Basic requirements for structural design of plastic toy molds

Time：2019-05-24 08:25:59 / Popularity： / Source：

Processing method of plastic parts is mainly injection molding manufacturing process. Injection molding is the most commonly used method in plastic injection moulding process, and is suitable for all thermoplastic materials. Injection molding manufacturing process is driven by a screw (or a plunger) to inject a heated plasticized plastic material melt into a closed cavity, and after cooling, solidifying, and shaping, molded article is obtained by opening mold. Injection molding manufacturing process is a cyclic process. Each cycle consists of: dosing - melt plasticization - pressure injection - mold cooling - mold release.

Four main factors affecting injection molding manufacturing process are:

· Temperature

· Pressure

· Flow rate

· Cooling rate

These four factors interact with each other. Due to imbalance of flow rate during injection molding manufacturing process, imbalance of cooling shrinkage will produce a certain internal stress.
In addition to meeting requirements for use, structural design of plastic parts must also consider selection of materials, and must also consider moulding process. Moulding process of plastic parts has a direct relationship with mold design. Only structural design of plastic parts can meet requirements of moulding process, and a reasonable mold structure can be designed to produce qualified plastic parts.
Toys are more demanding on structure. They can't have sharp points, edges, shell bursts, parts falling off and loss of function during normal play, abuse (down, torque/tension, pressure) and reliability life test. Appearance of toys is also demanding. Many toy dealers have strict requirements on appearance of toys, and phenomenon of returning because of appearances is not uncommon. Because injection molding manufacturing process is affected by many factors, if it is improperly controlled, it is prone to defects. Due to high safety standards of toys and limited profit margin of toy manufacturers, higher requirements are placed on structural design of plastic toy molds. Following are some basic requirements.

Wall thickness requirement

1. Wall thickness of injection molded parts from plastic toy molds should not be too thin, otherwise it will not meet strength requirements of impact and pressure, and it cannot be suitable for requirements of plastic injection moulding process. If wall thickness is too thin, rubber will not fully flow into cavity, and black-black spots (burnt), black-brown stripes, and insufficient filling (unsatisfactory) will occur.
2. Injection molded parts from plastic toy molds should not be too thick. Too thick will affect shrinkage of injection molded parts and shrink marks (commonly known as shrinkage) appear. Second, mold cooling time is proportional to wall thickness.
Cooling time accounts for 75% of entire cycle of injection molding manufacturing process. It can be seen that excessive wall thickness will reduce production efficiency and increase production costs. In addition, excessive thickness of rubber will increase amount of rubber and increase cost. Wall thickness of plastic parts from plastic toy molds is generally controlled at 1.5 to 3 mm.
3. Wall thickness of toy should be uniform, otherwise it will cause deformation, shrinkage marks and flaking of injection molded parts from plastic toy molds due to uneven shrinkage. Among them, there are many factors in draping, injection pressure is too large, intermittent pauses in production operation, and improper venting causes rubber to leave cavity area and so on. After appearance of draping, it is necessary to increase manual removal of personnel, which undoubtedly increases processing cost.
4. In order to keep material thickness uniform, many rubber parts adopt hollowed out rubber and replace structure with bone position such as larger pieces of nylon / steel gear, joint position, corner of force and so on.

Molding angle of mold

1. Angle of plastic part directly affects ease of removal of plastic part from cavity. Angle of die is related to properties of rubber compound, shrinkage rate, and structural form of plastic part. If angle of plastic toy molds is too small, plastic parts will be stuck, plastic parts will be deformed, cracked, and dragged.
2. Most of appearance of toy depends on injection/printing (injection molded product is only one color). When spraying, it is necessary to cover non-spraying place with spraying. In order to facilitate loading of sparying mold, injection angle of injection molded parts that need to be sprayed should be larger. Fuel injection boundary should make full use of bone position, and decorative groove should be properly arranged to prevent oil flying phenomenon.

Reinforcement

1. Reinforcement (plastic bone position) is the most commonly used in toy structure design. Reinforcement has two major functions:
One is to increase strength and rigidity of injection molded parts. For example, in order to prevent screw stud from falling, it is necessary to add a reinforcing rib (commonly known as a rocket foot) at root of screw column; if battery door is not rigid enough, battery door will be deformed when battery is dropped, causing buckle to come out and battery to fall off. At this time, a rib is added inside battery door to increase rigidity to prevent deformation; also edge of shell is prone to crack when falling, and ribs are added to edge of shell to increase strength.
Second, reinforcing ribs can change direction of flow, make flow more balanced, improve bonding ability of rubber molecules and make stress distribution uniform, increase strength of injection molded parts.
There are many types of ribs that can be used. There are straight bones, u-shaped bones, and semi-circular (round) bones.

Fillet

1. So-called rounded corner refers to transition of each side or internal connection of plastic part with a circular arc. Fillet is a factor that must be considered during design of injection molded parts. Rounded corners avoid stress concentration, increase strength of injection molded parts, improve flow of injection molded parts, and facilitate demolding.
2. If right angle is too large or rounded corner is small, it is easy to appear shrinkage marks (commonly known as shrinkage), plastic mold sticking, cracks, weld lines, dragging marks, etc. during plastic injection moulding process. In dropping, torque/tensile test, it is easy to break rubber parts. "Increasing (or adding) rounded corners" is a common term for defect improvement.
3. Fillet radius of each joint of injection molded parts shall not be less than 0.5~1mm.

With advancement of science and production technology, many new plastic materials have emerged. However considering cost and other factors, toy case is mainly made of traditional plastic materials such as abs/hips; pp is often used as a toy inner part and an electric device shell because of difficulty in fuel injection treatment; pc is often used for high-performance transparent structural parts; Pomma is often used for light-transmitting parts; pom and pa-66 are often used for gear-type parts.
Above points are the most basic requirements for design of toy structure of injection molded parts, and also must be paid attention to when designing toy structure of plastic toy molds. Toy structure design has other requirements for plastic parts. In actual design, many specific cases will be encountered. It is necessary to combine testing to gradually improve structure of plastic parts and produce qualified toy products.

Gud Mould Industry Co., Ltd