is better to be careful and pay special attention when using this plastic container.
Cleaning measures to deal with bisphenol A: The higher temperature of bisphenol A remaining in PC, the more it will be released and the faster it will be released. Therefore, PC water bottles should not be used to hold hot water to avoid increasing release rate and concentration of bisphenol A (if any). If your kettle is numbered 7, following methods can reduce risk:

(1). Do not heat it when using it.

(2). Do not use a dishwasher or dishwasher to clean kettle.

(3). Do not expose kettle to direct sunlight.

(4). Before the first use, wash it with baking soda and warm water, and dry it naturally at room temperature. This is because bisphenol A will be released more during the first use and long-term use.

(5). If container is dropped or damaged in any way, it is recommended to stop using it, because if there are fine grooves on the surface of plastic products, it is easy to hide bacteria.

(6). Avoid repeated use of aging plastic utensils.

Editor's summary: Plastic cups are safer than paper cups; plastic products with code 5 are temperature resistant up to 130 degrees, can be filled with hot water and heated in microwaves; the darker color of straw, the less safe it is; it is not advisable to use plastic straws for hot drinks; use disposable chopsticks as little as possible; how to use plastic bowls and imitation porcelain bowls; do not use dark-colored plastic bowls; imitation porcelain tableware cannot hold acidic substances; mineral water bottles and purified water buckets are code "1"; plastic bottles can store dry items; water bottles with code 5 are resistant to high temperatures and can be reused; qualified plastic lunch boxes are marked with "5" PP and have a small ventilation hole on lid.