Thermosetting plastics are phenolic, amino (melamine, urea-formaldehyde), polyester, polypropylene phthalate, etc. Mainly used for compression moulding, extrusion, and injection moulding. Silicone, epoxy resin and other plastics are used mainly as low-pressure extrusion packaging electronic components and casting moulding.
1. Process characteristics
(A) Shrinkage rate
The size shrinkage occurs after the plastic part is taken out from the mold and cooled to room temperature. This property is called shrinkage since the shrinkage is not only the thermal expansion and contraction of the resin itself but also related to the molding factors, so the shrinkage of the moulded part after molding should be called molding shrinkage.
1) The form of forming shrinkage is mainly manifested in the following aspects.
- Shrinkage of the line size of the plastic parts due to thermal expansion and contraction, elastic recovery of the plastic components of the mold, plastic deformation, etc., resulting in the size of the plastic parts after cooling to room temperature, for which the cavity design must be considered to compensate.
- Shrinkage directional molecules are arranged in the direction of forming so that the plastic parts show anisotropy along the direction of material flow (i.e. parallel direction). The shrinkage is considerable. High strength and right-angle direction with the material flow (i.e. vertical direction) is minor shrinkage and low strength.
- In addition, the shrinkage is also uneven due to uneven density and packing distribution in each part of the moulded part. The shrinkage difference makes the plastic part prone to warping, deformation, and cracking, especially in extrusion and injection moulding is a more obvious direction.
- Therefore, the mold design should consider the shrinkage direction according to the shape of the plastic part. The law of material flow to select the shrinkage rate is appropriate.
- After shrinkage of plastic molding, due to forming pressure, shear stress, anisotropy, density uneven, uneven packing distribution, uneven mould temperature, uneven hardening, plastic deformation and other factors, causing a series of pressure in the viscous flow state can not all disappear. So the plastic parts in the stress state when forming residual stress exists.
- When the mold is released, due to the pressure tends to equilibrium and the influence of storage conditions, the residual stress changes and makes the plastic part shrink again, called post-shrinkage. Generally, the plastic parts change the most within 10 hours after demolding, and after 24 hours, the fundamental shape is set, but the final stability is 30-60 days.
- Usually, the post-shrinkage of thermoplastic is more significant than that of thermosetting, and the extrusion and injection molding are more critical than that of compression moulding.
- Post-treatment shrinkage sometimes plastic parts according to the performance and process requirements. After forming heat treatment, treatment will also lead to changes in the size of plastic parts. Therefore, the mold design of high-precision plastic parts should be considered after the shrinkage and post-treatment shrinkage of the error and be compensated.
2) Shrinkage rate calculation plastic forming shrinkage can express the shrinkage rate, as shown in the formula (1-1) and procedure (1-2).
- (1-1) Q real = (a – b) / b × 100
- (1-2) Q count = (c – b) / b × 100
In the formula:
- Q real – the actual shrinkage rate (%)
- Q count – calculated shrinkage rate (%)
- a – plastic part at forming one-way temperature size (mm)
- b – plastic parts at room temperature one-way size (mm)
- c – mould at room temperature one-way size (mm)
The actual shrinkage rate is the substantial shrinkage of the plastic part because its value is minimal from the calculated shrinkage, so the mold design to Q design parameters to calculate the cavity and core size.
3) The factors affecting the change in shrinkage rate in the actual molding are not only different varieties of plastic shrinkage rate varies.
And different batches of the same type of plastic or various parts of the same plastic parts, the shrinkage values are often different. The main factors affecting the change in shrinkage rate are the following aspects.
- Plastic varieties of various plastics have their shrinkage range. The same type of plastic due to filler, molecular weight and ratio of different, the shrinkage rate and anisotropy are also different.
- Plastic parts characteristics of the shape, size, wall thickness, with or without inserts, the number of inserts and layout of the size of the shrinkage rate also have a significant impact.
- Mold structure mould parting surface and pressure direction, the form of the pouring system, layout and size of the shrinkage rate and directional impact is also more significant, especially in extrusion and injection molding more obvious.
- Forming process Extrusion and injection molding process generally has a significant shrinkage rate and obvious directionality. Preheating, forming temperature, forming pressure, holding time, filling the form, and hardening uniformity impact the shrinkage rate and directionality.
As mentioned above, the mold design should be based on the shrinkage range provided in the specifications of various plastics and according to the shape of the plastic part, size, wall thickness, with or without inserts, parting surface and pressure forming direction, mold structure and inlet form size and location, molding process and other factors to consider the selection of shrinkage value.
For extrusion or injection molding, it is often necessary to select different shrinkage rates according to the shape, size, wall thickness and other characteristics of each part of the plastic part.
In addition, the forming shrinkage is also affected by various forming factors but is mainly determined by the plastic variety, shape and size of the plastic part. Therefore, the molding conditions can be adjusted to change the shrinkage of the plastic parts.
The ability of plastic to fill the cavity under a specific temperature and pressure is called fluidity. This is a critical process parameter that must be considered when designing the mold.
The flowability is easy to cause too much overflow material, filling the cavity is not dense, plastic parts loose organization.
Resin, filler sub-head accumulation, easy to sticky mold, mould release and clean-up difficulties, premature hardening and other ills. But the liquidity is tiny, is not enough to fill, is not easy to form, and forming pressure is significant.
Therefore, the fluidity of the selected plastic must be adapted to the requirements of plastic parts, moulding process and molding conditions. Mould design should be based on liquidity to consider the pouring system, parting surface, feeding direction, etc. The fluidity of bakelite plastics is usually expressed in terms of Rasig fluidity (in millimetres). If the value is considerable, the fluidity is good, and each variety of plastic is generally divided into three different fluidity levels for other plastic parts and molding processes.
Generally, the plastic part area is large, more inserts, cores and inserts are weak, narrow, deep grooves, and a thin-walled complex shape is not conducive to filling. It should be used when the liquidity of the better plastic. When extrusion molding should be chosen, use the plastic with a flowability of 150mm or more, and when injection molding should be applied to the plastic with a flowability of 200mm or more.
To ensure that each batch of plastic has the same liquidity, in practice, commonly used and batch methods to regulate, that is, the same species and liquidity differences in the plastic to be used, so that each batch of plastic liquidity to compensate for each other, to ensure the quality of plastic parts. Commonly used plastic Rasig fluidity values are detailed in Table 1-1. Still, it must be pointed out that the plastic injectivity, in addition to determining the plastic varieties, in filling the cavity, is often affected by a variety of factors and the ability of the plastic to fill the actual cavity changes.
Such as fine particle size (incredibly round granules), high humidity, moisture and volatiles, preheating and moulding conditions, mold surface finish, mold structure, etc., are conducive to improving mobility. On the contrary, poor preheating or molding conditions, mold structure, flow resistance or plastic storage are too long. Overdue, high storage temperature (especially for amino plastics), etc., will lead to plastic filling cavity when the actual liquidity performance decreases and cause poor filling.
(C) specific volume and compression rate
The compression ratio is the volume or volume of the plastic powder to the plastic part (the value is always greater than 1). They can both be used to determine the size of the compression mould charge chamber. A significant value requires a large volume of the charging section.
At the same time, it means that the plastic powder is inflated with a lot of air, which makes it difficult to exhaust the air, resulting in long molding cycles and low productivity. A small specific volume is the opposite and is conducive to pressing ingots, pressing. The exact capacity of various plastics is detailed in Table 1-1. Still, the value of the specific power is often due to the size of the plastic and particle size and unevenness of the error.
(D) hardening characteristics
Bakelite plastics in the molding process in the heating under pressure into a plastic viscous flow state, followed by an increase in liquidity to fill the cavity.
And at the same time, the occurrence of the condensation reaction, cross-linking density increasing, liquidity decreases rapidly, and melt material gradually cured.
Mould design for fast hardening speed, maintain the flow state of the short material, should pay attention to facilitating the loading, loading and unloading inserts, and choose reasonable forming conditions and operations to avoid premature hardening or insufficient hardening in the poor molding of plastic parts.
Hardening speed can generally be analyzed from the holding time, related to the plastic variety, wall thickness, the shape of the plastic parts, mold temperature. But also by other factors and changes, especially with the preheating state, the appropriate preheating should be maintained so that the plastic can play the maximum liquidity conditions and improve its hardening speed. The general preheating temperature is high.
A long time (within the allowable range) will accelerate the hardening speed, especially the pre-pressing ingot billet by high-frequency preheating is significantly faster hardening speed. In addition, the forming temperature is high, and the pressurization time is extended, then the hardening speed also increases. Therefore, the hardening speed can also be adjusted to preheat or forming conditions to be controlled appropriately.
Hardening speed should also be suitable for forming method requirements. Such as injection, extrusion molding should be required in the plasticization, filling when the chemical reaction is slow, and slow hardening should be maintained for a long time in the flow state. But when filled with the cavity at a high temperature, high pressure should be fast hardening.
(E) moisture and volatile matter content
Various plastics contain different degrees of moisture, volatile content, too much when the liquidity increases, and easy to overflow material. Keep a long time, shrinkage increases, prone to ripples, warping, and other ills, affecting plastic parts’ mechanical and electrical properties.
But when the plastic is too dry will also lead to poor liquidity forming difficulties, so different plastics should be preheated and dried according to the requirements of the moisture-absorbing material, especially in the wet season. Even the preheated material should be prevented from re-absorbing moisture.
Since various plastics contain different components of water and volatiles, and water condensation occurs during the condensation reaction, these components need to be turned into gases and discharged from the mold during moulding. Some gases have a corrosive effect on the mold and stimulate the human body.
For this reason, mold design should be a variety of plastic characteristics of understanding and take appropriate measures, such as preheating, mould chrome plating, open exhaust slot, or moulding set exhaust process.
2. The molding characteristics.
The mold design must master the molding characteristics of the plastic used and the molding process characteristics.
Various plastic forming characteristics and each plastic species, but also with the filler species and particle size and uniformity related. Delicate material fluidity is good, but preheating is not accessible to uniform, filled with more air is not easy to discharge, poor heat transfer, and long forming time.
Coarse material plastic parts are not glossy, easy to occur surface unevenness. Too rough or fine also directly affects the specific volume and compression rate, mould filling room volume. If the particles are not uniform, the formability is not good, and the hardening is not constant. Meanwhile, using the capacity method to add material is not suitable.