GB2371514A

GB2371514A - Spreading of moulding material

Info

Publication number: GB2371514A
Application number: GB0102026A
Authority: GB
Inventors: Peter Reginald Clarke
Original assignee: Coraltech Ltd
Current assignee: Coraltech Ltd
Priority date: 2001-01-26
Filing date: 2001-01-26
Publication date: 2002-07-31
Also published as: GB0102026D0

Description

MOULDING

The present invention relates to injection moulding of plastics material.

It is known that large, thin articles are difficult to form by injection moulding. The reason is that the gap between the two parts of the mould is small and the distance that the material has to travel is too long for the pressure applicable by the moulding machine to be available at the far end of the gap from the injection point for driving the plastics to fill the mould. In short, the"flow path thickness ratio"is too long.

Conventionally, thin articles are formed by vacuum or pressure forming where a sheet of plastics material is stretched to conform to the shape of a mould. Such techniques are limited in their application as they cannot produce articles of even wall thickness or articles that have regions of increased or reduced wall thickness. This is because only one surface of the article is being moulded and the thickness at any point is determined exclusively by the thickness of the original sheet and the extent of its deformation.

The present invention seeks therefore to provide a method suitable for moulding an article of thin wall section in which all the surfaces of the article are defined by the wall surfaces of a mould cavity.

In its broadest aspect, the invention a method of moulding a plastics material in a mould cavity which relies primarily on a substantial movement of a part of the mould to provide the pressure necessary to force the plastics material melt to fill all the parts of the mould cavity.

The invention may thus broadly be regarded as applying to plastics material a technique similar to that used in metal forging.

It is known to move part of a mould in order to apply additional compression after having injected a plastics melt into a mould cavity in the conventional manner. This process, which is known as injection compression moulding (ICM) offers advantages of longer flow lengths, thinner walls and a lower level of material stresses. This makes the process suitable for moulding such articles as CD's and DVD's (because of improved internal stresses) and vehicle body and instrument panels (because of improved impact resistance).

The known ICM process differs from the present invention in the extent of the compression of the plastics melt by the closing of the mould cavity. In a typical embodiment of the present invention, the relative displacement of the mould parts is in excess of ten times the final mould thickness and may be as great as two hundred times the final moulding thickness. This is to be contrasted with a corresponding movement of typically twice the final wall thickness used in injection compression moulding.

As a consequence of the wide separation of the mould parts at the time of injection, in the present invention injection meets with little resistance and the area of the mould cavity covered by the dose of plastics material injected into the mould is only a small fraction of the total area of the cavity. Furthermore, the injected dose may make little or even no contact with the moving part of the mould. By contrast, in injection compression moulding, the plastics material meets significant resistance as it is injected, and at the end of the injection, the plastics material will have spread to contact both parts of the mould and to cover at least half of the mould cavity.

A further important difference between the invention and conventional injection compression moulding resides in the speed of closing the mould and the rate of pressure increase within the mould cavity during the closing process.

In the present invention, the mould is closed and maximum pressure is reached within the cavity within a period of less than 0.5 seconds and preferably less than 0.3 seconds.

By contrast, in injection compression moulding, after the plastics material has been injected under pressure to fill a major part of the mould and cavity, the pressure is ramped up progressively to flow the plastics material to fill the remainder of the mould.

According to a second aspect the invention, there is provided a method of moulding a plastics article which comprises the steps of: providing a mould cavity between relatively movable parts of a mould, injecting into the mould prior to full closure of the mould cavity a predetermined dose of plastics material sufficient to form the finish formed article, the injected plastics material covering only a minor proportion of the mould cavity to a thickness substantially greater than that of the finish formed article; and fully closing the mould cavity by moving the mould parts relative to one another, the closing of the mould providing the necessary pressure to cause the plastics material to flow so as to cover the remaining area of the mould cavity and provide the article with its finish formed thickness.

The parts of the mould may either be moved relative to one another in two stages or in one continuous movement.

Hence, in one embodiment of the invention, the cavity is partially closed during a first stage to a predetermined separation between the parts of the mould, the dose of plastics material is injected while the two mould parts

remain substantially stationary relative to one another and this is followed by a second stage of movement to close the cavity fully. As an alternative however, the cavity is closed by a single continuous movement but the injection of the dose of plastics material is timed to be completed while the separation of the mould parts is still significantly greater than the thickness of the finish formed article. In both the above embodiments of the invention, the plastics injection meets with little resistance and the majority of the force required for flowing the plastics material to fill the mould cavity is derived from the closing of the mould cavity.

The mould can have an additional part such as a movable core face or cavity bottom for providing the closure to the final mould cavity thickness. In this case, the two principal parts of the mould may be fully closed by the injection moulding machine for the injection step and final closure is provided by additional actuator (s) acting on the additional part.

Alternatively, the mould may be a conventionally configured two part mould which is not fully closed for the injection step. In this case, the mould is held slightly open at the injection step and fully closed for the final closure.

One can envisage an implementation of the invention using a moulding machine with a hydraulic lock (i. e. one that uses only hydraulic actuators to apply all the necessary pressure to the mould parts). Using such an approach, it is more difficult to achieve the necessary speed of closure of the mould cavity as well as the rapid rise in pressure that is needed as the mould parts approach their closed position. This may therefore require the use of purpose built machines. In this respect, it should be noted that the plastics material is cooled and sets as it

makes contact with the mould surfaces and it vital for this reason that the flowing of the plastics material by the closing of the mould parts should be completed as quickly as possible. An injection moulding machine with a toggle closure mechanism has however been found to be well adapted to provide the substantial force require to effect the final closure.

Since the pressure in the plastics material on final closure is substantial, the injection gate through which the plastics material is injected into the mould is preferably closed by a valve prior to final mould closure. This is to avoid expulsion of material from the tool during final closure.

Preferably, the final closure involves reduction of the mould part gap over a substantial portion of the surface area of the finish formed article, whereby movement of the plastics material through a thin mould part, which might otherwise be regarded as too thin, occurs only during the last part of-the final closure.

Because the plastics material in the present invention meets with little resistance during injection, in the absence of special steps, it is difficult to achieve injection of an equal mass of plastics material into all the mould cavities of a multi-cavity tool. This is because in the absence of increased resistance from each cavity at the end of injection, all the plastics material will tend to flow into the cavity offering the least resistance.

In order to mitigate the above disadvantage, it is preferred to provide a hot-runner system that comprises a manifold incorporating a shooting pot for each of the cavities, each shooting pot being connected to a common pressurised supply of molten plastics material by way of a

respective non-return valve and being connected to the associated cavity by way of a gate valve.

Each shooting pot is a variable volume chamber bounded by a piston which acts to store the required dose of plastics material for its associated cavity. When the plastics material is injected from the pressure source into the shooting pots it will first flow to the shooting pot offering the least resistance but when this pot is full the plastics material will meet resistance and will be diverted to another of the shooting pots until all the shooting pots are full. An adjustable stop may be provided for each of the pistons of the shooting pots to allow fine adjustment of the quantity of plastics material delivered to each cavity.

When the gate valves are opened and the pistons of the shooting pots are moved in a direction to reduce the volume of the working chambers, the stored plastics material is force past the gate valve into the cavity and is prevented from moving in the opposite direction by the non-return valves.

The invention will now be described further, by way of example with reference to the accompanying drawings, in which: Figure 1 is a plan view of an injection moulding machine that may be used to implement the invention, Figure 2 is a graphical representation of the applied hydraulic pressure, the clamp position, the movement of the injection screw and the position of the valve gate at different stages in an operating cycle of the moulding machine, Figures 3a to 3d show the positions of the mould parts, the injection screw and the valve gate at the different stages of the operating cycle, Figure 4 is generally similar to Figure 2 but shows an alternative method in accordance with the invention suitable for an injection moulding machine with a hydraulic clamp,

Figure 5 shows in a perspective view of the hot runner manifold and a more detailed cut away view of one of the shooting pots in the manifold that enable equal melt dosage in a mould having multiple cavities, and Figures 6a and 6d show the positions of the shooting pots at the different stages of each machine operating cycle.

The moulding machine shown in Figure 1 is generally conventional and will therefore only be described in the detail necessary to understand the Injection Impact Compression (IIC) method of the invention. The moulding machine 10 comprises two stationary bulkheads 12 and 14 connected to one another by four tie bars 16. The mould 18 is formed of two parts namely a stationary part 18a mounted on the bulkhead 12 and a movable parts 18b mounted on a platen 20 that can slide along the tie bars 16. The platen 20 is moved towards and away from the bulkhead 12 by a hydraulic ram 22 that is mounted on the bulkhead 14 and is connected to the platen 20 by a toggle mechanism comprises levers 24a pivoted on the bulkhead 14, levers 24b pivoted on the platen 20 and levers 24c pivoted on the ram 22 the other ends of all three levers 24a, 24b and 24c being pivoted to another. The levers of the toggle mechanism are shown in their position when the mould cavity is open and to close the mould cavity, the ram moves to the right as viewed so that the levers 24c move into a more vertical position and acts on the levers 24a and 24b to move them into alignment with one another, thus moving the platen 20 and the mould part 18 towards the closed position.

A heated screw feed mechanism 30 heats and compresses granules drawn from a hopper by rotation of the screw to form a plastics melt and the screw can also be moved axially to inject the melt into the mould cavity through a set of runners.

As earlier indicated, the machine of Figure 1 is already known for injection moulding. Conventionally, the mould cavity is closed and the injection screw is advanced to provide all the necessary pressure to inject sufficient melt to fill the cavity. After the plastics material has set in the mould, it is opened, the formed article is ejected and a new cycle is commenced.

This known method of operation has its limitation and cannot be used to form articles have a very thin wall section. This is because as the plastics material is injected, it cools very rapidly on contact with the mould surface and creates a large back pressure that prevents the plastics material from filling the entire cavity.

In the present invention, the injection screw is not relied upon to produce enough pressure to fill a closed mould cavity. Instead, the screw is used to inject a dose of the melt into the mould while the mould is still at least partially open and subsequently the mould parts 18a and 18b are brought together rapidly using the ram 22 to"forge"the plastics material and force it rapidly to fill every part of the mould cavity.

An operating cycle will now be considered into greater detail with reference to Figures 2 and 3.

The operating cycle has been divided in Figure 2 into eight stages to identify the logical points in the process method.

The cycle commences at instant 1 with the application of hydraulic pressure to close of the clamp from its fully open position until the instant 2 when the mould parts reach a partially open or"prestaged"position at which there is a predetermined mould face gap. Throughout this period, the

machine barrel is at the"screw back position"and hence is loaded with material at the required melt temperature.

At the instant 2, the mould parts are held still at the prestaged position, the valve gate valve gate controlling the flow of the melt into the mould cavity is opened and the screw is advanced in the barrel to inject the molten plastics material into the mould face gap. At the instant 3, the injection is terminated by closing the valve gate and discontinuing the advance of the screw in the barrel. A timer maintains the mould face gap position for a period sufficient overall to enable the action of the valve gate open/close and the material injection.

After the injection is terminated, the mould face gap timer completes its cycle and on reaching the instant 4, mould closure at very high speed is initiated by raising the hydraulic clamp pressure.

During the period between the instants 4 and 5, the mould is fully closed at high speed/pressure (impact compression) to spread the hot material from the injected 'slug'throughout the'mould cavity'.

With the cavity fully closed, the clamp pressure is release at the instant 5 but the toggle mechanism maintains the cavity closed until the instant 6. Thus, during this period, the clamp pressure is maintained allowing the moulded component to cool. At the same time, the screw is activated to recharge the barrel for the next cycle.

At the instant 7, hydraulic pressure is again applied to the hydraulic ram 22 but this time in a direction to separate the two parts of the mould fully. The formed component is ejected from the mould during or after opening.

The mould open time is kept as short as possible consistent

with maintaining smooth operation during the opening and closing phases.

It should be noted that: a) It is essential for the"Injection Impact Compression" (IIC) cycle that the mould tool should be partly open during injection of the melt and that it is not fully closed until after injection of the dose of the plastics material melt. b) The impact compression (second closing) essentially has to be carried out as rapidly as'possible in order to transfer the material throughout the mould cavity. c) A degree of overlap is available through finetuning of the settings including any necessary fine-tuning of the valve-gate timing. d) With a valve gated feed system, it is practical to recharge the barrel at any time following the closure of the valve-gate. e) The sequence of tool actions can be achieved by using either a toggle locking machine as shown in Figure 1, utilising the geometry of the toggle system and the driving hydraulics to achieve the Impact Compression closure or by a hydraulic lock machine with secondary hydraulic activation of the impact compression closure in-built within either the machine or the mould.

The following are some of the advantages that may be achieved by the IIC method of the invention: a) Flow-length to Thickness ratios are achievable beyond those normally accepted as maximum for the other cyclic moulding processes. b) Lower levels of in-moulded stress are achieved due to the speed of cycle and the management of characteristic polymer flow in-mould filling (fountain-flow). c) Cycle times are reduced as a result of shorter injection period and the sympathy between component cooling time and wall thickness.

d) A wide range of available thermoplastic polymers can be processed. Polypropylene (PP) has been converted in both natural and fibre reinforced grades. e) Other polymers that have been used successfully include: Acetal (POM), Polycarbonate (PC), Polyester (PBT), Elastomers (TPE). Acetal and polycarbonate can again be used in both natural and fibre reinforced grades.

Figure 2 details a two stage approach where the tool is pre-staged at a'gap'at which injection takes place and is closely followed by impact-close. As an alternative to this approach it is possible to use the method represented in Figure 4 where instead of maintaining a fixed mould gap during the injection of the plastics material melt, the plastics injection takes place at the same time as the closing of the mould gap.

In the case of the method represented in Figure 4, the mould tool is set to close with an increasing speed such that maximum force is applied during the final stages of closure. The injection time for the material dose is known and the start point for injection before full closure is set to be slightly greater than the injection time. The end of injection time coincides with the start of maximum clamp speed and pressure. The full material dose is achieved as the tool approaches final close and the injection occurs whilst the tool closure is in process.

The benefits of this alternative implementation of the invention include: * Maximised material temperature.

* Minimised cycle time.

* Reduced moulded in stress.

* Reduced machine stress due to'programmed continuous' closing action.

As the injection phase is likely to start at a significant mould face'gap', the process is likely to be best effected on a vertical machine (horizontal split line), in order to offset any gravitation effects on the material slug.

The decision as to the use of'two-stage'or 'programmed'close would be directly based on the format of the product being moulded. Flat products lend themselves better to the use of'two-stage'close. Deep products such as drinking cups, on the other hand, benefit from 'programmed'close). The initial control of the slug helps in determining the choice.

The IIC method of the invention has been found to be suitable in the production of a soup cup having the following parameters: * Top Outside Diameter: 83mm * Bottom Outside diameter: 59mm * height: 100mm * Sidewall thickness: 0.50mm * Base thickness: 0. 65mm 'Material : Polypropylene * Sidewall draft angle: 7.0 degrees * Weight of the finished product: 14.0 grams The following is a details explanation of how the cavity is filled under IIC conditions: First, 14.0 gms of polypropylene is injected into the partially closed mould. The mould then closes rapidly to transfer the material throughout the cavity. This transfer of the material involves the following steps 1) The mould closes first to form the material into a 'pancake'covering the bottom of the cavity and this is effectively a'pancake'of 5.7mm thickness (14.0 grams @ 0.91 gms/cm2 in a disc 59mm diameter x 5.70mm thick)

2) As the tool continues to close down to the final 0. 65mm thickness of the base, the'pancake'is progressively reduced in thickness and the displaced material is moved into the sidewall area of the cavity.

3) The core to cavity sidewall gap is dependent on the draft angle of the cavity and the closing distance mould tool. Typically at 5. 70mm, the sidewall'gap'is approx.

1. 12mm compared to the final'gap'of 0.50mm with mould closed.

4) If we consider the progressive closure and filling at 5 points in its change that is at 20% increments it is found that each step is a displacement of (5.7mm-0. 65mm) /5 = 1.05mm and the material displaced is a volume equivalent to 59mm diameter x 1.05mm.

5) In the 0 20% stage the material fills an annular section of the sidewall (1.12 +1.0)/2 average thickness.

This equates to a proportion of the eventual 100mm height of 14. 4mm.

6) As the tool closes, the sidewall'gap'reduces at the following rate: Step Height Sidewall Section Diameter 0% Omm height 1.12mm 59. 0dia 20% 20mm height 1. 00mm 62.8dia 40% 40mm height 0.87mm 67.6dia 60% 60mm height 0. 7 5mm 72.4dia 80% 80mm height 0.62mm 77.2dia 100% 100mm height 0. 50mm 82. dia 7) In the 80 100% stage where the sidewall'gap'is 0.5mm the equal amount of material fills the final 22mm of the height.

8) The reducing sidewall gap increases the pressure needed to flow the material forward. But this is in sympathy with the way closing pressure applied to the mould is being built up to maximise as the tool reaches final close. This is directly related to the conventional'toggle'mechanism and also to the application of hydraulic pressure in conventional Injection moulding machines.

9) The'pre-stage'tool position at which injection is made has to be greater than the'pancake'dimension such that tool closing acceleration can be achieved before that dimension is reached and the fastest material distribution obtained. A likely dimension for pre-stage is two to three times the'pancake'height.

The travel from'pre-stage'to'pancake'allows platen/tool mass acceleration, such that on reaching the

'pancake'position the machine is generating impact and building to maximum compression. pressure builds throughout the reduction of the'pancake'and the transfer of material throughout the cavity.

In practice the action of the'toggle-lock'machine generates extremely high pressures due to its geometry and, in fact, centring the toggle mechanism stretches the machine tie-bars in proportion to the programmed tonnage.

The above description applies to containers of significant height relative to their diameter. In the case of shallow items i. e.: trays/plates it is suggested that a secondary core be used to control and generate reduction of the'pancake'simultaneously promoting flow into the extremes of the closed or closing cavity.

When the plastics melt is injected into the partially open mould cavity, there is little back pressure encountered. This is of no consequence with a single

impression mould but in the case of a mould having several cavities a problem is encountered in metering equal doses to all the cavities. Conventionally, when one cavity of a multi-impression tool is full, the plastics material is automatically diverted to another cavity but with IIC no cavity is ever filled by the screw and the melt will continue to flow to the cavity offering least resistance.

To avoid this problem, it is possible to use a manifold as shown in Figure 5 that includes shooting pots. The manifold block 50 has a intake passage 52 that leads to the screw/barrel 30 and four discharge runner 54 that lead to four respective cavities, each incorporating a valve gate. A shooting pot 56 is associated with each cavity and acts as a reservoir to store the dose of plastics material required by the cavity.

Each shooting pot includes a piston 58 that has an adjustable limit position and forms a movable wall of the shooting pot 56. The working chamber of the shooting pot 56 is connected to the common intake passage 52 by a non-return valve 60 and to its respective cavity by way of its discharge runner 54 and valve gate 62.

In operation, when the plastics material is forced into the intake passage by advancing the screw in the barrel, the plastics material will flow past a non-return valve 60 to the shooting pot offering the least resistance. As the valve gate is closed, the working chamber of that shooting pot will fill with plastics material and force the piston 58 back until it reaches its end stop. At this point, the resistance from that pot will increase and the flow will be diverted to another pot. This is repeated until all four shooting pots are completely filled.

The manifold includes heating elements 64 to maintain the plastics material in a molten state until the time that

the plastics material dose stored in the pot is to be transferred to the associated mould cavity. This is carried out by moving the piston 58 back into the manifold 50 using a suitable actuator. The piston forces the plastics material out of the working chamber and at this time the valve gate 62 is opened to allow the displaced plastics material to flow through the runner 54 to the mould cavity. The nonreturn valve 60 prevents the plastics material from flowing back towards the barrel. In this way, each cavity receives the required dose stored in its shooting pot and the doses for the individual cavities can if necessary be adjusted individually.

Figures 6a and 6b show the manifold 50 fitted to a moulding machine used to implement the IIC method of the invention. In Figure 6a, the mould is fully open, the valve gates are closed and the shooting pots are full of molten plastics material.

In the top half of Figure 6b, the cavity is prestaged and the piston of the shooting pot is actuated to deliver a dose of plastics material to the cavity. Next, as shown in the lower half of Figure 6b, the valve gate is close and the mould is fully closed to spread the molten plastics material throughout the mould cavity. At the time, the shooting pot can be refilled by advancing the screw in its barrel to move the piston of the shooting back towards its limit position.

The adoption of a manifold containing individual shooting pots allows: a) improved and balanced filling of each cavity, b) optimised pressures per cavity, c) optimised cavity filling through closed-loop control of the rate of discharge of the shooting pot, d) recharge of the shooting pots can be by cyclic function of the conventional injection machine screw, and

e) recharge of the shooting pots can be by use of continuously rotating extruder screw (with by-pass loop) or cyclic function (without by-pass loop).

When combined with IIC the full benefits of thin wall moulding and wider range of materials become available together with improved product characteristics.

It is also practical to multi-feed a cavity or cavities and to time sequence the feeds to control filling of long or difficult component forms. This would currently'be achieved through the use of time sequenced multi valve-gate systems.

It is important to note that each cavity of a mould requires a specific machine clamp tonnage related directly to the plan area of the component (its projected area). The machine clamp tonnage for a multi-cavity tool is therefore the individual required tonnage multiplied by the number of cavities. The control of the shooting pots manifold is such that the four cavities of a four cavity tool could be filled sequentially and separately such that reduced tonnage could be used. This is also applicable through other multi-cavity lay-outs and also to stack tools.

Figure 6 also shows that the mould may include a spring biased containment ring 70 that closes the perimeter of each mould cavity in the prestaged position without hindering the full closing of the mould cavity. In this way, it is possible to contain the plastics material during the rapid compression and limit the extent that the dose of plastics material can spread radially away from its point of injection.

Claims

1. A method of moulding a plastics material in a mould cavity which relies primarily on a substantial movement of a part of the mould to provide the pressure necessary to force the plastics material melt to fill all the parts of the mould cavity.

2. A method as claimed in claim 1, wherein the movement of the mould parts serves to reduce the thickness of the plastics material injected into the mould by a factor of at least ten.

3. A method as claimed in claim 2, wherein the relative movement of the mould parts to close the cavity fully takes place in less than 0.5 sec. , preferably less than 0.3 sec.

4. A method of moulding a plastics article which comprises the steps of: providing a mould cavity between relatively movable parts of a mould, injecting into the mould prior to full closure of the mould cavity a predetermined dose of plastics material sufficient to form the finish formed article, the injected plastics material covering only a minor proportion of the mould cavity to a thickness substantially greater than that of the finish formed article; and fully closing the mould cavity by moving the mould parts relative to one another, the closing of the mould providing the necessary pressure to cause the plastics material to flow so as to cover the remaining area of the mould cavity and provide the article with its finish formed thickness.

5. A method as claimed in claim 4, which comprises the steps of partially closing the cavity during a first

stage to a predetermined separation between the parts of the mould, injecting the dose of plastics material while the two mould parts remain substantially stationary relative to one another and subsequently closing the cavity fully.

6. A method as claimed in claim 3, wherein the cavity is closed by a single continuous movement and the injection of the dose of plastics material is timed to be completed while the separation of the mould parts is still significantly greater than the thickness of the finish formed article.

7. A method as claimed in any one of claims 4 to 6, wherein the mould has an additional part, such as a movable core face or cavity bottom, for providing the closure to the final mould cavity thickness and wherein the two principal parts of the mould are fully closed by the injection moulding machine during the injection step and final closure is provided by movement of the additional part.

8. A method as claimed in any one of claims 4 to 6, wherein, the mould is a two part mould which is not fully closed for the injection step, the mould being held slightly open at the injection step and fully closed for the final closure.

9. A method as claimed in any of claims 4 to 8, wherein the injection moulding machine has a hydraulically actuated toggle closure mechanism.

10. A method as claimed in any of claims 4 to 9, wherein the injection machine has a mould with multiple cavities and is fitted with a hot-runner system that comprises a manifold incorporating a shooting pot for each of the cavities, each shooting pot being connected to a common pressurised supply of molten plastics material by way of a respective non-return valve and being connected to the associated cavity by way of a gate valve, the method comprising the steps of filling all the shooting pots with molten plastics material from the common pressurised supply and subsequently transferring the plastics material stored in each shooting pot to an associated cavity.